Saturday, May 16, 2009
18. CLEANING YOUR MOUSE
See the 'Easy Tips' section of the ComputerHelpAtoZ.com for complete instructions.
20. USE CUT, COPY AND PASTE TO MAKE FILE EDITING EASY
See the 'Easy Tips' section of the ComputerHelpAtoZ.com for complete instructions.
21. USE NOTEPAD TO CREATE SIMPLE TEXT FILES
Notepad is a limited word processor that comes with Windows. It allows you to create simple text documents with black only fonts. You can search for text strings in a document. All Notepad files have a text (.txt) file type.
To open Notepad, go to 'Start', then 'Programs' then 'Accessories'. Single click the Notepad icon and it opens with a blank document.
To open Notepad, go to 'Start', then 'Programs' then 'Accessories'. Single click the Notepad icon and it opens with a blank document.
22. USE WORDPAD TO CREATE SIMPLE WORD DOCUMENTS
Wordpad is a word processor that comes with Windows. It has all the features of Notepad plus more, like colored fonts, text alignment/formatting, and support for Word documents (.doc). A ruler is available to gauge text line lengths.
To open Wordpad, go to 'Start', then 'Programs' then 'Accessories' and single click the Wordpad Icon. Wordpad is a word processor that comes with Windows.
To open Wordpad, go to 'Start', then 'Programs' then 'Accessories' and single click the Wordpad Icon. Wordpad is a word processor that comes with Windows.
Tips 23-26 show how to change the view of running programs.
Tips 23-26 show how to change the view of running programs.
23. CONTROL THE VIEWS OF RUNNING PROGRAMS
Every running program is contained in a window on the desktop. In the upper right corner of each window there are usually three small control boxes that are used to control how the window is displayed.
24.. MINIMIZE A WINDOW
The left most control box has a minus [-] sign in it.
If you put the cursor over this box and click your mouse the application is 'minimized' and placed on the task bar as a small block.
To expand the minimized program, click it and it expands to become the active program on your desktop.
If you put the cursor over this box and click your mouse the application is 'minimized' and placed on the task bar as a small block.
To expand the minimized program, click it and it expands to become the active program on your desktop.
25. MAXIMIZE A WINDOW
The control box in the center has a square [ ] in it.
If you put the cursor over this box and click your mouse the application is 'maximized' and takes up the entire screen.
When maximized this control box changes to show one square in front of another square. Clicking this control box again restores the window back to about half size.
If you put the cursor over this box and click your mouse the application is 'maximized' and takes up the entire screen.
When maximized this control box changes to show one square in front of another square. Clicking this control box again restores the window back to about half size.
26. CLOSE AN ACTIVE WINDOW
The control box on the right has an [x] in it. If you
put the cursor over this box and click your mouse the application terminates.
To open it again you must click the program icon for it.
put the cursor over this box and click your mouse the application terminates.
To open it again you must click the program icon for it.
27. HOW TO ADD SHORTCUTS TO THE DESKTOP
A shortcut is an icon on the desktop that you click to open an application. This saves you from having to navigate the path to the actual location of the file. It's easy to add a shortcut:
* Right-click anywhere on the desktop. A pop-up menu appears. Select 'New', then 'Shortcut' and a 'Create Shortcut' window opens.
* Use the 'Browse' button to find the path to the application or program.
* Select the icon of the program you want and the path to it appears in the 'command line' box.
* Click 'next' and a 'Select a Title' box appears where you are asked to enter a name for the shortcut.
* Enter a name and then click 'finish'. The new shortcut appears on the desktop. Drag the shortcut icon to where you want it on the desktop.
* Right-click anywhere on the desktop. A pop-up menu appears. Select 'New', then 'Shortcut' and a 'Create Shortcut' window opens.
* Use the 'Browse' button to find the path to the application or program.
* Select the icon of the program you want and the path to it appears in the 'command line' box.
* Click 'next' and a 'Select a Title' box appears where you are asked to enter a name for the shortcut.
* Enter a name and then click 'finish'. The new shortcut appears on the desktop. Drag the shortcut icon to where you want it on the desktop.
28. USING THE CONTROL PANEL
The Control Panel is utility that lets you configure just about everything in your PC. To open the Control Panel:
* Go to 'Start', then 'Settings'.
* Select 'Control Panel' and the Control Panel Window appears on the desktop with many icons inside it.
* Go to 'Start', then 'Settings'.
* Select 'Control Panel' and the Control Panel Window appears on the desktop with many icons inside it.
28. USING THE CONTROL PANEL The Control Panel is utility that lets you configure just about everything in your PC. To open the Control Panel: *
28. USING THE CONTROL PANEL
The Control Panel is utility that lets you configure just about everything in your PC. To open the Control Panel:
* Go to 'Start', then 'Settings'.
* Select 'Control Panel' and the Control Panel Window appears on the desktop with many icons inside it.
The Control Panel is utility that lets you configure just about everything in your PC. To open the Control Panel:
* Go to 'Start', then 'Settings'.
* Select 'Control Panel' and the Control Panel Window appears on the desktop with many icons inside it.
29. ADD AND REMOVE INSTALLED PROGRAMS (Windows 98)
Double click the Add/Remove programs icon in the control panel and the 'Add/Remove Programs' window appears.
The top portion of the window allows you to install programs. When you install programs from a CD or floppy, the installation doesn't open the 'Add/Remove Programs' window and proceeds without it.
Removing Programs:
The bottom portion of the window shows all user installed programs loaded on the PC and allows you to remove them.
To remove a program, select it and then click the 'Add/Remove' button. You will be asked again if you want to proceed. Clicking 'yes' removes all the components of the selected program.
The top portion of the window allows you to install programs. When you install programs from a CD or floppy, the installation doesn't open the 'Add/Remove Programs' window and proceeds without it.
Removing Programs:
The bottom portion of the window shows all user installed programs loaded on the PC and allows you to remove them.
To remove a program, select it and then click the 'Add/Remove' button. You will be asked again if you want to proceed. Clicking 'yes' removes all the components of the selected program.
30. SET POWER USAGE OPTIONS
Double click the Power Options icon in the control panel and a window appears with a number of tabs across the top labeled:
Power Schemes, Alarms, Power Meter, Advanced and Hibernate. There may be variances in these options depending on which Windows operating system you have.
Power schemes is very useful, especially if you leave your PC on all the time. Double click on it to open it.
Two of the selections allow you to automatically turn off your monitor and hard drive(s) a selectable amount of time since the last keystroke or mouse movement. This saves your hard drive life by not having it continuously spin when it is not in use. You also save power by not having your display on when it is not in use.
To set the amount of time the hard drive runs before shutting down, select the time you want from the drop down menu next to 'Turn off hard disks'. The monitor shutdown time is set similarly in the 'Turn off monitor' drop down box.
The hard drive and monitor wake up when you initiate any activity that requires them.
Power Schemes, Alarms, Power Meter, Advanced and Hibernate. There may be variances in these options depending on which Windows operating system you have.
Power schemes is very useful, especially if you leave your PC on all the time. Double click on it to open it.
Two of the selections allow you to automatically turn off your monitor and hard drive(s) a selectable amount of time since the last keystroke or mouse movement. This saves your hard drive life by not having it continuously spin when it is not in use. You also save power by not having your display on when it is not in use.
To set the amount of time the hard drive runs before shutting down, select the time you want from the drop down menu next to 'Turn off hard disks'. The monitor shutdown time is set similarly in the 'Turn off monitor' drop down box.
The hard drive and monitor wake up when you initiate any activity that requires them.
31. SET YOUR MODEM'S MAX SPEED
* Double click the Modem icon in the control panel. The 'Modem Options' window appears. Two tabs should appear, 'General' and 'Diagnostics'.
* Select the 'General' tab, then select 'Properties'. which opens and has a 'Maximum Speed' box about half way down.
* Open the box and select 115,200, then 'OK'.
If you have a 56kb/s modem this setting won't hurt it, but a setting of 9600 could slow it down.
* Select the 'General' tab, then select 'Properties'. which opens and has a 'Maximum Speed' box about half way down.
* Open the box and select 115,200, then 'OK'.
If you have a 56kb/s modem this setting won't hurt it, but a setting of 9600 could slow it down.
32. ADD PROGRAM ICONS TO QUICK LAUNCH TOOLBAR
The Quick Launch toolbar can be opened on the task bar. It is used to hold the icons of programs you use most frequently. They are always visible and accessible on the toolbar.
To add a program icon from the desktop to the toolbar, drag it from the desktop to the end of the task bar near the 'Start' button. A copy of the program icon appears on the toolbar.
You can also add an icon by copying it to the Quick Launch folder which is found at:
C:\Windows\Application Data\Microsoft\Internet Explorer\Quick Launch.
To add a program icon from the desktop to the toolbar, drag it from the desktop to the end of the task bar near the 'Start' button. A copy of the program icon appears on the toolbar.
You can also add an icon by copying it to the Quick Launch folder which is found at:
C:\Windows\Application Data\Microsoft\Internet Explorer\Quick Launch.
33. ADD PROGRAM ICONS TO THE START MENU
When you click 'Start' there is an area above the 'Programs' folder where you can store program icons that you use fairly often. To add an icon here:
* Click 'Start', then 'Settings', then 'Taskbar and Start Menu'. The 'Taskbar and Start Menu' properties box opens.
* Click the 'advanced' tab, then select 'add'. A 'Create Shortcut' window opens where you select the path to the program icon.
* Select the icon and a 'Select Program' folder appears.
* Select the 'Start' folder, and a 'Select a Title' Folder appears where you can rename the icon if you wish.
* Click 'Finish' and the icon is added.
* Click 'Start', then 'Settings', then 'Taskbar and Start Menu'. The 'Taskbar and Start Menu' properties box opens.
* Click the 'advanced' tab, then select 'add'. A 'Create Shortcut' window opens where you select the path to the program icon.
* Select the icon and a 'Select Program' folder appears.
* Select the 'Start' folder, and a 'Select a Title' Folder appears where you can rename the icon if you wish.
* Click 'Finish' and the icon is added.
34. ADD WEB LINKS TO YOUR FAVORITES LIST
While viewing an Internet web page with your Internet Explorer browser press 'Alt' and 'D' together, and the link for the page is added to your favorites list.
If you get a message that says 'Are you sure you want to remove folder xxxx', it means you favorites window is active.
Click anywhere on the web page itself and try again. The web page link is stored even if your favorites list is not displayed.
To display your favorites, click the 'Favorites' button on the standard toolbar and they on the left side of your browser.
If you get a message that says 'Are you sure you want to remove folder xxxx', it means you favorites window is active.
Click anywhere on the web page itself and try again. The web page link is stored even if your favorites list is not displayed.
To display your favorites, click the 'Favorites' button on the standard toolbar and they on the left side of your browser.
35. PROTECT YOUR PC WITH A SURGE PROTECTOR
This is an area that is critical to the continued operation of your PC. Power surges and lightning strikes can sometimes get coupled onto your PC through the power or phone line. These surges can damage your PC.
Surge protected power strips have snubber circuits that filter out these potentially dangerous power line transients.
In addition to AC line protection many surge protectors also have protected phone line connectors. Use these to protect your PC's modem:
a) Plug a standard telephone cable between the phone wall connector and the 'In' connector on the surge protector.
b) Use a second cable to connect your modem to the 'Out' connector.
Your modem and PC are now protected from dangerous phone line transients.
Surge protectors with these features cost around $20 dollars and are well worth every bit of it!
Surge protected power strips have snubber circuits that filter out these potentially dangerous power line transients.
In addition to AC line protection many surge protectors also have protected phone line connectors. Use these to protect your PC's modem:
a) Plug a standard telephone cable between the phone wall connector and the 'In' connector on the surge protector.
b) Use a second cable to connect your modem to the 'Out' connector.
Your modem and PC are now protected from dangerous phone line transients.
Surge protectors with these features cost around $20 dollars and are well worth every bit of it!
36. DEFRAGMENT YOUR HARD DRIVE
A heavily fragmented drive can slow down your PC. Running this maintenance task can help keep your PC running at top speed.
See the 'Easy Tips' section of the ComputerHelpAtoZ.com for complete instructions.
See the 'Easy Tips' section of the ComputerHelpAtoZ.com for complete instructions.
37. HOW TO CLOSE RUNNING PROGRAMS (Windows 98)
Your PC can get hung up when applications get lost or stalled. With 'Close Program' you can usually identify and close the errant program.
All Windows operating systems beyond Windows 95 have a 'Close Program' box that lets you see all running programs. To close a running program:
* Open 'Close Program' by simultaneously pressing the 'Ctrl-Alt-Delete' keys. Usually the program that is hung up says (not responding) next to it.
* To close it or any other program, select it and then press the 'End Task' button.
If the program remains hung up, repeat these two steps until it closes. In some cases, depending on type of error, it is necessary to reset the PC.
All Windows operating systems beyond Windows 95 have a 'Close Program' box that lets you see all running programs. To close a running program:
* Open 'Close Program' by simultaneously pressing the 'Ctrl-Alt-Delete' keys. Usually the program that is hung up says (not responding) next to it.
* To close it or any other program, select it and then press the 'End Task' button.
If the program remains hung up, repeat these two steps until it closes. In some cases, depending on type of error, it is necessary to reset the PC.
38. ADJUST TEXT SIZE IN YOUR INTERNET EXPLORER
38. ADJUST TEXT SIZE IN YOUR INTERNET EXPLORER
When viewing web pages in your browser the text size may not be suitable for you. This is easily changed.
* Select 'View' on the menu bar found at the top of the browser. From the drop-down menu select 'Text Size' and another menu opens.
* This menu has five options: largest, larger,medium, smaller and smallest. One of them is identified as the current option.
* Select a size that moves the text size in the direction you desire and all body text on the current web page is accordingly adjusted.
Text that is explicitly set in the web page may not change size.
When viewing web pages in your browser the text size may not be suitable for you. This is easily changed.
* Select 'View' on the menu bar found at the top of the browser. From the drop-down menu select 'Text Size' and another menu opens.
* This menu has five options: largest, larger,medium, smaller and smallest. One of them is identified as the current option.
* Select a size that moves the text size in the direction you desire and all body text on the current web page is accordingly adjusted.
Text that is explicitly set in the web page may not change size.
Internal storage
Hardware that keeps data inside the computer for later use and remains persistent even when the computer has no power.
* Hard disk - for medium-term storage of data.
* Solid-state drive - a device similar to hard disk, but containing no moving parts and stores data in a digital format.
* RAID array controller - a device to manage several internal or external hard disks and optionally some peripherals in order to achieve performance or reliability improvement in what is called a RAID array.
[edit] Sound card
Main article: Sound card
Enables the computer to output sound to audio devices, as well as accept input from a microphone. Most modern computers have sound cards built-in to the motherboard, though it is common for a user to install a separate sound card as an upgrade. Most sound cards, either built-in or added, have surround sound capabilities.
* Hard disk - for medium-term storage of data.
* Solid-state drive - a device similar to hard disk, but containing no moving parts and stores data in a digital format.
* RAID array controller - a device to manage several internal or external hard disks and optionally some peripherals in order to achieve performance or reliability improvement in what is called a RAID array.
[edit] Sound card
Main article: Sound card
Enables the computer to output sound to audio devices, as well as accept input from a microphone. Most modern computers have sound cards built-in to the motherboard, though it is common for a user to install a separate sound card as an upgrade. Most sound cards, either built-in or added, have surround sound capabilities.
Power supply
Main article: Power supply unit (computer)
Includes power cord, switch, and cooling fan. Supplies power at appropriate voltages to the motherboard and internal disk drives.
[edit] Video display controller
Main article: Graphics card
Produces the output for the visual display unit. This will either be built into the motherboard or attached in its own separate slot (PCI, PCI-E, PCI-E 2.0, or AGP), in the form of a graphics card.
[edit] Removable media devices
Main article: Computer storage
* CD (compact disc) - the most common type of removable media, suitable for music and data.
o CD-ROM Drive - a device used for reading data from a CD.
o CD Writer - a device used for both reading and writing data to and from a CD.
* DVD (digital versatile disc) - a popular type of removable media that is the same dimensions as a CD but stores up to 12 times as much information. It is the most common way of transferring digital video, and is popular for data storage.
o DVD-ROM Drive - a device used for reading data from a DVD.
o DVD Writer - a device used for both reading and writing data to and from a DVD.
o DVD-RAM Drive - a device used for rapid writing and reading of data from a special type of DVD.
* Blu-ray Disc - a high-density optical disc format for data and high-definition video. Can store 70 times as much information as a CD.
o BD-ROM Drive - a device used for reading data from a Blu-ray disc.
o BD Writer - a device used for both reading and writing data to and from a Blu-ray disc.
* HD DVD - a discontinued competitor to the Blu-ray format.
* Floppy disk - an outdated storage device consisting of a thin disk of a flexible magnetic storage medium. Used today mainly for loading RAID drivers.
* Zip drive - an outdated medium-capacity removable disk storage system, first introduced by Iomega in 1994.
* USB flash drive - a flash memory data storage device integrated with a USB interface, typically small, lightweight, removable, and rewritable. Capacities vary, from hundreds of megabytes (in the same ballpark as CDs) to tens of gigabytes (surpassing, at great expense, Blu-ray discs).
* Tape drive - a device that reads and writes data on a magnetic tape, used for long term storage and backups.
Includes power cord, switch, and cooling fan. Supplies power at appropriate voltages to the motherboard and internal disk drives.
[edit] Video display controller
Main article: Graphics card
Produces the output for the visual display unit. This will either be built into the motherboard or attached in its own separate slot (PCI, PCI-E, PCI-E 2.0, or AGP), in the form of a graphics card.
[edit] Removable media devices
Main article: Computer storage
* CD (compact disc) - the most common type of removable media, suitable for music and data.
o CD-ROM Drive - a device used for reading data from a CD.
o CD Writer - a device used for both reading and writing data to and from a CD.
* DVD (digital versatile disc) - a popular type of removable media that is the same dimensions as a CD but stores up to 12 times as much information. It is the most common way of transferring digital video, and is popular for data storage.
o DVD-ROM Drive - a device used for reading data from a DVD.
o DVD Writer - a device used for both reading and writing data to and from a DVD.
o DVD-RAM Drive - a device used for rapid writing and reading of data from a special type of DVD.
* Blu-ray Disc - a high-density optical disc format for data and high-definition video. Can store 70 times as much information as a CD.
o BD-ROM Drive - a device used for reading data from a Blu-ray disc.
o BD Writer - a device used for both reading and writing data to and from a Blu-ray disc.
* HD DVD - a discontinued competitor to the Blu-ray format.
* Floppy disk - an outdated storage device consisting of a thin disk of a flexible magnetic storage medium. Used today mainly for loading RAID drivers.
* Zip drive - an outdated medium-capacity removable disk storage system, first introduced by Iomega in 1994.
* USB flash drive - a flash memory data storage device integrated with a USB interface, typically small, lightweight, removable, and rewritable. Capacities vary, from hundreds of megabytes (in the same ballpark as CDs) to tens of gigabytes (surpassing, at great expense, Blu-ray discs).
* Tape drive - a device that reads and writes data on a magnetic tape, used for long term storage and backups.
Motherboard
Motherboard
The motherboard is the "body"[citation needed] of the computer. Components directly attached to the motherboard include:
* The central processing unit (CPU) performs most of the calculations which enable a computer to function, and is sometimes referred to as the "brain" of the computer. It is usually cooled by a heat sink and fan.
* The chipset mediates communication between the CPU and the other components of the system, including main memory.
* RAM Stores all running processes (applications) and the current running OS. RAM Stands for Random Access Memory
* The BIOS includes boot firmware and power management. The Basic Input Output System tasks are handled by operating system drivers.
* Internal Buses connect the CPU to various internal components and to expansion cards for graphics and sound.
o Current
+ The northbridge memory controller, for RAM and PCI Express
# PCI Express, for graphics cards
+ PCI, for other expansion cards
+ SATA, for disk drives
o Obsolete
+ ATA (superseded by SATA)
+ AGP (superseded by PCI Express)
+ VLB VESA Local Bus (superseded by AGP)
+ ISA (expansion card slot format obsolete in PCs, but still used in industrial computers)
* External Bus Controllers support ports for external peripherals. These ports may be controlled directly by the southbridge I/O controller or based on expansion cards attached to the motherboard through the PCI bus.
o USB
o FireWire
o eSATA
The motherboard is the "body"[citation needed] of the computer. Components directly attached to the motherboard include:
* The central processing unit (CPU) performs most of the calculations which enable a computer to function, and is sometimes referred to as the "brain" of the computer. It is usually cooled by a heat sink and fan.
* The chipset mediates communication between the CPU and the other components of the system, including main memory.
* RAM Stores all running processes (applications) and the current running OS. RAM Stands for Random Access Memory
* The BIOS includes boot firmware and power management. The Basic Input Output System tasks are handled by operating system drivers.
* Internal Buses connect the CPU to various internal components and to expansion cards for graphics and sound.
o Current
+ The northbridge memory controller, for RAM and PCI Express
# PCI Express, for graphics cards
+ PCI, for other expansion cards
+ SATA, for disk drives
o Obsolete
+ ATA (superseded by SATA)
+ AGP (superseded by PCI Express)
+ VLB VESA Local Bus (superseded by AGP)
+ ISA (expansion card slot format obsolete in PCs, but still used in industrial computers)
* External Bus Controllers support ports for external peripherals. These ports may be controlled directly by the southbridge I/O controller or based on expansion cards attached to the motherboard through the PCI bus.
o USB
o FireWire
o eSATA
PC Case by John Anthony
PC Case
by John Anthony
The PC case is a thin sheet metal enclosure that houses the motherboard, power supply and various drives (HDD, FDD, CD, DVD).
* Cases are offered in two styles, desktop and tower. Today the tower type is predominant. It stands upright and is much taller than it is wide. It is usually placed on the floor next to, or under a desk. The desktop has a pizza box profile and usually sits on the desktop.
* Tower cases are offered in two basic sizes, one that can fit ATX (12" wide) motherboards and one that can accommodate ATX mini (8.5" wide) motherboards. The number of drive bays offered also varies depending on manufacturer.
* The motherboard and power supply mount to the floor at the rear of the case. The drives (hard, floppy and CD/DVD) mount in enclosures called drive bays at the front of the case.
* Cases run from $20 to $70 depending on size (ATX or ATX mini), number of drive bays and the wattage of the power supply.
by John Anthony
The PC case is a thin sheet metal enclosure that houses the motherboard, power supply and various drives (HDD, FDD, CD, DVD).
* Cases are offered in two styles, desktop and tower. Today the tower type is predominant. It stands upright and is much taller than it is wide. It is usually placed on the floor next to, or under a desk. The desktop has a pizza box profile and usually sits on the desktop.
* Tower cases are offered in two basic sizes, one that can fit ATX (12" wide) motherboards and one that can accommodate ATX mini (8.5" wide) motherboards. The number of drive bays offered also varies depending on manufacturer.
* The motherboard and power supply mount to the floor at the rear of the case. The drives (hard, floppy and CD/DVD) mount in enclosures called drive bays at the front of the case.
* Cases run from $20 to $70 depending on size (ATX or ATX mini), number of drive bays and the wattage of the power supply.
Monday, March 9, 2009
ROM Works
From How ROM Works
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Read-only memory is essential to your computer. You'll also find ROM in video games, cell phones and microwaves. Learn about the different types.. Similar to RAM, ROM chips (Figure 1) contain a grid of columns and rows. But where the columns and rows intersect, ROM chips are fundamentally...
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* > Memory
ROM Types
From How ROM Works
Read-only memory is essential to your computer. You'll also find ROM in video games, cell phones and microwaves. Learn about the different types.. There are five basic ROM types: ROM PROM EPROM EEPROM Flash memory Each type has unique characteristics, which you'll learn about in this article...
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* > Memory
Introduction to How ROM Works
Read-only memory is essential to your computer. You'll also find ROM in video games, cell phones and microwaves. Learn about the different types.. Computer Memory Image Gallery2008 HowStuffWorksROM is used in most electronic devices. See more computer memory pictures.Read-only memory (ROM...
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PROM
From How ROM Works
Read-only memory is essential to your computer. You'll also find ROM in video games, cell phones and microwaves. Learn about the different types.. Creating ROM chips totally from scratch is time-consuming and very expensive in small quantities. For this reason, mainly, developers created a type...
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* > Memory
EEPROMs and Flash Memory
From How ROM Works
Read-only memory is essential to your computer. You'll also find ROM in video games, cell phones and microwaves. Learn about the different types.. byte at a time. See How Flash Memory Works to learn more about this type of ROM and its applications. For more information on ROM and other types...
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EPROM
From How ROM Works
Read-only memory is essential to your computer. You'll also find ROM in video games, cell phones and microwaves. Learn about the different types.. Working with ROMs and PROMs can be a wasteful business. Even though they are inexpensive per chip, the cost can add up over time. Erasable...
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Installing the Hard Drive
From How to Build a Computer
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The hard drive sits in a bracket and is connected directly to the power supply. Find out how to install a hard drive and learn where the CD-ROM drive.. The last steps are installing the hard drive and the CD-ROM drive. The case has a removable bracket with four rubber grommets on it, which line...
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* > Desktops
Microprocessor Instructions
From How Microprocessors Work
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that RAM starts at address 128 in this processor, and ROM (which contains the assembly language program) starts at address 0, then for our simple...
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Why can I play some DVD movies, but not others, on my computer DVD-ROM?
* Images
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Why can I play some DVD movies, but not others, on my computer DVD-ROM?.. player or DVD-ROM drive sold in the country where the DVD movie was sold. The actual region code is in one byte on the DVD (digital versatile disc...
* Computer
* > CD/DVD Drives & Burners
Microprocessor Memory
From How Microprocessors Work
Microprocessor memory helps with the microprocessor's basic functions. Learn about microprocessor memory, ROM, BIOS and what the boot sector is...The previous section talked about the address and data buses, as well as the RD and WR lines. These buses and lines connect either to RAM or ROM...
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RAM WORKING
From How RAM Works
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Types of RAM have changed tremendously over time. Learn about the types of RAM and what kind of RAM your computer needs... The following are some common types of RAM: SRAM: Static random access memory uses multiple transistors, typically four to six, for each memory...
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How to Install RAM
From How RAM Works
A RAM upgrade can greatly extend your computer's lifespan. Learn how to install RAM and find the best prices on RAM upgrades...Most of the time, installing RAM is a very simple and straightforward procedure. The key is to do your research. Here's what you need to know...
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Static RAM
From How RAM Works
Static RAM offers premium performance at a premium price. Learn about static RAM and memory modules and printed circuit boards... Static RAM uses a completely different technology. In static RAM, a form of flip-flop holds each bit of memory (see How Boolean Logic Works...
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RAM Quiz
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With the RAM Quiz on HowStuffWorks, see how much you know about this type of computer memory. Explore RAM technology and applications with the RAM..You've probably heard about RAM memory when people talk about computers -- how much RAM a desktop might need, whether or not someone is adding more...
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Introduction to How RAM Works
RAM is the best known form of computer memory and easy to upgrade. Learn how RAM works, what kind of RAM to buy, and how to install it... Computer Hardware Image GalleryRAM is the best known form of computer memory and easy to upgrade. See more computer hardware pictures.Random access...
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How Much RAM Do You Need?
From How RAM Works
How much RAM is enough? Find out much RAM you need for any task, learn about the types of RAM and find the best prices for RAM upgrades...It's been said that you can never have enough money, and the same holds true for RAM, especially if you do a lot of graphics-intensive work...
* Computer
* > Memory
Memory Cells and DRAM
From How RAM Works
that include identifying the type, speed and amount of memory and checking for errors.Static RAM works differently from DRAM. We'll look at how in the next...
* Computer
* > Memory
Memory Modules
From How RAM Works
The type of board and connector used for RAM in desktop computers has evolved over the past few years. The first types were proprietary, meaning...
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* > Memory
Choosing RAM
From How to Add RAM to Your Laptop
* Images
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When choosing RAM for your laptop, you should take into account a few factors. Get advice for choosing RAM for your laptop... � iStockphoto.com/ilmwa555Everyone has their own needs for their laptop, so what should you look for when buying RAM?When most people refer to a...
* Computer
* > Memory
Adding RAM
From How to Add RAM to Your Laptop
Adding RAM to your laptop isn't as hard as you might think. See these tips and instructions for adding RAM to your laptop... Once you've purchased the necessary RAM module, you're ready to add more memory to your computer. Before you start anything, make sure the laptop...
* Computer
* > Memory
Memory hierarchy of RAM
Many computer systems have a memory hierarchy consisting of CPU registers, on-die SRAM caches, external caches, DRAM, paging systems, and virtual memory or swap space on a hard drive. This entire pool of memory may be referred to as "RAM" by many developers, even though the various subsystems can have very different access times, violating the original concept behind the random access term in RAM. Even within a hierarchy level such as DRAM, the specific row, column, bank, rank, channel, or interleave organization of the components make the access time variable, although not to the extent that rotating storage media or a tape is variable. The overall goal of using a memory hierarchy is to obtain the higher possible average access performance while minimizing the total cost of entire memory system. (Generally, the memory hierarchy follows the access time with the fast CPU registers at the top and the slow hard drive at the bottom.)
In many modern personal computers, the RAM comes in an easily upgraded form of modules called memory modules or DRAM modules about the size of a few sticks of chewing gum. These can quickly be replaced should they become damaged or too small for current purposes. As suggested above, smaller amounts of RAM (mostly SRAM) are also integrated in the CPU and other ICs on the motherboard, as well as in hard-drives, CD-ROMs, and several other parts of the computer system.
Did You Know . . . ?
# Social Media: Tips for Small Business Marketing
The Internet
* Social Media: Tips for Small Business Marketing
* Traceroute and Ping: What's the Difference?
* The Difference Between a Virus, Worm and Trojan Horse
* SSL Basics and Beyond
* Ad Network Primer: Tips for Targeted Campaigns
* Five SEO Tips for Driving Search Engine Traffic
* The Difference Between LinkedIn and Facebook
* Antivirus and Antispyware Software: What's The Difference?
* Laptops and Notebooks — What's the Difference?
* The Difference Between VoIP and PSTN Systems
* All About Online Forums
* How to Deal With Pop-Ups
* What Do Temporary Internet Files Do?
* 8 Terms to Watch in 2008
* What Is The Semantic Web?
* What Is a Widget
* Understanding and Resolving IP Address Conflicts
* What Makes A Virtual Private Network Private?
* All About Cookies and You
* How Affiliate Marketing Works
* Inside Rouge Wireless Device Problems
* How to Send Big Files Over The Internet
* Defend Yourself Against Identity Theft
* All About Spam, Spim and Spit
* What Can VoIP Do for You?
* Understanding Internet Governance
* Internet Grammar Lessons
* How Web Search Engines Work
* VoIP and the Small Business
* Understanding Denial of Service Attacks
* A Guide to Internet-based Communications
* SSL: Your Key to E-commerce Security
* Cable vs. DSL
* All About Peer-To-Peer Architecture
* All About Multiplayer PC Gaming
* All About Web Site Hosting
* VoIP Meets WiFi
* All About Phishing
* Is Your Child's Information Protected Online?
* When Is Downloading Music on the Internet Illegal?
* The Difference Between Adware & Spyware
* Are Cable Internet Connections Vulnerable to Hackers?
* The Birth of the Internet
* Choosing an ISP
* Do Cookies Compromise Security?
* Getting Rid of Spam
* The History of the @ Sign
* How Can I Be Spamming Myself?
* How Web Servers Work
* Internet Grammar Concerns
* JPG vs. GIF vs. PNG
* Registering a Domain Name
* Understanding IP Addressing
* Who Owns the Internet?
* Why E-Mails Bounce
Hardware/Software
* Netbooks: A Growing Trend — With Issues
* Trace the Five Generations of Computers
* How to Format a Hard Disk Drive
* iPhone and iPhone 3G — What's The Difference?
* All About Open Source
* Tips for Using Fonts
* Smartphone, Cell Phone, PDA: What's the Difference?
* DVD Formats Explained
* iPod vs. MP3 Players: What's the Diff?
* How Powerline Networking Works
* Preventative Maintenance Tips for Your PC
* The Differences Between Hubs, Switches and Routers
* All About Software
* Thick, Thin & Smart: An overview of client applications
* The Differences Between Thick and Thin Clients
* What Makes a Server a 'Storage Server'?
* Understanding Storage
* Wireless Tips for Road Warriors
* Understanding PC Buses
* AMD & Intel Processor Lineups
* DVD Formats Explained
* How to Choose a Color Printer
* Understanding Video and Graphics Adapters
* What Does Your Windows System Registry Do?
* All About Dual-Core Processors
* From AT to BTX: Motherboard Form Factors
* All About DirectX
* What Is a Router?
* All About Monitors: CRT vs. LCD
* What Is a Server?
* The Differences and Features of Hardware & Software Firewalls?
* Is Software Ownership the Same as Licensing?
* Are Deleted Files Completely Erased?
* Northern vs. Southern Hemisphere Monitors
* The System Boot Process Explained
* Understanding CD Burner Speeds
* What's Inside a Hard Drive?
Computer Science
* The Difference Between Laptop and Desktop Memory
* How to Network Your Files With NFS
* Getting Started With Green Computing
* How Wireless Networks Work
* SDLC — Understanding Software Development Lifecycle
* How to Defrag a Hard Drive
* Data Backup Basics
* What Is Enterprise Search?
* Different Types of Memory Cards
* Understanding LAMP and Its Effect on Web Development
* Metadata: Data About Data
* Storage Virtualization: Myths, Realities and Other Considerations
* The Differences Between WEP and WPA
* The Difference Between SCSI, ATA, SAS and SATA
* Understanding Hardware-Assisted Virtualization
* How to Completely Erase a Hard Disk Drive
* Ten Terms to Watch in 2007
* Understanding IPv6
* All About Network Access Controls
* Understanding NAT and PAT
* iSCSI vs. Fibre Channel
* Understanding Rootkits
* The Benefits of Tiered Storage
* Understanding OSPF Routing
* What is a Virtual Tape Library
* Understanding Routers
* What Is Perpendicular Hard Drive Technology?
* Understanding TCP, the Protocol
* Top 10 Terms to Know for the New Year
* In the OSI Model — Understanding the Data Layer
* The OSI Reference Model — Understanding Layers
* How Biometrics Security Works
* What's the Best Network Operating System?
* The Many Flavors of OFDMA
* How Surround Sound Works
* All About Digital TV — DTV and HDTV
* Deciphering the Latest Wireless Acronyms
* Understanding Web Services
* Digital Audio Formats
* What Is CPU Overclocking?
* What Is USB?
* Pardon the Intrusion: All About IPS & IDS
* All About PDF Technology
* Tech Terms That Are Strange but True
* All About RFID
* What Is 64-bit Computing?
* How Fingerprint Scanners Work
* Is It Possible to Eavesdrop on Electromagnetic Radiation?
* The Science of Color
* Static Electricity and Computers
The Internet
* Social Media: Tips for Small Business Marketing
* Traceroute and Ping: What's the Difference?
* The Difference Between a Virus, Worm and Trojan Horse
* SSL Basics and Beyond
* Ad Network Primer: Tips for Targeted Campaigns
* Five SEO Tips for Driving Search Engine Traffic
* The Difference Between LinkedIn and Facebook
* Antivirus and Antispyware Software: What's The Difference?
* Laptops and Notebooks — What's the Difference?
* The Difference Between VoIP and PSTN Systems
* All About Online Forums
* How to Deal With Pop-Ups
* What Do Temporary Internet Files Do?
* 8 Terms to Watch in 2008
* What Is The Semantic Web?
* What Is a Widget
* Understanding and Resolving IP Address Conflicts
* What Makes A Virtual Private Network Private?
* All About Cookies and You
* How Affiliate Marketing Works
* Inside Rouge Wireless Device Problems
* How to Send Big Files Over The Internet
* Defend Yourself Against Identity Theft
* All About Spam, Spim and Spit
* What Can VoIP Do for You?
* Understanding Internet Governance
* Internet Grammar Lessons
* How Web Search Engines Work
* VoIP and the Small Business
* Understanding Denial of Service Attacks
* A Guide to Internet-based Communications
* SSL: Your Key to E-commerce Security
* Cable vs. DSL
* All About Peer-To-Peer Architecture
* All About Multiplayer PC Gaming
* All About Web Site Hosting
* VoIP Meets WiFi
* All About Phishing
* Is Your Child's Information Protected Online?
* When Is Downloading Music on the Internet Illegal?
* The Difference Between Adware & Spyware
* Are Cable Internet Connections Vulnerable to Hackers?
* The Birth of the Internet
* Choosing an ISP
* Do Cookies Compromise Security?
* Getting Rid of Spam
* The History of the @ Sign
* How Can I Be Spamming Myself?
* How Web Servers Work
* Internet Grammar Concerns
* JPG vs. GIF vs. PNG
* Registering a Domain Name
* Understanding IP Addressing
* Who Owns the Internet?
* Why E-Mails Bounce
Hardware/Software
* Netbooks: A Growing Trend — With Issues
* Trace the Five Generations of Computers
* How to Format a Hard Disk Drive
* iPhone and iPhone 3G — What's The Difference?
* All About Open Source
* Tips for Using Fonts
* Smartphone, Cell Phone, PDA: What's the Difference?
* DVD Formats Explained
* iPod vs. MP3 Players: What's the Diff?
* How Powerline Networking Works
* Preventative Maintenance Tips for Your PC
* The Differences Between Hubs, Switches and Routers
* All About Software
* Thick, Thin & Smart: An overview of client applications
* The Differences Between Thick and Thin Clients
* What Makes a Server a 'Storage Server'?
* Understanding Storage
* Wireless Tips for Road Warriors
* Understanding PC Buses
* AMD & Intel Processor Lineups
* DVD Formats Explained
* How to Choose a Color Printer
* Understanding Video and Graphics Adapters
* What Does Your Windows System Registry Do?
* All About Dual-Core Processors
* From AT to BTX: Motherboard Form Factors
* All About DirectX
* What Is a Router?
* All About Monitors: CRT vs. LCD
* What Is a Server?
* The Differences and Features of Hardware & Software Firewalls?
* Is Software Ownership the Same as Licensing?
* Are Deleted Files Completely Erased?
* Northern vs. Southern Hemisphere Monitors
* The System Boot Process Explained
* Understanding CD Burner Speeds
* What's Inside a Hard Drive?
Computer Science
* The Difference Between Laptop and Desktop Memory
* How to Network Your Files With NFS
* Getting Started With Green Computing
* How Wireless Networks Work
* SDLC — Understanding Software Development Lifecycle
* How to Defrag a Hard Drive
* Data Backup Basics
* What Is Enterprise Search?
* Different Types of Memory Cards
* Understanding LAMP and Its Effect on Web Development
* Metadata: Data About Data
* Storage Virtualization: Myths, Realities and Other Considerations
* The Differences Between WEP and WPA
* The Difference Between SCSI, ATA, SAS and SATA
* Understanding Hardware-Assisted Virtualization
* How to Completely Erase a Hard Disk Drive
* Ten Terms to Watch in 2007
* Understanding IPv6
* All About Network Access Controls
* Understanding NAT and PAT
* iSCSI vs. Fibre Channel
* Understanding Rootkits
* The Benefits of Tiered Storage
* Understanding OSPF Routing
* What is a Virtual Tape Library
* Understanding Routers
* What Is Perpendicular Hard Drive Technology?
* Understanding TCP, the Protocol
* Top 10 Terms to Know for the New Year
* In the OSI Model — Understanding the Data Layer
* The OSI Reference Model — Understanding Layers
* How Biometrics Security Works
* What's the Best Network Operating System?
* The Many Flavors of OFDMA
* How Surround Sound Works
* All About Digital TV — DTV and HDTV
* Deciphering the Latest Wireless Acronyms
* Understanding Web Services
* Digital Audio Formats
* What Is CPU Overclocking?
* What Is USB?
* Pardon the Intrusion: All About IPS & IDS
* All About PDF Technology
* Tech Terms That Are Strange but True
* All About RFID
* What Is 64-bit Computing?
* How Fingerprint Scanners Work
* Is It Possible to Eavesdrop on Electromagnetic Radiation?
* The Science of Color
* Static Electricity and Computers
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* Modems
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Data
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* Data Formats
* Data Management
* Data Sizes
* Data Structures
* Databases
* Encryption
* Error Correction
* Special Characters
Graphics
* 3-D Graphics
* Animation
* Business Presentation
* Digital Cameras
* Digital Photography
* Color
* Graphics Software
* Graphics Standards
Hardware
* Adapters/Cards
* Addressing
* Buses
* Connectors, Plugs and Sockets
* Data Storage
* Hardware Companies
* Input Devices
* Integrated Circuits (ICs)
* Keyboards
* Memory
* Mice
* Microprocessors
* Modems
* Monitors
* Motherboards
* Networking Hardware
* Output Devices
* Performance
* Peripheral Devices
* Ports and Interfaces
* Printers
Internet
* Chat
* Convergence
* Domain Name System
* eBay and Auction Alternatives
* Electronic Commerce
* Electronic Mail
* File Transfer
* Internet and Online Services
* Internet Telephony
* Newsgroups
* Online Advertising
* Online Selling
* Online Services
* Security
* Slang
* Videoconferencing
* Web 2.0
* World Wide Web
Mobile Computing
* Flat-Panel displays
* Handhelds
* Modems
* Notebook Computers
* Wireless Computing
Multimedia
* Animation
* Audio
* CD-ROMs
* Computer Entertainment
* Consumer Electronics
* Convergence
* Data Compression
* Digital Television
* DVD
* Gaming
* Home Entertainment
* Hypermedia
* Video
* Videoconferencing
* Virtual Reality
Networks
* Data Transfer Rates
* Directory Services
* Electronic Mail
* Ethernet
* File Transfer
* Groupware
* Internet Access
* Network Interface Cards (NICs)
* Network Management
* Network Protocols
* Network Topologies
* Networking Companies
* Networking Hardware
* Networking Standards
* Routing
* Security
* Videoconferencing
Open Source (OSRC)
* OSRC Databases
* OSRC Initiatives
* OSRC Operating Systems
* OSRC Platforms
* OSRC Programming Languages
* OSRC Servers
Operating Systems
* DOS
* File Management
* Mac OS
* OSRC Operating Systems
* UNIX
* User Interfaces
* Virtual Memory
* Windows
Programming
* Addressing
* Application Failures
* Application Programming Interfaces (APIs)
* Aspect-oriented Programming
* Compiling, Binding and Linking
* Data Structures
* Expressions
* Object-Oriented Programming
* Operators and Operations
* Procedures, Functions and Routines
* Programming Languages
* Programming Tools
* Vocations
* Web Development
* XML
Software
* Applications
* Browsers
* Business Presentation
* Communications Software
* Databases
* Desktop Publishing
* Graphics Software
* Groupware
* Networking Software
* Software Companies
* Spreadsheets
* User Interfaces
* Word Processing
Standards
* Communications Standards
* Data Formats
* Display Standards
* Formatting Standards
* Graphics Standards
* Miscellaneous Standards
* Network Protocols
* Networking Standards
* Standards Organizations
* Video Formats
Types of Computers
* Macintosh
* Notebook Computers
* PCs
* Servers
* Supercomputers
* Workstations
World Wide Web
* Browsers
* HTML
* Markup Languages
* Search Engines
* Social Media
* Web Development
How many processors do you need for a Snipe Hunt?
Posted by Cameron Hughes, Tracey Hughes 11:53 PM | February 16, 2009
Snipe hunts, fool's gold, solutions to AI-complete problems, I've been victimized by them all. Although I have gotten over the snipe hunts and fool's gold, I haven't been able to quite shake the AI-complete part yet. Why can't I just throw more processors at the problem? Why can't the fastest beat the hardest? Oops, let me regress for a moment. I said I would have to stir up what I meant by AI-complete, so first let me do so
Snipe hunts, fool's gold, solutions to AI-complete problems, I've been victimized by them all. Although I have gotten over the snipe hunts and fool's gold, I haven't been able to quite shake the AI-complete part yet. Why can't I just throw more processors at the problem? Why can't the fastest beat the hardest? Oops, let me regress for a moment. I said I would have to stir up what I meant by AI-complete, so first let me do so
Security Reminder, or Who's That Looking Over Your Shoulder?
Security Reminder, or Who's That Looking Over Your Shoulder?
Posted by Jon Erickson 05:56 PM | February 25, 2009
It doesn't matter which coffee shop -- Henry's, Z's, or the Bourgeois Pig -- I end up at, it's always the same ever since free WiFi came on the scene. There are more laptops than coffee mugs, and never enough places to sit. But I'm as bad as the rest. What with free wireless Internet access and a cell phone, well, I haven't been in the office for year or so now, and even the concept of a home-office is becoming more remote.
Posted by Jon Erickson 05:56 PM | February 25, 2009
It doesn't matter which coffee shop -- Henry's, Z's, or the Bourgeois Pig -- I end up at, it's always the same ever since free WiFi came on the scene. There are more laptops than coffee mugs, and never enough places to sit. But I'm as bad as the rest. What with free wireless Internet access and a cell phone, well, I haven't been in the office for year or so now, and even the concept of a home-office is becoming more remote.
When Seeing Is Believing
Posted by Jon Erickson 10:26 AM | February 26, 2009
So you aren't ready to buy into this multicore and parallelization stuff until you're able to see its benefits with your own eyes? If that's the case, then take a couple of minutes to watch this pair of YouTube videos that Gaston Hillar has prepared as part of his book C# 2008 and 2005 Threaded Programming: Beginner's Guide.
So you aren't ready to buy into this multicore and parallelization stuff until you're able to see its benefits with your own eyes? If that's the case, then take a couple of minutes to watch this pair of YouTube videos that Gaston Hillar has prepared as part of his book C# 2008 and 2005 Threaded Programming: Beginner's Guide.
Posted by Jack Woehr
Open Sourcer Explores Intel MulticoreTools on Windows, Barely Lives to Tell Tale
An inveterate open-sourcer, I'm making an uneasy peace with Microsoft Windows for the purpose of exploring the tools Intel has made available for parallelization of applications running on multicore platforms. I have used and verbally abused Windows in every release since Windows 2.1. Thus I am no stranger, but Windows has not been a C/C++ development environment for me since the 1990's, though I test and run my Java stuff on Windows.
So, d'oh, of course I'm looking for the simplest way to do this ...
An inveterate open-sourcer, I'm making an uneasy peace with Microsoft Windows for the purpose of exploring the tools Intel has made available for parallelization of applications running on multicore platforms. I have used and verbally abused Windows in every release since Windows 2.1. Thus I am no stranger, but Windows has not been a C/C++ development environment for me since the 1990's, though I test and run my Java stuff on Windows.
So, d'oh, of course I'm looking for the simplest way to do this ...
Going Parallell
Part 2: So who's really writing parallel applications?
Posted by Stephen Blair-chappell 05:20 AM | March 4, 2009
In a crazy moment sometime ago I forked out a few UK Pounds registering a url how-parallel-is-your-software.com (not the real address). The plan was to use the site to let people register any applications they found that ran parallel. Perhaps even run a competition to see who could find the most parallel commercial application. Maybe this would be the route to me becoming the next dotcom millionaire! I never had the nerve to bring the idea to life. I suspect that the moment I published anything every lawyer in town would be knocking at my door.
Posted by Stephen Blair-chappell 05:20 AM | March 4, 2009
In a crazy moment sometime ago I forked out a few UK Pounds registering a url how-parallel-is-your-software.com (not the real address). The plan was to use the site to let people register any applications they found that ran parallel. Perhaps even run a competition to see who could find the most parallel commercial application. Maybe this would be the route to me becoming the next dotcom millionaire! I never had the nerve to bring the idea to life. I suspect that the moment I published anything every lawyer in town would be knocking at my door.
search Space
search Space ... the final frontier
Posted by Cameron Hughes, Tracey Hughes 02:56 PM | March 4, 2009
NP/NP complete and AI-complete problems are problems with huge or even infinite search or state spaces. The search or state space is a graph (or other representation) that contains all of the possible states (including the initial and goal states of the problem) of the domain of the initial problem. An example of a huge search space is all the nodes on the Internet ...
Posted by Cameron Hughes, Tracey Hughes 02:56 PM | March 4, 2009
NP/NP complete and AI-complete problems are problems with huge or even infinite search or state spaces. The search or state space is a graph (or other representation) that contains all of the possible states (including the initial and goal states of the problem) of the domain of the initial problem. An example of a huge search space is all the nodes on the Internet ...
Multicore and Power Consumption:
Ask James About It
Posted by Jon Erickson 01:52 PM | March 6, 2009
Power naps. Now there's something I can relate to. No, not because I get a few minutes of shut-eye in the middle of the afternoon, but because I was just reading about this topic in The Problem of Power Consumption in Servers. The problem, of course, is that data centers are energy hogs, costing money and squander resources.
Posted by Jon Erickson 01:52 PM | March 6, 2009
Power naps. Now there's something I can relate to. No, not because I get a few minutes of shut-eye in the middle of the afternoon, but because I was just reading about this topic in The Problem of Power Consumption in Servers. The problem, of course, is that data centers are energy hogs, costing money and squander resources.
Events
* The U.S. government lodges a protest with the Chinese government after five Chinese ships "harassed" the USNS Impeccable in the South China Sea. (BBC)
* United States President Barack Obama lifts Bush administration restrictions on embryonic stem cell research. (Reuters)
* Argentina and Chile reject British claims over Antarctic territory and the seabed. (Sky News)
* North Korea promises to retaliate if its "satellite" launch is intercepted, saying interference would mean war. (CNN)
* Sudanese opposition leader Hassan Turabi is released after previously suggesting President Omar al-Bashir should give himself up. (AP)
* The Sri Lankan military claims it has killed 180 LTTE fighters carrying out a wave of counterattacks. (Reuters)
* Bombs explode in Qinghai, China, damaging police cars after a clash with several people. (AP)
* The Japanese economy posts a record deficit of 172.8 billion yen. (Press Association)
* Zimbabwean Prime Minister Morgan Tsvangirai rules out foul play in a car accident that killed his wife. (Guardian)
* Former Israeli President Moshe Katsav is to be charged with sex offences. (Bloomberg)
* Madagascar's government announces it will take "military measures" against mutinying soldiers. (Reuters)
* An Ilyushin Il-76 carrying equipment to the African Mission in Somalia (AMSOM) crashes into Lake Victoria shortly after takeoff from Entebbe International Airport, likely killing all eleven aboard. (UGpulse)
* Work begins to remove the wreckage of Turkish Airlines Flight 1951 from Amsterdam's Schiphol Airport. (World Bulletin)
* The loss of silicon-carrying ship MV Ibn al-Battuta off Safaga, Egypt leaves around a dozen missing or dead, with reports on numbers conflicting. (AFP) (CNN) (Xinhua)
* United States President Barack Obama lifts Bush administration restrictions on embryonic stem cell research. (Reuters)
* Argentina and Chile reject British claims over Antarctic territory and the seabed. (Sky News)
* North Korea promises to retaliate if its "satellite" launch is intercepted, saying interference would mean war. (CNN)
* Sudanese opposition leader Hassan Turabi is released after previously suggesting President Omar al-Bashir should give himself up. (AP)
* The Sri Lankan military claims it has killed 180 LTTE fighters carrying out a wave of counterattacks. (Reuters)
* Bombs explode in Qinghai, China, damaging police cars after a clash with several people. (AP)
* The Japanese economy posts a record deficit of 172.8 billion yen. (Press Association)
* Zimbabwean Prime Minister Morgan Tsvangirai rules out foul play in a car accident that killed his wife. (Guardian)
* Former Israeli President Moshe Katsav is to be charged with sex offences. (Bloomberg)
* Madagascar's government announces it will take "military measures" against mutinying soldiers. (Reuters)
* An Ilyushin Il-76 carrying equipment to the African Mission in Somalia (AMSOM) crashes into Lake Victoria shortly after takeoff from Entebbe International Airport, likely killing all eleven aboard. (UGpulse)
* Work begins to remove the wreckage of Turkish Airlines Flight 1951 from Amsterdam's Schiphol Airport. (World Bulletin)
* The loss of silicon-carrying ship MV Ibn al-Battuta off Safaga, Egypt leaves around a dozen missing or dead, with reports on numbers conflicting. (AFP) (CNN) (Xinhua)
recycle time Term of the Day
Previous Terms-of-the-Day
Last modified: Thursday, December 18, 2008
The phrase used to mean the amount of time a digital camera requires to reset the flash between one photo and the next. A longer recycle time makes it more difficult to capture action or time lapse shots. May also be called lag time or refresh rate.
See also "The Digital Camera Picture" in Webopedia's Quick Reference section.
Last modified: Thursday, December 18, 2008
The phrase used to mean the amount of time a digital camera requires to reset the flash between one photo and the next. A longer recycle time makes it more difficult to capture action or time lapse shots. May also be called lag time or refresh rate.
See also "The Digital Camera Picture" in Webopedia's Quick Reference section.
Processor
Processor may refer to:
In computing:
* Central processing unit (CPU), the part of a computer that interprets instructions, in case of a multi-core processor it refers to multiple CPU's.
* Microprocessor, a CPU on one chip as part of a microcomputer
* Graphics processing unit (GPU / VPU), a dedicated graphics rendering device for a personal computer or game console
VPU (Visual / Video Processing Unit)
* Physics processing unit (PPU), a dedicated microprocessor designed to handle the calculations of physics
* Digital signal processor, a specialized microprocessor designed specifically for digital signal processing
* Network processor, a microprocessor specifically targeted at the networking application domain
* Front end processor, a helper processor for communication between a host computer and other devices
* Coprocessor
o Arithmetic logic unit
o Floating point unit
* Data processor, a system that translates or converts between different data formats
* Word processor, a computer application used for the production of printable material
* Audio processor, used in studios and radio stations
In computing:
* Central processing unit (CPU), the part of a computer that interprets instructions, in case of a multi-core processor it refers to multiple CPU's.
* Microprocessor, a CPU on one chip as part of a microcomputer
* Graphics processing unit (GPU / VPU), a dedicated graphics rendering device for a personal computer or game console
VPU (Visual / Video Processing Unit)
* Physics processing unit (PPU), a dedicated microprocessor designed to handle the calculations of physics
* Digital signal processor, a specialized microprocessor designed specifically for digital signal processing
* Network processor, a microprocessor specifically targeted at the networking application domain
* Front end processor, a helper processor for communication between a host computer and other devices
* Coprocessor
o Arithmetic logic unit
o Floating point unit
* Data processor, a system that translates or converts between different data formats
* Word processor, a computer application used for the production of printable material
* Audio processor, used in studios and radio stations
TYPES OF RAM
Top L-R, DDR2 with heat-spreader, DDR2 without heat-spreader, Laptop DDR2, DDR, Laptop DDR
1 Megabit chip - one of the last models developed by VEB Carl Zeiss Jena in 1989
Modern types of writable RAM generally store a bit of data in either the state of a flip-flop, as in SRAM (static RAM), or as a charge in a capacitor (or transistor gate), as in DRAM (dynamic RAM), EPROM, EEPROM and Flash. Some types have circuitry to detect and/or correct random faults called memory errors in the stored data, using parity bits or error correction codes. RAM of the read-only type, ROM, instead uses a metal mask to permanently enable/disable selected transistors, instead of storing a charge in them.
As both SRAM and DRAM are volatile, other forms of computer storage, such as disks and magnetic tapes, have been used as persistent storage in traditional computers. Many newer products instead rely on flash memory to maintain data when not in use, such as PDAs or small music players. Certain personal computers, such as many rugged computers and netbooks, have also replaced magnetic disks with flash drives. With flash memory, only the NOR type is capable of true random access, allowing direct code execution, and is therefore often used instead of ROM; the lower cost NAND type is commonly used for bulk storage in memory cards and solid-state drives.
[edit] Memory hierarchy
Many computer systems have a memory hierarchy consisting of CPU registers, on-die SRAM caches, external caches, DRAM, paging systems, and virtual memory or swap space on a hard drive. This entire pool of memory may be referred to as "RAM" by many developers, even though the various subsystems can have very different access times, violating the original concept behind the random access term in RAM. Even within a hierarchy level such as DRAM, the specific row, column, bank, rank, channel, or interleave organization of the components make the access time variable, although not to the extent that rotating storage media or a tape is variable. The overall goal of using a memory hierarchy is to obtain the higher possible average access performance while minimizing the total cost of entire memory system. (Generally, the memory hierarchy follows the access time with the fast CPU registers at the top and the slow hard drive at the bottom.)
In many modern personal computers, the RAM comes in an easily upgraded form of modules called memory modules or DRAM modules about the size of a few sticks of chewing gum. These can quickly be replaced should they become damaged or too small for current purposes. As suggested above, smaller amounts of RAM (mostly SRAM) are also integrated in the CPU and other ICs on the motherboard, as well as in hard-drives, CD-ROMs, and several other parts of the computer system.
[edit] Swapping
If a computer becomes low on RAM during intensive application cycles, the computer can perform an operation known as "swapping". When this occurs, the computer temporarily uses hard drive space as additional memory. Constantly relying on this type of backup memory is called thrashing, which is generally undesirable because it lowers overall system performance.
[edit] Other uses of the "RAM" term
Other physical devices with read–write capability can have "RAM" in their names: for example, DVD-RAM. "Random access" is also the name of an indexing method: hence, disk storage is often called "random access"[citation needed] because the reading head can move relatively quickly from one piece of data to another, and does not have to read all the data in between. However the final "M" is crucial: "RAM" (provided there is no additional term as in "DVD-RAM") always refers to a solid-state device.
[edit] RAM disks
Software can "partition" a portion of a computer's RAM, allowing it to act as a much faster hard drive that is called a RAM disk. Unless the memory used is non-volatile, a RAM disk loses the stored data when the computer is shut down. However, volatile memory can retain its data when the computer is shut down if it has a separate power source, usually a battery.
[edit] Shadow RAM
Sometimes, the contents of a ROM chip are copied to SRAM or DRAM to allow for shorter access times (as ROM may be slower). The ROM chip is then disabled while the initialized memory locations are switched in on the same block of addresses (often write-protected). This process, sometimes called shadowing, is fairly common in both computers and embedded systems.
As a common example, the BIOS in typical personal computers often has an option called “use shadow BIOS” or similar. When enabled, functions relying on data from the BIOS’s ROM will instead use DRAM locations (most can also toggle shadowing of video card ROM or other ROM sections). Depending on the system, this may or may not result in increased performance. On some systems the benefit may be hypothetical because the BIOS is not used after booting in favor of direct hardware access. Of course, somewhat less free memory is available when shadowing is enabled.[2]
[edit] Recent developments
Several new types of non-volatile RAM, which will preserve data while powered down, are under development. The technologies used include carbon nanotubes and the magnetic tunnel effect. In summer 2003, a 128 KB (128 × 210 bytes) magnetic RAM (MRAM) chip was manufactured with 0.18 µm technology. In June 2004, Infineon Technologies unveiled a 16 MB (16 × 220 bytes) prototype again based on 0.18 µm technology. Nantero built a functioning carbon nanotube memory prototype 10 GB (10 × 230 bytes) array in 2004. Whether some of these technologies will be able to eventually take a significant market share from either DRAM, SRAM, or flash-memory technology, however, remains to be seen.
Since 2006, "Solid-state drives" (based on flash memory) with capacities exceeding 642 gigabytes and performance far exceeding traditional disks have become available. This development has started to blur the definition between traditional random access memory and "disks", dramatically reducing the difference in performance. Also in development is research being done in the field of plastic magnets, which switch magnetic polarities based on light.
[edit] Memory wall
The "memory wall" is the growing disparity of speed between CPU and memory outside the CPU chip. An important reason for this disparity is the limited communication bandwidth beyond chip boundaries. From 1986 to 2000, CPU speed improved at an annual rate of 55% while memory speed only improved at 10%. Given these trends, it was expected that memory latency would become an overwhelming bottleneck in computer performance. [3]
Currently, CPU speed improvements have slowed significantly partly due to major physical barriers and partly because current CPU designs have already hit the memory wall in some sense. Intel summarized these causes in their Platform 2015 documentation (PDF)
“First of all, as chip geometries shrink and clock frequencies rise, the transistor leakage current increases, leading to excess power consumption and heat (more on power consumption below). Secondly, the advantages of higher clock speeds are in part negated by memory latency, since memory access times have not been able to keep pace with increasing clock frequencies. Third, for certain applications, traditional serial architectures are becoming less efficient as processors get faster (due to the so-called Von Neumann bottleneck), further undercutting any gains that frequency increases might otherwise buy. In addition, partly due to limitations in the means of producing inductance within solid state devices, resistance-capacitance (RC) delays in signal transmission are growing as feature sizes shrink, imposing an additional bottleneck that frequency increases don't address.”
The RC delays in signal transmission were also noted in Clock Rate versus IPC: The End of the Road for Conventional Microarchitectures which projects a maximum of 12.5% average annual CPU performance improvement between 2000 and 2014. The data on Intel Processors clearly shows a slowdown in performance improvements in recent processors. However, Intel's new processors, Core 2 Duo (codenamed Conroe) show a significant improvement over previous Pentium 4 processors; due to a more efficient architecture, performance increased while clock rate actually decreased.
[edit] Security concerns
Contrary to simple models (and perhaps common belief), the contents of modern SDRAM modules are not lost immediately when the computer is shut down; instead, the contents fade away, a process that takes only seconds at room temperatures, but which can be extended to minutes at low temperatures. It is therefore possible to get hold of an encryption key if it were stored in ordinary working memory (i.e. the SDRAM modules).[4] This is sometimes referred to as a cold boot attack.
[edit] See also
Sister project Wikimedia Commons has media related to: RAM
* CAS latency (CL)
* Dual-channel architecture
* ECC (Error-correcting code)
* Registered/buffered memory
* RAM parity
[edit] Notes and references
1. ^ Strictly speaking, modern types of DRAM are therefore not truly (or technically) random access, as data are read in burst; the name DRAM has stuck however.
2. ^ "Shadow Ram" (HTML). http://hardwarehell.com/articles/shadowram.htm. Retrieved on 2007-07-24.
3. ^ The term was coined in Hitting the Memory Wall: Implications of the Obvious (PDF).
4. ^ Cold Boot Attacks on Encryption Keys
[edit] External links
Retrieved from "http://en.wikipedia.org/wiki/Random-access_memory"
Categories: Computer memory
Hidden categories: All articles with unsourced statements | Articles with unsourced statements since March 2009
Views
* Article
* Discussion
* Edit this page
* History
1 Megabit chip - one of the last models developed by VEB Carl Zeiss Jena in 1989
Modern types of writable RAM generally store a bit of data in either the state of a flip-flop, as in SRAM (static RAM), or as a charge in a capacitor (or transistor gate), as in DRAM (dynamic RAM), EPROM, EEPROM and Flash. Some types have circuitry to detect and/or correct random faults called memory errors in the stored data, using parity bits or error correction codes. RAM of the read-only type, ROM, instead uses a metal mask to permanently enable/disable selected transistors, instead of storing a charge in them.
As both SRAM and DRAM are volatile, other forms of computer storage, such as disks and magnetic tapes, have been used as persistent storage in traditional computers. Many newer products instead rely on flash memory to maintain data when not in use, such as PDAs or small music players. Certain personal computers, such as many rugged computers and netbooks, have also replaced magnetic disks with flash drives. With flash memory, only the NOR type is capable of true random access, allowing direct code execution, and is therefore often used instead of ROM; the lower cost NAND type is commonly used for bulk storage in memory cards and solid-state drives.
[edit] Memory hierarchy
Many computer systems have a memory hierarchy consisting of CPU registers, on-die SRAM caches, external caches, DRAM, paging systems, and virtual memory or swap space on a hard drive. This entire pool of memory may be referred to as "RAM" by many developers, even though the various subsystems can have very different access times, violating the original concept behind the random access term in RAM. Even within a hierarchy level such as DRAM, the specific row, column, bank, rank, channel, or interleave organization of the components make the access time variable, although not to the extent that rotating storage media or a tape is variable. The overall goal of using a memory hierarchy is to obtain the higher possible average access performance while minimizing the total cost of entire memory system. (Generally, the memory hierarchy follows the access time with the fast CPU registers at the top and the slow hard drive at the bottom.)
In many modern personal computers, the RAM comes in an easily upgraded form of modules called memory modules or DRAM modules about the size of a few sticks of chewing gum. These can quickly be replaced should they become damaged or too small for current purposes. As suggested above, smaller amounts of RAM (mostly SRAM) are also integrated in the CPU and other ICs on the motherboard, as well as in hard-drives, CD-ROMs, and several other parts of the computer system.
[edit] Swapping
If a computer becomes low on RAM during intensive application cycles, the computer can perform an operation known as "swapping". When this occurs, the computer temporarily uses hard drive space as additional memory. Constantly relying on this type of backup memory is called thrashing, which is generally undesirable because it lowers overall system performance.
[edit] Other uses of the "RAM" term
Other physical devices with read–write capability can have "RAM" in their names: for example, DVD-RAM. "Random access" is also the name of an indexing method: hence, disk storage is often called "random access"[citation needed] because the reading head can move relatively quickly from one piece of data to another, and does not have to read all the data in between. However the final "M" is crucial: "RAM" (provided there is no additional term as in "DVD-RAM") always refers to a solid-state device.
[edit] RAM disks
Software can "partition" a portion of a computer's RAM, allowing it to act as a much faster hard drive that is called a RAM disk. Unless the memory used is non-volatile, a RAM disk loses the stored data when the computer is shut down. However, volatile memory can retain its data when the computer is shut down if it has a separate power source, usually a battery.
[edit] Shadow RAM
Sometimes, the contents of a ROM chip are copied to SRAM or DRAM to allow for shorter access times (as ROM may be slower). The ROM chip is then disabled while the initialized memory locations are switched in on the same block of addresses (often write-protected). This process, sometimes called shadowing, is fairly common in both computers and embedded systems.
As a common example, the BIOS in typical personal computers often has an option called “use shadow BIOS” or similar. When enabled, functions relying on data from the BIOS’s ROM will instead use DRAM locations (most can also toggle shadowing of video card ROM or other ROM sections). Depending on the system, this may or may not result in increased performance. On some systems the benefit may be hypothetical because the BIOS is not used after booting in favor of direct hardware access. Of course, somewhat less free memory is available when shadowing is enabled.[2]
[edit] Recent developments
Several new types of non-volatile RAM, which will preserve data while powered down, are under development. The technologies used include carbon nanotubes and the magnetic tunnel effect. In summer 2003, a 128 KB (128 × 210 bytes) magnetic RAM (MRAM) chip was manufactured with 0.18 µm technology. In June 2004, Infineon Technologies unveiled a 16 MB (16 × 220 bytes) prototype again based on 0.18 µm technology. Nantero built a functioning carbon nanotube memory prototype 10 GB (10 × 230 bytes) array in 2004. Whether some of these technologies will be able to eventually take a significant market share from either DRAM, SRAM, or flash-memory technology, however, remains to be seen.
Since 2006, "Solid-state drives" (based on flash memory) with capacities exceeding 642 gigabytes and performance far exceeding traditional disks have become available. This development has started to blur the definition between traditional random access memory and "disks", dramatically reducing the difference in performance. Also in development is research being done in the field of plastic magnets, which switch magnetic polarities based on light.
[edit] Memory wall
The "memory wall" is the growing disparity of speed between CPU and memory outside the CPU chip. An important reason for this disparity is the limited communication bandwidth beyond chip boundaries. From 1986 to 2000, CPU speed improved at an annual rate of 55% while memory speed only improved at 10%. Given these trends, it was expected that memory latency would become an overwhelming bottleneck in computer performance. [3]
Currently, CPU speed improvements have slowed significantly partly due to major physical barriers and partly because current CPU designs have already hit the memory wall in some sense. Intel summarized these causes in their Platform 2015 documentation (PDF)
“First of all, as chip geometries shrink and clock frequencies rise, the transistor leakage current increases, leading to excess power consumption and heat (more on power consumption below). Secondly, the advantages of higher clock speeds are in part negated by memory latency, since memory access times have not been able to keep pace with increasing clock frequencies. Third, for certain applications, traditional serial architectures are becoming less efficient as processors get faster (due to the so-called Von Neumann bottleneck), further undercutting any gains that frequency increases might otherwise buy. In addition, partly due to limitations in the means of producing inductance within solid state devices, resistance-capacitance (RC) delays in signal transmission are growing as feature sizes shrink, imposing an additional bottleneck that frequency increases don't address.”
The RC delays in signal transmission were also noted in Clock Rate versus IPC: The End of the Road for Conventional Microarchitectures which projects a maximum of 12.5% average annual CPU performance improvement between 2000 and 2014. The data on Intel Processors clearly shows a slowdown in performance improvements in recent processors. However, Intel's new processors, Core 2 Duo (codenamed Conroe) show a significant improvement over previous Pentium 4 processors; due to a more efficient architecture, performance increased while clock rate actually decreased.
[edit] Security concerns
Contrary to simple models (and perhaps common belief), the contents of modern SDRAM modules are not lost immediately when the computer is shut down; instead, the contents fade away, a process that takes only seconds at room temperatures, but which can be extended to minutes at low temperatures. It is therefore possible to get hold of an encryption key if it were stored in ordinary working memory (i.e. the SDRAM modules).[4] This is sometimes referred to as a cold boot attack.
[edit] See also
Sister project Wikimedia Commons has media related to: RAM
* CAS latency (CL)
* Dual-channel architecture
* ECC (Error-correcting code)
* Registered/buffered memory
* RAM parity
[edit] Notes and references
1. ^ Strictly speaking, modern types of DRAM are therefore not truly (or technically) random access, as data are read in burst; the name DRAM has stuck however.
2. ^ "Shadow Ram" (HTML). http://hardwarehell.com/articles/shadowram.htm. Retrieved on 2007-07-24.
3. ^ The term was coined in Hitting the Memory Wall: Implications of the Obvious (PDF).
4. ^ Cold Boot Attacks on Encryption Keys
[edit] External links
Retrieved from "http://en.wikipedia.org/wiki/Random-access_memory"
Categories: Computer memory
Hidden categories: All articles with unsourced statements | Articles with unsourced statements since March 2009
Views
* Article
* Discussion
* Edit this page
* History
history of RAM
History
An early type of widespread writable random access memory was the magnetic core memory, developed from 1949 to 1952, and subsequently used in most computers up until the development of the static and dynamic integrated RAM circuits in the late 1960s and early 1970s. Before this, computers used relays, delay line memory or various kinds of vacuum tube arrangements to implement "main" memory functions (i.e., hundreds or thousands of bits), some of which were random access, some not. Latches built out of vacuum tube triodes, and later, out of discrete transistors, were used for smaller and faster memories such as registers and (random access) register banks. Prior to the development of integrated ROM circuits, permanent (or read-only) random access memory was often constructed using semiconductor diode matrices driven by address decoders.
RAM
Random-access memory is a form of computer data storage. Today it takes the form of integrated circuits that allows the stored data to be accessed in any order (i.e., at random). The word random thus refers to the fact that any piece of data can be returned in a constant time, regardless of its physical location and whether or not it is related to the previous piece of data.[1]
This contrasts with storage mechanisms such as tapes, magnetic discs and optical discs, which rely on the physical movement of the recording medium or a reading head. In these devices, the movement takes longer than the data transfer, and the retrieval time varies depending on the physical location of the next item.
The word RAM is mostly associated with volatile types of memory (such as DRAM memory modules), where the information is lost after the power is switched off. However, many other types of memory are RAM as well (i.e., Random Access Memory), including most types of ROM and a kind of flash memory called NOR-Flash.
This contrasts with storage mechanisms such as tapes, magnetic discs and optical discs, which rely on the physical movement of the recording medium or a reading head. In these devices, the movement takes longer than the data transfer, and the retrieval time varies depending on the physical location of the next item.
The word RAM is mostly associated with volatile types of memory (such as DRAM memory modules), where the information is lost after the power is switched off. However, many other types of memory are RAM as well (i.e., Random Access Memory), including most types of ROM and a kind of flash memory called NOR-Flash.
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CPU operation
The fundamental operation of most CPUs, regardless of the physical form they take, is to execute a sequence of stored instructions called a program. The program is represented by a series of numbers that are kept in some kind of computer memory. There are four steps that nearly all CPUs use in their operation: fetch, decode, execute, and writeback.
The first step, fetch, involves retrieving an instruction (which is represented by a number or sequence of numbers) from program memory. The location in program memory is determined by a program counter (PC), which stores a number that identifies the current position in the program. In other words, the program counter keeps track of the CPU's place in the current program. After an instruction is fetched, the PC is incremented by the length of the instruction word in terms of memory units.[3] Often the instruction to be fetched must be retrieved from relatively slow memory, causing the CPU to stall while waiting for the instruction to be returned. This issue is largely addressed in modern processors by caches and pipeline architectures (see below).
The instruction that the CPU fetches from memory is used to determine what the CPU is to do. In the decode step, the instruction is broken up into parts that have significance to other portions of the CPU. The way in which the numerical instruction value is interpreted is defined by the CPU's instruction set architecture(ISA).[4] Often, one group of numbers in the instruction, called the opcode, indicates which operation to perform. The remaining parts of the number usually provide information required for that instruction, such as operands for an addition operation. Such operands may be given as a constant value (called an immediate value), or as a place to locate a value: a register or a memory address, as determined by some addressing mode. In older designs the portions of the CPU responsible for instruction decoding were unchangeable hardware devices. However, in more abstract and complicated CPUs and ISAs, a microprogram is often used to assist in translating instructions into various configuration signals for the CPU. This microprogram is sometimes rewritable so that it can be modified to change the way the CPU decodes instructions even after it has been manufactured.
After the fetch and decode steps, the execute step is performed. During this step, various portions of the CPU are connected so they can perform the desired operation. If, for instance, an addition operation was requested, an arithmetic logic unit (ALU) will be connected to a set of inputs and a set of outputs. The inputs provide the numbers to be added, and the outputs will contain the final sum. The ALU contains the circuitry to perform simple arithmetic and logical operations on the inputs (like addition and bitwise operations). If the addition operation produces a result too large for the CPU to handle, an arithmetic overflow flag in a flags register may also be set.
The final step, writeback, simply "writes back" the results of the execute step to some form of memory. Very often the results are written to some internal CPU register for quick access by subsequent instructions. In other cases results may be written to slower, but cheaper and larger, main memory. Some types of instructions manipulate the program counter rather than directly produce result data. These are generally called "jumps" and facilitate behavior like loops, conditional program execution (through the use of a conditional jump), and functions in programs.[5] Many instructions will also change the state of digits in a "flags" register. These flags can be used to influence how a program behaves, since they often indicate the outcome of various operations. For example, one type of "compare" instruction considers two values and sets a number in the flags register according to which one is greater. This flag could then be used by a later jump instruction to determine program flow.
After the execution of the instruction and writeback of the resulting data, the entire process repeats, with the next instruction cycle normally fetching the next-in-sequence instruction because of the incremented value in the program counter. If the completed instruction was a jump, the program counter will be modified to contain the address of the instruction that was jumped to, and program execution continues normally. In more complex CPUs than the one described here, multiple instructions can be fetched, decoded, and executed simultaneously. This section describes what is generally referred to as the "Classic RISC pipeline," which in fact is quite common among the simple CPUs used in many electronic devices (often called microcontroller). It largely ignores the important role of CPU cache, and therefore the access stage of the pipeline.
[edit] Design and implementation
Main article: CPU design
[edit] Integer range
The way a CPU represents numbers is a design choice that affects the most basic ways in which the device functions. Some early digital computers used an electrical model of the common decimal (base ten) numeral system to represent numbers internally. A few other computers have used more exotic numeral systems like ternary (base three). Nearly all modern CPUs represent numbers in binary form, with each digit being represented by some two-valued physical quantity such as a "high" or "low" voltage.[6]
MOS 6502 microprocessor in a dual in-line package, an extremely popular 8-bit design.
Related to number representation is the size and precision of numbers that a CPU can represent. In the case of a binary CPU, a bit refers to one significant place in the numbers a CPU deals with. The number of bits (or numeral places) a CPU uses to represent numbers is often called "word size", "bit width", "data path width", or "integer precision" when dealing with strictly integer numbers (as opposed to floating point). This number differs between architectures, and often within different parts of the very same CPU. For example, an 8-bit CPU deals with a range of numbers that can be represented by eight binary digits (each digit having two possible values), that is, 28 or 256 discrete numbers. In effect, integer size sets a hardware limit on the range of integers the software run by the CPU can utilize.[7]
Integer range can also affect the number of locations in memory the CPU can address (locate). For example, if a binary CPU uses 32 bits to represent a memory address, and each memory address represents one octet (8 bits), the maximum quantity of memory that CPU can address is 232 octets, or 4 GiB. This is a very simple view of CPU address space, and many designs use more complex addressing methods like paging in order to locate more memory than their integer range would allow with a flat address space.
Higher levels of integer range require more structures to deal with the additional digits, and therefore more complexity, size, power usage, and general expense. It is not at all uncommon, therefore, to see 4- or 8-bit microcontrollers used in modern applications, even though CPUs with much higher range (such as 16, 32, 64, even 128-bit) are available. The simpler microcontrollers are usually cheaper, use less power, and therefore dissipate less heat, all of which can be major design considerations for electronic devices. However, in higher-end applications, the benefits afforded by the extra range (most often the additional address space) are more significant and often affect design choices. To gain some of the advantages afforded by both lower and higher bit lengths, many CPUs are designed with different bit widths for different portions of the device. For example, the IBM System/370 used a CPU that was primarily 32 bit, but it used 128-bit precision inside its floating point units to facilitate greater accuracy and range in floating point numbers (Amdahl et al. 1964). Many later CPU designs use similar mixed bit width, especially when the processor is meant for general-purpose usage where a reasonable balance of integer and floating point capability is required.
[edit] Clock rate
Main article: Clock rate
Most CPUs, and indeed most sequential logic devices, are synchronous in nature.[8] That is, they are designed and operate on assumptions about a synchronization signal. This signal, known as a clock signal, usually takes the form of a periodic square wave. By calculating the maximum time that electrical signals can move in various branches of a CPU's many circuits, the designers can select an appropriate period for the clock signal.
This period must be longer than the amount of time it takes for a signal to move, or propagate, in the worst-case scenario. In setting the clock period to a value well above the worst-case propagation delay, it is possible to design the entire CPU and the way it moves data around the "edges" of the rising and falling clock signal. This has the advantage of simplifying the CPU significantly, both from a design perspective and a component-count perspective. However, it also carries the disadvantage that the entire CPU must wait on its slowest elements, even though some portions of it are much faster. This limitation has largely been compensated for by various methods of increasing CPU parallelism (see below).
However architectural improvements alone do not solve all of the drawbacks of globally synchronous CPUs. For example, a clock signal is subject to the delays of any other electrical signal. Higher clock rates in increasingly complex CPUs make it more difficult to keep the clock signal in phase (synchronized) throughout the entire unit. This has led many modern CPUs to require multiple identical clock signals to be provided in order to avoid delaying a single signal significantly enough to cause the CPU to malfunction. Another major issue as clock rates increase dramatically is the amount of heat that is dissipated by the CPU. The constantly changing clock causes many components to switch regardless of whether they are being used at that time. In general, a component that is switching uses more energy than an element in a static state. Therefore, as clock rate increases, so does heat dissipation, causing the CPU to require more effective cooling solutions.
One method of dealing with the switching of unneeded components is called clock gating, which involves turning off the clock signal to unneeded components (effectively disabling them). However, this is often regarded as difficult to implement and therefore does not see common usage outside of very low-power designs.[9] Another method of addressing some of the problems with a global clock signal is the removal of the clock signal altogether. While removing the global clock signal makes the design process considerably more complex in many ways, asynchronous (or clockless) designs carry marked advantages in power consumption and heat dissipation in comparison with similar synchronous designs. While somewhat uncommon, entire asynchronous CPUs have been built without utilizing a global clock signal. Two notable examples of this are the ARM compliant AMULET and the MIPS R3000 compatible MiniMIPS. Rather than totally removing the clock signal, some CPU designs allow certain portions of the device to be asynchronous, such as using asynchronous ALUs in conjunction with superscalar pipelining to achieve some arithmetic performance gains. While it is not altogether clear whether totally asynchronous designs can perform at a comparable or better level than their synchronous counterparts, it is evident that they do at least excel in simpler math operations. This, combined with their excellent power consumption and heat dissipation properties, makes them very suitable for embedded computers (Garside et al. 1999).
[edit] Parallelism
Main article: Parallel computing
Model of a subscalar CPU. Notice that it takes fifteen cycles to complete three instructions.
The description of the basic operation of a CPU offered in the previous section describes the simplest form that a CPU can take. This type of CPU, usually referred to as subscalar, operates on and executes one instruction on one or two pieces of data at a time.
This process gives rise to an inherent inefficiency in subscalar CPUs. Since only one instruction is executed at a time, the entire CPU must wait for that instruction to complete before proceeding to the next instruction. As a result the subscalar CPU gets "hung up" on instructions which take more than one clock cycle to complete execution. Even adding a second execution unit (see below) does not improve performance much; rather than one pathway being hung up, now two pathways are hung up and the number of unused transistors is increased. This design, wherein the CPU's execution resources can operate on only one instruction at a time, can only possibly reach scalar performance (one instruction per clock). However, the performance is nearly always subscalar (less than one instruction per cycle).
Attempts to achieve scalar and better performance have resulted in a variety of design methodologies that cause the CPU to behave less linearly and more in parallel. When referring to parallelism in CPUs, two terms are generally used to classify these design techniques. Instruction level parallelism (ILP) seeks to increase the rate at which instructions are executed within a CPU (that is, to increase the utilization of on-die execution resources), and thread level parallelism (TLP) purposes to increase the number of threads (effectively individual programs) that a CPU can execute simultaneously. Each methodology differs both in the ways in which they are implemented, as well as the relative effectiveness they afford in increasing the CPU's performance for an application.[10]
[edit] Instruction level parallelism
Main articles: Instruction pipelining and Superscalar
Basic five-stage pipeline. In the best case scenario, this pipeline can sustain a completion rate of one instruction per cycle.
One of the simplest methods used to accomplish increased parallelism is to begin the first steps of instruction fetching and decoding before the prior instruction finishes executing. This is the simplest form of a technique known as instruction pipelining, and is utilized in almost all modern general-purpose CPUs. Pipelining allows more than one instruction to be executed at any given time by breaking down the execution pathway into discrete stages. This separation can be compared to an assembly line, in which an instruction is made more complete at each stage until it exits the execution pipeline and is retired.
Pipelining does, however, introduce the possibility for a situation where the result of the previous operation is needed to complete the next operation; a condition often termed data dependency conflict. To cope with this, additional care must be taken to check for these sorts of conditions and delay a portion of the instruction pipeline if this occurs. Naturally, accomplishing this requires additional circuitry, so pipelined processors are more complex than subscalar ones (though not very significantly so). A pipelined processor can become very nearly scalar, inhibited only by pipeline stalls (an instruction spending more than one clock cycle in a stage).
Simple superscalar pipeline. By fetching and dispatching two instructions at a time, a maximum of two instructions per cycle can be completed.
Further improvement upon the idea of instruction pipelining led to the development of a method that decreases the idle time of CPU components even further. Designs that are said to be superscalar include a long instruction pipeline and multiple identical execution units. [Huynh 2003] In a superscalar pipeline, multiple instructions are read and passed to a dispatcher, which decides whether or not the instructions can be executed in parallel (simultaneously). If so they are dispatched to available execution units, resulting in the ability for several instructions to be executed simultaneously. In general, the more instructions a superscalar CPU is able to dispatch simultaneously to waiting execution units, the more instructions will be completed in a given cycle.
Most of the difficulty in the design of a superscalar CPU architecture lies in creating an effective dispatcher. The dispatcher needs to be able to quickly and correctly determine whether instructions can be executed in parallel, as well as dispatch them in such a way as to keep as many execution units busy as possible. This requires that the instruction pipeline is filled as often as possible and gives rise to the need in superscalar architectures for significant amounts of CPU cache. It also makes hazard-avoiding techniques like branch prediction, speculative execution, and out-of-order execution crucial to maintaining high levels of performance. By attempting to predict which branch (or path) a conditional instruction will take, the CPU can minimize the number of times that the entire pipeline must wait until a conditional instruction is completed. Speculative execution often provides modest performance increases by executing portions of code that may or may not be needed after a conditional operation completes. Out-of-order execution somewhat rearranges the order in which instructions are executed to reduce delays due to data dependencies.
In the case where a portion of the CPU is superscalar and part is not, the part which is not suffers a performance penalty due to scheduling stalls. The original Intel Pentium (P5) had two superscalar ALUs which could accept one instruction per clock each, but its FPU could not accept one instruction per clock. Thus the P5 was integer superscalar but not floating point superscalar. Intel's successor to the Pentium architecture, P6, added superscalar capabilities to its floating point features, and therefore afforded a significant increase in floating point instruction performance.
Both simple pipelining and superscalar design increase a CPU's ILP by allowing a single processor to complete execution of instructions at rates surpassing one instruction per cycle (IPC).[11] Most modern CPU designs are at least somewhat superscalar, and nearly all general purpose CPUs designed in the last decade are superscalar. In later years some of the emphasis in designing high-ILP computers has been moved out of the CPU's hardware and into its software interface, or ISA. The strategy of the very long instruction word (VLIW) causes some ILP to become implied directly by the software, reducing the amount of work the CPU must perform to boost ILP and thereby reducing the design's complexity.
[edit] Thread level parallelism
Another strategy of achieving performance is to execute multiple programs or threads in parallel. This area of research is known as parallel computing. In Flynn's taxonomy, this strategy is known as Multiple Instructions-Multiple Data or MIMD.
One technology used for this purpose was multiprocessing (MP). The initial flavor of this technology is known as symmetric multiprocessing (SMP), where a small number of CPUs share a coherent view of their memory system. In this scheme, each CPU has additional hardware to maintain a constantly up-to-date view of memory. By avoiding stale views of memory, the CPUs can cooperate on the same program and programs can migrate from one CPU to another. To increase the number of cooperating CPUs beyond a handful, schemes such as non-uniform memory access (NUMA) and directory-based coherence protocols were introduced in the 1990s. SMP systems are limited to a small number of CPUs while NUMA systems have been built with thousands of processors. Initially, multiprocessing was built using multiple discrete CPUs and boards to implement the interconnect between the processors. When the processors and their interconnect are all implemented on a single silicon chip, the technology is known as a multi-core microprocessor.
It was later recognized that finer-grain parallelism existed with a single program. A single program might have several threads (or functions) that could be executed separately or in parallel. Some of earliest examples of this technology implemented input/output processing such as direct memory access as a separate thread from the computation thread. A more general approach to this technology was introduced in the 1970s when systems were designed to run multiple computation threads in parallel. This technology is known as multi-threading (MT). This approach is considered more cost-effective than multiprocessing, as only a small number of components within a CPU is replicated in order to support MT as opposed to the entire CPU in the case of MP. In MT, the execution units and the memory system including the caches are shared among multiple threads. The downside of MT is that the hardware support for multithreading is more visible to software than that of MP and thus supervisor software like operating systems have to undergo larger changes to support MT. One type of MT that was implemented is known as block multithreading, where one thread is executed until it is stalled waiting for data to return from external memory. In this scheme, the CPU would then quickly switch to another thread which is ready to run, the switch often done in one CPU clock cycle. Another type of MT is known as simultaneous multithreading, where instructions of multiple threads are executed in parallel within one CPU clock cycle.
For several decades from the 1970s to early 2000s, the focus in designing high performance general purpose CPUs was largely on achieving high ILP through technologies such as pipelining, caches, superscalar execution, out-of-order execution, etc. This trend culminated in large, power-hungry CPUs such as the Intel Pentium 4. By the early 2000s, CPU designers were thwarted from achieving higher performance from ILP techniques due to the growing disparity between CPU operating frequencies and main memory operating frequencies as well as escalating CPU power dissipation owing to more esoteric ILP techniques.
CPU designers then borrowed ideas from commercial computing markets such as transaction processing, where the aggregate performance of multiple programs, also known as throughput computing, was more important than the performance of a single thread or program.
This reversal of emphasis is evidenced by the proliferation of dual and multiple core CMP (chip-level multiprocessing) designs and notably, Intel's newer designs resembling its less superscalar P6 architecture. Late designs in several processor families exhibit CMP, including the x86-64 Opteron and Athlon 64 X2, the SPARC UltraSPARC T1, IBM POWER4 and POWER5, as well as several video game console CPUs like the Xbox 360's triple-core PowerPC design, and the PS3's 8-core Cell microprocessor.
[edit] Data parallelism
Main articles: Vector processor and SIMD
A less common but increasingly important paradigm of CPUs (and indeed, computing in general) deals with data parallelism. The processors discussed earlier are all referred to as some type of scalar device.[12] As the name implies, vector processors deal with multiple pieces of data in the context of one instruction. This contrasts with scalar processors, which deal with one piece of data for every instruction. Using Flynn's taxonomy, these two schemes of dealing with data are generally referred to as SISD (single instruction, single data) and SIMD (single instruction, multiple data), respectively. The great utility in creating CPUs that deal with vectors of data lies in optimizing tasks that tend to require the same operation (for example, a sum or a dot product) to be performed on a large set of data. Some classic examples of these types of tasks are multimedia applications (images, video, and sound), as well as many types of scientific and engineering tasks. Whereas a scalar CPU must complete the entire process of fetching, decoding, and executing each instruction and value in a set of data, a vector CPU can perform a single operation on a comparatively large set of data with one instruction. Of course, this is only possible when the application tends to require many steps which apply one operation to a large set of data.
Most early vector CPUs, such as the Cray-1, were associated almost exclusively with scientific research and cryptography applications. However, as multimedia has largely shifted to digital media, the need for some form of SIMD in general-purpose CPUs has become significant. Shortly after floating point execution units started to become commonplace to include in general-purpose processors, specifications for and implementations of SIMD execution units also began to appear for general-purpose CPUs. Some of these early SIMD specifications like Intel's MMX were integer-only. This proved to be a significant impediment for some software developers, since many of the applications that benefit from SIMD primarily deal with floating point numbers. Progressively, these early designs were refined and remade into some of the common, modern SIMD specifications, which are usually associated with one ISA. Some notable modern examples are Intel's SSE and the PowerPC-related AltiVec (also known as VMX).[13]
The first step, fetch, involves retrieving an instruction (which is represented by a number or sequence of numbers) from program memory. The location in program memory is determined by a program counter (PC), which stores a number that identifies the current position in the program. In other words, the program counter keeps track of the CPU's place in the current program. After an instruction is fetched, the PC is incremented by the length of the instruction word in terms of memory units.[3] Often the instruction to be fetched must be retrieved from relatively slow memory, causing the CPU to stall while waiting for the instruction to be returned. This issue is largely addressed in modern processors by caches and pipeline architectures (see below).
The instruction that the CPU fetches from memory is used to determine what the CPU is to do. In the decode step, the instruction is broken up into parts that have significance to other portions of the CPU. The way in which the numerical instruction value is interpreted is defined by the CPU's instruction set architecture(ISA).[4] Often, one group of numbers in the instruction, called the opcode, indicates which operation to perform. The remaining parts of the number usually provide information required for that instruction, such as operands for an addition operation. Such operands may be given as a constant value (called an immediate value), or as a place to locate a value: a register or a memory address, as determined by some addressing mode. In older designs the portions of the CPU responsible for instruction decoding were unchangeable hardware devices. However, in more abstract and complicated CPUs and ISAs, a microprogram is often used to assist in translating instructions into various configuration signals for the CPU. This microprogram is sometimes rewritable so that it can be modified to change the way the CPU decodes instructions even after it has been manufactured.
After the fetch and decode steps, the execute step is performed. During this step, various portions of the CPU are connected so they can perform the desired operation. If, for instance, an addition operation was requested, an arithmetic logic unit (ALU) will be connected to a set of inputs and a set of outputs. The inputs provide the numbers to be added, and the outputs will contain the final sum. The ALU contains the circuitry to perform simple arithmetic and logical operations on the inputs (like addition and bitwise operations). If the addition operation produces a result too large for the CPU to handle, an arithmetic overflow flag in a flags register may also be set.
The final step, writeback, simply "writes back" the results of the execute step to some form of memory. Very often the results are written to some internal CPU register for quick access by subsequent instructions. In other cases results may be written to slower, but cheaper and larger, main memory. Some types of instructions manipulate the program counter rather than directly produce result data. These are generally called "jumps" and facilitate behavior like loops, conditional program execution (through the use of a conditional jump), and functions in programs.[5] Many instructions will also change the state of digits in a "flags" register. These flags can be used to influence how a program behaves, since they often indicate the outcome of various operations. For example, one type of "compare" instruction considers two values and sets a number in the flags register according to which one is greater. This flag could then be used by a later jump instruction to determine program flow.
After the execution of the instruction and writeback of the resulting data, the entire process repeats, with the next instruction cycle normally fetching the next-in-sequence instruction because of the incremented value in the program counter. If the completed instruction was a jump, the program counter will be modified to contain the address of the instruction that was jumped to, and program execution continues normally. In more complex CPUs than the one described here, multiple instructions can be fetched, decoded, and executed simultaneously. This section describes what is generally referred to as the "Classic RISC pipeline," which in fact is quite common among the simple CPUs used in many electronic devices (often called microcontroller). It largely ignores the important role of CPU cache, and therefore the access stage of the pipeline.
[edit] Design and implementation
Main article: CPU design
[edit] Integer range
The way a CPU represents numbers is a design choice that affects the most basic ways in which the device functions. Some early digital computers used an electrical model of the common decimal (base ten) numeral system to represent numbers internally. A few other computers have used more exotic numeral systems like ternary (base three). Nearly all modern CPUs represent numbers in binary form, with each digit being represented by some two-valued physical quantity such as a "high" or "low" voltage.[6]
MOS 6502 microprocessor in a dual in-line package, an extremely popular 8-bit design.
Related to number representation is the size and precision of numbers that a CPU can represent. In the case of a binary CPU, a bit refers to one significant place in the numbers a CPU deals with. The number of bits (or numeral places) a CPU uses to represent numbers is often called "word size", "bit width", "data path width", or "integer precision" when dealing with strictly integer numbers (as opposed to floating point). This number differs between architectures, and often within different parts of the very same CPU. For example, an 8-bit CPU deals with a range of numbers that can be represented by eight binary digits (each digit having two possible values), that is, 28 or 256 discrete numbers. In effect, integer size sets a hardware limit on the range of integers the software run by the CPU can utilize.[7]
Integer range can also affect the number of locations in memory the CPU can address (locate). For example, if a binary CPU uses 32 bits to represent a memory address, and each memory address represents one octet (8 bits), the maximum quantity of memory that CPU can address is 232 octets, or 4 GiB. This is a very simple view of CPU address space, and many designs use more complex addressing methods like paging in order to locate more memory than their integer range would allow with a flat address space.
Higher levels of integer range require more structures to deal with the additional digits, and therefore more complexity, size, power usage, and general expense. It is not at all uncommon, therefore, to see 4- or 8-bit microcontrollers used in modern applications, even though CPUs with much higher range (such as 16, 32, 64, even 128-bit) are available. The simpler microcontrollers are usually cheaper, use less power, and therefore dissipate less heat, all of which can be major design considerations for electronic devices. However, in higher-end applications, the benefits afforded by the extra range (most often the additional address space) are more significant and often affect design choices. To gain some of the advantages afforded by both lower and higher bit lengths, many CPUs are designed with different bit widths for different portions of the device. For example, the IBM System/370 used a CPU that was primarily 32 bit, but it used 128-bit precision inside its floating point units to facilitate greater accuracy and range in floating point numbers (Amdahl et al. 1964). Many later CPU designs use similar mixed bit width, especially when the processor is meant for general-purpose usage where a reasonable balance of integer and floating point capability is required.
[edit] Clock rate
Main article: Clock rate
Most CPUs, and indeed most sequential logic devices, are synchronous in nature.[8] That is, they are designed and operate on assumptions about a synchronization signal. This signal, known as a clock signal, usually takes the form of a periodic square wave. By calculating the maximum time that electrical signals can move in various branches of a CPU's many circuits, the designers can select an appropriate period for the clock signal.
This period must be longer than the amount of time it takes for a signal to move, or propagate, in the worst-case scenario. In setting the clock period to a value well above the worst-case propagation delay, it is possible to design the entire CPU and the way it moves data around the "edges" of the rising and falling clock signal. This has the advantage of simplifying the CPU significantly, both from a design perspective and a component-count perspective. However, it also carries the disadvantage that the entire CPU must wait on its slowest elements, even though some portions of it are much faster. This limitation has largely been compensated for by various methods of increasing CPU parallelism (see below).
However architectural improvements alone do not solve all of the drawbacks of globally synchronous CPUs. For example, a clock signal is subject to the delays of any other electrical signal. Higher clock rates in increasingly complex CPUs make it more difficult to keep the clock signal in phase (synchronized) throughout the entire unit. This has led many modern CPUs to require multiple identical clock signals to be provided in order to avoid delaying a single signal significantly enough to cause the CPU to malfunction. Another major issue as clock rates increase dramatically is the amount of heat that is dissipated by the CPU. The constantly changing clock causes many components to switch regardless of whether they are being used at that time. In general, a component that is switching uses more energy than an element in a static state. Therefore, as clock rate increases, so does heat dissipation, causing the CPU to require more effective cooling solutions.
One method of dealing with the switching of unneeded components is called clock gating, which involves turning off the clock signal to unneeded components (effectively disabling them). However, this is often regarded as difficult to implement and therefore does not see common usage outside of very low-power designs.[9] Another method of addressing some of the problems with a global clock signal is the removal of the clock signal altogether. While removing the global clock signal makes the design process considerably more complex in many ways, asynchronous (or clockless) designs carry marked advantages in power consumption and heat dissipation in comparison with similar synchronous designs. While somewhat uncommon, entire asynchronous CPUs have been built without utilizing a global clock signal. Two notable examples of this are the ARM compliant AMULET and the MIPS R3000 compatible MiniMIPS. Rather than totally removing the clock signal, some CPU designs allow certain portions of the device to be asynchronous, such as using asynchronous ALUs in conjunction with superscalar pipelining to achieve some arithmetic performance gains. While it is not altogether clear whether totally asynchronous designs can perform at a comparable or better level than their synchronous counterparts, it is evident that they do at least excel in simpler math operations. This, combined with their excellent power consumption and heat dissipation properties, makes them very suitable for embedded computers (Garside et al. 1999).
[edit] Parallelism
Main article: Parallel computing
Model of a subscalar CPU. Notice that it takes fifteen cycles to complete three instructions.
The description of the basic operation of a CPU offered in the previous section describes the simplest form that a CPU can take. This type of CPU, usually referred to as subscalar, operates on and executes one instruction on one or two pieces of data at a time.
This process gives rise to an inherent inefficiency in subscalar CPUs. Since only one instruction is executed at a time, the entire CPU must wait for that instruction to complete before proceeding to the next instruction. As a result the subscalar CPU gets "hung up" on instructions which take more than one clock cycle to complete execution. Even adding a second execution unit (see below) does not improve performance much; rather than one pathway being hung up, now two pathways are hung up and the number of unused transistors is increased. This design, wherein the CPU's execution resources can operate on only one instruction at a time, can only possibly reach scalar performance (one instruction per clock). However, the performance is nearly always subscalar (less than one instruction per cycle).
Attempts to achieve scalar and better performance have resulted in a variety of design methodologies that cause the CPU to behave less linearly and more in parallel. When referring to parallelism in CPUs, two terms are generally used to classify these design techniques. Instruction level parallelism (ILP) seeks to increase the rate at which instructions are executed within a CPU (that is, to increase the utilization of on-die execution resources), and thread level parallelism (TLP) purposes to increase the number of threads (effectively individual programs) that a CPU can execute simultaneously. Each methodology differs both in the ways in which they are implemented, as well as the relative effectiveness they afford in increasing the CPU's performance for an application.[10]
[edit] Instruction level parallelism
Main articles: Instruction pipelining and Superscalar
Basic five-stage pipeline. In the best case scenario, this pipeline can sustain a completion rate of one instruction per cycle.
One of the simplest methods used to accomplish increased parallelism is to begin the first steps of instruction fetching and decoding before the prior instruction finishes executing. This is the simplest form of a technique known as instruction pipelining, and is utilized in almost all modern general-purpose CPUs. Pipelining allows more than one instruction to be executed at any given time by breaking down the execution pathway into discrete stages. This separation can be compared to an assembly line, in which an instruction is made more complete at each stage until it exits the execution pipeline and is retired.
Pipelining does, however, introduce the possibility for a situation where the result of the previous operation is needed to complete the next operation; a condition often termed data dependency conflict. To cope with this, additional care must be taken to check for these sorts of conditions and delay a portion of the instruction pipeline if this occurs. Naturally, accomplishing this requires additional circuitry, so pipelined processors are more complex than subscalar ones (though not very significantly so). A pipelined processor can become very nearly scalar, inhibited only by pipeline stalls (an instruction spending more than one clock cycle in a stage).
Simple superscalar pipeline. By fetching and dispatching two instructions at a time, a maximum of two instructions per cycle can be completed.
Further improvement upon the idea of instruction pipelining led to the development of a method that decreases the idle time of CPU components even further. Designs that are said to be superscalar include a long instruction pipeline and multiple identical execution units. [Huynh 2003] In a superscalar pipeline, multiple instructions are read and passed to a dispatcher, which decides whether or not the instructions can be executed in parallel (simultaneously). If so they are dispatched to available execution units, resulting in the ability for several instructions to be executed simultaneously. In general, the more instructions a superscalar CPU is able to dispatch simultaneously to waiting execution units, the more instructions will be completed in a given cycle.
Most of the difficulty in the design of a superscalar CPU architecture lies in creating an effective dispatcher. The dispatcher needs to be able to quickly and correctly determine whether instructions can be executed in parallel, as well as dispatch them in such a way as to keep as many execution units busy as possible. This requires that the instruction pipeline is filled as often as possible and gives rise to the need in superscalar architectures for significant amounts of CPU cache. It also makes hazard-avoiding techniques like branch prediction, speculative execution, and out-of-order execution crucial to maintaining high levels of performance. By attempting to predict which branch (or path) a conditional instruction will take, the CPU can minimize the number of times that the entire pipeline must wait until a conditional instruction is completed. Speculative execution often provides modest performance increases by executing portions of code that may or may not be needed after a conditional operation completes. Out-of-order execution somewhat rearranges the order in which instructions are executed to reduce delays due to data dependencies.
In the case where a portion of the CPU is superscalar and part is not, the part which is not suffers a performance penalty due to scheduling stalls. The original Intel Pentium (P5) had two superscalar ALUs which could accept one instruction per clock each, but its FPU could not accept one instruction per clock. Thus the P5 was integer superscalar but not floating point superscalar. Intel's successor to the Pentium architecture, P6, added superscalar capabilities to its floating point features, and therefore afforded a significant increase in floating point instruction performance.
Both simple pipelining and superscalar design increase a CPU's ILP by allowing a single processor to complete execution of instructions at rates surpassing one instruction per cycle (IPC).[11] Most modern CPU designs are at least somewhat superscalar, and nearly all general purpose CPUs designed in the last decade are superscalar. In later years some of the emphasis in designing high-ILP computers has been moved out of the CPU's hardware and into its software interface, or ISA. The strategy of the very long instruction word (VLIW) causes some ILP to become implied directly by the software, reducing the amount of work the CPU must perform to boost ILP and thereby reducing the design's complexity.
[edit] Thread level parallelism
Another strategy of achieving performance is to execute multiple programs or threads in parallel. This area of research is known as parallel computing. In Flynn's taxonomy, this strategy is known as Multiple Instructions-Multiple Data or MIMD.
One technology used for this purpose was multiprocessing (MP). The initial flavor of this technology is known as symmetric multiprocessing (SMP), where a small number of CPUs share a coherent view of their memory system. In this scheme, each CPU has additional hardware to maintain a constantly up-to-date view of memory. By avoiding stale views of memory, the CPUs can cooperate on the same program and programs can migrate from one CPU to another. To increase the number of cooperating CPUs beyond a handful, schemes such as non-uniform memory access (NUMA) and directory-based coherence protocols were introduced in the 1990s. SMP systems are limited to a small number of CPUs while NUMA systems have been built with thousands of processors. Initially, multiprocessing was built using multiple discrete CPUs and boards to implement the interconnect between the processors. When the processors and their interconnect are all implemented on a single silicon chip, the technology is known as a multi-core microprocessor.
It was later recognized that finer-grain parallelism existed with a single program. A single program might have several threads (or functions) that could be executed separately or in parallel. Some of earliest examples of this technology implemented input/output processing such as direct memory access as a separate thread from the computation thread. A more general approach to this technology was introduced in the 1970s when systems were designed to run multiple computation threads in parallel. This technology is known as multi-threading (MT). This approach is considered more cost-effective than multiprocessing, as only a small number of components within a CPU is replicated in order to support MT as opposed to the entire CPU in the case of MP. In MT, the execution units and the memory system including the caches are shared among multiple threads. The downside of MT is that the hardware support for multithreading is more visible to software than that of MP and thus supervisor software like operating systems have to undergo larger changes to support MT. One type of MT that was implemented is known as block multithreading, where one thread is executed until it is stalled waiting for data to return from external memory. In this scheme, the CPU would then quickly switch to another thread which is ready to run, the switch often done in one CPU clock cycle. Another type of MT is known as simultaneous multithreading, where instructions of multiple threads are executed in parallel within one CPU clock cycle.
For several decades from the 1970s to early 2000s, the focus in designing high performance general purpose CPUs was largely on achieving high ILP through technologies such as pipelining, caches, superscalar execution, out-of-order execution, etc. This trend culminated in large, power-hungry CPUs such as the Intel Pentium 4. By the early 2000s, CPU designers were thwarted from achieving higher performance from ILP techniques due to the growing disparity between CPU operating frequencies and main memory operating frequencies as well as escalating CPU power dissipation owing to more esoteric ILP techniques.
CPU designers then borrowed ideas from commercial computing markets such as transaction processing, where the aggregate performance of multiple programs, also known as throughput computing, was more important than the performance of a single thread or program.
This reversal of emphasis is evidenced by the proliferation of dual and multiple core CMP (chip-level multiprocessing) designs and notably, Intel's newer designs resembling its less superscalar P6 architecture. Late designs in several processor families exhibit CMP, including the x86-64 Opteron and Athlon 64 X2, the SPARC UltraSPARC T1, IBM POWER4 and POWER5, as well as several video game console CPUs like the Xbox 360's triple-core PowerPC design, and the PS3's 8-core Cell microprocessor.
[edit] Data parallelism
Main articles: Vector processor and SIMD
A less common but increasingly important paradigm of CPUs (and indeed, computing in general) deals with data parallelism. The processors discussed earlier are all referred to as some type of scalar device.[12] As the name implies, vector processors deal with multiple pieces of data in the context of one instruction. This contrasts with scalar processors, which deal with one piece of data for every instruction. Using Flynn's taxonomy, these two schemes of dealing with data are generally referred to as SISD (single instruction, single data) and SIMD (single instruction, multiple data), respectively. The great utility in creating CPUs that deal with vectors of data lies in optimizing tasks that tend to require the same operation (for example, a sum or a dot product) to be performed on a large set of data. Some classic examples of these types of tasks are multimedia applications (images, video, and sound), as well as many types of scientific and engineering tasks. Whereas a scalar CPU must complete the entire process of fetching, decoding, and executing each instruction and value in a set of data, a vector CPU can perform a single operation on a comparatively large set of data with one instruction. Of course, this is only possible when the application tends to require many steps which apply one operation to a large set of data.
Most early vector CPUs, such as the Cray-1, were associated almost exclusively with scientific research and cryptography applications. However, as multimedia has largely shifted to digital media, the need for some form of SIMD in general-purpose CPUs has become significant. Shortly after floating point execution units started to become commonplace to include in general-purpose processors, specifications for and implementations of SIMD execution units also began to appear for general-purpose CPUs. Some of these early SIMD specifications like Intel's MMX were integer-only. This proved to be a significant impediment for some software developers, since many of the applications that benefit from SIMD primarily deal with floating point numbers. Progressively, these early designs were refined and remade into some of the common, modern SIMD specifications, which are usually associated with one ISA. Some notable modern examples are Intel's SSE and the PowerPC-related AltiVec (also known as VMX).[13]
Discrete transistor and IC CPUs
CPU, core memory, and external bus interface of a DEC PDP-8/I. made of medium-scale integrated circuits
The design complexity of CPUs increased as various technologies facilitated building smaller and more reliable electronic devices. The first such improvement came with the advent of the transistor. Transistorized CPUs during the 1950s and 1960s no longer had to be built out of bulky, unreliable, and fragile switching elements like vacuum tubes and electrical relays. With this improvement more complex and reliable CPUs were built onto one or several printed circuit boards containing discrete (individual) components.
During this period, a method of manufacturing many transistors in a compact space gained popularity. The integrated circuit (IC) allowed a large number of transistors to be manufactured on a single semiconductor-based die, or "chip." At first only very basic non-specialized digital circuits such as NOR gates were miniaturized into ICs. CPUs based upon these "building block" ICs are generally referred to as "small-scale integration" (SSI) devices. SSI ICs, such as the ones used in the Apollo guidance computer, usually contained transistor counts numbering in multiples of ten. To build an entire CPU out of SSI ICs required thousands of individual chips, but still consumed much less space and power than earlier discrete transistor designs. As microelectronic technology advanced, an increasing number of transistors were placed on ICs, thus decreasing the quantity of individual ICs needed for a complete CPU. MSI and LSI (medium- and large-scale integration) ICs increased transistor counts to hundreds, and then thousands.
In 1964 IBM introduced its System/360 computer architecture which was used in a series of computers that could run the same programs with different speed and performance. This was significant at a time when most electronic computers were incompatible with one another, even those made by the same manufacturer. To facilitate this improvement, IBM utilized the concept of a microprogram (often called "microcode"), which still sees widespread usage in modern CPUs (Amdahl et al. 1964). The System/360 architecture was so popular that it dominated the mainframe computer market for the decades and left a legacy that is still continued by similar modern computers like the IBM zSeries. In the same year (1964), Digital Equipment Corporation (DEC) introduced another influential computer aimed at the scientific and research markets, the PDP-8. DEC would later introduce the extremely popular PDP-11 line that originally was built with SSI ICs but was eventually implemented with LSI components once these became practical. In stark contrast with its SSI and MSI predecessors, the first LSI implementation of the PDP-11 contained a CPU composed of only four LSI integrated circuits (Digital Equipment Corporation 1975).
Transistor-based computers had several distinct advantages over their predecessors. Aside from facilitating increased reliability and lower power consumption, transistors also allowed CPUs to operate at much higher speeds because of the short switching time of a transistor in comparison to a tube or relay. Thanks to both the increased reliability as well as the dramatically increased speed of the switching elements (which were almost exclusively transistors by this time), CPU clock rates in the tens of megahertz were obtained during this period. Additionally while discrete transistor and IC CPUs were in heavy usage, new high-performance designs like SIMD (Single Instruction Multiple Data) vector processors began to appear. These early experimental designs later gave rise to the era of specialized supercomputers like those made by Cray Inc.
[edit] Microprocessors
Main article: Microprocessor
The integrated circuit from an Intel 8742, an 8-bit microcontroller that includes a CPU running at 12 MHz, 128 bytes of RAM, 2048 bytes of EPROM, and I/O in the same chip.
Intel 80486DX2 microprocessor in a ceramic PGA package.
The introduction of the microprocessor in the 1970s significantly affected the design and implementation of CPUs. Since the introduction of the first microprocessor (the Intel 4004) in 1970 and the first widely used microprocessor (the Intel 8080) in 1974, this class of CPUs has almost completely overtaken all other central processing unit implementation methods. Mainframe and minicomputer manufacturers of the time launched proprietary IC development programs to upgrade their older computer architectures, and eventually produced instruction set compatible microprocessors that were backward-compatible with their older hardware and software. Combined with the advent and eventual vast success of the now ubiquitous personal computer, the term "CPU" is now applied almost exclusively to microprocessors.
Previous generations of CPUs were implemented as discrete components and numerous small integrated circuits (ICs) on one or more circuit boards. Microprocessors, on the other hand, are CPUs manufactured on a very small number of ICs; usually just one. The overall smaller CPU size as a result of being implemented on a single die means faster switching time because of physical factors like decreased gate parasitic capacitance. This has allowed synchronous microprocessors to have clock rates ranging from tens of megahertz to several gigahertz. Additionally, as the ability to construct exceedingly small transistors on an IC has increased, the complexity and number of transistors in a single CPU has increased dramatically. This widely observed trend is described by Moore's law, which has proven to be a fairly accurate predictor of the growth of CPU (and other IC) complexity to date.
While the complexity, size, construction, and general form of CPUs have changed drastically over the past sixty years, it is notable that the basic design and function has not changed much at all. Almost all common CPUs today can be very accurately described as von Neumann stored-program machines. As the aforementioned Moore's law continues to hold true, concerns have arisen about the limits of integrated circuit transistor technology. Extreme miniaturization of electronic gates is causing the effects of phenomena like electromigration and subthreshold leakage to become much more significant. These newer concerns are among the many factors causing researchers to investigate new methods of computing such as the quantum computer, as well as to expand the usage of parallelism and other methods that extend the usefulness of the classical von Neumann model.
The design complexity of CPUs increased as various technologies facilitated building smaller and more reliable electronic devices. The first such improvement came with the advent of the transistor. Transistorized CPUs during the 1950s and 1960s no longer had to be built out of bulky, unreliable, and fragile switching elements like vacuum tubes and electrical relays. With this improvement more complex and reliable CPUs were built onto one or several printed circuit boards containing discrete (individual) components.
During this period, a method of manufacturing many transistors in a compact space gained popularity. The integrated circuit (IC) allowed a large number of transistors to be manufactured on a single semiconductor-based die, or "chip." At first only very basic non-specialized digital circuits such as NOR gates were miniaturized into ICs. CPUs based upon these "building block" ICs are generally referred to as "small-scale integration" (SSI) devices. SSI ICs, such as the ones used in the Apollo guidance computer, usually contained transistor counts numbering in multiples of ten. To build an entire CPU out of SSI ICs required thousands of individual chips, but still consumed much less space and power than earlier discrete transistor designs. As microelectronic technology advanced, an increasing number of transistors were placed on ICs, thus decreasing the quantity of individual ICs needed for a complete CPU. MSI and LSI (medium- and large-scale integration) ICs increased transistor counts to hundreds, and then thousands.
In 1964 IBM introduced its System/360 computer architecture which was used in a series of computers that could run the same programs with different speed and performance. This was significant at a time when most electronic computers were incompatible with one another, even those made by the same manufacturer. To facilitate this improvement, IBM utilized the concept of a microprogram (often called "microcode"), which still sees widespread usage in modern CPUs (Amdahl et al. 1964). The System/360 architecture was so popular that it dominated the mainframe computer market for the decades and left a legacy that is still continued by similar modern computers like the IBM zSeries. In the same year (1964), Digital Equipment Corporation (DEC) introduced another influential computer aimed at the scientific and research markets, the PDP-8. DEC would later introduce the extremely popular PDP-11 line that originally was built with SSI ICs but was eventually implemented with LSI components once these became practical. In stark contrast with its SSI and MSI predecessors, the first LSI implementation of the PDP-11 contained a CPU composed of only four LSI integrated circuits (Digital Equipment Corporation 1975).
Transistor-based computers had several distinct advantages over their predecessors. Aside from facilitating increased reliability and lower power consumption, transistors also allowed CPUs to operate at much higher speeds because of the short switching time of a transistor in comparison to a tube or relay. Thanks to both the increased reliability as well as the dramatically increased speed of the switching elements (which were almost exclusively transistors by this time), CPU clock rates in the tens of megahertz were obtained during this period. Additionally while discrete transistor and IC CPUs were in heavy usage, new high-performance designs like SIMD (Single Instruction Multiple Data) vector processors began to appear. These early experimental designs later gave rise to the era of specialized supercomputers like those made by Cray Inc.
[edit] Microprocessors
Main article: Microprocessor
The integrated circuit from an Intel 8742, an 8-bit microcontroller that includes a CPU running at 12 MHz, 128 bytes of RAM, 2048 bytes of EPROM, and I/O in the same chip.
Intel 80486DX2 microprocessor in a ceramic PGA package.
The introduction of the microprocessor in the 1970s significantly affected the design and implementation of CPUs. Since the introduction of the first microprocessor (the Intel 4004) in 1970 and the first widely used microprocessor (the Intel 8080) in 1974, this class of CPUs has almost completely overtaken all other central processing unit implementation methods. Mainframe and minicomputer manufacturers of the time launched proprietary IC development programs to upgrade their older computer architectures, and eventually produced instruction set compatible microprocessors that were backward-compatible with their older hardware and software. Combined with the advent and eventual vast success of the now ubiquitous personal computer, the term "CPU" is now applied almost exclusively to microprocessors.
Previous generations of CPUs were implemented as discrete components and numerous small integrated circuits (ICs) on one or more circuit boards. Microprocessors, on the other hand, are CPUs manufactured on a very small number of ICs; usually just one. The overall smaller CPU size as a result of being implemented on a single die means faster switching time because of physical factors like decreased gate parasitic capacitance. This has allowed synchronous microprocessors to have clock rates ranging from tens of megahertz to several gigahertz. Additionally, as the ability to construct exceedingly small transistors on an IC has increased, the complexity and number of transistors in a single CPU has increased dramatically. This widely observed trend is described by Moore's law, which has proven to be a fairly accurate predictor of the growth of CPU (and other IC) complexity to date.
While the complexity, size, construction, and general form of CPUs have changed drastically over the past sixty years, it is notable that the basic design and function has not changed much at all. Almost all common CPUs today can be very accurately described as von Neumann stored-program machines. As the aforementioned Moore's law continues to hold true, concerns have arisen about the limits of integrated circuit transistor technology. Extreme miniaturization of electronic gates is causing the effects of phenomena like electromigration and subthreshold leakage to become much more significant. These newer concerns are among the many factors causing researchers to investigate new methods of computing such as the quantum computer, as well as to expand the usage of parallelism and other methods that extend the usefulness of the classical von Neumann model.
History of CPUs
Main article: History of general purpose CPUs
EDVAC, one of the first electronic stored program computers.
Prior to the advent of machines that resemble today's CPUs, computers such as the ENIAC had to be physically rewired in order to perform different tasks. These machines are often referred to as "fixed-program computers," since they had to be physically reconfigured in order to run a different program. Since the term "CPU" is generally defined as a software (computer program) execution device, the earliest devices that could rightly be called CPUs came with the advent of the stored-program computer.
The idea of a stored-program computer was already present during ENIAC's design, but was initially omitted so the machine could be finished sooner. On June 30, 1945, before ENIAC was even completed, mathematician John von Neumann distributed the paper entitled "First Draft of a Report on the EDVAC." It outlined the design of a stored-program computer that would eventually be completed in August 1949 (von Neumann 1945). EDVAC was designed to perform a certain number of instructions (or operations) of various types. These instructions could be combined to create useful programs for the EDVAC to run. Significantly, the programs written for EDVAC were stored in high-speed computer memory rather than specified by the physical wiring of the computer. This overcame a severe limitation of ENIAC, which was the large amount of time and effort it took to reconfigure the computer to perform a new task. With von Neumann's design, the program, or software, that EDVAC ran could be changed simply by changing the contents of the computer's memory. [1]
While von Neumann is most often credited with the design of the stored-program computer because of his design of EDVAC, others before him such as Konrad Zuse had suggested similar ideas. Additionally, the so-called Harvard architecture of the Harvard Mark I, which was completed before EDVAC, also utilized a stored-program design using punched paper tape rather than electronic memory. The key difference between the von Neumann and Harvard architectures is that the latter separates the storage and treatment of CPU instructions and data, while the former uses the same memory space for both. Most modern CPUs are primarily von Neumann in design, but elements of the Harvard architecture are commonly seen as well.
Being digital devices, all CPUs deal with discrete states and therefore require some kind of switching elements to differentiate between and change these states. Prior to commercial acceptance of the transistor, electrical relays and vacuum tubes (thermionic valves) were commonly used as switching elements. Although these had distinct speed advantages over earlier, purely mechanical designs, they were unreliable for various reasons. For example, building direct current sequential logic circuits out of relays requires additional hardware to cope with the problem of contact bounce. While vacuum tubes do not suffer from contact bounce, they must heat up before becoming fully operational and eventually stop functioning altogether.[2] Usually, when a tube failed, the CPU would have to be diagnosed to locate the failing component so it could be replaced. Therefore, early electronic (vacuum tube based) computers were generally faster but less reliable than electromechanical (relay based) computers.
Tube computers like EDVAC tended to average eight hours between failures, whereas relay computers like the (slower, but earlier) Harvard Mark I failed very rarely (Weik 1961:238). In the end, tube based CPUs became dominant because the significant speed advantages afforded generally outweighed the reliability problems. Most of these early synchronous CPUs ran at low clock rates compared to modern microelectronic designs (see below for a discussion of clock rate). Clock signal frequencies ranging from 100 kHz to 4 MHz were very common at this time, limited largely by the speed of the switching devices they were built with.
EDVAC, one of the first electronic stored program computers.
Prior to the advent of machines that resemble today's CPUs, computers such as the ENIAC had to be physically rewired in order to perform different tasks. These machines are often referred to as "fixed-program computers," since they had to be physically reconfigured in order to run a different program. Since the term "CPU" is generally defined as a software (computer program) execution device, the earliest devices that could rightly be called CPUs came with the advent of the stored-program computer.
The idea of a stored-program computer was already present during ENIAC's design, but was initially omitted so the machine could be finished sooner. On June 30, 1945, before ENIAC was even completed, mathematician John von Neumann distributed the paper entitled "First Draft of a Report on the EDVAC." It outlined the design of a stored-program computer that would eventually be completed in August 1949 (von Neumann 1945). EDVAC was designed to perform a certain number of instructions (or operations) of various types. These instructions could be combined to create useful programs for the EDVAC to run. Significantly, the programs written for EDVAC were stored in high-speed computer memory rather than specified by the physical wiring of the computer. This overcame a severe limitation of ENIAC, which was the large amount of time and effort it took to reconfigure the computer to perform a new task. With von Neumann's design, the program, or software, that EDVAC ran could be changed simply by changing the contents of the computer's memory. [1]
While von Neumann is most often credited with the design of the stored-program computer because of his design of EDVAC, others before him such as Konrad Zuse had suggested similar ideas. Additionally, the so-called Harvard architecture of the Harvard Mark I, which was completed before EDVAC, also utilized a stored-program design using punched paper tape rather than electronic memory. The key difference between the von Neumann and Harvard architectures is that the latter separates the storage and treatment of CPU instructions and data, while the former uses the same memory space for both. Most modern CPUs are primarily von Neumann in design, but elements of the Harvard architecture are commonly seen as well.
Being digital devices, all CPUs deal with discrete states and therefore require some kind of switching elements to differentiate between and change these states. Prior to commercial acceptance of the transistor, electrical relays and vacuum tubes (thermionic valves) were commonly used as switching elements. Although these had distinct speed advantages over earlier, purely mechanical designs, they were unreliable for various reasons. For example, building direct current sequential logic circuits out of relays requires additional hardware to cope with the problem of contact bounce. While vacuum tubes do not suffer from contact bounce, they must heat up before becoming fully operational and eventually stop functioning altogether.[2] Usually, when a tube failed, the CPU would have to be diagnosed to locate the failing component so it could be replaced. Therefore, early electronic (vacuum tube based) computers were generally faster but less reliable than electromechanical (relay based) computers.
Tube computers like EDVAC tended to average eight hours between failures, whereas relay computers like the (slower, but earlier) Harvard Mark I failed very rarely (Weik 1961:238). In the end, tube based CPUs became dominant because the significant speed advantages afforded generally outweighed the reliability problems. Most of these early synchronous CPUs ran at low clock rates compared to modern microelectronic designs (see below for a discussion of clock rate). Clock signal frequencies ranging from 100 kHz to 4 MHz were very common at this time, limited largely by the speed of the switching devices they were built with.
CPU" redirects here.
For other uses, see CPU (disambiguation).
Die of an Intel 80486DX2 microprocessor (actual size: 12×6.75 mm) in its packaging.
A central processing unit (CPU) is an electronic circuit that can execute computer programs. This broad definition can easily be applied to many early computers that existed long before the term "CPU" ever came into widespread usage. The term itself and its initialism have been in use in the computer industry at least since the early 1960s (Weik 1961). The form, design and implementation of CPUs have changed dramatically since the earliest examples, but their fundamental operation has remained much the same.
Early CPUs were custom-designed as a part of a larger, sometimes one-of-a-kind, computer. However, this costly method of designing custom CPUs for a particular application has largely given way to the development of mass-produced processors that are suited for one or many purposes. This standardization trend generally began in the era of discrete transistor mainframes and minicomputers and has rapidly accelerated with the popularization of the integrated circuit (IC). The IC has allowed increasingly complex CPUs to be designed and manufactured to tolerances on the order of nanometers. Both the miniaturization and standardization of CPUs have increased the presence of these digital devices in modern life far beyond the limited application of dedicated computing machines. Modern microprocessors appear in everything from automobiles to cell phones to children's toys.
Die of an Intel 80486DX2 microprocessor (actual size: 12×6.75 mm) in its packaging.
A central processing unit (CPU) is an electronic circuit that can execute computer programs. This broad definition can easily be applied to many early computers that existed long before the term "CPU" ever came into widespread usage. The term itself and its initialism have been in use in the computer industry at least since the early 1960s (Weik 1961). The form, design and implementation of CPUs have changed dramatically since the earliest examples, but their fundamental operation has remained much the same.
Early CPUs were custom-designed as a part of a larger, sometimes one-of-a-kind, computer. However, this costly method of designing custom CPUs for a particular application has largely given way to the development of mass-produced processors that are suited for one or many purposes. This standardization trend generally began in the era of discrete transistor mainframes and minicomputers and has rapidly accelerated with the popularization of the integrated circuit (IC). The IC has allowed increasingly complex CPUs to be designed and manufactured to tolerances on the order of nanometers. Both the miniaturization and standardization of CPUs have increased the presence of these digital devices in modern life far beyond the limited application of dedicated computing machines. Modern microprocessors appear in everything from automobiles to cell phones to children's toys.
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