Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!rpi!uupsi!grebyn!ckp
From: ckp@grebyn.com (Checkpoint Technologies)
Newsgroups: comp.arch
Subject: Re: Let's pretend
Keywords: Intel, 586, windows
Message-ID: <24124@grebyn.com>
Date: 19 Dec 90 05:38:11 GMT
References: <3042@crdos1.crd.ge.COM> <1990Dec18.082623.16648@kithrup.COM> <15145@ogicse.ogi.edu> <24117@grebyn.com> <9876@pasteur.Berkeley.EDU>
Reply-To: ckp@grebyn.UUCP (Checkpoint Technologies)
Organization: Grebyn Timesharing, Vienna, VA, USA
Lines: 23

In article <9876@pasteur.Berkeley.EDU> jbuck@galileo.berkeley.edu (Joe Buck) writes:
>What you do is you get an illegal instruction trap
>when the wierd instruction is run, and the trap handler then emulates
>the instruction.  The chip-maker releases the code for the trap-handler
>(makes it public) and the PC-clone folks put it in their BIOS ROMs and
>the Unix-port people put it in their kernels.

You're right.  I believed BIOS compatibility would be an issue too, but
maybe not.

You know, Motorola has been getting away with exactly this.  The 68010
took away the user-level MOVE SR,dest instruction.  The 68030 took away
the user-mode CALLM and RETM instructions (good riddance, I say)
introduced on the 68020.  But you know what else? No system I know of
traps and emulates those for backward compatability. Now the 68040
removes the user-mode trig instructions in the FPU, and replaces them
with emulation support. I suspect these will emulated in real systems,
unlike the others.
-- 
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