emailler/client/drivers/a2timer.s

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; timer routines
;
; unfortunately the standard Apple 2 has no CIA or VBI, so for the moment, we will
; make each call to 'timer_read' delay for a little while
; this kludge will make the polling loops work at least
;
; timer_read is meant to return a counter with millisecond resolution
.include "../inc/common.i"
.export timer_init
.export timer_read
.bss
current_time_value: .res 2
.code
;reset timer to 0
;inputs: none
;outputs: none
timer_init:
ldax #0
stax current_time_value
rts
;this SHOULD just read the current timer value
;but since a standard apple 2 has no dedicated timing circuit,
;each call to this function actually delays a while, then updates the current value
; inputs: none
; outputs: AX = current timer value (roughly equal to number of milliseconds since the last call to 'timer_init')
timer_read:
lda #111
jsr $fca8 ;wait for about 33ms
clc
lda #33
adc current_time_value
sta current_time_value
bcc :+
inc current_time_value+1
:
ldax current_time_value
rts