Can't get output of assembly language code

user_12475578 0
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I am newbie in assembly language. I am trying to get a string of numbers from user terminated by Enter or the length of the string reaching 20. When I executed the program it didn't show any error, but neither did it show any output nor did it terminate when the string exceeded the 20 characters limit.

My code is:

.model small
.stack 100h
.data 
    var1 db 100 dup('$')
.code
main proc
mov ax, @data
mov dx, ax
mov si, offset var1

l1:
mov ah, 1
int 21h
cmp al,20
je programend
mov [si], al
inc si
jmp l1

programend:
mov dx,offset var1
mov ah,9
int 21h
mov ah, 4ch
int 21h

main endp
end main
masm x86-16 dos assembly
2 Answers
mov ax, @data
mov dx, ax

You want to initialize the DS segment register here but have erroneously written DX. Honest typo, but your code will have corrupted the Program Segment Prefix in this manner.

I am trying to get a string of numbers from user terminated by ENTER key or the length of string reaches 20

It's clear that you need a loop to do this and that you will need 2 tests to decide about when to stop!

  1. test the character in AL to see if it is 13
  2. test a counter (e.g. CX) to see if it reached 20
  xor cx, cx           ; Empty counter
  mov si, offset var1
TheLoop:
  mov ah, 01h          ; DOS.GetCharacter
  int 21h              ; -> AL
  cmp al, 13
  je  programend
  mov [si], al
  inc si
  inc cx
  cmp cx, 20
  jb  TheLoop
programend:

But wait, didn't the task say that it had to be a string of numbers? You need to make sure that the input is indeed a number.
Numbers "0" through "9" have ASCII codes 48 through 57.

  xor cx, cx           ; Empty counter
  mov si, offset var1
TheLoop:
  mov ah, 01h          ; DOS.GetCharacter
  int 21h              ; -> AL
  cmp al, 13
  je  programend
  cmp al, 48
  jb  TheLoop          ; Not a number
  cmp al, 57
  ja  TheLoop          ; Not a number
  mov [si], al
  inc si
  inc cx
  cmp cx, 20
  jb  TheLoop
programend:

Without using a separate counter and using the assembler's ability to translate characters into codes:

  mov si, offset var1
TheLoop:
  mov ah, 01h          ; DOS.GetCharacter
  int 21h              ; -> AL
  cmp al, 13
  je  programend
  cmp al, "0"
  jb  TheLoop          ; Not a number
  cmp al, "9"
  ja  TheLoop          ; Not a number
  mov [si], al
  inc si
  cmp si, offset var1 + 20
  jb  TheLoop
programend:
user_31447700

See the comments in the code below. Remember not to confuse the data read pair ds:si with dx:si. One could also argue that ds:di would be more appropriate here.

main proc
    mov ax, @data
    mov ds, ax
    mov si, offset var1
    
    xor cl, cl ; set CX to zero
    
    l1:
        mov ah, 1
        int 21h
        ; AL now contains the character read
        mov [si], al ; put the character into our variable
        inc si ; move up the memory cursor
        
        inc cl
        cmp cl, 20
        jl l1
        ; if the number of iterations is less than 20,
        ; do all of the above again
        ; otherwise proceed to program end
    
    programEnd:
        mov dx, offset var1
        mov ah, 9
        int 21h
        ; at this point we have printed the string read
        
        mov ah, 4ch
        int 21h
        ; and now we've terminated the program

main endp
end main
user_9489090
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