INT 16调用的问题

assume cs:code
code segment
start : mov ah,0
int 16h
mov ah,1
cmp al,'r'
je red
cmp al,'g'
je green
cmp al.'b'
je blue
jmp short sret
red: shl ah,1
green: shl ah,1
blue: mov bx,0b800h
mov es,bx
mov bx,1
mov cx,2000
s: and byte ptr es:[bx],11111000b
or es:[bx],ah
add bx,2
loop s
sret: mov ax.4c00h
int 21h
code ends
end start

请问画线处的程序是用什么比较技巧?