適当にやってみる

 % cat tmp.s
 mov %rax, %rdx
 mov %eax, %edx
 % gcc -c tmp.s && objdump -d tmp.o | grep mov
   0:   '''48 89 c2'''                mov    %rax,%rdx
   3:   89 c2                   mov    %eax,%edx

• REX: 0x48 == 0100_1_0_0_0 (0100 REX.w REX.r REX.x REX.b)
• mov: 0x89
• Mod R/M: 0xc2 == 11_000_010 (Mod Reg R/M)