[TUHS] [TUHS} PDP-11, Unix, octal?

Doug McIlroy doug at cs.dartmouth.edu
Tue Jan 17 12:23:07 AEST 2017


> 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.

Octal predates the 6-bit byte. Dumps on Whirwind II, a 16-bit machine,
were issued in octal. And to help with arithmetic, the computer lab
had an octal Friden (IIRC) desk calculator. One important feature of
octal is you don't have to learn new numerals and their addition
and multiplication tables, 2.5x the size of decimal tables.

Established early, octal was reinforced by a decade of 6-bit bytes.
Perhaps the real question is why did IBM break so completely to hex
for the 360? (Absent actual knowledge, I'd hazard a guess that it
was eased in on the 7030.)

Doug> 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.

Octal predates the 6-bit byte. Dumps on Whirwind II, a 16-bit machine,
were issued in octal. And to help with arithmetic, the computer lab
had an octal Friden (IIRC) desk calculator. One important feature of
octal is you don't have to learn new numerals and their addition
and multiplication tables, 2.5x the size of decimal tables.

Established early, octal was reinforced by a decade of 6-bit bytes.
Perhaps the real question is why did IBM break so completely to hex
for the 360? (Absent actual knowledge, I'd hazard a guess that it
was eased in on the 7030.)

Doug
=


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