.Z80 FILES:
    -----------

    The old .Z80 snapshot format (for version 1.45 and below) looks like
    this:

        Offset  Length  Description

        0       1       A register
        1       1       F register
        2       2       BC register pair (LSB, i.e.  C, first)
        4       2       HL register pair
        6       2       Program counter
        8       2       Stack pointer
        10      1       Interrupt register
        11      1       Refresh register (Bit 7 is not significant!)
        12      1       Bit 0  : Bit 7 of the R-register
                        Bit 1-3: Border colour
                        Bit 4  : 1=Basic SamRom switched in
                        Bit 5  : 1=Block of data is compressed
                        Bit 6-7: No meaning
        13      2       DE register pair
        15      2       BC' register pair
        17      2       DE' register pair
        19      2       HL' register pair
        21      1       A' register
        22      1       F' register
        23      2       IY register (Again LSB first)
        25      2       IX register
        27      1       Interrupt flipflop, 0=DI, otherwise EI
        28      1       IFF2 (not particularly important...)
        29      1       Bit 0-1: Interrupt mode (0, 1 or 2)
                        Bit 2  : 1=Issue 2 emulation
                        Bit 3  : 1=Double interrupt frequency
                        Bit 4-5: 1=High video synchronisation
                                 3=Low video synchronisation
                                 0,2=Normal
                        Bit 6-7: 0=Cursor/Protek/AGF joystick
                                 1=Kempston joystick
                                 2=Sinclair 1 joystick
                                 3=Sinclair 2 joystick

    Because of compatibility, if byte 12 is 255, it has to be regarded as
    being 1.  After this header block of 30 bytes the 48K bytes of Spectrum
    memory follows in a compressed format (if bit 5 of byte 12 is one).
    The compression method is very simple: it replaces repetitions of at
    least five equal bytes by a four-byte code ED ED xx yy, which stands
    for "byte yy repeated xx times".  Only sequences of length at least 5
    are coded. The exception is sequences consisting of ED's; if they are
    encountered, even two ED's are encoded into ED ED 02 ED.  Finally,
    every byte directly following a single ED is not taken into a block,
    for example ED 6*00 is not encoded into ED ED ED 06 00 but into ED 00
    ED ED 05 00. The block is terminated by an end marker, 00 ED ED 00.

    That's the format of .Z80 files as used by versions up to 1.45.
    Starting from version 2.0, a different format is used, since from then
    on also 128K snapshots had to be supported.  This new format is used
    for all snapshots, either 48K or 128K.  However, the emulator still
    understands the old format.

    Also note that, although old .Z80 file formats are still understood,
    the emulator will never procude an old format .Z80 file, and also 48K
    snapshots are written in the new format.  But, then again, new .Z80
    file formats can be translated back to the old one (provided that it is
    an 48K snapshot of course) by using ConvZ80.

    Version 2.01 and 3.0 .Z80 files start with the same 30 byte header as
    old .Z80 files used.  Bit 4 and 5 of the flag byte have no meaning
    anymore, and the program counter (byte 6 and 7) are zero to signal a
    version 2.01 or version 3.0 snapshot file.

    After the first 30 bytes, the additional header follows:


        Offset  Length  Description

      * 30      2       Length of additional header block (see below)
      * 32      2       Program counter
      * 34      1       Hardware mode (see below)
      * 35      1       If in SamRam mode, bitwise state of 74ls259.
                        For example, bit 6=1 after an OUT 31,13 (=2*6+1)
                        If in 128 mode, contains last OUT to 7ffd
      * 36      1       Contains 0xFF if Interface I rom paged
      * 37      1       Bit 0: 1 if R register emulation on
                        Bit 1: 1 if LDIR emulation on
      * 38      1       Last OUT to fffd (soundchip register number)
      * 39      16      Contents of the sound chip registers
        55      2       Low T state counter
        57      1       Hi T state counter
        58      1       Flag byte used by Spectator (QL spec. emulator)
                        Ignored by Z80 when loading, zero when saving
        59      1       0FF if MGT Rom paged
        60      1       0FF if Multiface Rom paged. Should always be 0.
        61      1       0FF if 0-8191 is RAM
        62      1       0FF if 8192-16383 is RAM
        63      10      5x ascii word: keys for user defined joystick
        73      10      5x keyboard mappings corresponding to keys above
        83      1       MGT type: 0=Disciple+Epson,1=Discipls+HP,16=Plus D
        84      1       Disciple inhibit button status: 0=out, 0ff=in
        85      1       Disciple inhibit flag: 0=rom pageable, 0ff=not


    The value of the word at position 30 is 23 for version 2.01 files, and
    54 for version 3.0 files.  The starred fields are the ones that
    constitute the version 2.01 header, and their interpretation has
    remained unchanged except for byte 34:

        Value:          Meaning in v2.01        Meaning in v3.0

        0               48k                     48k
        1               48k + If.1              48k + If.1
        2               SamRam                  48k + M.G.T.
        3               128k                    SamRam
        4               128k + If.1             128k
        5               -                       128k + If.1
        6               -                       128k + M.G.T.

    The hi T state counter counts up modulo 4.  Just after the ULA
    generates its once-in-every-20-ms interrupt, it is 3, and is increased
    by one every 5 emulated milliseconds.  In these 1/200s intervals, the
    low T state counter counts down from 17472 to 0, which make a total of
    69888 T states per frame.

    Byte 60 must be zero, because the contents of the Multiface RAM is not
    saved in the snapshot file.  If the Multiface was paged when the
    snapshot was saved, the emulated program will most probably crash when
    loaded back.

    Bytes 61 and 62 are a function of the other flags, such as byte 34, 59,
    60 and 83.

    Hereafter a number of memory blocks follow, each containing the
    compressed data of a 16K block.  The compression is according to the
    old scheme, except for the end-marker, which is now absent.  The
    structure of a memory block is:


        Byte    Length  Description

        0       2       Length of data (without this 3-byte header)
        2       1       Page number of block
        3       [0]     Compressed data


    The pages are numbered, depending on the hardware mode, in the
    following way:


        Page    In '48 mode     In '128 mode    In SamRam mode

         0      48K rom         rom (basic)     48K rom
         1      Interface I, Disciple or Plus D rom, according to setting
         2      -               rom (reset)     samram rom (basic)
         3      -               page 0          samram rom (monitor,..)
         4      8000-bfff       page 1          Normal 8000-bfff
         5      c000-ffff       page 2          Normal c000-ffff
         6      -               page 3          Shadow 8000-bfff
         7      -               page 4          Shadow c000-ffff
         8      4000-7fff       page 5          4000-7fff
         9      -               page 6          -
        10      -               page 7          -
        11      Multiface rom   Multiface rom   -


    In 48K mode, pages 4,5 and 8 are saved.  In SamRam mode, pages 4 to 8
    are saved.  In '128 mode, all pages from 3 to 10 are saved.  This
    version saves the pages in numerical order.  There is no end marker.