implement $EA7E entry point and fix $EA31 entry point. Issue #20

c64/kernal/irq_handler.s

@@ -0,0 +1,23 @@
+irq_handler:
+	;; The IRQ is a commonly messed with thing on the C64,
+	;; so we need to handle entry points that are commonly
+	;; relied upon, including:
+	;; $EA31 - Standard IRQ routine
+	;; $EA61 - Check keyboard, but don't update timer? https://github.com/cc65/cc65/issues/324
+	;; $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
+	pha
+	txa
+	pha
+	tya
+	pha
+
+	;; Call interrupt routine
+	;; (but only if initialised)
+	lda $0314
+	ora $0315
+	beq default_irq_handler
+	jmp ($0314)