MASM - Why doesn't this code cause the computer to beep?

I've read about the code to make the computer beep but I can't get it to work. The following code asks the user to enter digits. The digits are displayed on the screen but if the user types a non-digit character it's supposed to beep. When I run the program and type a non-digit character, it just crashes.

INCLUDE Irvine32.inc

.data

enterDigits     BYTE        'Please type some digits: ', 0

.code

main PROC

    mov EDX, OFFSET enterDigits
    call    WriteString

L1:</b>

    call    ReadChar
    cmp AL, 0Dh
    je  FINISHED
    cmp AL, 30h
    jl  BEEP
    cmp AL, 39h
    jg  BEEP
    call    WriteChar
    loop    L1  

BEEP:</b>

    mov AH, 02h
    mov DL, 07h
    int 21h
    jmp L1

FINISHED:</b>

    call    CRLF
    call    CRLF

exit</b>

main ENDP</b>

END main


If I replace BEEP with:

BEEP:</b>

    mov AL, 33h
    call    WriteChar
    jmp L1

It will print a 3 when you type a non-digit character. Don't know if that makes any difference or not.

Solution

If you look at the source for WriteChar here, you can see that it is invoking the Windows API WriteConsole method rather than use an MSDOS API. I believe you aren't going to be able to call a DOS API method because you're running in a mode incompatible with the kernel managing the hardware and I/O functions.

You might try adjusting the compatibility mode of your exe to use Windows 95 (right click on EXE, go to Compatibility tab).