Reproducing old machines with newer technology (Re: PDP-12 at the RICM)
paulkoning at comcast.net
Wed Jul 15 13:58:42 CDT 2015
> On Jul 15, 2015, at 2:14 PM, Chuck Guzis <cclist at sydex.com> 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 http://www.cs.utexas.edu/users/EWD/transcriptions/OtherDocs/NN041.html (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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