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ZX0

ZX0 is an optimal data compressor for a custom LZ77/LZSS based compression format, that provides a tradeoff between high compression ratio, and extremely simple fast decompression. Therefore it's especially appropriate for low-end platforms, including 8-bit computers like the ZX Spectrum.

A comparison with other compressors (courtesy of introspec/spke) can be seen here.

WARNING: The ZX0 file format was changed in version 2. This new format allows decompressors to be slightly smaller and run slightly faster. If you need to compress a file to the old "classic" file format from version 1, then execute ZX0 compressor using parameter "-c".

Usage

To compress a file, use the command-line compressor as follows:

zx0 Cobra.scr

This will generate a compressed file called "Cobra.scr.zx0".

Afterwards you can choose a decompressor routine in assembly Z80, according to your requirements for speed and size:

"Standard" routine: 68 bytes only
"Turbo" routine: 126 bytes, about 21% faster
"Fast" routine: 187 bytes, about 25% faster
"Mega" routine: 673 bytes, about 28% faster

Finally compile the chosen decompressor routine and load the compressed file somewhere in memory. To decompress data, just call the routine specifying the source address of compressed data in HL and the target address in DE.

For instance, if you compile the decompressor routine to address 65000, load "Cobra.scr.zx0" at address 51200, and you want to decompress it directly to the screen, then execute the following code:

    LD    HL, 51200  ; source address (put "Cobra.scr.zx0" there)
    LD    DE, 16384  ; target address (screen memory in this case)
    CALL  65000      ; decompress routine compiled at this address

It's also possible to decompress data into a memory area that partially overlaps the compressed data itself (only if you won't need to decompress it again later, obviously). In this case, the last address of compressed data must be at least "delta" bytes higher than the last address of decompressed data. The exact value of "delta" for each case is reported by ZX0 during compression. See image below:

                       |------------------|    compressed data
    |---------------------------------|       decompressed data
  start >>                            <--->
                                      delta

For convenience, there's also a command-line decompressor that works as follows:

dzx0 Cobra.scr.zx0

Performance

The ZX0 optimal compressor algorithm is fairly complex, thus compressing typical files can take a few seconds. During development, you can speed up this process simply using ZX0 in "quick" mode. This will produce a non-optimal larger compressed file but execute almost instantly:

zx0 -q Cobra.scr

This way, you can repeatedly modify your files, then quickly compress and test them. Later, when you finish changing these files, you can compress them again without "quick" mode for maximum compression. Notice that using "quick" mode will only affect the size of the compressed file, not its format. Therefore all decompressor routines will continue to work exactly the same way.

Fortunately all complexity lies on the compression process only. The ZX0 compression format itself is very simple and efficient, providing a high compression ratio that can be decompressed quickly and easily. The provided ZX0 decompressor routines in assembly Z80 are small and fast, they only use main registers (BC, DE, HL, AF), consume very little stack space, and do not require additional decompression buffer.

The provided ZX0 decompressor in C writes the output file while reading the compressed file, without keeping it in memory. Therefore it always use the same amount of memory, regardless of file size. Thus even large compressed files can be decompressed in very small computers with limited memory, even if it took considerable time and memory to compress it originally. It means decompressing within asymptotically optimal space and time O(n) only, using storage space O(n) for input and output files, and only memory space O(w) for processing.

File Format

The ZX0 compressed format is very simple. There are only 3 types of blocks:

Literal (copy next N bytes from compressed file)

    0  Elias(length)  byte[1]  byte[2]  ...  byte[N]

Copy from last offset (repeat N bytes from last offset)

    0  Elias(length)

Copy from new offset (repeat N bytes from new offset)

    1  Elias(MSB(offset)+1)  LSB(offset)  Elias(length-1)

ZX0 needs only 1 bit to distinguish between these blocks, because literal blocks cannot be consecutive, and reusing last offset can only happen after a literal block. The first block is always a literal, so the first bit is omitted.

The offset MSB and all lengths are stored using interlaced Elias Gamma Coding. When offset MSB equals 256 it means EOF. The offset LSB is stored using 7 bits instead of 8, because it produces better results in most practical cases.

Advanced Features

The ZX0 compressor contains a few extra "hidden" features, that are slightly harder to use properly, and not supported by the ZX0 decompressor in C. Please read carefully these instructions before attempting to use any of them!

COMPRESSING BACKWARDS

When using ZX0 for "in-place" decompression (decompressing data to overlap the same memory area storing the compressed data), you must always leave a small margin of "delta" bytes of compressed data at the end. However it won't work to decompress some large data that will occupy all the upper memory until the last memory address, since there won't be even a couple bytes left at the end.

A possible workaround is to compress and decompress data backwards, starting at the last memory address. Therefore you will only need to leave a small margin of "delta" bytes of compressed data at the beginning instead. Technically, it will require that lowest address of compressed data should be at least "delta" bytes lower than lowest address of decompressed data. See image below:

 compressed data    |------------------|
decompressed data       |---------------------------------|
                    <--->                            << start
                    delta

To compress a file backwards, use the command-line compressor as follows:

zx0 -b Cobra.scr

To decompress it later, you must call one of the supplied "backwards" variants of the Assembly decompressor, specifying last source address of compressed data in HL and last target address in DE.

For instance, if you compile a "backwards" Assembly decompressor routine to address 64000, load backwards compressed file "Cobra.scr.zx0" (with size 2202 bytes) to address 51200, and want to decompress it directly to the ZX Spectrum screen (with 6912 bytes), then execute the following code:

    LD    HL, 51200+2202-1  ; source (last address of "Cobra.scr.zx0")
    LD    DE, 16384+6912-1  ; target (last address of screen memory)
    CALL  64000             ; backwards decompress routine

Notice that compressing backwards may sometimes produce slightly smaller compressed files in certain cases, slightly larger compressed files in others. Overall it shouldn't make much difference either way.

COMPRESSING WITH PREFIX

The LZ77/LZSS compression is achieved by "abbreviating repetitions", such that certain sequences of bytes are replaced with much shorter references to previous occurrences of these same sequences. For this reason, it's harder to get very good compression ratio on very short files, or in the initial parts of larger files, due to lack of choices for previous sequences that could be referenced.

A possible improvement is to compress data while also taking into account what else will be already stored in memory during decompression later. Thus the compressed data may even contain shorter references to repetitions stored in some previous "prefix" memory area, instead of just repetitions within the decompressed area itself.

An input file may contain both some prefix data to be referenced only, and the actual data to be compressed. An optional parameter can specify how many bytes must be skipped before compression. See below:

                                        compressed data
                                     |-------------------|
         prefix             decompressed data
    |--------------|---------------------------------|
                 start >>
    <-------------->                                 <--->
          skip                                       delta

As usual, if you want to decompress data into a memory area that partially overlaps the compressed data itself, the last address of compressed data must be at least "delta" bytes higher than the last address of decompressed data.

For instance, if you want the first 6144 bytes of a certain file to be skipped (not compressed but possibly referenced), then use the command-line compressor as follows:

zx0 +6144 Cobra.cbr

In practice, suppose an action game uses a few generic sprites that are common for all levels (such as player graphics), and other sprites are specific for each level (such as enemies). All generic sprites must stay always accessible at a certain memory area, but any level specific data can be only decompressed as needed, to the memory area immediately following it. In this case, the generic sprites area could be used as prefix when compressing and decompressing each level, in an attempt to improve compression. For instance, suppose generic graphics are loaded from file "generic.gfx" to address 56000, occupying 2500 bytes, and level specific graphics will be decompressed immediately afterwards, to address 58500. To compress each level using "generic.gfx" as a 2500 bytes prefix, use the command-line compressor as follows:

copy /b generic.gfx+level_1.gfx prefixed_level_1.gfx
zx0 +2500 prefixed_level_1.gfx

copy /b generic.gfx+level_2.gfx prefixed_level_2.gfx
zx0 +2500 prefixed_level_2.gfx

copy /b generic.gfx+level_3.gfx prefixed_level_3.gfx
zx0 +2500 prefixed_level_3.gfx

To decompress it later, you simply need to use one of the normal variants of the Assembly decompressor, as usual. In this case, if you loaded compressed file "prefixed_level_1.gfx.zx0" to address 48000 for instance, decompressing it will require the following code:

    LD    HL, 48000  ; source address (put "prefixed_level_1.gfx.zx0" there)
    LD    DE, 58500  ; target address (level specific memory area in this case)
    CALL  65000      ; decompress routine compiled at this address

However decompression will only work properly if exactly the same prefix data is present in the memory area immediately preceding the decompression address. Therefore you must be extremely careful to ensure the prefix area does not store variables, self-modifying code, or anything else that may change prefix content between compression and decompression. Also don't forget to recompress your files whenever you modify a prefix!

In certain cases, compressing with a prefix may considerably help compression. In others, it may not even make any difference. It mostly depends on how much similarity exists between data to be compressed and its provided prefix.

COMPRESSING BACKWARDS WITH SUFFIX

Both features above can be used together. A file can be compressed backwards, with an optional parameter to specify how many bytes should be skipped (not compressed but possibly referenced) from the end of the input file instead. See below:

       compressed data
    |-------------------|
                 decompressed data             suffix
        |---------------------------------|--------------|
                                     << start
    <--->                                 <-------------->
    delta                                       skip

As usual, if you want to decompress data into a memory area that partially overlaps the compressed data itself, lowest address of compressed data must be at least "delta" bytes lower than lowest address of decompressed data.

For instance, if you want to skip the last 768 bytes of a certain input file and compress everything else (possibly referencing this "suffix" of 768 bytes), then use the command-line compressor as follows:

zx0 -b +768 Cobra.cbr

In previous example, suppose the action game now stores level-specific sprites in the memory area from address 33000 to 33511 (512 bytes), just before generic sprites that are stored from address 33512 to 34535 (1024 bytes). In this case, these generic sprites could be used as suffix when compressing and decompressing level-specific data as needed, in an attempt to improve compression. To compress each level using "generic.gfx" as a 1024 bytes suffix, use the command-line compressor as follows:

copy /b level_1.gfx+generic.gfx level_1_suffixed.gfx
zx0 -b +1024 level_1_suffixed.gfx

copy /b level_2.gfx+generic.gfx level_2_suffixed.gfx
zx0 -b +1024 level_2_suffixed.gfx

copy /b level_3.gfx+generic.gfx level_3_suffixed.gfx
zx0 -b +1024 level_3_suffixed.gfx

To decompress it later, use the backwards variant of the Assembly decompressor. In this case, if you compile a "backwards" decompressor routine to address 64000, and load compressed file "level_1_suffixed.gfx.zx0" (with 217 bytes) to address 39000 for instance, decompressing it will require the following code:

    LD    HL, 39000+217-1  ; source (last address of "level_1_suffixed.gfx.zx0")
    LD    DE, 33000+512-1  ; target (last address of level-specific data)
    CALL  64000            ; backwards decompress routine

Analogously, decompression will only work properly if exactly the same suffix data is present in the memory area immediately following the decompression area. Therefore you must be extremely careful to ensure the suffix area does not store variables, self-modifying code, or anything else that may change suffix content between compression and decompression. Also don't forget to recompress your files whenever you modify a suffix!

Also if you are using "in-place" decompression, you must leave a small margin of "delta" bytes of compressed data just before the decompression area.

License

The ZX0 data compression format and algorithm was designed and implemented by Einar Saukas. Special thanks to introspec/spke for several suggestions and improvements, and together with uniabis for providing the "Fast" decompressor. Also special thanks to Urusergi for additional ideas and improvements.

The optimal C compressor is available under the "BSD-3" license. In practice, this is relevant only if you want to modify its source code and/or incorporate the compressor within your own products. Otherwise, if you just execute it to compress files, you can simply ignore these conditions.

The decompressors can be used freely within your own programs (either for the ZX Spectrum or any other platform), even for commercial releases. The only condition is that you must indicate somehow in your documentation that you have used ZX0.