Question

I need to declare a dummy absolute address in the code of a self-modifying program. This pointer works similar to a relocation for a linker – it only reserves appropriate space in the instruction and is updated with a valid address later on at runtime. This has worked fine for me on x86-32:

movups xmm0, [0xDEADBEEF]

This assembles and works as expected at runtime. However, when I try to do this in x86-64 code:

movups xmm0, [0xDEADC0DEDEADBEEF]

It assembles with the following warning:

warning: dword data exceeds bounds

And promptly crashes at runtime because the next instruction is overwritten with the rest of the address, which happens to be garbage instruction-wise.

Any address longer than 32 bits fails to assemble without a warning, even a minimally longer one than 32 bits:

movups xmm0, [0xADEADBEEF] ; 36-bit address

How should I go about declaring a constant, absolute 64-bit pointer? Or is there just no way around it and I need to drop a RIP-relative, 32-bit pointer in there?

Answer 1

I may be wrong because I haven’t done NASM in a long time, but I don’t think you can use a 64 bit immediate value with any register other than AL, AX, EAX, RAX. Your 64 bit address has to be declared as a QWORD.

Solution:

mov rax , 0xDEADCODEDEADBEEF
movups xmm0, [rax]

See comments below for explanation.

This reference: NASM Manual