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comp.os.msdos.programmer FAQ part 2/5
Section - - What is the SysRq key for?

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 There is no standard use for the key. The BIOS keyboard routines in INT
 16 simply ignore it; therefore so do the DOS input routines in INT 21 as
 well as the keyboard routines in libraries supplied with high-level
 languages.

 When you press or release a key, the keyboard triggers hardware line
 IRQ1, and the CPU calls INT 9. INT 9 reads the scan code from the
 keyboard and the shift states from the BIOS data area.

 What happens next depends on whether your PC's BIOS supports an enhanced
 keyboard (101 or 102 keys). If so, INT 9 calls INT 15 AH=4F to translate
 the scan code. If the translated scan code is 54 hex (for the SysRq key)
 then INT 9 calls INT 15 AH=85 and doesn't put the keystroke into the
 keyboard buffer. The default handler of that function does nothing and
 simply returns. (If your PC has an older BIOS that doesn't support the
 extended keyboards, INT 15 AH=4F is not called. Early ATs have 84-key
 keyboards, so their BIOS calls INT 15 AH=85 but not 4F.)

 Thus your program is free to use SysRq for its own purposes, but at the
 cost of some programming. You could hook INT 9, but it's probably easier
 to hook INT 15 AH=85, which is called when SysRq is pressed or released.

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Top Document: comp.os.msdos.programmer FAQ part 2/5
Previous Document: - How can I speed up the keyboard's auto-repeat?
Next Document: - How can my program tell what kind of keyboard is on the system?

Part1 - Part2 - Part3 - Part4 - Part5 - Single Page

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Send corrections/additions to the FAQ Maintainer:
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Last Update March 27 2014 @ 02:11 PM