dos33fsprogs/music/ksp_theme/interrupt_test.s

; Test the 6522 on a Mockingboard running in interrupt mode
; Trying to set it to 50Hz

.include	"zp.inc"

	;=============================
	; Print message
	;=============================
	jsr     HOME
	jsr     TEXT

	lda	#0
	sta	DRAW_PAGE
	sta	CH
	sta	CV
	lda	#<mocking_message
	sta	OUTL
	lda	#>mocking_message
	sta	OUTH
	jsr	move_and_print

	jsr	mockingboard_detect_slot4
	cpx	#$1
	beq	mockingboard_found

	lda	#<not_message
	sta	OUTL
	lda	#>not_message
	sta	OUTH
	inc	CV
	jsr	move_and_print

	jmp	forever_loop

mockingboard_found:
	lda     #<found_message
	sta     OUTL
	lda     #>found_message
	sta     OUTH
	inc     CV
	jsr     move_and_print

	;=========================
	; Setup Interrupt Handler
	;=========================
	; Vector address goes to 0x3fe/0x3ff
	; FIXME: should chain any existing handler

	lda	#<interrupt_handler
	sta	$03fe
	lda	#>interrupt_handler
	sta	$03ff

	;============================
	; Enable 50Hz clock on 6522
	;============================

	lda	#$40		; Generate continuous interrupts, don't touch PB7
	sta	$C40B		; ACR register
	lda	#$7F		; clear all interrupt flags
	sta	$C40E		; IER register (interrupt enable)

	lda	#$C0
	sta	$C40D		; IFR: 1100, enable interrupt on timer one oflow
	sta	$C40E		; IER: 1100, enable timer one interrupt

	lda	#$E7
	sta	$C404		; write into low-order latch
	lda	#$4f
	sta	$C405		; write into high-order latch,
				; load both values into counter
				; clear interrupt and start counting

	; 4fe7 / 1e6 = .020s, 50Hz

	;============================
	; Enable 6502 interrupts
	;============================
	;
	cli		; clear interrupt mask


	;============================
	; Loop forever
	;============================
	lda	#64
forever_loop:
;	clc
;	adc	#1
	sta	$0401
	jmp	forever_loop



	;=============================
	; simple 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

interrupt_handler:
	pha		; save A

	bit		$C404	; can clear interrupt by reading T1C-L


	inc		$0410	; increment text char on screen

	pla		; restore A
	rti



;=========
;routines
;=========
.include	"../../asm_routines/gr_offsets.s"
.include	"../../asm_routines/text_print.s"
.include	"../../asm_routines/mockingboard.s"

;=========
; strings
;=========
mocking_message:	.asciiz "LOOKING FOR MOCKINGBOARD IN SLOT #4"
not_message:		.byte   "NOT "
found_message:		.asciiz "FOUND"
done_message:		.asciiz "DONE PLAYING"