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Most of the time octal causes me pain, but octal is definitely the way to understand Intel instruction sets. In hex you can sort of see some patterns, but in octal things are trivial.

The octal encoding goes back to the Datapoint 2200, which use BCD decoder chips (7442) for instruction decoding. These were used to decode three instruction bits at a time, so the instruction set of the Datapoint 2200 (and thus the 8008) was based on groups of three bits. Among other things, that's why the 8008 has 7 registers (A, B, C, D, E, H, L) - the 8th value was used to indicate a memory access.

Also see "x86 is octal": http://www.dabo.de/ccc99/www.camp.ccc.de/radio/help.txt

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