segment .text
global main
main:
push rbp
mov rbp, rsp
sub rsp, 32 ; shaow register space
mov rax, 0x12345678; Inditial value for rax
ror rax, 8 ; Preserve bits 7-0
shr rax, 4 ; Shift out original 11-8
shl rax, 4 ; Bits 3-0 are 0's
or rax, 1010b ; Set the field to 1010b
rol rax, 8 ; Bring back bits 7-0
xor rax, rax
leave
ret