Blinking Arduino Uno with Assembly

.ORG 0x0000            ; the next instruction has to be written to
                       ; address 0x0000
rjmp START             ; the reset vector: jump to "main"
START:
    ldi r16, low(RAMEND)   ; set up the stack
    out SPL, r16
    ldi r16, high(RAMEND)
    out SPH, r16
    ldi r16, 0xFF          ; load register 16 with 0xFF (all bits 1)
    out DDRB, r16          ; write the value in r16 (0xFF) to Data
                       ; Direction Register B
LOOP:
    sbi PortB, 5         ; switch off the LED
    rcall delay_05       ; wait for half a second
    cbi PortB, 5         ; switch it on
    rcall delay_05       ; wait for half a secon
    rjmp LOOP            ; jump to loop

DELAY_05:              ; the subroutine:
    ldi r16, 31          ; load r16 with 31

OUTER_LOOP:            ; outer loop label
    ldi r24, low(1021)   ; load registers r24:r25 with 1021, our new
                        ; init value
    ldi r25, high(1021)  ; the loop label
DELAY_LOOP:            ; "add immediate to word": r24:r25 are
                       ; incremented
    adiw r24, 1          ; if no overflow ("branch if not equal"), go
                        ; back to "delay_loop"
    brne DELAY_LOOP
    dec r16              ; decrement r16
    brne OUTER_LOOP      ; and loop if outer loop not finished
    ret                  ; return from subroutine

Ref:
ref1
ref2