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  Function 03H (decimal 3) reads one character into AL from AUX the standard
  auxiliary device. The default auxiliary device is COM1, the first RS-232
  serial communications port. You can, however, use the DOS MODE command to
  assign other devices, such as COM2, to the auxiliary device.

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  NOTE:
    This function waits for input. It does not report status information
    about the many miseries that a serial port can suffer. If you want to
    know the status of the serial port, use the ROM BIOS communications-port
    services.
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