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46 lines
1.0 KiB
ArmAsm
46 lines
1.0 KiB
ArmAsm
; 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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