implemented BRK handling routine (not yet connected to IRQ)

c64/kernal/interrupts/irq_handler.s

@@ -1,3 +1,4 @@
+
 irq_handler:
 	;; The IRQ is a commonly messed with thing on the C64,
 	;; so we need to handle entry points that are commonly
@@ -7,17 +8,18 @@ irq_handler:
 	;; $EA81 - https://www.lemon64.com/forum/viewtopic.php?t=2112&sid=6ea01982b26da69783120a7923ca46fb
 	;; Also the $0314 vector is widely used (e.g., https://www.lemon64.com/forum/viewtopic.php?t=2112&sid=6ea01982b26da69783120a7923ca46fb)
 
-	;; Save registers
-	;; Sequence according to Compute's Mapping the 64 p73
+	;; Save registers, sequence according to Compute's Mapping the Commodore 64, page 73
 	pha
 	txa
 	pha
 	tya
 	pha
 
-	;; Call interrupt routine
-	;; (but only if initialised)
+	;; Call interrupt routine (only if initialised)
 	lda CINV
 	ora CINV+1
-	beq default_irq_handler
+	bne +
 	jmp (CINV)
+*
+	;; Vector not initialized - call default interrupt routine
+	jmp default_irq_handler