asm prime

; prime.asm
; sample program to demonstrate procedures
; calulates and prints prime numbers from 1 to 20

include 'emu8086.inc'

org  100h; set location counter to 100h

jmp CodeStart

DataStart:
   max dw 20
   space db " ", 0

CodeStart:
   mov bx, 2;set this value to the number you want to start at I used 2 because 1 is not a prime.

   LoopStart:



 ;call procedure IsPrime to test whether the number is
 ;prime or not
       call IsPrime
       cmp dx, 0
 ;if dx is equal to 0 jump
       je Equal

       mov ax, bx
       call print_num

 ; print a space
       mov si, offset space
       call print_string

    Equal:
       add bx, 1
       cmp bx, max

   jle LoopStart

   ret

   IsPrime PROC
 ; uses a loop to determine if number in bx is prime
 ; upon return if bx not prime dx will be 0, otherwise dx > 0

 ; we only have to test divisors from 2 to bx/2

 ; prepare to divide dx:ax / 2
       mov ax, bx
       mov dx, 0
       mov cx, 2
       div cx

 ; move result into si for loop
       mov si, ax

 ; assume the value is prime
       mov dx, 1

 ; start loop count at 2
       mov cx, 2

    PrimeLoop:

          ; compare loop count(in cx) and max loop value (in si)
           cmp cx, si

          ; jump out of loop if count(cx) > si
           ja StopLabel

          ; divide test value (in bx) by loop count (in cx)
           mov ax, bx
           mov dx, 0
           div cx

          ; check remainder (in dx), if zero then we found a divisor
          ; and the number cannot be prime
           cmp dx, 0

          ; if dx = 0 then we found a divisor and can stop looking
           je StopLabel

          ; increment count
           add cx, 1



     jmp PrimeLoop

     StopLabel:

     ret
   IsPrime ENDP

DEFINE_PRINT_STRING
DEFINE_SCAN_NUM
DEFINE_PRINT_NUM
DEFINE_PRINT_NUM_UNS