; set VIA address
PORTB = $6000
PORTA = $6001
DDRB = $6002
DDRA = $6003
T1CL = $6004
T1CH = $6005
ACR = $600B
IFR = $600D
IER = $600E

; define variables
MODE = $0300
HOLD = $0301
COUNT = $0302

E  = %01000000
RW = %00100000
RS = %00010000

	.org $8000

reset:
	sei ;disable interrupts

	ldx #$FF
	txs ;inlisize stack pointer

	lda #$FF
	sta DDRB ; set all port b pins to output
	sta DDRA ; set all port a pins to output

	lda #%01000000
	sta ACR ; enable timer 1 continuous mode

	lda #%11000000
	sta IER ; enable timer 1 interrupts

	stz HOLD ; set hold to 0
	stz MODE ; set mode to 0

	lda #$01
	sta COUNT ; set count to 1
	sta PORTA ; set port a to output 1

	jsr lcd_init
	lda #%00101000 ; Set 4-bit mode; 2-line display; 5x8 font
	jsr lcd_instruction
	lda #%00001110 ; Display on; cursor on; blink off
	jsr lcd_instruction
	lda #%00000110 ; Increment and shift cursor; don't shift display
	jsr lcd_instruction
	lda #$00000001 ; Clear display
	jsr lcd_instruction

	cli ; enable interrupts

	lda #$FF
	sta T1CL ; load timer 1 counter low with 255
	sta T1CH ; load timer 1 counter high with 255, and start the count

loop:
	; do nothing loop
	jmp loop

lcd_wait:
	pha
	lda #%11110000  ; LCD data is input
	sta DDRB
lcdbusy:
	lda #RW
	sta PORTB
	lda #(RW | E)
	sta PORTB
	lda PORTB       ; Read high nibble
	pha             ; and put on stack since it has the busy flag
	lda #RW
	sta PORTB
	lda #(RW | E)
	sta PORTB
	lda PORTB       ; Read low nibble
	pla             ; Get high nibble off stack
	and #%00001000
	bne lcdbusy

	lda #RW
	sta PORTB
	lda #%11111111  ; LCD data is output
	sta DDRB
	pla
	rts

lcd_init:
	lda #%00000010 ; Set 4-bit mode
	sta PORTB
	ora #E
	sta PORTB
	and #%00001111
	sta PORTB
	rts

lcd_instruction:
	jsr lcd_wait
	pha
	lsr
	lsr
	lsr
	lsr            ; Send high 4 bits
	sta PORTB
	ora #E         ; Set E bit to send instruction
	sta PORTB
	eor #E         ; Clear E bit
	sta PORTB
	pla
	and #%00001111 ; Send low 4 bits
	sta PORTB
	ora #E         ; Set E bit to send instruction
	sta PORTB
	eor #E         ; Clear E bit
	sta PORTB
	rts

print_char:
	jsr lcd_wait
	pha
	lsr
	lsr
	lsr
	lsr             ; Send high 4 bits
	ora #RS         ; Set RS
	sta PORTB
	ora #E          ; Set E bit to send instruction
	sta PORTB
	eor #E          ; Clear E bit
	sta PORTB
	pla
	and #%00001111  ; Send low 4 bits
	ora #RS         ; Set RS
	sta PORTB
	ora #E          ; Set E bit to send instruction
	sta PORTB
	eor #E          ; Clear E bit
	sta PORTB
	rts

nmi:
	rti
irq:
	pha
	bit HOLD
	bne slow ; jump to slow if HOLD is not zero

	lda COUNT ; load count into a register

	bit MODE
	bmi invert ; jump to invert if MODE bit 7 is a 1

	inc ; increment the counter
	jmp finish

invert:
	dec ; decrement the counter

finish:
	beq switch ; if the counter is now 0, jump to switch
	sta PORTA ; output the counter to port A LEDs
	sta COUNT ; save new counter value
	jmp exit_irq_hold

switch:
	sta COUNT ; save counter without outputting
	lda #%10000000
	eor MODE ; flip bit 7 of MODE
	sta MODE ; save new MODE value

exit_irq_hold:
	lda #$02
	sta HOLD ; save a 2 into HOLD
	jmp exit_irq

slow:
	dec HOLD ; decrement HOLD

exit_irq:
	bit T1CL ; read timer 1 counter low to clear interrupt
	pla
	rti ; return from interrupt

	.org $FFFA
	.word nmi
	.word reset
	.word irq