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. 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 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 RAM
62 1 0FF if 8192-16383 is 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=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.
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 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.