regsize.asm

section .data
section .text

global .start
start:
        ; When you want to put the value 0 into a register, the fastest way is to use XOR.
        ; XORing a value againsnt it self yields 0.
        xor rbx, rbx
        ; Yes, you could use `mov rbx, 0` instnad, but that has to go out to memory to load the immediate value 0
        ; XORing register against it self doesn't go out of the CPU for either the source or the destination, so its a little bit faster.
        xor rcx, rcx

        mov rax, 067feh
        mov rbx, rax
        mov cl, bh
        mov ch, bl

        xchg cl, ch

section .bass