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                        .Z80 (Z80) [from Z80 documentation]

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 2 Left joystick (or user
                                   defined, for version 3 .Z80 files)
                                 3=Sinclair 2 Right 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.

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 0FF 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 ROM, 0 if RAM
        62      1       0FF if 8192-16383 is ROM, 0 if RAM
        63      10      5x keyboard mappings for user defined joystick
        73      10      5x ascii word: keys corresponding to mappings above
        83      1       MGT type: 0=Disciple+Epson,1=Disciple+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 v3.0x
        --------------------------------------------------------
        0               48k                     48k
        1               48k + If.1              48k + If.1
        2               SamRam                  SamRam
        3               128k                    48k + M.G.T.
        4               128k + If.1             128k
        5               -                       128k + If.1
        6               -                       128k + M.G.T.

The documenation for versions 3.00 to 3.02 of Z80 had the entries for 'SamRam' 
and '48k + M.G.T.' in the second column of the above table reversed; also 
bytes 61 and 62 of the format were wrong up to version 3.04. (The snaps 
produced by the older versions of Z80 still follow what is above; the 
documentation for the older versions is wrong).

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 17471 to 0 (17726 in 128K modes), which make a total of 
69888 (70908) T states per frame.

The 5 ASCII words (high byte always 0) at 73-82 are the keys corresponding 
to the joystick directions left, right, down (!), up (!), fire respectively. 
Shift, Symbol Shift, Enter and Space are denoted by [,],/,\ respectively. 
The ascii values are used only to display the joystick keys; the information 
in the 5 keyboard mapping words determine which key is actually pressed (and 
should correspond to the ascii values). The low byte is in the range 0-7 and 
determines the keyboard row. The high byte is a mask byte and determines the 
column. Enter for example is stored as 0x0106 (row 6 and column 1) and 'g' 
as 0x1001 (row 1 and column 4).

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 compressed data (without this 3-byte header)
                        If length=0xffff, data is 16384 bytes long and not compressed
        2       1       Page number of block
        3       [0]     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. There is no end marker.

Warajevo writes v2 .Z80 files, but with some extensions to deal with its 
Timex 2068 emulation:

    Byte 34: 128 signifies Timex 2068 emulation is active.
    Byte 35: If Timex 2068 is active, last OUT to port 244.
    Byte 36: If Timex 2068 is active, last OUT to port 255. 

xzx has also made various extensions to the .Z80 format:

    The AY registers are always saved into .Z80 snapshots, independent 
    of the emulated machine. Bit 2 of byte 37 being set signifies that 
    this feature is in use.
    +3 snapshots: The hardware field is written as '7', and a 55th byte 
    is added to the header, which stores the last byte output to #1FFD. 
    This 55th byte is added to all snapshots, which causes many other 
    emulators to reject them. If you have this problem, SnapConv may be 
    able to help. Also, some versions of xzx could write a value of 8 to 
    hardware field, but this should also be treated as a +3.
    A hardware field of '9' signifies Pentagon emulation, whilst '10' 
    signifies Scorpion emulation. The Pentagon snapshots are (to quote 
    from xzx's ChangeLog) "pretty much the same as standard 128K ones", 
    whilst those for the Scorpion consist of 16 RAM pages.