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80 lines
3.0 KiB
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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
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unfortunately the standard Apple 2 has no CIA or VBI, so for the moment, we will
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make each call to 'timer_read' delay for a little while
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this kludge will make the polling loops work at least
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timer_read is meant to return a counter with millisecond resolution
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</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
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inputs: none
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outputs: none
</pre></td></tr><tr><td id="timer_read">timer_read</td><td><pre>this SHOULD just read the current timer value
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but since a standard apple 2 has no dedicated timing circuit,
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each call to this function actually delays a while, then updates the current value
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inputs: none
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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
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;
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; unfortunately the standard Apple 2 has no CIA or VBI, so for the moment, we will
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; make each call to 'timer_read' delay for a little while
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; this kludge will make the polling loops work at least
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;
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; timer_read is meant to return a counter with millisecond resolution
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.include "../inc/common.i"
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.export timer_init
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.export timer_read
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.bss
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current_time_value: .res 2
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.code
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;reset timer to 0
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;inputs: none
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;outputs: none
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timer_init:
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ldax #0
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stax current_time_value
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rts
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;this SHOULD just read the current timer value
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;but since a standard apple 2 has no dedicated timing circuit,
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;each call to this function actually delays a while, then updates the current value
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; inputs: none
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; outputs: AX = current timer value (roughly equal to number of milliseconds since the last call to 'timer_init')
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timer_read:
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lda #111
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jsr $fca8 ;wait for about 33ms
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clc
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lda #33
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adc current_time_value
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sta current_time_value
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bcc :+
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inc current_time_value+1
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:
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ldax current_time_value
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rts
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;-- LICENSE FOR a2timer.s --
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; The contents of this file are subject to the Mozilla Public License
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; Version 1.1 (the "License"); you may not use this file except in
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; compliance with the License. You may obtain a copy of the License at
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; http://www.mozilla.org/MPL/
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;
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; Software distributed under the License is distributed on an "AS IS"
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; basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See the
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; License for the specific language governing rights and limitations
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; under the License.
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;
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; The Original Code is ip65.
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;
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; The Initial Developer of the Original Code is Jonno Downes,
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; jonno@jamtronix.com.
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; Portions created by the Initial Developer are Copyright (C) 2009
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; Jonno Downes. All Rights Reserved.
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; -- LICENSE END --
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</pre></body></html>
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