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

[Jay Jaeger]

Sigh.  Again, the difference is between how OPERANDS were formatted vs.
INSTRUCTIONS.  As I said, I agree that lots of machines had variable
length operands (including a couple at the bit level, which the 1400
series did not do except for an individual character).  But darn few had
variable length INSTRUCTIONS, things like operand address chaining, and
the like.

And again and again - it isn't about what is better or anything like that.

And again and again and again - it isn't about what survived into
current architectures.

Certainly the variable length instruction idea was about conserving memory.

On 7/15/2015 1:14 PM, Chuck Guzis 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.
> 
> --Chuck
> 
> 
>