; Arduino sketch .S subroutine written in AVR assembly code
#define __SFR_OFFSET 0x20
#include "avr/io.h"

.global start
.global forever

start:
  LDI R16, 0xFF     ; Setting all PORTB as output
  STS DDRB, R16
  LDI R17, 0x00
  STS PORTB, R17    ; Writing initial value 0 to PORTB
  LDI R16, 0x00
  STS TCCR1A, R16   ; Setting all bits of TCCR1A as 0
  RET

forever: 
  LDI R16, 0xF8
  STS TCNT1H, R16   ; Writing loop delay 0xF8 to TCNT1H (8bit)
  LDI R16, 0xFB
  STS TCNT1L, R16   ; Writing loop delay 0xFB to TCNT1L (8bit)
  LDI R16, 0x05
  STS TCCR1B, R16   ; Writing 0x05 into TCCR1B
L:LDS R0, TIFR1     ; Load the value of TIFR1 into R0
  SBRS R0, 0        ; Skip the next statement if overflow 
  RJMP L            ; Loop until overflow occurs.
  LDI R16, 0x00
  STS TCCR1B, R16   ; Stop the Timer/Counter1
  LDI R16, 0x01
  STS TIFR1, R16    ; Clear the overflow flag by writing 1 
  
  ; increment PORTB blinks led
  INC R17           ; Increment R17 register
  STS PORTB, R17    ; Toggle the LED output
  RET               ; return to label forever: