Upper To Lower n vice versa

AIM:-Write a program to convert the string from upper case to lower case and vice versa.

.model small

.data

str1 db 25 , ? , 25 dup(‘$’)

str2 db 25 , ? , 25 dup(‘$’)

str3 db 25 , ? , 25 dup(‘$’)

msg1 db 0ah,0dh,’menu $’

msg21 db 0ah,0dh,’accept the string $’

msg6 db 0ah,0dh,’string is :  $’

msg7 db 0ah,0dh,’  $’

.code

mov      ax,@data

mov      ds,ax

mov      ah,0ah

lea        dx,str1

int         21h

endd:   mov ah,4ch

int 21h

lea        si,str1+2                       ; source string

lea        di,str2+2                       ; destination string

mov      cl,str1+1

back:   mov       al,byte ptr[si]

cmp      al,41h                           ; check if letter

jge        bigg

jmp       stop

smal :  cmp       al,61h                           ; check if letter is lower case

jge subb

bigg :  cmp al,5ah                                  ; check if letter is upper case

jge        smal                             ; if greater letter is small case

add       al,20h                           ; make letter lower case

branc:  mov byte ptr[di],al                       ; move letter to destination

inc        di

inc        si

dec       cl                                  ; decrememt count

cmp      cl, 00h

jne        back                             ; check next character

jmp            stop

subb:  cmp        al,7ah                           ; if small case letter

jge             stop                              ; if not letter display as is

sub            al,20h                           ; make letter upper case

jmp branc

stop:

mov           ah,09h                          ; display result

lea             dx,str2+2

int 21h

end