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From: stever@videovax.Tek.COM (Steven E. Rice, P.E.)
Newsgroups: comp.sys.amiga
Subject: Re: 68020
Message-ID: <3830@videovax.Tek.COM>
Date: Tue, 11-Nov-86 15:33:22 EST
Article-I.D.: videovax.3830
Posted: Tue Nov 11 15:33:22 1986
Date-Received: Wed, 12-Nov-86 10:16:57 EST
References: <1150@cit-vax.Caltech.Edu>
Reply-To: stever@videovax.UUCP (Steven E. Rice, P.E.)
Distribution: net
Organization: Tektronix, Inc., Wilsonville, OR
Lines: 47
Keywords: 68020, 68881

In article <1150@cit-vax.Caltech.Edu>, Lyle N. Scheer
(lyles@tybalt.caltech.edu.UUCP) writes:

> A few questions.  First, is it truely possible to use a 68020 replacing
> the 68000(ie, are there any problems, bugs, etc(I have a program that is
> said to make a 68010 work with the amiga, but I don't know if it will do
> the same thing with the 68020).  Second, what is the relation of the
> 68881 to the 68020, and, if you can replace the 68000 with the 68020, is
> there a spot for the 68881?

1. There are a couple of problems in replacing a 68000 with a 68020, one
   of which is also present when replacing a 68000 with a 68010.  In the
   68000, the MOVE SR,<ea> (Move from Status Register) instruction is a
   non-privileged instruction.  In the 68010 and 68020, this instruction
   was made privileged (because visibility of the "S" bit, indicating
   supervisor state, would prevent a virtual memory system from being
   transparently layered over an existing operating system).  The 68010
   and 68020 added the MOVE CCR,<ea> (Move from Condition Code Register)
   instruction to allow user-mode access to the condition codes.  The
   program you have is designed to paper over this problem in software
   that uses the MOVE SR,<ea> instruction by substituting MOVE CCR,<ea>
   instructions.  There is a call in the Amiga operating system that
   gets the condition codes the correct way, no matter which processor
   is used.

   The second problem is physical.  The 68000 (and the 68010) come in a
   64-pin DIP ("aircraft carrier").  The 68020 is packaged in a 114-pin
   Pin Grid Array (PGA).  There needs to be a socket adapter added.

2. The 68020 and the 68881 share a common interface -- the Coprocessor
   Interface.  This is a protocol, rather than extra pins, and it can
   be emulated by the 68000 or 68010 (or, for that matter, the 68008 or
   a Z80!).  However, it is *much* faster with the 68020 because the
   protocol is built into the processor's microcode.  Something else
   that helps speed up the 68020-68881 combination is that they can
   communicate over a 32-bit-wide bus, while the 68000 and 68010 must
   perform almost twice as many data transfers to accomplish the same
   results.

   The 68881 is packaged in a 68-pin PGA.  It can be connected more-or-
   less directly to the bus, so adding it to a 68020 adapter card, along
   with a few control chips, would be the most effective solution.

					Steve Rice

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