Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!decwrl!sgi!rpw3@rigden.wpd.sgi.com
From: rpw3@rigden.wpd.sgi.com (Rob Warnock)
Newsgroups: comp.arch
Subject: Re: Looking for a really odd computer
Message-ID: <71383@sgi.sgi.com>
Date: 8 Oct 90 02:51:29 GMT
References: <2721@crdos1.crd.ge.COM> <2515@ns-mx.uiowa.edu>
Sender: guest@sgi.sgi.com
Reply-To: rpw3@sgi.com (Rob Warnock)
Organization: Silicon Graphics, Inc., Mountain View, CA
Lines: 38

In article <2515@ns-mx.uiowa.edu> jones@pyrite.cs.uiowa.edu
(Douglas W. Jones,201H MLH,3193350740,3193382879) writes:
+---------------
| Back in the days of the DEC-10, a 36 bit machine, the byte size was
| whatever you wanted (the DEC-10 had byte manipulation instructions that
| allowed any size, although 6 and 9 were the popular ones because they
| divided evenly into 36.
+---------------

6 & 9 were used for some things, as was even 8, occasionally, but the
overwhelming majority of all byte operations were on *7* bit bytes, which
is what the PDP-10 used for ordinary ASCII text files.

Yes, packing five 7-bit bytes into a 36-bit word wastes a bit. There were
early PDP-10 editors that tried to make use of this bit to flag a word as
containing a 5-digit "line number" -- five decimal digits in ASCII -- but
that hack died out... eventually. The 36th bit was just left fallow.

The second most common size was 6, which is how file names were encoded in
directories. (Which explains why DEC file names have 6 chars and a 3-char
"extension": FOOBAR.BAZ) There was lots of 6 <-> 7 conversion going on, but
the hardware byte load/store were indirect through "byte pointers" which
contained the byte size, so conversions were cheap (load/store loop).

8-bit bytes were used when writing "industry-compatible" magtapes, and when
talking to the PDP-11 communications front-end.

I don't recall any production software that used 9-bit bytes, though at least
one attempt at an experimental C compiler used them.


-Rob

-----
Rob Warnock, MS-9U/510		rpw3@sgi.com		rpw3@pei.com
Silicon Graphics, Inc.		(415)335-1673		Protocol Engines, Inc.
2011 N. Shoreline Blvd.
Mountain View, CA  94039-7311