Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!aplcen!uakari.primate.wisc.edu!zaphod.mps.ohio-state.edu!tut.cis.ohio-state.edu!ucbvax!hplabs!hp-pcd!hpcvlx!bill
From: bill@hpcvlx.cv.hp.com (Bill Frolik)
Newsgroups: comp.binaries.ibm.pc.d
Subject: Re: Sys Req - key.
Message-ID: <102760025@hpcvlx.cv.hp.com>
Date: 18 Dec 89 17:35:27 GMT
References: <187@nmtvax.nmt.edu>
Organization: Hewlett-Packard Co., Corvallis, OR, USA
Lines: 38


>>This is at least partially false.  On my machine, an XT clone, the sys-req
>>key DOES use int 09.  (int 15h is for cassette io).  It generates ascii 00, 
>>...
>The SysReq key first appeared on the AT, thus it's operation on the PC, XT,
>and clones on them is non-defined. The scan codes don't matter. The BIOS
>processes the scan codes, and that is where an AT BIOS would use INT 15h.
>(remember, the AT didn't have a cassette port). 
>...

Int 9h is the keyboard HARDWARE interrupt vector -- ALL keys go
through it in true PC-compatible systems.  On any transition (up or
down), the BIOS Int 9h handler retrieves the key's scancode from
the keyboard controller, notes whether it was an up or down
transition, and performs an appropriate action.  Most of the time
that means converting the scancode into a keycode if necessary and 
stuffing the results into a queue, where it can be later read using 
Int 16h.  Int 9h treats some keys special: instead of getting
stuffed in the keyqueue, SYS REQ causes the Int 9h handler to turn
around and perform an Int 15h, allowing whoever is fielding Int 15h
requests to handle the transition.  Modifier keys such as Ctrl, Shift,
Alt, and CapsLock set/clear flags in the BIOS RAM "shift state" byte, 
so that subsequent calls to the Int 9h handler will properly convert 
scancode to keycode.  This "shift state" byte can also be read via Int 16h.
Ctrl-Alt-Del gets trapped in the Int 9h handler, causing a "warm boot"
flag (1234h) to be stuffed into the word at 40:72, generally followed 
by a far jump to FFFF:0, causing a reboot.

If you want to see ALL keyboard activity, you write your own Int 9h
handler.  If you simply want to WATCH all keyboard activity without 
changing the way things are currently handled, intercept Int 9h, 
examine the keyboard registers without doing anything, and then pass 
the interrupt on to the real Int 9h handler.

________________________________________

Bill Frolik	Hewlett-Packard Co.
hp-pcd!bill	Corvallis, OR