assembly concatenate two strings

Question

I want to concatenate two strings but in my output instead of getting the final concatenated string, I get a line of weird characters and spaces, maybe someone could help me a bit. I want to save the result in s3. Here is the code

DATA SEGMENT
    STR1 DB "ENTER FIRST STRING HERE ->$"
    STR2 DB "ENTER SECOND STRING HERE ->$"
    STR3 DB "CONCATEnatedD STRING :->$"
    STR11 DB "FIRST STRING : ->$"
    STR22 DB "SECOND STRING: ->$"

    s1 DB 20 DUP("$")
    s2 DB 20 DUP("$")
    s3 db 40 dup(?)
    NEWLINE DB 10,13,"$"

DATA ENDS

CODE SEGMENT

    ASSUME DS:DATA,CS:CODE
START:

    MOV AX,DATA
    MOV DS,AX


lea dx,str1
mov ah,09h
int 21h

lea si,s1

l1: mov ah,01h
int 21h

mov [si],al
inc si
cmp al,31h
jne l1

dec si

lea dx,str2
mov ah,09h
int 21h

lea di,s2

l2: mov ah,01h
int 21h

mov [di],al
inc di
cmp al,32h
jnz l2

lea si,s3
lea di,s1


lea si,s3
lea di,s1
l3: mov al,[di]
mov [si],al
inc si
inc di
cmp al,31h
jnz l3

dec si

lea di,s2

l4: mov al,[di]
mov [si],al
inc si
inc di
cmp al,32h
jnz l4

lea dx,str3
mov ah,09h
int 21h

l5: mov dl,[si]
cmp dl,32h
jz l6
mov ah,02h
int 21h
inc si
jmp l5


l6:MOV AH,4CH
    INT 21H


CODE ENDS
END START

I have found the solution, hope it is a good one without any mistake: data segment msg db 0Dh, 0Ah, "String1: $" msg2 db 0Dh, 0Ah, "String2: $" rev db 0Dh, 0Ah, "Result: $"

buffer label byte  
buffer2 label byte
str_maxlen db 255
str_length db 0

str_string db 255 dup(0)
str_string2 db 255 dup(0)
result db 255 dup('$')
num db 0
data ends

code segment
assume cs:code, ds:data
start:
mov ax,data
mov ds,ax

lea dx,msg
mov ah,09h
int 21h

lea dx,buffer
mov ah,0Ah
int 21h

mov cl,str_length
mov bh,cl
mov ch,0
lea si,str_string
lea di,result

op1:    mov bl,[si]
mov [di],bl
inc di
inc si
loop op1

mov cl,bh
op3:    dec si
loop op3

lea dx,msg2
mov ah,09h
int 21h

lea dx,buffer2
mov ah,0Ah
int 21h
mov cl,str_length

lea si,str_string
mov cl,str_length

mov ax,0
op2:    mov al,[si]
mov [di],al
inc di
inc si
loop op2

lea dx,rev
mov ah,09h
int 21h

lea dx,result
mov ah,09h
int 21h

mov ah,4Ch
int 21h
code ends

end start

Why do you keep comparing the character being copied to `31h` as the termination condition for the copy loop? `31h` represents the ASCII for the numeral `1`. — lurker, Aug 08 '16 at 12:32
I'm voting to close for I believe this is an homework assignment and I can't see how OP made any effort to solve their problem. Maybe they were encouraged by their [first successful attempt for a cheap solution](https://stackoverflow.com/questions/38824670/assembly-reverse-a-string)? — Margaret Bloom, Aug 08 '16 at 12:59
This is not a homework, just a hobby, and I ask here as a last resort when I don't find the solution. And who has this kind of homework in the summer? Those who study assembly do so in college during the normal school time not August. — user3848412, Aug 08 '16 at 13:44
@user3848412 Glad to know you enjoy programming. :) But then, why you don't want to learn how to use a debugger? That's just as essential as the assembler is. — Margaret Bloom, Aug 08 '16 at 14:02
`REP SCASB`(for zero term) and `REP MOVSB` could be your friends ;-) — zx485, Aug 08 '16 at 14:43
Post answers as answers; don't edit them into the question. Post your solution as an answer and roll back the edit to the question. — Peter Cordes, Aug 08 '16 at 17:38
If nobody's forcing you to learn 16-bit DOS system calls as well as 16-bit assembly, I'd suggest learning 32-bit or 64-bit in the first place. You don't have to worry about segments or anything, and the system calls / library functions are the same ones you use from normal programs in C. See http://stackoverflow.com/a/34918617/224132, and also learning resources in the [x86 tag wiki](http://stackoverflow.com/tags/x86/info). — Peter Cordes, Aug 08 '16 at 17:41

score 1 · Accepted Answer · answered Aug 08 '16 at 19:51

In your first code, you have to set si to point to the beginning of string s3 right before label l5: (because si was pointing to the end of s3 after loop l4:) :

.
.
.
lea dx,str3
mov ah,09h
int 21h

lea si,s3          ;◄■■■■■■■■■■■  
l5: mov dl,[si]
cmp dl,32h
jz l6
.
.
.

assembly concatenate two strings

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