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From: hwe@beta.UUCP (Skip Egdorf)
Newsgroups: comp.arch
Subject: Re: *Why* do modern machines mostly hav
Message-ID: <8974@beta.UUCP>
Date: Fri, 14-Aug-87 18:55:32 EDT
Article-I.D.: beta.8974
Posted: Fri Aug 14 18:55:32 1987
Date-Received: Sun, 16-Aug-87 05:30:07 EDT
References: <2807@phri.UUCP> <101200004@datacube> <1332@killer.UUCP>
Organization: Los Alamos Natl Lab, Los Alamos, N.M.
Lines: 37
Summary: DEC-10 and 20, 36-bit bytes

In article <1332@killer.UUCP>, molly@killer.UUCP (Molly Fredericks) writes:
> 
> DEC-System 10's and 20's actually had 36 bits in the word, all of which
> where addressable.  The addresses where 18 bits long, which I think
> explains the 18 bit Unibus addresses on early PDP-11's
> 
> The 10's had 2 or 3 character modes.  The first has 6 bit.  Six characters
> were packed into a 36 bit word.  That gave 6 characters per word, no
> parity.
> 
> The next mode was 7 bit ascii.  5 characters were packed into 36 bits with
> the last bit wasted.  This was used as I understand for storing data in
> datafiles.  The six bit stuff tended to shift all of your characters into
> upper case, so that wasn't so hot for text files and anything else you
> didn't want to become monocase.
> 
> There may have been an 8 bit ascii mode, but I don't ever recall doing
> anything with it.

They also had 1-bit modes, 2-bit modes, ..., 36-bit modes.

The Dec-10s (and 20s, same machine architecture, different OS support)
had two byte handeling instructions: load byte, and deposit byte.
Each used a "byte pointer" as an operand.
This byte pointer had the normal 18-bit word address and a byte length
so that any size byte could be handeled within a word.
6-bit and 7-bit bytes were common, (7-bits * 5 bytes = 35 bits + one left
over) but some things (anyone from Stanford out there??) used 9-bit
bytes at 4 per word.
TOPS-10 did its IO by transfering to a buffer and giving you a byte pointer
to the buffer. This was a 7-bit byte pointer by default, but could be
changed if you wanted to use a file with weird bytes.

This is also the classic C example of a normal (int, float, etc) pointer
at 18-bits being shorter than a 'char *' at 36 bits.

				Skip Egdorf