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@mercury.lcs.mit.edu (Noel" <Chiappa)>

To:
<tuhs@minnie.tuhs.org>
Cc:
<jnc@mercury.lcs.mit.edu>
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 for
> 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 which
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 register
numbers are 4 bits.

As to why the 360 had 8-bit bytes, according to "IBM's 360 and Early 370
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) statistics
showed that more customer data was numbers, rather than text, and storing
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- and
lower-case.

Noel