;
; file: prime.asm
; This program calculates prime numbers
;
; To create executable:
; nasm -f coff prime.asm
; gcc -o prime prime.o driver.c asm_io.o
;
; Works like the following C program:
; #include <stdio.h>
;
;int main()
;{
;  unsigned guess;          /* current guess for prime      */
;  unsigned factor;         /* possible factor of guess     */
;  unsigned limit;          /* find primes up to this value */
;
;  printf("Find primes up to: ");
;  scanf("%u", &limit);
;
;  printf("2\n");    /* treat first two primes as special case */
;  printf("3\n");
;
;  guess = 5;        /* initial guess */
;  while ( guess <= limit ) {
;    /* look for a factor of guess */
;    factor = 3;
;    while ( factor*factor < guess && guess % factor != 0 )
;      factor += 2;
;    if ( guess % factor != 0 )
;      printf("%d\n", guess);
;    guess += 2;    /* only look at odd numbers */
;  }
;  return 0;
;}
;

%include "asm_io.inc"

segment .data
Message         db      "Find primes up to: ", 0


segment .bss
Limit           resd    1               ; find primes up to this limit
Guess           resd    1               ; the current guess for prime

 

segment .text
        global  _asm_main
_asm_main:
        enter   0,0               ; setup routine
        pusha

        mov     eax,  Message
        call    print_string
        
        call    read_int             ; scanf("%u", & limit );
        mov     [Limit], eax

        mov     eax, 2               ; printf("2\n");
        call    print_int
        call    print_nl
        mov     eax, 3               ; printf("3\n");
        call    print_int
        call    print_nl

        mov     dword [Guess], 5     ; Guess = 5;

while_limit:                         ; while ( Guess <= Limit )
        mov     eax,[Guess]
        cmp     eax, [Limit]
        jnbe    end_while_limit      ; use jnbe since numbers are unsigned

        mov     ebx, 3               ; ebx is factor = 3;
while_factor:
        mov     eax,ebx
        mul     eax                  ; edx:eax = eax*eax
        jo      end_while_factor     ; if answer won't fit in eax alone
        cmp     eax, [Guess]
        jnb     end_while_factor     ; if !(factor*factor < guess)
        mov     eax,[Guess]
        mov     edx,0
        div     ebx                  ; edx = edx:eax % ebx
        cmp     edx, 0
        je      end_while_factor     ; if !(guess % factor != 0)

        add     ebx,2                ; factor += 2;
        jmp     while_factor
end_while_factor:
        je      end_if               ; if !(guess % factor != 0)
        mov     eax,[Guess]          ; printf("%u\n")
        call    print_int
        call    print_nl
end_if:
        mov     eax,[Guess]
        add     eax, 2
        mov     [Guess], eax         ; guess += 2
        jmp     while_limit
end_while_limit:

        popa
        mov     eax, 0            ; return back to C
        leave                     
        ret