Reproducing old machines with newer technology (Re: PDP-12 at the RICM)
cclist at sydex.com
Wed Jul 15 13:14:05 CDT 2015
On 07/15/2015 10:48 AM, Jay Jaeger wrote:
> Lots of machines supported variable length operands (like the machine
> you reference in the link, IBM S/360, Burroughs, etc. etc. However,
> machines with variable length instructions not split into any kind of
> word boundary are not as common.
Sure, but that doesn't mean that they didn't exist. As a matter of
fact, the machine I cited was *bit*-addressable. That doesn't imply
that any datum was absolved of some sort of alignment. But yes, you
could have bit fields overlapping word boundaries--let's see your 1410
I really don't see much of a fundamental distinction between the 1401,
1410, 7080 or 1620 or any other variable word-length machine of the
time. One really have to ask oneself "why variable word-length?" when
it costs so much in terms of performance. I believe that it's mostly
because memory was very expensive and it was viewed as a way of coping
with that issue.
FWIW, Dijkstra disliked the 1620 immensely. I don't recall his opinion
of the 1401.
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