Int 17h, 00h            Send One Byte to Printer

    Sends one character to the specified parallel printer and returns
    the current status of the printer.

        Entry   AH = 00h
                AL = Character to print
                DX = Parallel printer number (0 - LPT1, 1 - LPT2, etc.)

        Return  AH = Printer status (see Service 02h)

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    For the AT, XT-286, and PC Convertible, Int 15h, Service 90h
    (Device Busy), is called when the printer is busy. This notifies
    the operating system that a time-out loop is going to begin.