Scancode

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A scancode (or scan code) is the data that most computer keyboards send to a computer to report which keys have been pressed. A number, or sequence of numbers, is assigned to each key on keyboard.

Mapping key positions by row and column requires less complex computer hardware; therefore, in the past, using software or firmware to translate the scancodes to text characters was less expensive than wiring the keyboard by text character.[citation needed] This cost difference is not as profound as it used to be. However, many types of computers still use their traditional scancodes to maintain backward compatibility.

Some keyboard standards include a scancode for each key being pressed and a different one for each key being released. In addition, many keyboard standards (for example, IBM PC compatible standards) allow the keyboard itself to generate "typematic" repeating keys by having the keyboard itself generate the pressed-key scancode repeatedly while the key is held down, with the release scancode sent once when the key is released.

Contents

[edit] Scancode sets

[edit] USB

USB keyboards use a new set of scancodes, mostly specified in the USB standard. All computers that recognize USB keyboards recognize these new scancodes.

[edit] PC compatibles

Scancodes on IBM PC compatible keyboards are sets of 1 to 3 bytes send by the keyboard. Most character keys have a single byte scancode; keys that perform special functions have 2-byte or 3-byte scancodes, usually beginning with the byte (in hexidecimal) E0, E1, or E2. In addition, a few keys send longer scancodes, effectively emulating a series of keys to make it easier for different types of software to process.

PC-compatibles have used three scancode sets. The most common are the "XT" ("set 1") scancodes, used by the IBM PC XT and earlier; and the "AT" ("set 2") scancodes, used by the IBM PC AT and later, keyboards including PS/2 keyboards. There is also a PS/2-specific "set 3" that is rarely used.[1]

On all PC-compatibles with IBM PC AT or PS/2 keyboards, the computer translates AT (set 2) scancodes into XT (set 1) scancodes for backwards compatibility unless sent a code to do otherwise. Therefore, whether an engineer will encounter AT scancodes or XT scancodes on a modern PC-compatible depends on how the keyboard is being accessed.

[edit] Example PC compatible scancodes

Set 2 (IBM PC AT) scancodes
key press release
A (normal letter) 1C F0 1C
Return / Enter (main keyboard) 5A F0 5A
Enter (numeric keypad) E0 5A F0 5A
Left Windows key E0 1F E0 F0 1F
Right Windows key E0 27 E0 F0 27

[edit] References

  1. ^ Keyboard scancodes (section 10, "Keyboard internal scancodes"), Andries Brouwer. Version 1.2e, 2004-05-20, accessed 2006-11-15.

[edit] See also

[edit] External links


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