Re: is Vax addressing sane today

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.

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