That's good for the instruction set, but for the memory management hardware, etc you'll really want one of the {/44, /45, /70, /73, etc} group, since only those models support split I+D. You mean '16 bits', right? :-) Only the '407' is functional. (IIRC, in early UNIX versions, the OS didn't strip the header on loading a new program into memory, so the 407 was actually executed.) The others are just magic numbers, inspired by the '407' - the code always starts at byte 020 in the file. The _real_ reason octal is optimal for PDP-11 is that when looking at core, most instructions are more easily understood in octal, because the PDP-11 has 8 registers (3 bits), and 3 bits worth of mode modifier, and the fields are aligned to show up in octal. I.e. in the double-op instruction '0awxyz', the 'a' octit gives the opcode, 'w' is the mode for the first operand, 'x' is the register for the first operand, and 'y' and 'z' similarly for the second operand. So '12700' is 'MOV (PC)+, R0' - AKA 'MOV #bbb, R0', where 'bbb' is the contents of the word after the '12700'. Noel