Assembly Lanuage Program, which converts string lower case

Source: http://cssimplified.com/computer-organisation-and-assembly-language-programming/an-assembly-lanuage-program-which-converts-string-lower-case-characters-to-upper-case-characters-and-upper-case-characters-to-lower-case-characters

DATA SEGMENT
    MSG1 DB 10,13,'ENTER ANY STRING :- $'
    MSG2 DB 10,13,'ENTERED STRING IS :- $'
    MSG3 DB 10,13,'CONVERTED STRING IS : $'

    P1 LABEL BYTE
    M1 DB 0FFH
    L1 DB ?
    P11 DB 0FFH DUP ('$')
DATA ENDS
DISPLAY MACRO MSG
    MOV AH,9
    LEA DX,MSG
    INT 21H
ENDM
CODE SEGMENT
    ASSUME CS:CODE,DS:DATA
START:
        MOV AX,DATA
        MOV DS,AX

        DISPLAY MSG1

        LEA DX,P1
        MOV AH,0AH
        INT 21H

        DISPLAY MSG2

        DISPLAY P11

        DISPLAY MSG3

        LEA SI,P11

        MOV CL,L1
        MOV CH,0
CHECK:
        CMP [SI],41H
        JB DONE

        CMP [SI],5BH
        JB LWR

        CMP [SI],61H
        JB DONE

        CMP [SI],7BH
        JG DONE

UPR:    SUB [SI],20H
        JMP DONE
LWR:    ADD [SI],20H

DONE:   INC SI
        LOOP CHECK

        DISPLAY P11

        MOV AH,4CH
        INT 21H
CODE ENDS
END START