Load from a 64-bit address into other register than rax

Question

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?

Answer 1

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

MOVABS is the GAS opcode name for the MOV opcode forms MOV Areg, [Offs64] and MOV [Offs64], Areg. In Yasm's NASM syntax mode, you can get this form by saying MOV AX, [qword xxx]. Yasm's GAS syntax mode accepts MOVABS (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