atrtimer.s, vic20timer.s: use decimal mode in interrupt handler

Using decimal mode (SED/CLD) reduces the memory footprint and execution time
of the interrupt routine. Some comments were added or fixed, too.

drivers/atrtimer.s

@@ -83,7 +83,7 @@ timer_read:
   ldax current_time_value
   rts
 
-; tick over the current timer value - should be called 60 times per second
+; tick over the current timer value - should be called 50 or 60 times per second, depending on the TV system
 ; inputs: none
 ; outputs: none (all registers preserved, but carry flag can be modified)
 timer_vbl_handler:
@@ -97,28 +97,15 @@ timer_vbl_handler:
 : inc current_jiffies
   lda current_jiffies
   cmp timer_freq                ; full second?
-  bne @done
-  lda #0
+  bne @done                     ; no, we're done
+  lda #0                        ; yes, increment "seconds" counter
   sta current_jiffies
-  inc current_seconds
-  ; we don't want to mess around with decimal mode in an IRQ handler
-  lda current_seconds
-  cmp #$0a
-  bne :+
-  lda #$10
-: cmp #$1a
-  bne :+
-  lda #$20
-: cmp #$2a
-  bne :+
-  lda #$30
-: cmp #$3a
-  bne :+
-  lda #$40
-: cmp #$4a
-  bne :+
-  lda #$50
-: cmp #$5a
+  sed
+  clc
+  lda #$01
+  adc current_seconds
+  cld
+  cmp #$60
   bne :+
   lda #$00
 : sta current_seconds

drivers/vic20timer.s

@@ -68,29 +68,16 @@ timer_vbl_handler:
   inc current_time_value+1
 : inc current_jiffies
   lda current_jiffies
-  cmp #60
-  bne @done
-  lda #0
+  cmp #60                       ; full second?
+  bne @done                     ; no, we're done
+  lda #0                        ; yes, increment "seconds" counter
   sta current_jiffies
-  inc current_seconds
-  ; we don't want to mess around with decimal mode in an IRQ handler
-  lda current_seconds
-  cmp #$0a
-  bne :+
-  lda #$10
-: cmp #$1a
-  bne :+
-  lda #$20
-: cmp #$2a
-  bne :+
-  lda #$30
-: cmp #$3a
-  bne :+
-  lda #$40
-: cmp #$4a
-  bne :+
-  lda #$50
-: cmp #$5a
+  sed
+  clc
+  lda #$01
+  adc current_seconds
+  cld
+  cmp #$60
   bne :+
   lda #$00
 : sta current_seconds