Octal

On 8/31/06, aliensrcooluk at yahoo.co.uk <aliensrcooluk at yahoo.co.uk> wrote: In some contexts, it can make some patterns more apparent than using decimal or hex. One example is the PDP-11 instruction set - if you look at the way the register selection and addressing modes are specified, it's much clearer when you view the instructions in octal rather than hex, even though it's a 16-bit machine. (cf - http://en.wikipedia.org/wiki/PDP-11 and http://www.village.org/pdp11/faq.pages/PDPinst.html ) In binary, many instructions are of the form "biiissssssdddddd", where 'b' is byte vs word, 'iii' is the operand (instruction type), 'ssssss' is the source operand, and 'dddddd' is the destination operand. In octal, it would be represented as 6 digits (000000- 177777) and you can think of the instructions more like 'BISSDD' where 'B' is still byte vs word, 'I' is a single octal digit for the operand, and 'SS' and 'DD' are the source and destination as two octal digits - one being the addressing mode type, the other the register to apply the addressing mode to. As you can see, it's not tough to look at, say, a MOV instruction and see what it does... 010304 copies the contents of R3 to R4. In hex, that would be 0x40C4 and not at all obvious what it's doing at first glance. There are other applications, such as using 7-segment displays where it might be easier to represent things in octal than hex or decimal. It's just another number base, but not as commonly used anymore as hex. -ethan | | | : | : | | | Op | Source | Dest |