2 Questions About Cpus...

[name9902]

1) If i am not mistaken, high-order bits are used to begin commands, tho the low-order bits are still used in these commands...

But the thing i dont understand is this....i am told that low-order bits are used to store data, can anyone explain exactly how this works or if i am mistaken, try to explain how it works, i am a little lost.

2) What is the difference between the CPU "codebook" and the CPU instruction set?

[CurlingSteve]

How the bits in an instruction are used vary depending on the instruction.

This may explain it much better than I can:

Intel Archetecture

[jcl]

1) If i am not mistaken, high-order bits are used to begin commands, tho the low-order bits are still used in these commands...

As CurlingSteve said, it depends on the instruction and also of course on the architecture. IA-32 has the opcode more or less in the low-order byte(s) of the instructions.

But the thing i dont understand is this....i am told that low-order bits are used to store data, can anyone explain exactly how this works or if i am mistaken, try to explain how it works, i am a little lost.

Data is encoded in the instruction. Simple as that. For example, the instruction

mov eax, 1

is encoded as

b8 01 00 00 00

a one byte opcode (B8, or "MOV reg32, imm32" with EAX as the target register) followed by a four byte integer representing 1.

2) What is the difference between the CPU "codebook" and the CPU instruction set?

Never heard the term 'codebook' used in reference to processors.

Edited March 15, 2005 by jcl