Programming the internal timer

  ; start of code
  push  ds
  pop   es
  mov   bx,offset Flag	; must point to a BYTE in your data segment (make sure es=ds)
  mov   ax,8300h	; (AH) Service 83h, --  (AL)=0 to start timer, 1 to cancel timer
  mov   cx,004Ch	; 5,000,000  (or 5 sec)
  mov   dx,4B40h	;  cx = high order word of interval time, dx = low order word
  int   15h		; 1,000,000 = 1 sec (cx:dx = how many microseconds to wait)

  mov   al,Flag		; Check to see if flag set (80h)
  or    al,al		; if it is set then do code
  jnz   TimerUp    	;
  . do what ever code you wanted to do while timer was tickn'
  jmp   short LoopIt
TimerUp: ;  do what ever code you wanted to do when timer was up

Be careful. If you put some complicated code inside the loop that is running while the timer
is tickn', then you might not catch the timer on time. If you test for the 'timer up' at the
top of the loop, then have some code that takes 16 cycles to execute before you get back to
test the flag, you could be off by 16 cycles. So make it short!