4_1

    org 100h

    ESC_ equ 01bh ;ascii code escape
    COLON_ equ 58 ;ascii code :
    AT_ equ 64 ;ascii code @ (< > ? ; - are between : and @)
    BELL_ equ 7 ;ascii codering

start_:
    mov ah, 09h ;output - helloStr_
    mov dx, helloStr_
    int 21h
main_:
    mov ah, 08h ;no echo input with waiting, output al - ascii code of pressed button
    int 21h
    cmp al, ESC_ ;was it escape?
    jne other_ ;if no go to other_
    jmp exit_ ;if yes exit
other_:
    cmp al, COLON_ ;if letter is less than : it's incorrect, go to err_
    jb err_ ;jump if below
    cmp al, AT_ ;if letter is greater than @ it's incorrect, go to err_
    ja err_ ;jump if above
    mov dl, al ;else just display that letter
    mov ah, 02h
    int 21h
    jmp main_ ;go back to start
err_:
    mov ah, 02h ;play ring
    mov dl, BELL_
    int 21h
    jmp main_ ;go back to start
exit_:
    mov ah, 09h ;output - byeStr
    mov dx, byeStr
    int 21h
    mov ah, 07h ;wait for input
    int 21h
    ret
helloStr_ db "This program filters input text according to", $0d, $0a, ":, ;, <, =, >, ?, @", $0d, $0a, "You can exit on ESC", $0d, $0a, "Try to input something:", $0d, $0a, "$"
byeStr db 0dh, 0ah, "The program has now terminated. Press anything to continue...$"