From: jnc@mercury.lcs.mit.edu (Noel Chiappa)
Subject: [TUHS] PDP-11, Unix, octal?
Date: Mon, 16 Jan 2017 15:45:08 -0500 (EST) [thread overview]
Message-ID: <20170116204508.CFD1918C085@mercury.lcs.mit.edu> (raw)
> From: Warren Toomey
> Something I've been meaning to ask for a while: why Unix and octal on
> the PDP-11? Because of the DEC documentation?
Yeah, DEC did it all in octal.
> I understand why other DEC architectures (e.g. PDP-7) were octal: 18b
> is a multiple of 3. But PDP-11 is 16b, multiple of 4.
Look at PDP-11 machine code. Two-op instructions look like this (bit-wise):
oooossssssdddddd
where 'ssssss' and 'dddddd' (source and destination) have the same format:
mmmrrr
where 'mmm' is the mode (things like R, @Rn, etc) and 'rrr' is the register
number. All on octal boundaries. So if you see '010011' in a dump (or when
looking at memory through the front console switches :-), you know
immediately that means:
MOV R0, @R1
Much harder in hex... :-)
Noel
next reply other threads:[~2017-01-16 20:45 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-16 20:45 Noel Chiappa [this message]
-- strict thread matches above, loose matches on Subject: below --
2017-01-16 19:10 [TUHS] Article on 'not meant to understand this' Steve Johnson
2017-01-16 20:15 ` [TUHS] PDP-11, Unix, octal? Warren Toomey
2017-01-16 20:25 ` Lars Brinkhoff
2017-01-16 22:31 ` ron minnich
2017-01-16 20:31 ` Erik E. Fair
2017-01-17 1:09 ` Steve Johnson
2017-01-17 1:33 ` William Pechter
2017-01-17 14:28 ` Joerg Schilling
2017-01-17 15:14 ` Nelson H. F. Beebe
2017-01-17 16:53 ` William Cheswick
2017-01-17 14:12 ` Joerg Schilling
2017-01-17 15:28 ` ron minnich
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