I think that line is specific to the basic instructions only (if it weren't, "...and is the lower six bits" would be false). "For Non-basic opcodes, 'a' is act

I think that line is specific to the basic instructions only (if it weren't, "...and is the lower six bits" would be false). "For Non-basic opcodes, 'a' is actually the opcode" Actually it's the single argument -- 'o' is the opcode.