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

Paul Koning paulkoning at
Wed Jul 15 13:58:42 CDT 2015

> On Jul 15, 2015, at 2:14 PM, Chuck Guzis <cclist at> wrote:
> 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.


I don’t know what he thought of the 1401.  He did reject the 7040 when it was proposed to the university as its main computer.  That analysis is in (in Dutch).  When comparing with the machine they ended up with (Electrologica EL-X8) I have to concur with his judgment, not that the EL-X8 is flawless, but in key spots it gets things right that IBM gets wrong.


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