added panic screen, added versioning mechanism proposal

c64/kernal/panic.s

@@ -3,19 +3,29 @@
 // Something terrible happened - show the panic screen
 //
 
-// .A = 0 - just panic, no extra message
-// .A = 1 - ROM version mismatch
+
+#if CONFIG_PANIC_SCREEN
 
 panic:
 
+	// Disable interrupts, store error code on stack
+
 	sei
 	pha
 
+	// Reinitialize the hardware
+
 	jsr IOINIT
 	jsr setup_vicii
 	jsr clear_screen
 
-	// XXX disable NMIs
+	// Disable NMIs - our routine checks for NULL vector
+
+	lda #$00
+	sta NMINV+0
+	sta NMINV+1
+
+	// Display KERNAL PANIC message
 
 	ldx #$14
 	ldy #$0E
@@ -24,6 +34,8 @@ panic:
 	ldx #__MSG_KERNAL_PANIC
 	jsr print_kernal_message
 
+	// Display error code and message (if known)
+
 	pla
 	beq kernal_panic_infinite_loop
 	pha
@@ -37,12 +49,14 @@ panic:
 	jsr print_hex_byte
 
 	pla
-	cmp #$01
+	cmp #P_ERR_ROM_MISMATCH
 	bne kernal_panic_infinite_loop
 
-	ldx #__MSG_KERNAL_PANIC_VERSION
+	ldx #__MSG_KERNAL_PANIC_ROM_MISMATCH
 	jsr print_kernal_message
 
+	// Display some raster effect in the infinitew loop
+
 kernal_panic_infinite_loop:
 	ldx #$00
 	stx VIC_EXTCOL
@@ -59,3 +73,5 @@ kernal_panic_infinite_loop:
 	bne !-
 
 	beq kernal_panic_infinite_loop // branch always
+
+#endif // CONFIG_PANIC_SCREEN