[x86 Assembly] Password and menu example

.model small
.stack 100h

print macro txt
    mov ah, 9h
    lea dx, txt
    int 21h
endm

read macro buffer
    mov ah, 08h
    int 21h

endm


buffsize equ 08h
hide     equ '*'

.data
    pasPrompt db 10,13,'Enter password:$'
    passWrong db 10,13,'Access Denied.$'
    inpComp   db 10,13,'Input completed.$'
    selPrompt db 10,13,'Select an option:$'
    optionOne db 10,13,  '[1] One.$'
    oOneMessage   db 10,13,'You have selected the first option.$'
    oTwoMessage   db 10,13,'You have selected the second option.$'
    oInvalid db 10,13,'Invalid selection.'
    oExiting db 10,13,'Exiting...$'
    optionTwo db 10,13,  '[2] Two.$'
    optionThree db 10,13,'[3] Exit.$'
    pass db 'password'
    buffLen  db 0
    passbuff db 8 DUP(0)
.code
    mov ax, @data
    mov ds, ax

    ; We need to initialize our ES as well as our DS, the ES is used in REP instructions.
    mov es, ax

    Main:

    print pasPrompt

    readPass:
       ; First, we need to reset out strings length.
       ; Next, we load the source index of our buffer.
       ; This index will later be used to insert chars into byte by byte.
       xor cl, cl
       lea si, passbuff

    readPass_Loop:
       ; Read char by char until CX is max buffer size
       ; Results go into 'passbuff'
       ; Display hidden char each input
       ; 0D = enter key

       ; Now lets get a char as the start of our password input.
       mov ah, 08h
       int 21h

       ; If char==enter then
       ;   compare the passwords
       cmp al, 0Dh
       je readPass_Done

       ; else
       ;   If we are not pressing enter
       ;   Move the char into our buffer
       ;   Increment the char read.
       ; Store char into buffer.
       mov [si], al

       ;Check if we are at the last char
       ; If so, check password.
       cmp cl, buffSize
       je readPass_Done

       ; Increment length.
       ; Move pointer to next char in buffer.
       inc cl
       inc si

       ; Print password "star"
       mov ah, 02h
       mov dl, hide
       int 21h

       jmp readPass_Loop

       readPass_Done:
          ; Load the length of our read string into a variable.
          mov [buffLen], cl

          ; Make sure we are comparing from password length
          ; and NOT buffer length, if we used our buffer length
          ; simply entering the first letter of our password would let us in.
          xor cx, cx
          mov cl, buffSize

          ; Set direction of comparison.
          cld

          ; Load our 2 passwords to be checked by CMPSB <Compare Strings by Bytes>
          lea si, [pass]
          lea di, [passbuff]

          ; Check if password is correct.
          ; Using REP CMPSB <Repeat Compare String Bytes>
          rep cmpsb
          ; If equal go to menus
          ; If not, display "Access Denied"
          je Menus
          jne Denied

       Denied:
          print passWrong
       jmp exit

       Menus:
          ; Clear password buffer.
          ; Clear counter register(s), retrieve pointer to password input buffer.
          xor cx, cx
          lea si, passbuff
          ; Clear buffer char by char until it is completely cleared.
          clearloop:
            mov [si], 0h
            inc si
            inc cl
            cmp cl, buffsize
            jne clearloop
          ; Print "GUI"
          print selPrompt
          print optionOne
          print optionTwo
          print optionThree
          ; Prompt for an input, check against ASCII for 1,2,3
          getOption:
            mov ah, 8h
            int 21h
            cmp al, 31h
            je opOne
            cmp al, 32h
            je opTwo
            cmp al, 33h
            je opThree
            ; If the previous 3 chars were not pressed proceed
            ; to tell the user that the option was invalid.
            ; Then go back to the menu.
            jne opInvalid
          opOne:
            print oOneMessage
          jmp Menus

          opTwo:
            print oTwoMessage
          jmp Menus

          opThree:
            print oExiting
          jmp Main

          opInvalid:
            print oInvalid
          jmp Menus


      exit:
        mov ah, 4ch
        int 21h
end