Assembly TZ

.MODEL SMALL
.STACK 100H
.DATA
    PROMPT_TRY_MSG DB 'Illegal character, Enter 0..9 or A..F: $'
    PRMPT_MSG_1 DB 'ENTER A HEX DIGIT: $'
    OUTPUT_PRMPT DB 0AH,0DH,'In Decimal it is: $'
    ASK_PROMPT DB 0AH,0DH,'Do you want to continue?: $'
.CODE
MAIN PROC

    MOV AX,@DATA
    MOV DS,AX

    TOP:
        MOV AH,9
        LEA DX,PRMPT_MSG_1
        INT 21H

        MOV AH,1
        INT 21H
        MOV BL,AL

        CMP BL,30H
        JL  TRY_MSG

        CMP BL,46H
        JG  TRY_MSG

        CMP BL,39H
        JLE DIGIT

        MOV AH,9
        LEA DX,OUTPUT_PRMPT
        INT 21H

        SUB BL,11H

        MOV AH,2
        MOV DL,31H
        INT 21H
        MOV DL,BL
        INT 21H
    AGAIN:
        MOV AH,9
        LEA DX,ASK_PROMPT
        INT 21H

        MOV AH,1
        INT 21H
        MOV BH,AL
        MOV AH,2
        MOV DL,0AH
        INT 21H
        MOV DL,0DH
        INT 21H

        CMP BH,'Y'
        JE TOP
        CMP BH,'y'
        JE TOP
        JMP END_
    DIGIT:
        MOV AH,9
        LEA DX,OUTPUT_PRMPT
        INT 21H

        MOV AH,2
        MOV DL,BL
        INT 21H

        JMP AGAIN
    TRY_MSG:
        MOV AH,9
        LEA DX,PROMPT_TRY_MSG
        INT 21H
        JMP TOP
    END_:

    MOV AH,4CH
    INT 21H

    MAIN ENDP
END MAIN