🚀 go-pugleaf

In article <44627835.555851238.7102...@news.eternal-september.org>,
Peter Flass  <peter...@yahoo.com> wrote:
>>> And the epitome of the CISC processor, the VAX-11.
>> 
>> Sixteen addressing modes, single instructions to calculate CRCs and polynomials,
>> string translation instructions... the VAX had just about everything except an
>> opcode for "phase of the moon."  Too many bells and whistles to be considered
>> elegant, IMO.

It suffered from a surprising degree of technical myopia.  The VAX was
designed to be implemented in serial microcode, and was quite painful
to overlap or pipline.  The instruction format was an opcode and a series
of variable length operand descriptors which had to be decoded in order since
one decriptor could modify the register used in a subsequent one, e.g.

 ADDW2 (R3)+,(R3)

It can't fetch the second operand until it's decoded the first one,
since the first one modifies R3.  There were also a lot of absurdly
complex instructions like evaluate polynomial which would have made
a lot more sense as subroutines, at least if they had a lighter
weight subroutine call instruction.

I gather the later Vaxen did complex pipelines with interlocks but it
was a lot of effort to solve a problem that they needn't have created
in the first place.  Compare it to the 360/370 which is carefully
designed so that as soon as you've looked at the opcode you can
evaluate all of the operands in parallel.

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