Question

I have an application I have written in C where I really need to modify the value of one of the processor registers before calling a function. Normally I would do this with inline assembly, but as we all know that has been removed for 64 bit applications. I also cannot do this in a separate .asm file that is compiled with ml64 due to certain project constraints. So basically I need to execute the equivalent of the following code inline:

_asm mov r10d, 0xDEADBEEF

Does anyone know of a creative method or some other compiler intrinsic for x64 that will allow you to modify the value of a register inline?

Answer 1

Unfortunately, after looking at possible workarounds, it seems that Hans was right and it’s simply not possible to modify the contents of a register inline. There is no compiler intrinsic that exists to do it and the only alternative is to either write the entire function in 64 bit assembly as a separate .asm file and compile it with ml64, or do as Alexey suggested and allocate an executable block of memory before hand and write the opcodes to it. You can then create a function pointer and just call this code directly. So for example, if I wanted to do the equivalent of:

mov r10d, ecx
ret

Just create an array to store the opcodes:

BYTE copyValueToR10[] = "\x44\x8B\xD1\xC3";

You can then VirtualAlloc memory for this tiny function with PAGE_EXECUTE protection. Next just create a function pointer and you’re good to go. Definitely a dirty way to do it, but given the constraints of not having inline asm or wanting to compile using ml64, this seems to be the only other way to do it.