[TUHS] PDP-11, Unix, octal?

Noel Chiappa 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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