4_2

    org 100h

    ESC_ equ 01bh ;ascii code of escape
    A_UPPER_ equ 65 ;ascii code of A
    Z_UPPER_ equ 90 ;ascii code of Z
    A_LOWER_ equ 97 ;ascii code of a
    Z_LOWER_ equ 122 ;ascii code of z
    BELL_ equ 7 ;ascii code of ring

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, A_UPPER_ ;if below A than it's incorrect, jump to err_
    jb err_ ;jump if below
    cmp al, Z_UPPER_ ;if below or equal Z it's uppercase, jump to convert_
    jbe convert_ ;jump if below or equal
    cmp al, A_LOWER_ ;if below a than it's incorrect, jump to err_
    jb err_ ;jump if below
    cmp al, Z_LOWER_ ;if below or equal z it lowercase, jump to display_
    jbe diplay_ ;jump if below or equal
    jmp err_ ;if it's here than it's incorrect, jump to err_
    convert_:
        add al, 32 ;to convert to lowercase you add 32 to ascii code
    diplay_:
        mov dl, al ;display letter
        mov ah, 02h
        int 21h
        jmp main_ ;go back
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 converts uppercase letters to lowercase", $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...$"