Reproducing old machines with newer technology (Re: PDP-12 at the RICM)

[Chuck Guzis]

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 
do that...

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.

--Chuck