> On May 25, 2017, at 10:41 AM, Noel Chiappa via cctalk <cctalk@classiccmp.org>
> wrote:
>
>> From: Anders Nelson
>
>> Heavens, why are the bit positions in descending order right to left in
>> that PCM-12?
>
> Numbering bits in descending order from right to left (AKA increasing order
> from left to right) used to be the standard - IBM S/360, PDP-10, etc, etc
> all did it that way.
For some definition of "standard". It seems that IBM did this, and DEC prior
to the PDP-11, but other machines of that time or earlier numbered bits
according to the power of 2 they represent, i.e., the "current standard". CDC
and Burroughs are examples.
I just went back a bit, and found something interesting: the Dutch machine ARRA
(around 1951, relay based, only worked once) has IBM style bit numbering. But
its successor ARRA II (1953, tubes) uses "current" ordering. Perhaps the
reason is that the description of ARRA II was written by Dijkstra.
paul
