uPPER TO LOWER & lOWER TO UPPER

.MODEL SMALL
.STACK 100H
.DATA

MSG1 DW "INPUT: $"
MSG2 DW "OUTPUT IS: $"

.CODE
MAIN PROC

    MOV AX,@DATA
    MOV DS,AX

    MOV AH,9
    LEA DX,MSG1
    INT 21H

    MOV AH,1
    INT 21H
    MOV BL,AL

    MOV AH,2
    MOV DL,0AH
    INT 21H
    MOV DL,0DH
    INT 21H

    CMP BL,'a'
    JGE CAPITAL

    CMP BL,'A'
    JG SMALL

    SMALL:
    CMP BL,'Z'
    JLE C
    C:
    MOV AH,9
    LEA DX,MSG2
    INT 21H
    ADD BL,32
    MOV AH,2
    MOV DL,BL
    INT 21H
    JMP END_IF

    CAPITAL:
    CMP BL,'z'
    JL S
    S:
    MOV AH,9
    LEA DX,MSG2
    INT 21H
    SUB BL,32
    MOV AH,2
    MOV DL,BL
    INT 21H
    JMP END_IF

    END_IF:
    MAIN ENDP
END MAIN