CODEWITHSUNDEEP
assemblyx86-16
user4197956
user4197956

Reputation:

Finding factorial of a number in 8086 assembly

Following is my code :

data segment
num db 05h
fact db 02h
data ends
code segment
assume cs:code, ds:data
start: mov dx,data
mov ds,dx
mov al,01h
mov bl,num
inc bl
call prc
prc proc
imul fact
inc fact
cmp fact,bl
jz endproc
call prc
endproc: mov fact,al
ret
prc endp
mov ah,4ch
int 21h
code ends
end start

I am aware that the program isn't correct. Here are the questions I have about this (I am trying to learn procedures) :

  1. To end a procedure, do we write directly endp or [name] endp (multiple books are giving me multiple answers)
  2. When it reaches the multiplication of 4*6, imul returns 18 in AL? How?
  3. After ret statement, the program jumps to the previous statement, not the statement after procedure end

And help is appreciated!

Upvotes: 1

Views: 11956

Answers (2)

Melaku from AAIT
Melaku from AAIT

Reputation: 1

;The following program is used to find the factorial of a number in 8086 assembly include emu8086.inc

org 100h


 call input
 call check
 call factorial
ret

input proc
  lea dx,msg
  mov ah,9  
  int 21h  ;to print the string

  mov ah,1
  int 21h    ;input from the user one character
  sub al,30h ;to change the ascii into our decimal number
  mov ch,al

  mov ah,1
  int 21h
  sub al,30h
  mov cl,al
  print '!'
 ret
  input endp

check proc
    cmp ch,0  ;ch-0
    je fact ;if they are equal jump to the factorial

    lea dx,msg2
    mov ah,9
    int 21h
     hlt
ret 
 check endp

fact: 
factorial proc
    lea dx,msg1
    mov ah,9
    int 21h

    mov ax,1 ;initialize
label:
    cmp cx,0
    je quit
    mul cx ;ax=ax*cx
    loop label
quit: 
  call print_num_uns ;to print the number found in ax register
ends   
factorial endp

mov ah,4ch    ;to return control to the operating system
int 21h 

msg db 'Enter the number: $'
msg1 db 10,13,'The factorial is: $'                                  
msg2 db 10,13,'The factorial is out of bound,sorry we can not help you $'
define_print_num_uns

end

Upvotes: 0

davmac
davmac

Reputation: 20631

To end a procedure, do we write directly endp or [name] endp

Try it and see... I believe either will work. Including the name improves clarity.

When it reaches the multiplication of 4*6, imul returns 18 in al? how?

Because 4 times 6 is 24 (in decimal) which is 18 in hexadecimal.

After ret statement, the program jumps to the previous statement, not the statement after procedure end

ret causes the execution to continue from the point at which the procedure was called. Since your procedure calls itself - i.e. it recurses - the ret instruction could cause it to return to the line just before the ret instruction simply because the corresponding call is the line before that. Probably, that (second) call instruction should really be a jmp instruction, to return to the start of the procedure without recursing. Then, the ret will always return to the instruction after the original call.

At that point, however, you have another problem. Since you have the procedure immediately after the call point, the ret instruction will cause execution to resume at the beginning of the procedure; when it reaches ret again your program will likely crash. The code you have written after the end of the procedure should instead be moved so that it is after the call point (i.e. it should be between the call point and the procedure).

Upvotes: 2

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