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

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

MODE = $0300
HOLD = $0301
COUNT = $0302

  .org $8000

reset:
  sei

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

  ldx #$ff
  txs

  lda #%11111111 ; Set all pins on port B to output
  sta DDRB
  lda #%11111111 ; Set all pins on port A to output
  sta DDRA

  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

  ldx #0
print:
  lda message,x
  beq do_count
  jsr print_char
  inx
  jmp print

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

  lda #%11000000
  sta IER ; enable timer 1 interrupts

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

  cli

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

  jmp loop

loop:

  jmp loop

message: .asciiz "Hello, World!"

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:
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 ; decrment 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