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From: John Levine <johnl@taugh.com>
Newsgroups: comp.arch
Subject: Re: is Vax adressing sane today
Date: Sat, 7 Sep 2024 21:17:42 -0000 (UTC)
Organization: Taughannock Networks
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According to Anton Ertl <anton@mips.complang.tuwien.ac.at>:
>Given modern OoO technology, even VAX can fly.  It does not matter
>whether, say,
>
>*a++ = *b++ + *c++;
>
>is encoded as 1 VAX instruction, or as 4 ARM A64 instructions, or as 7
>RISC-V instructions, what goes on inside the OoO engine is pretty
>similar in all cases, and so is the performance.

It is my impression that unwinding all the side effects if the reference to "c" causes a
page fault was painful. Particularly keeping in mind that b and c could be the same
register, and if the code were this:

 *a++ = *b++ - *b++

the order of increments and fetches matters.

If you split it into four ARM instructions, a fault just has to restart one of those
instructions which will have at most one register to fix up.

It is my impression that even when the Vax was designed, it was already becoming evident
that the Vax's super dense super encoded instruction set was not going to be a long term
winner. The IBM 801 project was well along in 1975 when they started designing the Vax.

-- 
Regards,
John Levine, johnl@taugh.com, Primary Perpetrator of "The Internet for Dummies",
Please consider the environment before reading this e-mail. https://jl.ly