Path: utzoo!utgpu!tmsoft!mason
From: mason@tmsoft.UUCP (Dave Mason)
Newsgroups: comp.arch
Subject: Re: Performance increase - a suggestion
Message-ID: <267@tmsoft.UUCP>
Date: 19 Jan 88 14:29:31 GMT
References: <8843@steinmetz.steinmetz.UUCP> <221@imagine.PAWL.RPI.EDU>
Reply-To: mason@tmsoft.UUCP (Dave Mason)
Followup-To: comp.arch
Organization: TM Software Associates, Toronto
Lines: 30

In article <221@imagine.PAWL.RPI.EDU> userfe0e@mts.rpi.edu (George Kyriazis) writes:
"In article <8843@steinmetz.steinmetz.UUCP> sunray!oconnor@steinmetz.UUCP writes:
">An article by physh@unicom.UUCP says:
"> [ proposes increasing computer performance by making busses
">   128 bits wide, while leaving data-paths 32-bits wide ]
">
"  I don't know what he says exactly, but he mentions expanding data paths to
"128 bits.  I don't know, but I thought of that, just a few days ago, but for
"fetching instructions only.
" ...
"  The only problem when doing that is jump instructions.  Assume that memory
"operates at its fastest possible speed.  If you meet a jump instruction
"in the middle of the 128-bit word, you'll have to (more or less) execute
"all the rest up till the end of the fetch.
" ... (Actually it turns
"out that the jump has to be in the first of the 4 instructions in the 128-bit
"word, so as the memory can get the right address.) 

Actually (like everything else) it's already been done.  In this case
by the Control Data 6000 series at least.  They had a mix of 15 and 30
bit instructions in a 60 bit word.  (Probably applies to all other
Cray designs as well, although I haven't programmed others.)

Any label (that could therefore be branched to) forced alignment on a
new word.  Also a 30 bit instruction when you only had 15 bits left in
the current word would go to the next word.  There wasn't a delayed
jump however, so there was no attempt to fill the rest of the word with
useful instructions, it was simply filled with NOPs.  Also a jump could
appear anywhere in a word.  (The FORTRAN compiler was pretty advanced
for its day, and I believe did some code migration to fill words)