Sieve of Eratosthenes in AT&T Assemby

Question

I have written a simple program in C to find all prime numbers within a given range using Sieve of Eratosthenes. In school, we are currently learning assembly in a class, but I can't figure out how to write the three loops. I have somewhat figured it out in NASM, which I have fiddled with before, but we have to use AT&T. Any suggestions what I can do with "void sieve()"?

#include <stdio.h>
#include <stdlib.h>

void sieve(int *, int);

int main()
{
    int *a, n, i;
    n = 46000;
    printf("Enter the range : ");
    scanf("%d", &n);
    a = malloc(sizeof(int)*n);
    sieve(a,n);

    printf("\nThe primes numbers from 1 to %d are: ", n);
    for(i=2; i<=n; i++)
    {
        if(a[i] == 1)
        printf("%d, ", i);
    }
    printf(".\n\n");

    return 0;
}

void sieve(int *a, int n)
{
    int i, j;
    for(i=2; i<=n; i++)
    a[i] = 1;
    for(i=2; i<=n; i++)
    {
        if(a[i] == 1)
        {
            for(j=i; (i*j)<=n; j++)
            a[(i*j)] = 0;
        }
    }
}

Answer 1

Here are some pseudo code in C and Assembly of the two most common loop constructions.

PseudoCode of While Loop in C:

while ([condition]) {
    [body]
}

PseudoCode of While Loop in Assembly:

begin_loop:
    [condition]
    test or cmp instruction
    jcc end_loop (conditional jmp according to the condition)
    [body]
    jmp begin_loop
end_loop:

PseudoCode of For Loop in C:

for ([initialization]; [condition]; [increment]) {
    [body]
}

PseudoCode of For Loop in Assembly:

    [initialization]
begin_loop:
    [condition]
    test or cmp instruction
    jcc end_loop (conditional jmp according to the condition)
    [body]
    [increment]
    jmp begin_loop
end_loop:

Some code of converting the outer loop of:

for(i=2; i<=n; i++)
{
    if(a[i] == 1)
    {
        for(j=i; (i*j)<=n; j++)
        a[(i*j)] = 0;
    }
}

to Assembly:

    mov ecx, 2
    mov eax, N (variable n)

begin_loop:
    cmp ecx, eax
    jg end_loop
    lea ebx, A (array)
    test [ebx + i*4], 1 (4 is assuming size of element of A is 4 bytes)
    jnz continue

    # the other loop

continue:
    inc ecx
    jmp begin_loop

end_loop:

Be careful with the use of register, if you need to use the register for other operations in the middle of the loop, save their current value (eg: to the stack with push reg), and when need to used the older value again restored (eg: with pop reg if it's in the stack).