Question

Implement the following functions with AVR assembly language 1) 2-byte addition (i.e, addition on 16-bit numbers) 2) 2-byte signed subtraction 3) 2-byte signed multiplication

Answer 1

1) 2-byte addition Code

.DEF  AL = R16         ;To hold low byte of value1
.DEF  AH = R17         ;To hold high byte of value1
.DEF  BL = R18         ;To hold low byte of value2
.DEF  BH = R19         ;To hold high byte of value2
add:
     ADD AL,BL         ;Add the lower bytes together
     ADC AH,BH         ;Add the higher bytes and include carry bit
     BRCS OVERFLOW
     ret

2) 2 byte signed subtraction

subtract:

      SUB AL,BL         ;Subtract two low bytes
      SBC AH,BH         ;Subtract two high bytes and include carry 
      BRCS UNDERFLOW
      ret

3) 2 byte signed multiplication

mul16x16_16:

mul   r22, r20       ; al * bl
movw   r17:r16, r1:r0
mul   r23, r20       ; ah * bl
add   r17, r0
mul   r21, r22       ; bh * al
add   r17, r0
ret