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From: robison@uiucdcsb.CS.UIUC.EDU
Newsgroups: net.arch
Subject: Re: VAX polyd instruction (and the
Message-ID: <5600044@uiucdcsb>
Date: Thu, 20-Mar-86 00:52:00 EST
Article-I.D.: uiucdcsb.5600044
Posted: Thu Mar 20 00:52:00 1986
Date-Received: Sat, 22-Mar-86 06:12:22 EST
References: <119@tekchips.UUCP>
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Nf-From: uiucdcsb.CS.UIUC.EDU!robison    Mar 19 23:52:00 1986


> Anyhow, why beat on the poor little vaxen?  They are but pale shadows compared
> to a real *C*ISC like a Burroughs B6700.  Kind of interesting when you review 
> the old claims about Burroughs architecture being designed for HLL's - what
> percentage of the instructions and whatnot did their compiler writers manage
> to use?

The Burroughs B6700 and its successors (B7700 and current A15) have a aspects
of both CISCS and RISCS.  There are relatively few instructions, but each
instruction has complex semantics.  For example, there are only two
non-immediate load instructions: "value call" and "name call", which
have automatic chain dereferencing and "thunk" evaluation.
For the code I've looked at, good use was made of most of the instruction set
by the ALGOL compiler.  (This is to be expected, since Burroughs ALGOL
has extensions based on the instruction set.  E.g. field extractions.)

The marketing advantage of the complex semantics is that it allows
a wide price range of machines with the same instruction set.
Because the instructions try to describe what to do, and not exactly
how to do it, the higher-priced machines can exploit more parallelism
by rearranging the computations at run-time (and in some cases not doing them!),
which compilers can not do.

The principle problem is other languages which the designers did not
anticipate.  E.g. there is no C compiler available because the current
hardware can not support C's pointers.

Arch D. Robison
University of Illinois