25 May
2017
25 May
'17
10:41 a.m.
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.
Noel
25 May
25 May
11:13 a.m.
On May 25, 2017, at 10:41 AM, Noel Chiappa via cctalk
<cctalk at classiccmp.org> wrote:
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
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. 
11:33 a.m.
On 05/25/2017 08:13 AM, Paul Koning via cctalk wrote:
.
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.
Not all CDC machines did; consider the Cyber 200 series:
http://bitsavers.informatik.uni-stuttgart.de/pdf/cdc/cyber/cyber_200/602560…
But that only made sense, as the Cyber 200 machines are bit-addressable.
For a time, my work straddled both the Cyber 70 and 200 systems, and
so had to adjust my thinking. Not just bit-ordering, but the 70/6000
used 6-bit display code, while the 200 used 8 bit ASCII.
Fun times.
--Chuck
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