timer now uses 60hz IRQ handler, rather than CIA

git-svn-id: http://svn.code.sf.net/p/netboot65/code@65 93682198-c243-4bdb-bd91-e943c89aac3b

client/drivers/c64timer.s

@@ -1,50 +1,47 @@
 ; timer routines
 ;
 ; the timer should be a 16-bit counter that's incremented by about
-; 1000 units per second. it doesn't have to be particularly accurate,
-; if you're working with e.g. a 60 Hz VBLANK IRQ, adding 17 to the
-; counter every frame would be just fine.
-
+; 1000 units per second. it doesn't have to be particularly accurate.
+; this C64 implementation requires the routine timer_vbl_handler be called 60 times per second
 
 	.include "../inc/common.i"
 
 
 	.export timer_init
 	.export timer_read
+  .export timer_vbl_handler
+  
+	.code
+  .bss
+  current_time_value: .res 2
   
 	.code
 
-;initialize timers
+;reset timer to 0
 ;inputs: none
 ;outputs: none
 timer_init:
-	lda #$80		; stop timers
-	sta $dd0e
-	sta $dd0f
-
-	ldax #999		; timer A to 1000 cycles
-	stax $dd04
-
-	ldax #$ffff		; timer B to max cycles
-	stax $dd06
-
-	lda #$81		; timer A in continuous mode
-	sta $dd0e
-
-	lda #$c1		; timer B to count timer A underflows
-	sta $dd0f
-
+  ldax  #0
+  stax current_time_value
 	rts
 
-
-;initialize timers
-;inputs: none
-;outputs: AX = count in milliseconds sent last call to timer_init
+;read the current timer value 
+; inputs: none
+; outputs: AX = current timer value (roughly equal to number of milliseconds since the last call to 'timer_init')
 timer_read:
-	lda $dd07		; cia counts backwards, return inverted value
-	eor #$ff
-	tax
-	lda $dd06
-	eor #$ff
-	rts
+  ldax  current_time_value
+  rts
 
+; tick over the current timer value - should be called 60 times per second
+; inputs: none
+; outputs: none (all registers preserved, by carry flag can be modified)
+timer_vbl_handler:
+  pha
+  lda #$11
+  adc current_time_value
+  sta current_time_value
+  bcc :+
+  inc current_time_value+1
+:
+  pla
+  rts