how to restrict input to 6 character string

Hi..everybody..
I have this simple assembly language program to read in string of user input then do the checking whether the string is a palindrome or not, and display the result of the checking.
I wanted to slightly change the program to make it read the string of exactly certain number of character long, for example 6. Can anybody help?? Give me some guide...since i am a begginers in Assembly..thank you very much..

This are the code..

Code:

org  100h
   
print_output macro
    mov ah, 9
    int 21h                                                               
    
  ; input the string:
  ; the interupt module does fill the third array with input from keyborad

endm                                                                                            
    
scan_input macro
    mov ah, 0ah
    int 21h
    
    ;get string size                        
    xor bx, bx             ; zero the bx register 
    mov bl, input[1]          ; then save the size of s1 to base lower register               
                                          
    ;print string                           
    mov input[bx+2],'$'        ;close the string with dollar sign   
        
  ; input the string:
  ; the interupt module does fill the third array with input from keyborad

endm             


is_input_empty macro 
    ;lea bx, input    
    ;mov dl, [bx+1] ; get string size.
    mov dl, input[1] 
    cmp dl, 0
    je stop
endm


part2:
    ;print the main message of program2
    mov dx, offset MSG_part2
    print_output
part2_loop:

    ;print the message to promt the user to enter the string
    mov dx, offset MSG_prompt_part2
    print_output
                  
    ;scan for input              
    mov dx, offset input
    scan_input  
     
    ;print newline 
    mov dx, offset newline
    print_output
    
    is_input_empty
    
    ;output the inputed string
    mov dx, offset MSG_urstris
    print_output
                                           
    ;print string                           
    mov dx, offset input+2
    print_output
       
    ;print newline 
    mov dx, offset newline
    print_output
    
    sub si, si           ;zero the si for increment   
    mov di, bx 
    dec di
     
    ;check for 1 character
    mov cx, bx;    
    cmp cx, 1
    je print_yes
                          
                          
    mov bx, offset input+2        ;get the string into bx    
    shr cx, 1                    ;divide cx by 2 by shifting to right 1 bit.

    myloop:
        mov al, [bx+di]
        mov dl, [bx+si]     
        cmp al, dl
        jne print_no
        inc si
        dec di  
    loop myloop  
                                           
    
print_yes:
    mov dx, offset MSG_isPalindrom
    print_output
    jmp part2_loop

print_no:
    mov dx, offset MSG_notPalindrom
    print_output
    jmp part2_loop
    
stop:
    ; wait for any key...
    
    mov dx, offset MSG_any_key
    print_output
    
    mov ax, 0 
    int 16h
    
    ret


MSG_prompt_part2        db  "-------------------------------------------------------------------------------",13,10,13,10
                    db  "Enter a String: "
                    db  "$"  

MSG_part2                db  "--Part 2--",13,10
                    db  "* This program to check palindrome string by :",13,10
                    db  "* Rusydi Hasan Makarim (0721051)*",13,10
                    db  "* Cutifa Safitri (072656)       *",13,10
                    db  "* Haroon Shoukat Ali (0629753)  *",13,10
                    db  "*                               *",13,10
                    db  "*Press <Enter> to exit          *",13,10
                    db "$"
MSG_urstris            db  "Your input string is $"
MSG_isPalindrom        db  "PALINDROME !",13,10,'$'
MSG_notPalindrom    db  "NOT A PALINDROME!",13,10,'$'
input                db  100,?, 100 dup(' ')  ;used for scan_input
newline                db  13,10,'$'
MSG_any_key            db "Press any key to terminate$"

end