.MDR FILES:
    -----------

    The emulator uses a cartridge file format identical to the 'Microdrive
    File' format of Carlo Delhez' Spectrum emulator Spectator for the QL,
    who devised the format.  This format is now also supported by XZX of
    Des Harriot.  The following information is adapted from Carlo's
    documentation.  It can also be found in the 'Spectrum Microdrive Book',
    by Ian Logan (co-writer of the excellent 'Complete Spectrum ROM
    Disassembly').

    A cartridge file contains 254 'sectors' of 543 bytes each, and a final
    byte flag which is non-zero is the cartridge is write protected, so the
    total length is 137923 bytes.  On the cartridge tape, after a GAP of
    some time the Interface I writes 10 zeros and 2 FF bytes (the
    preamble), and then a fifteen byte header-block-with-checksum.  After
    another GAP, it writes a preamble again, with a 15-byte record-
    descriptor-with-checksum (which has a structure very much like the
    header block), immediately followed by the data block of 512 bytes, and
    a final checksum of those 512 bytes.  The preamble is used by the
    Interface I hardware to synchronise, and is not explicitly used by the
    software.  The preamble is not saved to the microdrive file:

    offset length name    contents

      0      1   HDFLAG   Value 1, to indicate header block
      1      1   HDNUMB   sector number (values 254 down to 1)
      2      2            not used
      4     10   HDNAME   microdrive cartridge name (blank padded)
     14      1   HDCHK    header checksum (of first 14 bytes)

     15      1   RECFLG   - bit 0: always 0 to indicate record block
                          - bit 1: set for the EOF block
                          - bit 2: reset for a PRINT file
                          - bits 3-7: not used (value 0)
     16      1   RECNUM   data block sequence number (value starts at 0)
     17      2   RECLEN   data block length (<=512, LSB first)
     19     10   RECNAM   filename (blank padded)
     29      1   DESCHK   record descriptor checksum (of previous 14 bytes)
     30    512            data block
    542      1   DCHK     data block checksum (of all 512 bytes of data
                           block, even when not all bytes are used)
    ---------
    254 times


    (Actually, this information is 'transparent' to the emulator.  All it
    does is store 2 times 254 blocks in the .MDR file as it is OUTed,
    alternatingly of length 15 and 528 bytes.  The emulator does check
    checksums, see below; the other fields are dealt with by the emulated
    Interface I software.)

    A used record block is either an EOF block (bit 1 of RECFLG is 1) or
    contains 512 bytes of data (RECLEN=512, i.e.  bit 1 of MSB is 1).  An
    empty record block has a zero in bit 1 of RECFLG and also RECLEN=0.  An
    unusable block (as determined by the FORMAT command) is an EOF block
    with RECLEN=0.

    The three checksums are calculated by adding all the bytes together
    modulo 255; this will never produce a checksum of 255.  Possibly, this
    is the value that is read by the Interface I if there's no or bad data
    on the tape.

    In normal operation, all first-fifteen-byte blocks of each header or
    record block will have the right checksum.  If the checksum is not
    right, the block will be treated as a GAP.  For instance, if you type
    OUT 239,0 on a normal Spectrum with interface I, the microdrive motor
    starts running and the cartridge will be erased completely in 7
    seconds. CAT 1 will respond with 'microdrive not ready'.  Try it on the
    emulator...