Path: utzoo!attcan!uunet!auspex!guy
From: guy@auspex.auspex.com (Guy Harris)
Newsgroups: comp.arch
Subject: Re: Computer Architecture methodology
Keywords: A Series, B6700
Message-ID: <3630@auspex.auspex.com>
Date: 9 Jul 90 16:27:49 GMT
References: <844@tredysvr.Tredydev.Unisys.COM> <3627@auspex.auspex.com> <2328@l.cc.purdue.edu>
Distribution: comp.arch
Organization: Auspex Systems, Santa Clara
Lines: 36

>Admittedly there are weird instructions which are at least extremely difficult
>to justify.

Yup; I think the one I concocted is not just "extremely difficult" to
justify, but *impossible* to justify - at least I certainly tried to
make it so....

>In fact, I would even argue that some of the "instructions" on floating
>point chips, such as the transcendental functions, are nothing more
>that programs encoded in microcode.

I'd agree there; Motorola may agree, also, as, at least according to the
Microprocessor Report article on the 68040, they ripped those
instructions out of the on-chip floating point unit and did them in
software - the article says that "by eliminating microcoded algorithms
needed by transcendental functions, the 040's designers were able to
allot more transistors to the computational logic," and then ascribes
much of the speedup of the '040's floating point unit to doing so.

>But there are lots of reasonable hardware instructions which have either
>disappeared or were rarely implemented.

OK, *but* was the one that Unisys removed from the A-series one of them
- and, more importantly, can you tell whether it is without a
description of the instruction?  Complaining as soon as you hear of
somebody removing an instruction from an instruction set, as you
appeared to do in your reply, doesn't seem very sensible; it may well be
that they could have devoted the transistors used to implement that
instruction to implementing one of the ones you wanted, in which case,
had they actually done that, you should be *congratulating* them on
removing it.... 

Giving specific examples of instructions you want in hardware, as you
did in this article, is more useful than just lashing out when you hear
of somebody removing an instruction from an instruction set, as you did
in the posting to which I followed up.