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

Steve Johnson scj at yaccman.com
Wed Jan 18 13:06:28 AEST 2017

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 (Noel" <Chiappa)>
To:<tuhs at minnie.tuhs.org>
Cc:<jnc at 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
 > 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-


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