small startup/interrupt cleanup

c64/kernal/interrupts/hw_entry_irq.s

@@ -0,0 +1,55 @@
+
+hw_entry_irq:
+
+	// 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 Commodore 64, page 73
+	pha
+#if CONFIG_CPU_MOS_6502
+	txa
+	pha
+	tya
+	pha
+#else
+	phx
+	phy
+#endif
+
+#if CONFIG_CPU_MOS_6502 && CONFIG_BCD_SAFE_INTERRUPTS
+	cld // clear decimal flag to allow using it without disabling interrupts
+#endif
+
+	// Check if caused by BRK
+	tsx
+	lda $0104, x // get the pre-interrupt processor state
+	and #$10
+	bne irq_handler_brk
+
+	// Not caused by BRK - call interrupt routine (only if initialised)
+	lda CINV
+	ora CINV+1
+	beq !+
+	jmp (CINV)
+!:
+	// Vector not initialized - call default interrupt routine
+	jmp default_irq_handler
+
+irq_handler_brk:
+
+	// Store BRK adddress, to be displayed
+	sec
+	lda $0105, x
+	sbc #$02
+	sta CMP0+0
+	lda $0106, x
+	sbc #$00
+	sta CMP0+1
+
+	// Interrupt caused by BRK
+	jmp (CBINV)