How do I compute the 16-bit sum of the 8-bit values of an array in assembly?

Question

Feel like I've been asking a lot of these questions lately lol, but assembly is still pretty foreign to me.

Using an Arduino, I have to write a function in Atmel AVR Assembly for my computer science class that calculates the sum of the 8-bit values in an array and returns it as a 16-bit integer. The function is supposed to take in an array of bytes and a byte representing the length of the array as arguments, with those arguments stored in r24 and r22, respectively, when the function is called. I am allowed to use branching instructions and such.

The code is in this format:

.global sumArray
sumArray:
  //magic happens
  ret

I know how to make loops and increment the counter and things like that, but I am really lost as to how I would do this.

I am unsure as to how I would do this. Does anyone know how to write this function in Atmel AVR Assembly? Any help would be much appreciated!

Answer 1

Why don't you ask the question to your compiler?

#include <stdint.h>

uint16_t sumArray(uint8_t *val, uint8_t count)
{
    uint16_t sum = 0;
    for (uint8_t i = 0; i < count; i++)
        sum += val[i];
    return sum;
}

Compiling with avr-gcc -std=c99 -mmcu=avr5 -Os -S sum8-16.c generates the following assembly:

.global sumArray
sumArray:
    mov  r19, r24
    movw r30, r24
    ldi  r24, 0
    ldi  r25, 0
.L2:
    mov  r18, r30
    sub  r18, r19
    cp   r18, r22
    brsh .L5
    ld   r18, Z+
    add  r24, r18
    adc  r25,__zero_reg__
    rjmp .L2
.L5:
    ret

This may not be the most straight-forward solution, but if you study this code, you can understand how it works and, hopefully, come with your own version.