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From: aglew@ccvaxa.UUCP
Newsgroups: comp.arch
Subject: Re: RPM-40 microprocessor @ 40 MHz; dat
Message-ID: <28200112@ccvaxa>
Date: 26 Feb 88 16:52:00 GMT
References: <1642@mips.mips.COM>
Lines: 33
Nf-ID: #R:mips.mips.COM:1642:ccvaxa:28200112:000:1552
Nf-From: ccvaxa.UUCP!aglew    Feb 26 10:52:00 1988


..> Prefix instructions in the GE RPM-40

I like this idea.

(I should - I used it in a school project back in '84, before I knew details
of the Transputer - I think I got it from an earlier architecture, melded
with the 8088's PREFIX instructions.)

I particularly like how it begins to let the instruction set get independent
of the register size (so long as people do not expect 1<<32 == 0)

A question, though: how would you compare PREFIX to an instruction SHIFT and
OR --  SHOR r,lit ::== r := (r<<14)|lit? PREFIX always seems to eventually
require a specification for one of several literal fields it is extending,
plus it requires state to be saved on interrupts, which leans towards 
assembling the constant in a register.
    On the other hand, you can always build a decoder that never puts prefix
into a register at all, but takes prefix and the prefixed instruction as
one packet. This is nice, and makes it a pity to require the register write.

What do people (particularly the RPM-40 people) feel on this?



Andy "Krazy" Glew. Gould CSD-Urbana.    1101 E. University, Urbana, IL 61801   
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