blinky.S

#include <avr/io.h>

.org 0x0000
rjmp run

run:
	; Clear r1
	clr r1
	
	; Load 0x01 into r16 for use later
	ldi r16, 0x01
	
	; Load the data direction of DIRSET into r17
	lds r17, PORTD_DIRSET
	
	; OR with r16
	or r17, r16
	
	; Commit the data direction
	sts PORTD_DIRSET, r17
	
	; No interrupts
	cli
	
	; Loop
	rjmp loop

loop:
	; Load the OUTTGL register into r17
	lds r17, PORTD_OUTTGL
	
	; OR with r16
	or r17, r16
	
	; Commit
	sts PORTD_OUTTGL, r17
	
	; Delay loop
	rjmp delay
	
delay:
	; Load a 50 millisecond delay
	ldi r24, 0xa8  ; 168
	ldi r25, 0x7d  ; 125
	
	; Optimized delay loop 
	sbiw r24, 0x01
	brne .-4
	
	; Back to the loop
	rjmp loop