[TUHS] 6-bit, 7-bit and 9-bit byte UNIXes
cdl at mpl.ucsd.edu
Fri Oct 1 13:59:43 AEST 2004
> Subject: Re: [TUHS] 6-bit, 7-bit and 9-bit byte UNIXes
> From: Norman Wilson <norman at nose.cs.utoronto.ca>
> To: tuhs at tuhs.org
> Date: Thu, 30 Sep 2004 20:47:22 -0400
> The very first UNIX ran on the PDP-7, which had 18-bit words.
> I happen to have the assembly-language source code to parts
> of that system. Many programs contain error-handling code
> that does something like this:
> lac d1
> sys write; 1f; 1
> jmp somewhere
> 1: 077012
> d1: 1
> Evidently the system thought in words in those days
> (the second argument to sys write is presumably a word
> count), but the single word written is a strong clue
> that 9-bit bytes were used, and that a certain concise
> error message that people love to complain about was
> there from the beginning (and why not?).
I would say "the PDP7 computer was word-addressable". In this
context, characters seem to have been packed as 9-bit half-words
in a big-endian fashion. No 'bytes'.
Maybe tomorrow I will be near my DEC literature archives, and see
if I can find some clues about PDP7 instructions that might deal
with half-words. If it's anything like the PDP8 of similar vintage,
there aren't any. Late in its life the PDP8 got a BSW "byte swap"
instruction to swap the half-words in the AC register. 6 bits,
carl lowenstein marine physical lab u.c. san diego
clowenst at ucsd.edu
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