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hi everyone,

I have doubt whether r/m16 is just a superset of r16. But the intel doc(IA-32, part-2) seems to specify registers in both r16 and r/m16 as the same. But they use different opcodes for the two cases.

This is how they define it:
-----------------------------------------------------------------------------
r/m16 - A byte operand that is either the contents of a byte general-purpose register or a byte from memory.

r16 - One of the word general purpose registers.-----------------------------------------------------------------------------
Now what if I had something like


       mov ax, imm16       ;imm16 is immediate value of size 16 bits


What will be the opcode ? Which one would it ?

       1. mov r/m16, imm16
                     or

       2. mov r16, imm16


Thanking you in advace.
modshah

someone else feel free to correct me if i am wrong, but i belive that r/m16 refers to instructions like this:

mov word [16], 0x1234
mov word [bx], 0x5678

where as r16 is used when MOVing directly to a register like this:

mov bx, 0x9abc

the mod/rm byte looks like this:
7-6  5-3  2-0
Mod  Reg  R/M

So "mov word [16], 0x1234" would set Mod and R/M to [16], but "mov bx, 0x9abc" could be either. (Actually there's a shorter way for immed->reg). r16 (i think) refers to the reg field (which refers to a 16-bit register). Read page 33 on the Intel Manual 2: Instruction Set Reference.
ftp://download.intel.com/design/Pentium ... 547112.pdf


Ps. Btw, mov ax, imm16 would compile to "a8 xxxx". A0-A8 are special shorthands for loading registers with immediates.