I'm writing an assembler. I always thought that assembly had a 1:1 correspondence between a certain assembly instruction and corresponding instruction, so I thought it would be fairly easy to write one. But after actually reading the Intel 64 Software Dev Manual, I now realize that a command like
mov %rdx,%rbx
has a certain ambivalence to it. It could be translated to 48 89 D3 which corresponds to
REX.W + 89 /r - MOV r64,r/m64
from page 1235 of the Intel 64 Software Dev Manual.
But it could also be translated to 48 8B DA (REX.W + 8B /r - MOV r/m64,r64).
This is the output from a disassembly by objdump -d of the binary:
0: 48 89 d3 mov %rdx,%rbx
3: 48 8b da mov %rdx,%rbx
By reading the manual it is pretty obvious why this ambivalence exists. There are a handful of cases where there is ambivalence like this.
I have tested the code in some instances and it performed exactly the same.
Is there a benefit of using the one over the other (gnu assembler uses 48 89 D3 in all cases I looked at) or should I just pick one of them at random?