initial work towards burst IEC protocol support

c64/kernal/init/fda3.ioinit.s

@@ -110,8 +110,6 @@ IOINIT:
 	lda #$3F
 	sta CIA2_DDRA    // $DD02
 
-	// XXX detect TOD clock, see  https://codebase64.org/doku.php?id=base:efficient_tod_initialisation
-
 	// XXX how to initialize these timers???
 	// stx CIA1_CRB    // $DC0F
 	// stx CIA2_CRA    // $DD0E
@@ -123,13 +121,8 @@ IOINIT:
 	ldx #%00000111 // XXX adapt this to IEC idle state, JMP at the end won't be needed
 	stx CIA2_PRA    // $DD00
 
-	// 
-
-	ldy #<16421
-	ldx #>16421
-
-	sty CIA1_TIMALO    // $DC04
-	stx CIA1_TIMAHI    // $DC05
+	// Put something sane in the IRQ timer
+	jsr setup_irq_timer
 
 	// Enable timer A to run continuously (http://codebase64.org/doku.php?id=base:timerinterrupts)
 	ldx #$11

c64/kernal/init/setup_pal_ntsc.s

@@ -23,43 +23,39 @@ setup_pal_ntsc:
 
 setup_pal:
 
-	// Set timer interval to ~1/60th of a second
-
-	// (This value was calculated by running a custom IRQ handler on a C64
-	// with original KERNAL, and writing the values of $DC04/5 to the screen
-	// in the IRQ handler to see roughly what value the timers must be set to)
-	// ldy #<16380
-	// ldx #>16380
-
-	// PAL C64 (https://codebase64.org/doku.php?id=base:cpu_clocking),
-	// is clocked at 0.985248 MHz, so that 1/60s is 16421 ($4025) CPU cycles
-
-	ldy #<16421
-	ldx #>16421
-
 	lda #$01
+	skip_2_bytes_trash_nvz
+
+	// FALLTROUGH
+
+setup_ntsc:
+
+	lda #$00
 
 	// FALLTROUGH
 
 setup_pal_ntsc_end:
 
-	sty CIA1_TIMALO    // $DC04
-	stx CIA1_TIMAHI    // $DC05
-
 	sta TVSFLG
 
+	// FALLTROUGH
+
+setup_irq_timer: // entry point needed by UP9600 and CIA1 burst mod support
+
+	// Sets up IRQ timer in CIA1 to ~1/60th of a second,
+	// depending on the detected video system
+
+	// Retrieve video system (0 - NTSC, 1 - PAL), assume PAL for unknown values
+
+	ldx TVSFLG
+	beq !+
+	ldx #$01
+!:
+	// Setup interval for timer A in CIA1
+
+	lda irq_timing_lo, x
+	sta CIA1_TIMALO                    // $DC04
+	lda irq_timing_hi, x
+	sta CIA1_TIMAHI                    // $DC05
+
 	rts
-
-setup_ntsc:
-
-	// Set timer interval to ~1/60th of a second
-	
-	// NTSC C64 (https://codebase64.org/doku.php?id=base:cpu_clocking),
-	// is clocked at 1.022727 MHz, so that 1/60s is 17045 ($4295) CPU cycles
-
-	ldy #<17045
-	ldx #>17045
-
-	lda #$00
-
-	beq setup_pal_ntsc_end // branch always