Concat Two User Provided Strings

.model small
.stack 100h
.data

; Its recommended to display a message to user before every input and output

; Message for first input
msg1 db "Enter the first string: $"

; Message for second input
msg2 db 0ah, 0dh, "Enter the second string: $"

; Message for third input
msg3 db 0ah, 0dh, "New string is: $"

; For each buffered input, we have to reserve space such that
tsize1 db 10        ; First byte contains a value that indicates the number of characters to be received at maximum
asize1 db ?         ; Second byte is reserved for interrupt's use
array1 db 10 dup(?) ; The third byte and on for saving the input. The minimum amount of space required
                    ; here is equal to the value of first byte

; For second input
tsize2 db 10
asize2 db ?
array2 db 10 dup(?)

; Here both values would be concatenated. So, it must be sum of the sizes of both inputs long
array3 db 20 dup('$')

.code

main:

; Set the value of ds
mov ax, @data
mov ds, ax

; Display first message
mov ah, 09h
mov dx, offset msg1
int 21h

; Get first input
mov ah, 0ah
mov dx, offset tsize1
int 21h

; Display second message
mov ah, 09h
mov dx, offset msg2
int 21h

; Get second input
mov ah, 0ah
mov dx, offset tsize2
int 21h

; Copy first string

; Loop uses cx register
mov cx, 0 ; So clean it up first
mov cl, asize1 ; Put actual size of first input in cl

;	cx
;	0	7
;	ch	cl

; The value of cx is same as value of cl now

mov si, offset array1    ; Offset of first byte of first input is moved in si
mov di, offset array3    ; Offset of first byte of new string is moved in di ; Here the loop starts

start1:

mov al, [si]    ; Copy the value at the address of si in al (one byte)
mov [di], al    ; Copy this value at the address of di from al

inc si          ; Move to next byte of source
inc di          ; Move to next byte of target

loop start1

; loop subtracts 1 from cx and checks if its zero. Zero means break the loop
; So, the loop appears to run cx times which has number of characters in first input

; First string copied, start the second

mov cx, 0
mov cl, asize2

mov si, offset array2  ; Start copying from first byte of second string
                       ; But the target remains the same

start2:

mov al, [si]
mov [di], al
inc si
inc di

loop start2

; Copy complete

; Display message
mov ah, 09h
mov dx, offset msg3
int 21h

; Display the string
mov dx, offset array3
int 21h

; Exit
mov ah, 4ch
int 21h

end main