begin implementing IRQ framework

c64/kernal/ea81.s

@@ -0,0 +1,17 @@
+	;; 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)
+
+return_from_interrupt:
+	;; Restore registers and return
+	pla
+	tay
+	pla
+	tax
+	pla
+	rti
+	

c64/kernal/irq_handler.s

@@ -1,3 +1,25 @@
 irq_handler:
-	inc $d020
-	rti
+	;; 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
+	pha
+	txa
+	pha
+	tya
+	pha
+
+	;; Call interrupt routine
+	;; (but only if initialised)
+	lda $0314
+	ora $0315
+	beq no_handler_installed
+	jmp ($0314)
+no_handler_installed:
+	
+	jmp return_from_interrupt