The Interface I is quite complicated.  It uses three different I/O
    ports, and contains logic to page and unpage an 8K ROM if new commands
    are used.  I won't be very detailed here; you could refer to the source
    code of the emulator if you want to know some details, or read the
    'Spectrum Shadow ROM Disassembly' by Gianlura Carri, published by
    Melbourne House - but don't expect the same level of detail as of Ian
    Logan and Frank O'Hara in their Rom disassembly book.

    The ROM is paged if the processor executes the instruction at ROM
    address 0008 or 1708 hexadecimal, the error and close# routines.  It is
    inactivated when the Z80 executes the RET at address 0700.

    I/O Port E7 is used to send or receive data to and from the microdrive.
    Accessing this port will halt the Z80 until the Interface I has
    collected 8 bits from the microdrive head; therefore, it the microdrive
    motor isn't running, or there is no formatted cartridge in the
    microdrive, the Spectrum hangs.  This is the famous 'IN 0 crash'.

    Port EF is used for several things:


       Bit    7   6    5    4    3    2    1     0
            +---+---+----+----+-----+---+-----+-----+
        READ|   |   |    |busy| dtr |gap| sync|write|
            |   |   |    |    |     |   |     |prot.|
            +---+---+----+----+-----+---+-----+-----+
       WRITE|   |   |wait| cts|erase|r/w|comms|comms|
            |   |   |    |    |     |   | clk | data|
            +---+---+----+----+-----+---+-----+-----+


    Bits DTR and CTS are used by the RS232 interface.  The WAIT bit is used
    by the Network to synchronise, GAP, SYNC, WR_PROT, ERASE, R/_W, COMMS
    CLK and COMMS DATA are used by the microdrive system.  If the
    microdrive is not being used, the COMMS DATA output selects the
    function of bit 0 of out-port F7:


       Bit      7    6   5   4   3   2   1       0
            +------+---+---+---+---+---+---+-----------+
        READ|txdata|   |   |   |   |   |   |    net    |
            |      |   |   |   |   |   |   |   input   |
            +------+---+---+---+---+---+---+-----------+
       WRITE|      |   |   |   |   |   |   |net output/|
            |      |   |   |   |   |   |   |   rxdata  |
            +------+---+---+---+---+---+---+-----------+


    TXDATA and RXDATA are the input and output of the RS232 port.  COMMS
    DATA determines whether bit 0 of F7 is output for the RS232 or the
    network.