Path: utzoo!utgpu!water!watmath!ccplumb
From: ccplumb@watmath.waterloo.edu (Colin Plumb)
Newsgroups: comp.arch
Subject: Re: Prefix instructions (Was: RPM-40 microprocessor; data from ISSCC)
Summary: Is a prefix an instruction for skips or delay slots?
Message-ID: <17192@watmath.waterloo.edu>
Date: 2 Mar 88 00:10:25 GMT
References: <9651@steinmetz.steinmetz.UUCP> <9678@steinmetz.steinmetz.UUCP> <1199@microsoft.UUCP> <448@imagine.PAWL.RPI.EDU>
Reply-To: ccplumb@watmath.waterloo.edu (Colin Plumb)
Organization: U of Waterloo, Ontario
Lines: 58
Confusion: U. of Waterloo, Ontario

beowulf!lunge!steinmetz.UUCP!jesup wrote:
>In article <1199@microsoft.UUCP> ccplumb@watmath (Colin Plumb) writes:
>>The only funny question is the semantics: what does a prefix do to an
>>instruction without an immediate constant?
>
>In the RPM-40, nothing.  Goes to the great bit-bucket in the sky.
>In the more general case you mentioned, you COULD seperate prefixes from their
>instructions, but it would really make the reorganizer hard to write (I
>know, I've written them!)  Also, the prefix logic would be more
>complicated (and thus slower/bigger).

Well, what happens if you conditionally skip an instruction with
prefixes?  There are two appraoches:
Skip until the next non-prefix instruction, so

skip
prefix
operation

would work, or allow prefixes to span skips, so you could have

prefix
skip
operation

Both would take the same time, but I don't know which would be easier
to implement.

For chips with delayed jumps, prefix instructions are a pain, since you
can't put a prefix-using instruction into the delay slot.  Another
special case for the code reorganizer to worry about.

And can you put a prefix instruction into the delay slot?  If it's
the target of the jump, it's advantageous to move it back to the
no-op, but a bit confusing.

Essentially, prefix instructions add non-register context which is, as
a general rule, A Bad Thing.  However, they simplify instruction
decode, A Good Thing.  It really only becomes a problem when you're
dealing with skip instructions or delay slots, where you're applying
special-case logic to *one* instruction?  Is a prefix a full-fledged
instruction, or part of a larget unit?

I hope I've made my concerns clear; now can someone soothe them?
I'd love to see a really neat solution to all this.

>
>     //Randell Jesup			      Lunge Software Development
>    //	Dedicated Amiga Programmer            13 Frear Ave, Troy, NY 12180
> \\//	beowulf!lunge!jesup@steinmetz.UUCP    (518) 272-2942
>  \/   (uunet!steinmetz!beowulf!lunge!jesup) BIX: rjesup
>
>(-: The Few, The Proud, The Architects of the RPM40 40MIPS CMOS Micro :-)
--
	-Colin (watmath!ccplumb)

Zippy says:
I've got a COUSIN who works in the GARMENT DISTRICT...