monkey: was missing file

monkey/pt3_interrupt_handler.s

@@ -0,0 +1,68 @@
+	;================================
+	;================================
+	; mockingboard interrupt handler
+	;================================
+	;================================
+	; On Apple II/6502 the interrupt handler jumps to address in 0xfffe
+	; This is in the ROM, which saves the registers
+	;   on older IIe it saved A to $45 (which could mess with DISK II)
+	;   newer IIe doesn't do that.
+	; It then calculates if it is a BRK or not (which trashes A)
+	; Then it sets up the stack like an interrupt and calls 0x3fe
+
+	; Note: the IIc is much more complicated
+	;	its firmware tries to decode the proper source
+	;	based on various things, including screen hole values
+	;	we bypass that by switching out ROM and replacing the
+	;	$fffe vector with this, but that does mean we have
+	;	to be sure status flag and accumulator set properly
+
+interrupt_handler:
+	php			; save status flags
+	pha			; save A				; 3
+				; A is saved in $45 by firmware
+	txa
+	pha			; save X
+	tya
+	pha			; save Y
+
+
+
+;	inc	$0404		; debug (flashes char onscreen)
+
+
+.include "pt3_lib_irq_handler.s"
+
+	jmp	exit_interrupt
+
+	;=================================
+	; Finally done with this interrupt
+	;=================================
+
+quiet_exit:
+	stx	DONE_PLAYING
+	jsr	clear_ay_both
+
+	ldx	#$ff		; also mute the channel
+	stx	AY_REGISTERS+7	; just in case
+
+
+exit_interrupt:
+
+	pla
+	tay			; restore Y
+	pla
+	tax			; restore X
+	pla			; restore a				; 4
+
+	; on II+/IIe (but not IIc) we need to do this?
+interrupt_smc:
+	lda	$45		; restore A
+	plp
+
+	rti			; return from interrupt			; 6
+
+								;============
+								; typical
+								; ???? cycles
+