Re: [TUHS] PDP-11, Unix, octal? - TUHS

17 Jan 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):

  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