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From: stern@inmet.UUCP
Newsgroups: net.arch
Subject: RISC vs VAX
Message-ID: <1815@inmet.UUCP>
Date: Sat, 17-Nov-84 01:44:15 EST
Article-I.D.: inmet.1815
Posted: Sat Nov 17 01:44:15 1984
Date-Received: Mon, 19-Nov-84 02:46:17 EST
Lines: 37
Nf-ID: #N:inmet:2500011:000:2273
Nf-From: inmet!stern    Nov 15 10:14:00 1984


In response to "Is is possible to build a RISC in the same class as a VAX?"
I'd have to say the answer is yes and no.  If you want to talk about integer
computations, procedure entry/exit, and data movement, by all means yes.
In theory, it would be possible to design and implement a RISC with a
performance in the same class as a VAX -- meaning only that the RISC could
shuffle data, access memory with a *few* addressing modes (direct and register
indirect are probably the easiest) and zip through some integer computation   
code faster than a VAX 780.  RISC architecture is not suited for floating
point operations, or matrix multiplications, since the arithmetic steps
produce a very tight bottleneck.  In my somewhat young opinion, a RISC is
ideal for LISP hacking -- most of what you are doing is searching, matching,
and chasing pointers.  In this application, a RISC beats a VAX hands-down.

The answer to the original question is also "no," due to the problem of
i/o on a RISC.  Attaching an ACIA (or similar character-oriented i/o device)
to the RISC could be a big difficulty -- the cycle times of the two devices
are different by one or two orders of magnitude.  You can either halt the RISC
while doing character i/o (boo hiss) or use the RISC as an attached processor:
plug it into a spare UNIBUS slot and let a VAX share its memory.  The i/o
problem is then solved: whatever OS you have running on the VAX does the
i/o, and the RISC does the nasty computation.  Perhaps a better question 
would be "Do you want to run a RISC with the power of a VAX stand-alone?"
I think it might be better to plug eight or sixteen RISC boards into a VAX,
rehack UNIX to download microcode for appropriate tasks, and thereby 
save VAX cycles for cycle-hungry tasks.  Example:  Write a search routine
to get downloaded onto a RISC.  Put the relevant data in the common VAX/RISC
memory and let the RISC rip.  Mr. VAX goes and deals with somebody else's
process, and the search job finishes *faster* than if the VAX had done it.
This isn't just a marvel of parallel processing -- the RISC really would
complete the task faster than a VAX could.  They just can perform *every*
task faster than a VAX.


	Hal Stern
	Intermetrics, Inc
	{ihnp4, harpo, esquire, ima}!inmet!stern