Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!decwrl!decvax!ucbvax!pasteur!ames!hao!oddjob!gargoyle!ihnp4!homxb!whuts!mtune!mtund!bfb
From: bfb@mtund.ATT.COM (Barry Books)
Newsgroups: comp.arch
Subject: Re: 16 & 32 bit vs 32 bit only instructions for RISC.
Message-ID: <929@mtund.ATT.COM>
Date: 27 Feb 88 03:37:36 GMT
References: <9651@steinmetz.steinmetz.UUCP> <9678@steinmetz.steinmetz.UUCP> <2574@im4u.UUCP>
Reply-To: bfb@mtund.UUCP (Barry Books)
Organization: AT&T ISL Middletown NJ USA
Lines: 25
Keywords: CRISP

I wrote code for CRISP a while ago includes machine language ( no tools )
and was inpressed by the opcodes.  CRISP had only a few instructions but
calling it RISC may be stretching things since it had three operand instructions
and no registers.  However it had a 32 word stack cache which it seems to me
is better than registers ( or even register windows which at least allow future
versions to have more registers without recompiling to take advantage ).  Most
of the 16 bit instructions had 5 bit offsets so they could be used to access
the stack cache with small instructions.  This allowed instruction working on 
local variables to to be small.  The 32 bit instructions I belive had 10 bit
offsets so as the stack cache is made larger it still didn't take a large
address field.  These are also usefull for short jumps.  The 80 bit instructions
could take to address ( usefull for memcpy which seems to happen all to 
frequently ).  The thing that impressed me the most was it didn't take
a killer optimizing compiler ( read big, buggy and slow ) to make the
machine run fast.  It seems for stack based langauges a stack cache is
much more sensible than having a bunch of registers that the compiler
has to figure out what to do with.  Also since the subject of jumps seem
to be popular.  CRISP used a brach predition bit which if correct caused
jumps to take no cycles.  Brach predition is fairly easy for loops the
compiler I used always set it so the brach was taken.  This simple
approach works for loops assuming you go thru them more that once.
All in all a nice design.

Barry Books
mtund!bfb		better is bigger