emailler/docs/drivers_a2timer_s.html

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<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"/><link rel="stylesheet" type="text/css" href="ca65-doc-style.css"/></head><body><a href="ref_index.html"><h1>ip65 technical reference</h1></a><h1>File : drivers/a2timer.s</h1><pre> 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
</pre><h2 id="functions">functions</h2><table><tr><th>function</th><th>description</th></tr><tr><td id="timer_init">timer_init</td><td><pre>reset timer to 0
inputs: none
outputs: none </pre></td></tr><tr><td id="timer_read">timer_read</td><td><pre>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') </pre></td></tr></table><h2>implementation</h2><pre id="code">; 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
;-- LICENSE FOR a2timer.s --
; The contents of this file are subject to the Mozilla Public License
; Version 1.1 (the "License"); you may not use this file except in
; compliance with the License. You may obtain a copy of the License at
; http://www.mozilla.org/MPL/
;
; Software distributed under the License is distributed on an "AS IS"
; basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See the
; License for the specific language governing rights and limitations
; under the License.
;
; The Original Code is ip65.
;
; The Initial Developer of the Original Code is Jonno Downes,
; jonno@jamtronix.com.
; Portions created by the Initial Developer are Copyright (C) 2009
; Jonno Downes. All Rights Reserved.
; -- LICENSE END --
</pre></body></html>