From: scj@yaccman.com (Steve Johnson)
Subject: [TUHS] [TUHS} PDP-11, Unix, octal?
Date: Tue, 17 Jan 2017 19:06:28 -0800 [thread overview]
Message-ID: <50a7fbcbb6af280eb108fff1361c37ee1718bff0@webmail.yaccman.com> (raw)
In-Reply-To: <20170118023358.BE5C818C095@mercury.lcs.mit.edu>
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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 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
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
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next prev parent reply other threads:[~2017-01-18 3:06 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-18 2:33 Noel Chiappa
2017-01-18 3:06 ` Steve Johnson [this message]
2017-01-18 3:36 ` Dan Cross
2017-01-18 6:53 ` Angelo Papenhoff
2017-01-18 7:31 ` ron minnich
2017-01-18 8:09 ` Angelo Papenhoff
2017-01-18 21:04 ` Steve Johnson
2017-01-18 21:42 ` Charles Anthony
2017-01-18 6:04 ` Lars Brinkhoff
2017-01-18 18:47 ` Peter Jeremy
2017-01-18 18:58 ` Charles Anthony
-- strict thread matches above, loose matches on Subject: below --
2017-01-18 14:28 Nelson H. F. Beebe
2017-01-17 2:23 Doug McIlroy
2017-01-18 16:47 ` Clem Cole
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