Re: [TUHS] 6-bit, 7-bit and 9-bit byte UNIXes - TUHS

1 Oct 2004

...
  Subject: Re: [TUHS] 6-bit, 7-bit and 9-bit byte
UNIXes
 From: Norman Wilson &lt;norman(a)nose.cs.utoronto.ca&gt;
 To: tuhs(a)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,
of course.

    carl
-- 
    carl lowenstein         marine physical lab     u.c. san diego
                                                 clowenst(a)ucsd.edu