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From: gnu@hoptoad.uucp (John Gilmore)
Newsgroups: comp.sys.m68k
Subject: Re: MC68030 & MC68882 now out and available; comparisons with MC68020?
Message-ID: <3297@hoptoad.uucp>
Date: Fri, 6-Nov-87 23:16:25 EST
Article-I.D.: hoptoad.3297
Posted: Fri Nov  6 23:16:25 1987
Date-Received: Mon, 9-Nov-87 04:24:28 EST
References: <4733@elroy.Jpl.Nasa.Gov> <542@amc.UUCP> <3570@ccicpg.UUCP> <2274@mcdchg.UUCP>
Organization: Nebula Consultants in San Francisco
Lines: 41

> The 68882  *** IS *** a drop in replacement for the 68881.

Allen H. Brumm (allen@ccicpg.UUCP) writes:
> This may be true for hardware, but not software.  The internal state frames
> for the idle and busy states on the '882 are larger than that of the '881.

heiby@mcdchg.UUCP (Ron Heiby) wrote:
> If you are mucking around with the internal state frames of a chip in
> a way that violates the documented ways of using it, then you get what
> you deserve.

The point is that a Sun or Levco customer can't take a 68882 and 
plug it in where the 68881 goes, since the operating system software
provided by the manufacturers does not know to allocate that much space
for the stack frame.  No "mucking around" is required, simple things
like the space allocation for each process's floating point state will
kill you.  Users end up having to wait for the manufacturer
to update the software and/or firmware.

I used to write that kind of code for Sun and it's easy to set it up
*if you know the size of each stack frame*.  Unfortunately, the stack
frame format field is just 4 bits, assigned at random, so there's no
way a program can tell the size of a stack frame that wasn't designed
yet when the program was written.

I built a set of Sun-1 boot PROMs that would boot either a 68000 or a
68010, by causing a trap and seeing how much the stack pointer moved,
and adjusting the other places that needed to know.  But it's likely
that those PROMs would not be able to boot a 68020...or 68030.

> Obey the rules and you won't get burned.

For a manufacturer, this should read "obey the rules and you'll only
have to do a minor software update".  That's certainly better than
having to rewrite all your user programs to go from 8086 to 80286, but
it's not the whole story.

-- 
{pyramid,ptsfa,amdahl,sun,ihnp4}!hoptoad!gnu			  gnu@toad.com
Love your country but never trust its government.
		      -- from a hand-painted road sign in central Pennsylvania