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47 lines
832 B
ArmAsm
47 lines
832 B
ArmAsm
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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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; 1000 units per second. it doesn't have to be particularly accurate,
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; if you're working with e.g. a 60 Hz VBLANK IRQ, adding 17 to the
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; counter every frame would be just fine.
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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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.code
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; initialize timers
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timer_init:
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lda #$80 ; stop timers
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sta $dd0e
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sta $dd0f
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ldax #999 ; timer A to 1000 cycles
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stax $dd04
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ldax #$ffff ; timer B to max cycles
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stax $dd06
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lda #$81 ; timer A in continuous mode
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sta $dd0e
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lda #$c1 ; timer B to count timer A underflows
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sta $dd0f
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rts
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; return the current value
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timer_read:
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lda $dd07 ; cia counts backwards, return inverted value
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eor #$ff
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tax
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lda $dd06
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eor #$ff
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rts
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