What is the SysRq key for
Date: 5 Feb 2002 22:03:03 -0400There is no standard use for the key. The BIOS keyboard routines in INT16 simply ignore it; therefore so do the DOS input routines in INT 21 aswell as the keyboard routines in libraries supplied with high-levellanguages.When you press or release a key, the keyboard triggers hardware lineIRQ1, and the CPU calls INT 9. INT 9 reads the scan code from thekeyboard and the shift states from the BIOS data area.What happens next depends on whether your PC’s BIOS supports an enhancedkeyboard (101 or 102 keys). If so, INT 9 calls INT 15 AH=4F to translatethe 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 thekeyboard buffer. The default handler of that function does nothing andsimply returns. (If your PC has an older BIOS that doesn’t support theextended keyboards, INT 15 AH=4F is not called. Early ATs have 84-keykeyboards, so their BIOS calls INT 15 AH=85 but not 4F.
