org 100h
        ;output task
        mov ah, 9h
        mov dx, task1
        int 21h

        ;output task
        mov ah, 9h
        mov dx, task2
        int 21h

        ;output task
        mov ah, 9h
        mov dx, task3
        int 21h

        ;output task
        mov ah, 9h
        mov dx, task4
        int 21h

        ;input line
        mov ah,0Ah
        mov dx, my_str
        int 21h

        ;new line
        mov ah,9h
        mov dx, new_line
        int 21h


        mov bp, my_str+2 ; address of 1st char
        mov cl,[bp+3] ; 4th char
        mov ch, [bp+5] ; 6th char

        ;swap
        xchg ch,cl
        mov [bp+5],ch
        mov [bp+3],cl

        ;3rd - (9th - 1st)
        mov ch, [bp+8]
        sub ch, [bp]
        mov cl, [bp+2]
        sub cl, ch
        mov [bp+4], cl

        ;output
        mov ah,9h
        mov dx, res_str
        int 21h

        ;output res
        mov ah,9h
        mov dx, bp
        int 21h

        ;waiting
        mov ah, 8h
        int 21h

        ret

task1      db "This program swaps fourth and sixth elements of the string" ,$0d,$0a, "$"
task2      db "and performs arithmetic operations with ASKI codes of symbols" ,$0d,$0a, "$"
task3      db "numbered 1, 3, 9 and writes the result to symbol 5" ,$0d,$0a, "$"
task4      db "Input string (max. 10 elements)" ,$0d,$0a, "$"
res_str    db "Result:" ,$0d,$0a, "$"
my_str     db 11,0,11 dup('$')
new_line   db $0d, $0A, "$"