CODEWITHSUNDEEP
assemblyx86movinstruction-encoding
Gary Yin
Gary Yin

Reputation: 535

Puzzled by x86 MOV instruction

From Intel64 & IA-32 manual vol 2a, there are many possible usage for instruction mov. Such as:

mov r64, m64      # move m64 to r64

mov rax, moffs64  # move quadword at (offset) to RAX

I wrote code to test (gas, intel syntax):

movabs rax, label

movabs rax, offset label

...
label:
    .quad 0x112233445566

After assembly, linking and objdump, I got related assembly:

mov rax, qword ptr ds:0xffff80000000008e
mov rax, 0xffff80000000008e

Obviously, the second instruction I wrote is not the instruction expected as manual. What's the instruction for the format in the manual?

Upvotes: 0

Views: 1816

Answers (1)

Michael
Michael

Reputation: 58507

moffs denotes a memory operand. Here's the description from the Intel manual:

moffs8, moffs16, moffs32, moffs64 — A simple memory variable (memory offset) of type byte, word, or doubleword used by some variants of the MOV instruction. The actual address is given by a simple offset relative to the segment base. No ModR/M byte is used in the instruction. The number shown with moffs indicates its size, which is determined by the address-size attribute of the instruction.

The offset operator, which is used by assemblers like MASM and TASM, gives you "the offset into the relevant segment of [the operand]" (source). So what you get with offset label is the offset of label, which is an immediate value. So you're looking at two different things: getting the value stored at a particular address, versus getting the address itself.

I don't know what the GAS syntax would be to get the moffs64 variant, but with NASM you could write mov rax,[qword label] (note that this encoding would be longer than if you had simply written mov rax,[label] - i.e. mov r64, r/m64).

Upvotes: 3

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