real2prot:
cli
mov eax, cr0
or eax, 0x01
mov cr0, eax
sti
ret

prot2real:
cli
mov eax, cr0
xor eax, 0x01
mov cr0, eax
sti
ret

msg1 db "I shall go to the protect mode.", 0x0A, '$'
msg2 db "I returned from the protect mode.", 0x0A, '$'