Move quadword between xmm and general-purpose register in ml64?
assembly, gnu-assembler, masm, sse, x86-64
Solution
The `MOV` instruction cannot move data between a general-purpose register and an `xmm` register. The instruction you are looking for is `MOVQ` (like in the A&T syntax code you show), as defined in Intel's instruction set manuals. (HTML extract here: https://www.felixcloutier.com/x86/movd:movq)
The fact that ML64 does not accept `MOVQ` is in disagreement with Intel's manuals, and therefore - in my view at least - a bug (or at least an inconsistency).
ML64 does seem to use `MOVD` in its place, even for 64-bit registers. You can verify this by disassembling the code it generates.
Note that there are two different `movq` instructions (not counting load and store forms as separate):
One is `movq xmm, xmm/m64` form, the MMX/SSE2 instruction that copies between vector registers or loads/stores. This existed in 32-bit mode with MMX (and SSE2), and the opcode always implies a 64-bit transfer (zero-extending to 128 with an XMM destination). ML64 uses `movq` for this form.
The other is the 64-bit version of `movd xmm, r/m32` that can move data between XMM or MMX registers and GP-integer registers like RCX, or memory. This form is new with x86-64 (which includes MMX and SSE2); the opcode is the same as `movd`, with a REX.W prefix for 64-bit operand-size. ML64 apparently always uses `movd` for this form, regardless of the actual operand-size.
A 64-bit load or store between an XMM register and memory can use either opcode, but the first form is shorter, not needing a REX prefix.
(AT&T syntax `movq %rax, %rcx` is just `mov` with a `q` operand-size suffix; in that case the `q` is not part of the true mnemonic.)
Problem
In a simple program written for Microsoft's x64 assembler, I want to move a 64-bit value between an SSE register (say xmm0) and a general-purpose register (say rcx), as in <Intel syntax in MASM>: ``` mov xmm0, rcx ... mov rcx, xmm0 ``` These two lines generate the following error messages, respectively, from `ml64.exe`: - error A2152: coprocessor register cannot be first operand - error A2070: invalid instruction operands However, it is clearly possible to accomplish this simple task in x64. For example, the following is a functioning x64 program that I can assemble and run in GAS <AT&T syntax using GCC 4.8.2>: ``` .text .globl main main: movl $1, %ecx movq %rcx, %xmm0 movq %xmm0, %rax ret ``` As expected, the return value of this program is 1 and the `objdump` output for `main()` is: ``` 1004010d0: b9 01 00 00 00 mov $0x1,%ecx 1004010d5: 66 48 0f 6e c1 movq %rcx,%xmm0 1004010da: 66 48 0f 7e c0 movq %xmm0,%rax 1004010df: c3 retq ``` So my question is, how can I accomplish this in MASM given that `ml64.exe` is producing the above errors?