; 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, 1
  
   LoopStart:
  
    ; must be a prime
       mov ax, bx
       call print_num
      
    ; print a space
       mov si, offset space
       call print_string
      
       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 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

; 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, 1
  
   LoopStart:
            
                    
       mov ax, bx
   ;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 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  
          
       ; increment count
           add cx, 1
              
       ; check remainder (in dx), if zero then we found a divisor
       ; and the number cannot be prime
           cmp dx, 1
          
       ;if dx is not equal to 1 restart loop
           jne LoopStart            
                    
        
        
       jmp PrimeLoop
      
       StopLabel:
      
       ret
   IsPrime ENDP
  
DEFINE_PRINT_STRING
DEFINE_SCAN_NUM
DEFINE_PRINT_NUM
DEFINE_PRINT_NUM_UNS