From: jnc@mercury.lcs.mit.edu (Noel Chiappa)
Subject: [TUHS] [TUHS} PDP-11, Unix, octal?
Date: Tue, 17 Jan 2017 21:33:58 -0500 (EST) [thread overview]
Message-ID: <20170118023358.BE5C818C095@mercury.lcs.mit.edu> (raw)
> 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
next reply other threads:[~2017-01-18 2:33 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-18 2:33 Noel Chiappa [this message]
2017-01-18 3:06 ` Steve Johnson
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20170118023358.BE5C818C095@mercury.lcs.mit.edu \
--to=jnc@mercury.lcs.mit.edu \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).