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From: mitchalsup@aol.com (MitchAlsup1)
Newsgroups: comp.arch
Subject: Re: is Vax addressing sane today
Date: Mon, 9 Sep 2024 00:27:39 +0000
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On Sun, 8 Sep 2024 21:09:39 +0000, Lawrence D'Oliveiro wrote:

> On Sun, 8 Sep 2024 17:56:55 +0000, MitchAlsup1 wrote:
>
>> The problem with VAX was NOT that one could not put a lot of work in a
>> single instruction;
>>
>> no,
>>
>> The problem with VAX is that it made putting too much work in a single
>> instruction easy.
>
> Perhaps there is also the issue of the wildly-variable instruction
> length.
> A single VAX operand descriptor could be up to 6 bytes; I think the
> instruction with the most general-format operands could have 6 of them:
> so, plus opcode, such an instruction could be 37 bytes long.

I have not heard an argument that the complex things in VAX ISA are
a) desirable
b) performance helpful

I (sort of) think VAX ISA as a grown up PDP-11, ignoring all the
dastardly complicated instructions it inflicted upon itself. AND
it did inflict those things upon itself.

Restricting a new-VAX-like ISA to 1-2-3 Operand and 1-result with
at most 1 exception would result in a MUCH cleaner and easier to
build machine.

> While the shortest instruction could be just 1 byte.
>
> Even those who are talking about “post-RISC” are, I think, still in
> favour of RISC-style fixed instruction lengths.

I, for the record, are in favor of fixed length instruction-specifier
followed by constants the entirety is the instruction, while the
former minimizes your ability of shooting yourself in the foot.