Blink external led connected to PB4 (Pin 12 arduino uno). Assembly Atmega328p.

blink_external.ino

extern "C" {
  // function prototypes
  void start();
  void blink();
}

void setup() {
  start();
}

void loop() {
  blink();
}

blink_external.S

; Blink LED on PB4(Arduino Leonardo pin 8, Arduino Uno pin 12)
; http://forum.arduino.cc/index.php?topic=159572#msg1194604
; https://forum.arduino.cc/index.php?topic=413151.msg2844634#msg2844634

; https://gist.github.com/mhitza/8a4608f4dfdec20d3879


#define __SFR_OFFSET 0
  
#include "avr/io.h"

.global start
.global blink

start:
  sbi   DDRB,4  ; Set PB4 as output
  ret

blink: 
  ldi   r20,100 ; Set the delay duration in ms. Maximum value is 255.
  call  delay_n_ms
  sbi   PORTB,4 ; Set PB4 HIGH
  ldi   r20,250
  call  delay_n_ms
  cbi   PORTB,4 ; Set PB4 LOW
  ret

  
delay_n_ms:
  ; Delay about r20*1ms. Destroys r20, r30, and r31.
  ; One millisecond is about 16000 cycles at 16MHz.
  ; The basic loop takes about 5 cycles, so we need about 3000 loops.
  ldi   31, 3000>>8 ; high(3000)
  ldi   30, 3000&255  ; low(3000)
delaylp:
  sbiw    r30, 1
  brne    delaylp
  subi    r20, 1
  brne    delay_n_ms
  ret