File Archive

Output of file : TIMEOUT.ASM contained in archive : PKTD11B.ZIP

;put into the public domain by Russell Nelson, [email protected]

;we read the timer chip's counter zero. It runs freely, counting down
;from 65535 to zero. We sample the count coming in and subract the previous
;count. Then we double it and add it to our timeout_counter. When it overflows,
;then we've waited a tick of 27.5 ms.

timeout dw ? ;number of ticks to wait.
timeout_counter dw ? ;old counter zero value.
timeout_value dw ?

;enter with ax = number of ticks (36.4 ticks per second).
inc ax ;the first times out immediately.
mov cs:timeout,ax
mov cs:timeout_counter,0
call latch_timer
mov cs:timeout_value,ax

mov al,0 ;latch counter zero.
out 43h,al
in al,40h ;read counter zero.
mov ah,al
in al,40h
xchg ah,al

;call at *least* every 27.5ms when checking for timeout. Returns nz
;if we haven't timed out yet.
call latch_timer
xchg ax,cs:timeout_value
sub ax,cs:timeout_value
shl ax,1 ;keep timeout in increments of 27.5 ms.
add cs:timeout_counter,ax ;has the counter overflowed yet?
jnc do_timeout_1 ;no.
dec cs:timeout ;Did we hit the timeout value yet?
or sp,sp ;ensure nz.