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From: John Levine <johnl@taugh.com>
Newsgroups: comp.arch
Subject: Re: is Vax addressing sane today
Date: Mon, 9 Sep 2024 20:34:39 -0000 (UTC)
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According to Anton Ertl <anton@mips.complang.tuwien.ac.at>:
>Lawrence D'Oliveiro <ldo@nz.invalid> writes:
>>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.
>
>The regularity of the VAX operand formats may actually help build the
>decoder: Decode your byte stream as possible operands, and then let
>the instruction decoder pick the real operands from the potential
>operands.

Urrgh. Some of those bogus operands are indirect indexed auto-increment, so you
are going to be throwing away a whole lot of work.

Compare that to zSeries, where even after 50 years of sticking new instructions
into the holes in the S/360 instruction set, it can still tell the length of the
instruction from the first two bits and the operands from the first byte.

-- 
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