#include <avr/io.h>

; declarations for better readability
.equ LED_PORT, _SFR_IO_ADDR(PORTA)
.equ LED_DIR, _SFR_IO_ADDR(DDRA)
.equ LED_PIN, PA3
.equ BTN_PORT, _SFR_IO_ADDR(PORTA)
.equ BTN_DIR, _SFR_IO_ADDR(DDRA)
.equ BTN_PIN, PA7
.equ BTN_INTERRUPT, (1 << PCIE0)
.equ BTN_INTERRUPT_PIN, (1 << PCINT7)
; naming registers we use
.equ tmp, 17
.equ tmp1, 18
.equ pressed, 19

  .section .text

; jump to main program
rjmp main

  ; declaring the pin change interrupt routine
  .global PCINT0_vect
PCINT0_vect:
  cp pressed, 0
  brne skip
  ; if the button is already pressed, toggle LED
  in tmp, LED_PORT
  eor tmp, tmp1
  out LED_PORT, tmp
skip:
  ; toggle internal button state
  eor pressed, tmp1
  reti

  ; declaring the main routine
  .global main
main:
  ; initialize LED pin (i.e. output high)
  sbi LED_PORT, LED_PIN
  sbi LED_DIR, LED_PIN

  ; initialize button pin (i.e. input, pullup enabled)
  sbi BTN_PORT, BTN_PIN
  cbi BTN_DIR, BTN_PIN

  ; configure clock division to operate at 20MHz
  ldi tmp, (1 << CLKPCE)
  ldi tmp1, (0 << CLKPS3) | (0 << CLKPS2) | (0 << CLKPS1) | (0 << CLKPS0)
  out _SFR_IO_ADDR(CLKPR), tmp
  out _SFR_IO_ADDR(CLKPR), tmp1

  ; enable interrupts for the port of the button
  in tmp, _SFR_IO_ADDR(GIMSK)
  sbr tmp, BTN_INTERRUPT
  out _SFR_IO_ADDR(GIMSK), tmp

  ; enable interrupts for the pin of the button
  in tmp, _SFR_IO_ADDR(PCMSK0)
  sbr tmp, BTN_INTERRUPT_PIN
  out _SFR_IO_ADDR(PCMSK0), tmp

  ; globally enable interrupts
  sei

  ; turn LED off
  cbi LED_PORT, LED_PIN

  ; select "standby" sleep mode
  in tmp, _SFR_IO_ADDR(MCUCR)
  sbr tmp, (1 << SM1) | (1 << SM0)
  out _SFR_IO_ADDR(MCUCR), tmp

  ; initialize register with bit mask for pin A3
  sbr tmp1, (1 << LED_PIN)
  loop:
    ; set sleep enable bit
    in tmp, _SFR_IO_ADDR(MCUCR)
    sbr tmp, (1 << SE)
    out _SFR_IO_ADDR(MCUCR), tmp

    ; go to sleep
    sleep

    ; clear sleep enable bit
    in tmp, _SFR_IO_ADDR(MCUCR)
    cbr tmp, (1 << SE)
    out _SFR_IO_ADDR(MCUCR), tmp

    ; jump back 
    rjmp loop