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From: jgd@cix.co.uk (John Dallman)
Newsgroups: comp.arch
Subject: Re: is Vax adressing sane today
Date: Thu, 5 Sep 2024 22:55 +0100 (BST)
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In article <vbd6b9$g147$1@dont-email.me>, ggtgp@yahoo.com (Brett) wrote:

> It has been determined from trusted sources that add from memory 
> and add to memory as used in x86 are sane, and not much of a problem.
> 
> But Vax allows all three arguments to be in memory with different 
> pointers.
> 
> Is this sane, just a natural progression if you allow memory 
> operands?

Memory-to-memory instructions, in general, are hard to get to run fast
with today's processors and memory, simply because memory access times
are long enough for many register-to-register instructions to execute. A
lot of that time can be hidden with good caches and prefetchers, but if
your memory access patterns are complicated, those speedups can fail to
work. 

One reason for memory-to-memory instructions was to avoid the need to
dedicate registers to operands, but that's not much of a problem these
days, since we have space in the CPU for lots of registers and rename
systems for them. 

VAX was designed when heavy use of microcoding seemed like a good idea to
make a CPU at an economical price, and memory wasn't much slower than
registers. It was a backward-looking design in some ways, being a much
better computer for the 1970s, rather than looking ahead to new concepts.
VMS was the last large operating system written in assembly language (and
Bliss, which is somewhat higher-level, bit not much). 

DEC spent a lot of time and money trying to keep VAX competitive and took
too long to accept that was impractical. That was one of the seeds of
their downfall. 

John