org 100h

;output task

mov ah, 9h
mov dx, str1
int 21h

;output task

mov ah,9h
mov dx, str2
int 21h

;output task

mov ah,9h
mov dx, str3
int 21h

;input string

mov ah,0Ah
mov dx, my_str
int 21h

;new line break

mov ah,9h
mov dx, new_line
int 21h

;swap 4 and 6

mov bp, my_str+2 ;address 1st
mov cl,[bp+3] ;4th element
mov ch, [bp+5] ;6th element
xchg ch,cl ;swap
mov [bp+5],ch
mov [bp+3],cl

;9rd - 1th

mov ch, [bp+8]
sub ch, [bp]

;3rd - (9rd - 1th)

mov cl, [bp+2]
sub cl, ch
mov [bp+4], cl

;output result

mov ah,9h
mov dx, bp
int 21h

;waiting to press

mov ah, 8h
int 21h
ret

str1       db "This program swaps the 4s and 6s characters of the string. ",$0d,$0a, "$"
str2       db "Performs arithmetic operations: S3 - (S9 - S1). Result is written to S5. ",$0d,$0a, "$"
str3       db "Input string (max. 10 elements)", $0d, $0a, "$"
my_str     db 11,0,11 dup('$')
new_line   db $0d, $0A, "$"