.equ DDRB,   0x04
.equ PORTB,  0x05

.equ TCCR0B, 0x25
.equ TCNT0,  0x26
.equ TIMSK0, 0x6e

.equ SPL,    0x3d
.equ SPH,    0x3e
.equ SREG,   0x3f

.equ COUNTER_ADDR, 0x0100


.org 0x00
__reset:

  ; 0x00 RESET
  jmp __init

  ; 0x02 INT0
  jmp __bad_interrupt

  ; 0x04 INT1
  jmp __bad_interrupt

  ; 0x06 PCINT0
  jmp __bad_interrupt

  ; 0x08 PCINT1
  jmp __bad_interrupt

  ; 0x0a PCINT2
  jmp __bad_interrupt

  ; 0x0c WDT
  jmp __bad_interrupt

  ; 0x0e TIMER2_COMPA
  jmp __bad_interrupt

  ; 0x10 TIMER2_COMPB
  jmp __bad_interrupt

  ; 0x12 TIMER2_OVF
  jmp __bad_interrupt

  ; 0x14 TIMER2_CAPT
  jmp __bad_interrupt

  ; 0x16 TIMER1_COMPA
  jmp __bad_interrupt

  ; 0x18 TIMER1_COMPB
  jmp __bad_interrupt

  ; 0x1a TIMER1_OVF
  jmp __bad_interrupt

  ; 0x1c TIMER0_COMPA
  jmp __bad_interrupt

  ; 0x1e TIMER0_COMPB
  jmp __bad_interrupt

  ; 0x20 TIMER0_OVF
  jmp __timer0_overflow_handler

  ; 0x22 SPI_STC
  jmp __bad_interrupt

  ; 0x24 USART_RX
  jmp __bad_interrupt

  ; 0x26 USART_UDRE
  jmp __bad_interrupt

  ; 0x28 USART_TX
  jmp __bad_interrupt

  ; 0x2a ADC
  jmp __bad_interrupt

  ; 0x2c EE_READY
  jmp __bad_interrupt

  ; 0x2e ANALOG_COMP
  jmp __bad_interrupt

  ; 0x30 TWI
  jmp __bad_interrupt

  ; 0x32 SPM_READY
  jmp __bad_interrupt


__init:
  ; r1 = zero register
  eor	r1, r1

  ; SREG = 0
  out	SREG, r1

  ; SP = 0x08FF
  ldi	r28, 0xFF
  ldi	r29, 0x08
  out	SPH, r29
  out	SPL, r28

  call	main
  jmp	_exit


__bad_interrupt:
  jmp	__reset


__timer0_overflow_handler:
  ; prologue
  push	r1
  push	r0
  in	r0, SREG
  push	r0
  eor	r1, r1
  push	r24
  push	r28
  push	r29
  in	r28, SPL
  in	r29, SPH

  ; disable interrupts
  cli

  ; counter++
  lds	r24, COUNTER_ADDR
  subi	r24, 0xFF
  sts	COUNTER_ADDR, r24

  ; enable interrupts
  sei

  ; prologue
  nop
  pop	r29
  pop	r28
  pop	r24
  pop	r0
  out	SREG, r0
  pop	r0
  pop	r1
  reti


main:
  ; prologue: save Y; Y = SP
  push	r28
  push	r29
  in	r28, SPL
  in	r29, SPH

  ; TCCR0B = 0x05
  ldi	r24, 0x45
  ldi	r25, 0x00
  ldi	r18, 0x05
  movw	r30, r24
  st	Z, r18

  ; TIMSK0 = 0x01
  ldi	r24, 0x6E
  ldi	r25, 0x00
  ldi	r18, 0x01
  movw	r30, r24
  st	Z, r18

  ; counter = 0
  sts COUNTER_ADDR, r1

  ; enable interrupts
  sei

  ; DDRB = 0xFF
  ldi	r24, 0x24
  ldi	r25, 0x00
  ldi	r18, 0xFF
  movw	r30, r24
  st	Z, r18

  ; PORTB = 0x00
  ldi	r24, 0x25
  ldi	r25, 0x00
  ldi r18, 0x00
  movw	r30, r24
  st	Z, r18

_temp0:
  ; if (counter != 10) jmp _temp0
  lds	r24, COUNTER_ADDR
  cpi	r24, 0x0a
  brne _temp0

  ; counter = 0
  ldi r18, 0x00
  sts	COUNTER_ADDR, r18

  ; PORTB = ~PORTB
    ; read
  ldi	r24, 0x25
  ldi	r25, 0x00
  movw	r30, r24
  ld	r18, Z
    ; write
  ldi	r24, 0x25
  ldi	r25, 0x00
    ; negate
  com	r18
  movw	r30, r24
  st	Z, r18

  rjmp	_temp0


_exit:
  cli


__stop_program:
  rjmp	__stop_program