Adapt Atari timer code to PAL systems.

The vertical blank interrupt (which is used by the timer) runs with
60Hz on NTSC systems and 50Hz on PAL systems.
This commit also clears 'vbichain' in 'timer_exit', so a subsequent
'timer_init' would do the right thing.

drivers/atrtimer.s

@@ -2,7 +2,6 @@
 ;
 ; 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.
-; this Atari implementation requires the routine timer_vbl_handler be called 60 times per second
 
 .include "atari.inc"
 .include "../inc/common.i"
@@ -18,7 +17,8 @@
 current_time_value: .res 2
 current_seconds:    .res 1
 current_jiffies:    .res 1
-
+timer_freq:         .res 1      ; VBLANK frequency: 50 - PAL, 60 - NTSC
+timer_freq_reciproc:.res 1      ; reciprocal for timer_freq in ms: 20 - PAL, ~17 - NTSC
 
 .data
 
@@ -40,6 +40,22 @@ timer_init:
   lda #6                        ; STAGE 1 VBI
   jsr SETVBV                    ; vector to set VBLANK parameters
 @handler_installed:
+  lda PAL                       ; hardware register describing TV system of GTIA
+  and #$0e                      ; mask out irrelevant bits
+  bne @ntsc
+  ; PAL system
+  lda #50
+  sta timer_freq
+  lda #20
+  sta timer_freq_reciproc
+  bne @system_set
+@ntsc:
+  ; NTSC system
+  lda #60
+  sta timer_freq
+  lda #17
+  sta timer_freq_reciproc
+@system_set:
   lda #0
   sta current_time_value
   sta current_time_value+1
@@ -54,6 +70,9 @@ timer_exit:
   ldx vbichain+1
   lda #6                        ; STAGE 1 VBI
   jsr SETVBV                    ; vector to set VBLANK parameters
+  lda #0
+  sta vbichain
+  sta vbichain+1
 @handler_not_installed:
   rts
 
@@ -69,7 +88,7 @@ timer_read:
 ; outputs: none (all registers preserved, but carry flag can be modified)
 timer_vbl_handler:
   pha
-  lda #17                       ; 60 HZ =~ 17 ms per 'tick'
+  lda timer_freq_reciproc       ; ms per tick
   clc
   adc current_time_value
   sta current_time_value
@@ -77,7 +96,7 @@ timer_vbl_handler:
   inc current_time_value+1
 : inc current_jiffies
   lda current_jiffies
-  cmp #60
+  cmp timer_freq                ; full second?
   bne @done
   lda #0
   sta current_jiffies