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    Your comment about PDP-11 documentation is incorrect. The PDP-11 had 8 registers. R0-R5 were general purpose registers. R6 was the stack pointer, and R7 was the PC. The MOV instruction took two 3-bit register numbers and two 3-bit addressing modes. Octal captures the register and mode fields PERFECTLY. (I strongly doubt that this was an accident.) Using hex would have confused everything. PUSH and POP were just addressing modes that used R6. Immediate operand was an addressing mode that used R7. Commented Jan 18, 2014 at 5:53
  • @JohnR.Strohm Thanks for the comment re the 3-bit registers. I agree with your analysis and have updated my answer. Commented Jan 19, 2014 at 5:23
  • So DEC worked in OCT. Should've just used multiples of 3.32 bits to avoid confusion. Commented Aug 14, 2017 at 12:37
  • It’s funny how I never considered the Unix permissions to be an octal number, I always thought of it as three individual digits written together, each representing the three rwx bits, so they could be octal, decimal, or even hex. Commented Nov 17, 2017 at 5:33
  • "50 years ago" could be replaced by writing a decade to make this answer more future-proof. Commented Aug 4 at 12:58