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From: davet@oakhill.UUCP (Dave Trissel)
Newsgroups: net.arch,net.micro.68k
Subject: Re: 68k dbcc
Message-ID: <682@oakhill.UUCP>
Date: Sun, 13-Apr-86 04:10:38 EST
Article-I.D.: oakhill.682
Posted: Sun Apr 13 04:10:38 1986
Date-Received: Sun, 20-Apr-86 05:13:57 EST
References: <1511@decwrl.DEC.COM> <5100028@ccvaxa>
Reply-To: davet@oakhill.UUCP (Dave Trissel)
Organization: Motorola Inc. Austin, Tx
Lines: 34
Xref: linus net.arch:2819 net.micro.68k:1537

In article <3479@sun.uucp> kevin@sun.uucp (Kevin Sheehan) writes:
>
>	the loop mode works by using the instruction prefetch register to
>hold the instruction. This means only instructions that are 16 bits TOTAL
>can use loop mode - fortunately, that includes most of the useful ones.
>
>	the only answers I ever got to "why dont the other 16 bit instructions
>do it too" was something about not being able to do RMW write cycles, and
>"internal path/ALU limitations", although the latter still seems like a glib
>answer.  Anybody at MOT have an answer?
>

Sorry I don't have access to the microcoder at this moment, but from what
I remember there are indeed certain classes of instructions
which require some kind of special setup in the control
unit such that they could not be looped.

On the other hand, there is a bit in the microcode for the rest which indicate
whether loop mode is allowed.  I once coded a benchmark and found to  my
suprize a two instruction sequence which didn't run in loop mode.  It turns
out that the microcoder 'thought' nobody would ever code such a loop :-(.

This brings to mind another interesting story.  I wrote a MC6809 debugger
(ASSIST09) as my first 6809 code - and the very first instruction I coded
was STX ,X++ which was supposed to store the address of a table at the
first slot in the table itself.  To my suprize what got stored was the
address of table+2.  I hit one of the designers up (Terry Ritter) and to my
shock he said that they thought there was no reason for anybody to ever code
such an instruction! And that was the first 6809 instruction I coded!

The lesson here is don't ever say "Nobody will ever try X" with an instruction
set.

  --  Dave Trissel  {seismo,ihnp4}!ut-sally!im4u!oakhill!davet