assume cs:code,ds:data
data segment
dw 0,0
data ends
stack segment
db 16 dup (0)
code segment
start: cli
mov ax,data
mov ds,ax
mov ax,0
mov es,ax
mov ax,stack
mov ss,ax
mov sp,16
mov ax,es:[9*4]
mov ds:[0],ax
mov ax,es:[9*4+2]
mov ds:[2],ax
mov word ptr es:[9*4],offset int9
mov word ptr es:[9*4+2],cs
sti
mov ax,0b800h
mov es,ax
mov byte ptr es:[160*12+80],'a'
s0:cmp byte ptr es:[160*12+80],'z'
ja s1
call delay
add byte ptr es:[160*12+80],1
jmp s0
s1:mov ax,0
mov es,ax
mov ax,ds:[0]
mov es:[9*4],ax
mov ax,ds:[2]
mov es:[9*4+2],ax
mov ax,4c00h
int 21h
delay: push dx
push ax
mov ax,100h
mov ax,0
s3:sub ax,1
sbb dx,0
cmp ax,0
jne s3
cmp dx,0
jne s3
pop ax
pop dx
ret
int9: push ax
push es
in al,60h
pushf
call dword ptr ds:[0]
cmp al,1
jne s4
mov ax,0b800h
mov es,ax
add byte ptr es:[160*12+80+1],1
s4:pop es
pop ax
iret
code ends
end start