Loop input assembly x86 - loops

Question

The program needs to iterate trough the input that a user enters and replace every underscore character ("_") that it finds with "#".
The program does it, but just one time, then every underscore character that is found, stays the same.
.MODEL SMALL
.STACK
.DATA
string_input DB "Enter a string: $"
string_output DB "Your string is: $"
bufferLong db 250
inputLong db 0
buffer db 250, 0, 250 dup ("$"), "$"
.CODE
start:
mov ax, #data
mov ds, ax
mov dx, offset string_input
mov ah, 09h
int 21h
call inputString
;mov cl, al
mov dl, 0Ah
mov ah, 02h
int 21h
mov dl, 0Dh
mov ah, 02h
int 21h
mov dx, offset string_output
mov ah, 09h
int 21h
;mov al, cl
call outputString
mov ax, 4C00h
int 21h
inputString proc
mov dx, offset bufferLong
mov ah, 0Ah
int 21h
ret
inputString endp
outputString proc
mov dx, offset buffer
mov ah, 09h
mov si,0
mov cx, 0
mov bl, "_"
loop:
cmp bl, buffer[si]
je replace
inc cx
inc si
jne loop
replace:
mov buffer[si], "#"
int 21h
ret
outputString endp
end start

Answer 1

bufferLong db 250
inputLong db 0
buffer db 250, 0, 250 dup ("$"), "$"
This looks like you don't truly know what the required input structure for this DOS.BufferedInput function 0Ah should look like. Read more about it at How buffered input works.
This is the correct definition:
bufferLong db 250
inputLong db 0
buffer db 250 dup (0)
Upon the first replacement, you immediately print the result, where in fact you should continu the loop. Better change the conditional jump to the opposite like this (from je to jne):
xor si, si
loop:
mov al, buffer[si]
cmp al, 13
je print ; Exit the loop upon finding 13
cmp al, "_"
jne skip ; While skipping you stay in the loop
replace:
mov buffer[si], "#"
skip:
inc si
jmp loop ; Unconditionally to the top where a possible exit can get detected
print:
mov buffer[si], "$"
mov dx, offset buffer
mov ah, 09h
int 21h
ret
What your 'loop' was missing is a clear way to exit. The string that you process ends with a carriage return (13). Use that to stop iterating. This could happen from the start if the user did not type in characters and just used enter.
Next you need to provide the correct string termination so DOS can print the result. Preloading the input buffer with $ characters serves no purpose. Just store one $ character either replacing the byte 13 or else right behind the byte 13.