adc r3,r5  

(1) first case: carry flag is set
    r3=r3+r5+1

    if adc does not exist, this is the same as

    inc r3
    add r3,r5 ; carry bit is updated for the next operation
    

(2) second case: carry flag is cleared
    r3=r3+r5

    if adc does not exist,
  
    add r3,r5


Want to write general code to replace adc

- conditional: don't want to add one if the carry flag is cleared!
- need to execute the add instruction

    brcc  l1
    inc   r3
l1: add   r3,r5

================================
want to do something 20 times
    sbi PORTA,2
    cbi PORTA,2

useful instructions:

ldi to initialize a register for countdown purposes

dec to decrease the counter by one for each iteration

breq or brne to utilize the Z flag after dec

designate r16 as the counter

     ldi  r16,20 ; initialize counter to 20

L1:  sbi  PORTA,2
     cbi  PORTA,2
     dec  r16    ; decrement the counter because we just performed one more iteration
     brne L1


brne   L1 == rjmp L1 only if Z=0