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From: wang@m.cs.uiuc.edu (Eric Wang)
Newsgroups: comp.binaries.ibm.pc.d
Subject: Re: Sys Req - key.
Keywords: SysReq
Message-ID: <1989Dec5.190738.17084@ux1.cso.uiuc.edu>
Date: 5 Dec 89 19:07:38 GMT
References: <187@nmtvax.nmt.edu>
Sender: news@ux1.cso.uiuc.edu (News)
Reply-To: wang@cs.uiuc.edu (Eric Wang)
Organization: University of Illinois, Dept. of Comp. Science, Urbana
Lines: 47

In article <187@nmtvax.nmt.edu> jeff@nmtvax.nmt.edu (- Jeff -) writes:
>This is probably a useless question, but there is a mysterious key on my 
>XT-keyboard called the 'Sys Req' key. I've seen this key on many other 
>IBM and compatible keyboards and I am just curious if anyone knows what it 
>is for and how to access it? I can't seem to find anything in my DOS 3.3 
>manual about it. Anyone know what this key is for? Just curious...

This is only what little I know of SysReq.  It does not claim to be a
comprehensive, technical treatise.

The SysReq key is indeed a special key.  Of all keys on the keyboard, it is
the only one which is NOT processed through the INT 09 Keyboard interrupt.
Instead, it has a special interrupt all to itself (INT 15, I believe).  It
is a carryover from the days of pre-PC IBM terminals, when it was used to
send noninterruptable signals to e.g. an IBM mainframe, for use in 
establishing a connection and/or starting/ending/otherwise modifying a
login session.  Though it is little used today by most engineering, business,
or recreational programs (can't say about PC-mainframe connectivity programs,
as I've never seen one), it remains extremely useful for PC-host connection
programs.  Its primary strength is the fact that it is not affected by any
TSRs or the like that are chained to the keyboard interrupt, so that no
matter how badly your runaway program has messed up your keyboard layout, you
can always rely on SysReq to come through with its meaning unchanged.

One elegant and exceedingly useful use of SysReq was in an ANSI terminal
emulation program written by Doug Schafer for use on the rather specialized
Ethernet equipment that we had installed at Caltech.  This wonderful comm
program, which was called Ansi PopTerm,  was done as a TSR that would 
communicate directly with the Ethernet card in the machine when active, and
hide in the background when not active.  It used SysReq as its hotkey to 
toggle between the host and the PC.  The nice thing about this program was 
that you could run any application on the PC while you logged into a remote
host and ran another application(s) there, then toggle between the two at
the touch of a key.  My friends and I used it to play Empire, and it beat
logging in via Kermit or Cterm hands down, mostly because we could escape
back to the PC at the touch of a single key and edit a map file, then pop
back to the host, and do this so quickly that it made it feasible to
interactively map a scout's entire move.  To the best of my knowledge,
this program is alive and well and still widely used at Caltech.  

Let me state right now that I don't know how to write a program to use 
SysReq.

Eric Wang
wang@cs.uiuc.edu

"Birds of prey know they're cool."