Load from a 64-bit address into other register than rax
On x64, loading from a 64-bit absolute address (that is, dereferencing a 64-bit immediate) can be done by
movabs addr64, %rax
However, when the destination register is any other than rax the assembler gives an error message saying operand size mismatch for movabs. What am I missing?
From the MOV instruction document you can see that you can move a 64-bit immediate to any registers, but loads/stores involving a 64-bit immediate absolute address can only work with Areg
| Opcode | Instruction | Description |
|---|---|---|
| A0 | MOV AL,moffs8* | Move byte at (seg:offset) to AL. |
| REX.W + A0 | MOV AL,moffs8* | Move byte at (offset) to AL. |
| A1 | MOV AX,moffs16* | Move word at (seg:offset) to AX. |
| A1 | MOV EAX,moffs32* | Move doubleword at (seg:offset) to EAX. |
| REX.W + A1 | MOV RAX,moffs64* | Move quadword at (offset) to RAX. |
| A2 | MOV moffs8,AL | Move AL to (seg:offset). |
| REX.W + A2 | MOV moffs8***,AL | Move AL to (offset). |
| A3 | MOV moffs16*,AX | Move AX to (seg:offset). |
| A3 | MOV moffs32*,EAX | Move EAX to (seg:offset). |
| REX.W + A3 | MOV moffs64*,RAX | Move RAX to (offset). |
As you can see there's no ModR/M byte to encode the register number. Since this is less commonly used than moving a 64-bit immediate to a register it won't be a problem. If really needed it can be done in 2 instructions
MOVABSis the GAS opcode name for theMOVopcode formsMOV Areg, [Offs64]andMOV [Offs64], Areg. In Yasm's NASM syntax mode, you can get this form by sayingMOV AX, [qword xxx]. Yasm's GAS syntax mode acceptsMOVABS(for GAS compatibility). Note this form is only valid with Areg (AL/AX/EAX/RAX) as the source/destination register.http://cvs.tortall.net/pipermail/yasm-devel/2006-March/000579.html
For any register other than %rax one can replace it by two instructions:
48 bb f0 de bc 9a 78 56 34 12 mov $0x123456789abcdef0,%rbx
48 8b 1b mov (%rbx),%rbx
This is longer than the single one,
48 a1 f0 de bc 9a 78 56 34 12 mov 0x123456789abcdef0,%rax
hence you'd quite probably prefer movabs (the latter) if you can use it.