Question

Write a program for 8051 in assembly that upon startup writes 55H (0x55) to port 2 of the microcontroller and then AAH (0xAA) to port 2 after 25 mSec and stops. (Oscillator = 29.4MHz)

Answer 1

cli
push ds
push 0
pop ds
mov ax, word[ds:8h*4], ivt
mov dx, word[ds:8h*4+2]
mov word[intold], ax
mov word[intold+2], dx
mov word[ds:8h*4], ivt
mov word[ds:8h*4+2], cs
pop ds
sti
ivt:
pushf
pusha
mov al, 0x55
out 2, al
mov al, 0xaa
popf
db 0eah
intold dw 0, 0