Code

.model small
.stack 100h
.data

a db ?
msg db "Ok$"

.code
    main proc

        mov ax, @data
        mov ds, ax

        mov ah, 1
        int 21h
        mov a, al

        cmp a, 'A'
        jge  greaterEq

            greaterEq:

            cmp a, 'Z'
            jle lessEq
            jg  greater

                lessEq:
                cmp a, 'Z'
                je  Eq
                jne notEq

                    Eq:
                    mov ah, 9
                    lea dx, msg
                    int 21h
                    jmp Both

                    notEq:
                    mov ah, 2
                    mov bl, a
                    add bl, 33
                    mov dl, bl
                    int 21h
                    jmp Both

                greater:
                cmp a, 'a'
                jge greaterEq1

                    greaterEq1:
                    cmp a, 'z'
                    jle lessEq1

                        lessEq1:
                        cmp a, 'z'
                        je  Eq1
                        jne notEq1

                            Eq1:
                            mov ah, 9
                            lea dx, msg
                            int 21h
                            jmp Both

                            notEq1:
                            mov bl, a
                            sub bl, 31
                            mov dl, bl
                            mov ah, 2
                            int 21h
                            jmp Both


            Both:

        mov ah, 4ch
        int 21h
    main endp
end main