[TUHS] [TUHS} PDP-11, Unix, octal?

Steve Johnson scj at yaccman.com
Thu Jan 19 07:04:20 AEST 2017

The PDP-10 and the GE/Honeywell were the two machines I recall that
elicited Dennis' comment about 10-track tape drives.  When I ported C
to the Honeywell machine at the Murray Hill comp center, I used 9-bit
bytes as the default, and added a syntax `abcd` to create a constant
in the 6-bit character set.  Most of the OS calls used 6-bit
characters, although the time-sharing system was moving to 9-bits. 
And most of the use of C on the Honeywell was in the time-sharing

Quite a few years later, I discovered accidentally that the syntax
`abcd` was still accepted on the Sun compiler, that had been based on
PCC.  It drew some kind of error message like "GCOS characters not
supported", presumably because some switch was turned off in the
machine-dependent files...


----- Original Message -----
From: "Dan Cross" <crossd at gmail.com>
To:"Steve Johnson" <scj at yaccman.com>
Cc:"Noel Chiappa" <jnc at mercury.lcs.mit.edu>, "TUHS main list"
<tuhs at minnie.tuhs.org>
Sent:Tue, 17 Jan 2017 22:36:12 -0500
Subject:Re: [TUHS] [TUHS} PDP-11, Unix, octal?

A question about 36 bit machines....

In some of the historical accounts I've read, it seems that before the
PDP-11 a pitch was made for a PDP-10 to support the then-nascent Unix
efforts. This was shot down by labs management and sometime later the
PDP-11 arrived and within a decade or so the question of byte width
was the creatively settled for general purpose machines.

The question then is twofold: why a PDP-10 in the early 70s (instead
of, say, a 360 or something) and why later the aversion to
word-oriented machines? The PDP-7 was of course word oriented.

I imagine answers have to do with cost/performance for the former and
with regard to the latter, a) the question was largely settled by the
middle of the decade, and b) by then Unix had evolved so that a port
was considered rather different than a rewrite.  But I'd love to hear
from some of the players involved.

        - Dan C.

On Jan 18, 2017 10:06 AM, "Steve Johnson" <scj at yaccman.com [1]> wrote:
When we were considering what machine to port PDP-11 Unix to, there
were several 36-bit machines around and some folks were lobbying for
them.   Dennis' comment was quite characteristically succinct: "I'll
consider it if they throw in a 10-track tape drive...".    Just
thinking about Unix (and C!) on a machine where the byte size does not
evenly divide the word size is pretty painful...

(Historical note: before networking, magnetic tapes were essential for
backups and moving large quantities of data.  Data was stored in
magnetic dots running across the tape, and typically held a character
plus a parity bit.  Thus, there were 7-track drives for 6-bit
machines, and 9-track drives for 8-bit machines.  But nothing for
9-bit machines...)

----- Original Message -----
From: "jnc at mercury.lcs.mit.edu [2] (Noel" <Chiappa)>
To:<tuhs at minnie.tuhs.org [3]>
Cc:<jnc at mercury.lcs.mit.edu [4]>
Sent:Tue, 17 Jan 2017 21:33:58 -0500 (EST)
Subject:Re: [TUHS] [TUHS} PDP-11, Unix, octal?

 > From: Doug McIlroy

 > Perhaps the real question is why did IBM break so completely to hex
 > the 360?

 Probably because the 360 had 8-bit bytes?

 Unless there's something like the PDP-11 instruction format which
makes octal
 optimal, octal is a pain working with 8-bit bytes; anytime you're
looking at
 the higher bytes in a word, unless you are working through software
 will 'interpret' the bytes for you, it's a PITA.

 The 360 instruction coding doesn't really benefit from octal (well,
 instructions are in 4 classes, based on the high two bits of the
first byte,
 but past that, hex works better); opcodes are 8 or 16 bits, and
 numbers are 4 bits.

 As to why the 360 had 8-bit bytes, according to "IBM's 360 and Early
 Systems" (Pugh, Johnson, and Palmer, pp. 148-149), there was a big
fight over
 whether to use 6 or 8, and they finally went with 8 because i)
 showed that more customer data was numbers, rather than text, and
 decimal numbers in 6-bit bytes was inefficient (BCD does two digits
per 8-bit
 byte), and ii) they were looking forward to handling text with upper-


[1] mailto:scj at yaccman.com
[2] mailto:jnc at mercury.lcs.mit.edu
[3] mailto:tuhs at minnie.tuhs.org
[4] mailto:jnc at mercury.lcs.mit.edu

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