I'm working with x64 assembly and Visual C++ 2010/MASM is telling me that the instruction
movq mm0, rax
contains "invalid instruction operands".
Is this really illegal, or could it be a bug? How does one perform this procedure without using the instruction?
It's a bug, or you didn't specify 64 bit mode or MMX extensions.
movq mm0, rax is a valid instruction with machine code 48 0F 6E C0.
See the intel instruction set reference. If all else fails, you can include the machine code as data bytes.
That said, MMX is obsolete (especially since in 64 bit mode you by default have SSE2). Make sure you really want to use it.