2009-01-22 01:27:03 +00:00
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; timer routines
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;
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; the timer should be a 16-bit counter that's incremented by about
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2009-03-28 02:06:49 +00:00
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; 1000 units per second. it doesn't have to be particularly accurate.
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; this C64 implementation requires the routine timer_vbl_handler be called 60 times per second
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2009-01-22 01:27:03 +00:00
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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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2009-03-28 02:06:49 +00:00
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.export timer_vbl_handler
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.code
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.bss
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current_time_value: .res 2
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2009-01-22 01:27:03 +00:00
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.code
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2009-03-28 02:06:49 +00:00
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;reset timer to 0
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2009-03-21 06:40:53 +00:00
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;inputs: none
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;outputs: none
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2009-01-22 01:27:03 +00:00
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timer_init:
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2009-03-28 02:06:49 +00:00
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ldax #0
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stax current_time_value
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2009-01-22 01:27:03 +00:00
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rts
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2009-03-28 02:06:49 +00:00
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;read the current timer 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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2009-01-22 01:27:03 +00:00
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timer_read:
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2009-03-28 02:06:49 +00:00
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ldax current_time_value
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rts
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; tick over the current timer value - should be called 60 times per second
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; inputs: none
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; outputs: none (all registers preserved, by carry flag can be modified)
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timer_vbl_handler:
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pha
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lda #$11
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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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pla
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rts
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