Ending assembly program when user types period

Question

The program I am writing should read in user input from the keyboard. Instructions: If the character is an upper case letter (A-Z), blank (space) or period, then write it to the standard output. If its lowercase then convert it to uppercase and write to standard output. If the character is anything else just throw the character away. After processing the character, if the character is a period (2Eh) then end your program’s execution.

My code works until I enter a period at which point it prints the period, and then the cursor moves over one spot and blinks indefinitely.

.model small ;64k code and 64k data
.8086 ;only allow 8086 instructions

.stack 256                      ;
                                ;
.data                           ;
                                ;
.code                           ;
start:                          ;
    get_l:                      ;
            mov ah, 8           ; set ah=8 to request a char     w/o echo
            int 21h             ; [char] read is now in the al reg.
            mov dl, al          ; save the input in dl
    ;----------------------------------------------------------------
    ; test for period, if [char] is period the character prints
    ;----------------------------------------------------------------
            cmp dl, 2Eh         ; is [char] a period? 2Eh to 20h
            je exit_            ; if so, jump to exit
    ;----------------------------------------------------------------
    ; if input >= a(61h) && input <= z(7Ah) then subtract 20h
    ;----------------------------------------------------------------
        if_:                    ;
            cmp dl, 61h         ; compare input to a
            jb else_if          ; if [char]<a, jump to else_if
            cmp dl, 7Ah         ; compare input to z
            ja else_if          ; if [char]>z, jump to else_if
        then_1:                 ;
            sub dl, 20h         ; capitalize [char] by subtracting 20h 
            jmp print_          ; print_ [char] to console
    ;----------------------------------------------------------------
    ;  if input >= A(41h) && input <= Z(5Ah) then print
    ;----------------------------------------------------------------
        else_if:                ;
            cmp dl, 41h         ; compare [char] to A
            jb else_            ; if [char]<A, jump to else_
            cmp dl, 5Ah         ; compare [char] to Z
            ja else_            ; if [char]>Z, jump to else_
        then_2:                 ;
            jmp print_          ; print_ [char] to console
    ;----------------------------------------------------------------
    ; if input == *space*(20h) then print
    ;----------------------------------------------------------------
        else_:                  ;
            cmp dl, 20h         ; compare [char] to ' '
            je print_           ; print if space
            jne get_l           ; ignore and repeat loop if not space
        endif_:                 ;
    ;----------------------------------------------------------------
    ; print subroutine
    ;----------------------------------------------------------------
        print_:                 ;
            mov ah, 2           ; set ah=2 to req. [char] to be writ
            int 21h             ; call DOS and [char] is written
            jmp get_l           ; go back to start of loop
        exit_:                  ;
            mov ah, 2           ; if so, print and jump to exit_
            int 21h             ;
            end start           ;

Answer 1

Processing simply continues into whatever is in memory after exit_ has shown the dot.

To terminate the program, you need to explicitly execute a terminate program function. There's more than one, but

    exit_:                  ;
        mov ah, 2           ; if so, print and jump to exit_
        int 21h             ;
        mov ah,4CH          ; Terminate program
        int 21h             ; Execute
        end start           ; end -of-module with enntrypoint start

The value in AL when INT 21H/4ACH is executed appears as the ERRORLEVEL in batch... I've not set it, so it'll be whatever is in AL at the time...