Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!swrinde!ucsd!ucsdhub!hp-sdd!hplabs!hpfcso!hpldola!hpctdlb!hpctdls!wei
From: wei@hpctdls.HP.COM (Bill Ives)
Newsgroups: comp.sys.ibm.pc
Subject: Re: 'Sys Req' - key.
Message-ID: <2010019@hpctdls.HP.COM>
Date: 8 Dec 89 16:22:40 GMT
References: <189@nmtvax.nmt.edu>
Organization: Hewlett-Packard CTD, Colo. Spgs.
Lines: 32



>/ hpctdls:comp.sys.ibm.pc / kds10773@uxa.cso.uiuc.edu (Ken S.) / 11:10 am  Dec  5, 1989 /
>The SysReq key can be polled using entry point 0x85 of int 0x15 (system
>services), however I believe that's only implemented on AT's, so on your
>XT keyboard, it's most likely useless.  I'm afraid I also don't know
>of any less technical ways to access it.

>   - Ken Stillson

    Ken is correct... this is the only standard way of accessing the sys req
    key.   THe original poster also asked what it was used for -- since I
    have used it I may be able to fill in some details there.

    The sys req key was meant to activate the main tasker/controller in a
    multitasking system.  It was theorised at least in the documentation I
    have, that a tasker ( for 286 systems which support multi-tasking) could
    hook into sys req and provide ( or use ) ABIOS routines which give 
    tasks essentail functions in protected mode.  The sys req key is
    ideal for activating a main tasker since the keyboard does not have
    to be polled for an asynchronous sys req to come through ( unlike
    control-break) .  This would have been a good choice for OS/2's session
    manager shell ( the shell that doesn't include PM ), but alas they
    chose control-esc -- don't ask me why :-).   My use of it was to put
    in a hardware debug register manager for the 386 that would work behind
    any application -- even if that application went into protected mode (i.e
    Smalltalk V/286).  

    Thought you might be interested in its intended use - even if obscure.

    Bill Ives
    HP CTD