[TUHS] PDP-11, Unix, octal?
jnc at mercury.lcs.mit.edu
Tue Jan 17 06:45:08 AEST 2017
> 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):
where 'ssssss' and 'dddddd' (source and destination) have the same format:
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... :-)
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