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Because with 8 registers and 8 addressing modes octal makes for extremely readable machine code. There's another completely distinct proposal for the PDP-11 arc
by aap_ 6y ago
Because with 8 registers and 8 addressing modes octal makes for extremely readable machine code. There's another completely distinct proposal for the PDP-11 architecture which uses hex but it was never built.
- Sniffnoy 6y agoOh, good point -- the instructions break down nicely as one leading bit plus 5 octal digits. Branches are a little bit of an exception, but you're right that octal works overall better here. Still, that doesn't explain the use of octal at the top where it's not referring to instructions.
- aap_ 6y agoUsing two different number bases sounds painful, especially considering hex wasn't very common at the time. Also physical unibus addresses are 18 bits so they divide nicely into octal digits.