That's because you're using `mov rdi, rax` again. You keep changing `edi, eax` to `rdi, rax`. Why? The default operand size in 64-bit mode is, for most instruc

That's because you're using `mov rdi, rax` again. You keep changing `edi, eax` to `rdi, rax`. Why? The default operand size in 64-bit mode is, for most instructions, still 32 bits. So `mov edi, eax` encodes the same in 32- and 64-bit mode. For `mov rdi, rax` you need an extra REX prefix byte [1], that's the 48 you're seeing above, but you don't need it here. [1] https://wiki.osdev.org/X86-64_Instruction_Encoding#REX_prefix https://wiki.osdev.org/X86-64_Instruction_Encoding#REX_prefi...