This deserves a little more explanation. The PDP-11 had 8 registers and 8 addressing modes so in the days before debuggers you could dump the binary and pretty

This deserves a little more explanation. The PDP-11 had 8 registers and 8 addressing modes so in the days before debuggers you could dump the binary and pretty much debug straight from the octal without so much as referring to your handy-dandy pocket reference card. http://www.montagar.com/~patj/dec/pocket/pdp11_programmingcard_1975.pdf http://www.montagar.com/~patj/dec/pocket/pdp11_programmingca... EDIT: Also, the most significant bit was used for 3/4 of the instruction space as a flag for a byte vs. word operation (or add vs subtract) so having it alone in its own octal digit (0/1) made perfect sense. For example 01ssdd was MOV (word) and 11ssdd was MOVB. 06ssdd was ADD and 16ssdd was SUB.