INT 17h,  00h (0)        Send One Byte to Printer

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

       On entry:      AH         00h
                      AL         Character to print
                      DX         Parallel printer number (0 - LPT1, 1 - LPT2,
                                 etc)

       Returns:       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.