Path: utzoo!attcan!uunet!pdn!alan
From: alan@pdn.UUCP (Alan Lovejoy)
Newsgroups: comp.sys.m68k
Subject: Re: opcode emulation (Was: Re: 68020 in a *68010* socket?)
Message-ID: <3447@pdn.UUCP>
Date: 13 Jun 88 00:02:16 GMT
References: <17206@gatech.edu> <10123@mcdchg.UUCP> <17479@glacier.STANFORD.EDU> <6276@cup.portal.com> <17482@glacier.STANFORD.EDU> <182@elgar.UUCP>
Reply-To: alan@pdn.UUCP (0000-Alan Lovejoy)
Organization: Paradyne Corporation, Largo, Florida
Lines: 27

In article <182@elgar.UUCP> ford@kenobi.UUCP (Mike "Ford" Ditto) writes:
>What doesn't seem right is what happened for an instruction that used
>the new scaled index 68020 addressing mode.  The opcode looks almost
>the same, except where there were required zeroes on the 68010, the
>68020 has two bits to specify a scale amount for the index register.
>When I ran this opcode (with a scale value of 4, incompatible with the
>68010) on the 68010, the result was a *bus error*.

The 68010 was not implemented to detect scaled index addressing as an
unimplemented instruction exception.  The bus error you got was probably
just that (due to virtual to physical address translation of a bogus
logical address).  

Motorola fucked this up royally.  One of their rare lapses, so I guess
we shouldn't complain too loudly.  A little light ribbing may be in
order.

Howerver, there is the mc68012, which if I remember correctly is
completely software compatible with the 68020 but pin-compatible with
the 68010.  But my memory is rather vague on this part.  Perhaps someone
else has more authoritative info?

-- 
Alan Lovejoy; alan@pdn; 813-530-8241; Paradyne Corporation: Largo, Florida.
Disclaimer: Do not confuse my views with the official views of Paradyne
            Corporation (regardless of how confusing those views may be).
Motto: Never put off to run-time what you can do at compile-time!