A new data compressor called ZX0

[jimmy]

84 hours in, with the last dot (....) under the space between "Einar" and "Saukas" it stopped with "Error: Insufficient Memory".
I'm slightly confused about this because when I last looked in Task Manager it was only taking 1.6Gb of space - it had 8Gb to play with.

So I did another test using the -quick mode on the same file and the results were:
zx0 compressed from 33887 bytes to 17007 (non-quick mode is 16356)
zx5 compressed from 33887 bytes to 17223

Whilst watching this 'quick' zx5 compression I noticed that at the point where my first attempt failed, memory usage was around 40Mb. When the dot had moved across to the last letter "s", memory usage was not much higher. However the compressor spent most of it's time on this last dot of progress - nearly 2 hours! Memory usage also increased alarmingly to over 332Mb by the time it finished.

Based on this I don't think my first test would ever have worked as the 32-bit zx5.exe would unlikely be able to address more than 4Gb. The zx5 quick mode took just over 150 minutes to complete! Even if zx5 was 64-bit, it seems to imply I'd need at least 16Gb of RAM to perform this task.

Is it true to say that if a quick zx5 result is larger/greater than a quick zx0 result, then the 'full compression' versions will also come out as zx5 is larger/greater than zx0 output? (ie: is this a quick test to save wasting hours compressing a file that zx0 would do better anyway?)

I don't have Java 8 installed so I haven't been able to test the multi-threaded versions, however I will see what I can do this weekend.

[Einar Saukas]

84 hours in, with the last dot (....) under the space between "Einar" and "Saukas" it stopped with "Error: Insufficient Memory".

Ouch!

Based on this I don't think my first test would ever have worked as the 32-bit zx5.exe would unlikely be able to address more than 4Gb. The zx5 quick mode took just over 150 minutes to complete! Even if zx5 was 64-bit, it seems to imply I'd need at least 16Gb of RAM to perform this task.

AFAIK the memory limit for 32 bit applications is 2Gb only.

Is it true to say that if a quick zx5 result is larger/greater than a quick zx0 result, then the 'full compression' versions will also come out as zx5 is larger/greater than zx0 output? (ie: is this a quick test to save wasting hours compressing a file that zx0 would do better anyway?)

No, I don't think it's a reliable estimate.

I don't have Java 8 installed so I haven't been able to test the multi-threaded versions, however I will see what I can do this weekend.

It should run much faster. Don't forget to specify memory size, as described in the project documentation!

[Einar Saukas]

I just released a ZX0 multi-thread compressor in Java, if anyone's interested:

https://github.com/einar-saukas/ZX0-Java

You can download the compiled JAR file directly from here:

https://github.com/einar-saukas/ZX0-Java/releases

The execution time is exactly the same as the Kotlin version. It means I didn't need to learn Kotlin after all

[andydansby]

I've started using ZX0 in my development scheme. Loving it.

An example project that I've started playing with is at https://github.com/andydansby/Vortex2_P ... 2_compress to compress Vortex tracker songs. Here I am creating a buffer and uncompressing the song to it to play. Next to add multiple songs.

Nice compression. Takes a 11302 byte song and compresses it to 4019 bytes.

Thanks Einar.

[Einar Saukas]

I've started using ZX0 in my development scheme. Loving it.

An example project that I've started playing with is at https://github.com/andydansby/Vortex2_P ... 2_compress to compress Vortex tracker songs. Here I am creating a buffer and uncompressing the song to it to play. Next to add multiple songs.

Nice compression. Takes a 11302 byte song and compresses it to 4019 bytes.

Thanks Einar.

You are welcome!

Thanks a lot for publishing a tutorial about using zx0 with z88dk/sccz80. I highly recommend it:

https://zxspectrumcoding.wordpress.com/ ... -lesson-7/

[Bedazzle]

Hello guys, I need a help.
Read documentation on github about decompressing data that overlaps source.
Packed screen is 4253 bytes with 3 bytes delta.
When I'm trying to unpack 6912 bytes into address 16384 all works like a charm, but when I'm changing destination to 49152 something goes wrong - rubbish is copied to the screen, and if I explore memory at 49152, picture is unpacked incorrectly.

Code: Select all

	device ZXSPECTRUM48

MEM_END  EQU 65535
displace EQU MEM_END - file_end + 1

	org 50000
	display "start ", /A, $
start:
	ld sp, 24999

	; fill screen with pattern
	ld hl, 16384
	ld de, 16385
	ld bc, 6911
	ld (hl), e
	ldir

	; move packed data and unpacker to end of memory
	ld hl, file_end-1
	ld de, MEM_END
	ld bc, file_end - file_start
	lddr
	jp unpacker + displace


	; data and code from this point will be moved
file_start:
	incbin "Andy Green - Flashback (2018).scr_norm.zx0"
	include	"dzx0_standard.asm"

unpacker:
	ld hl, file_start + displace
	ld de, 16384
	;ld de, 49152	; <=== crash
	
	push de
	call dzx0_standard + displace
	pop hl

	; copy unpacked picture to screen
	ld de, 16384
	ld bc, 6912
	ldir

	; dead end to indicate all went ok
	xor a
loop:
	xor %111
	out(254),a
	jr loop

file_end:

	savesna "test.sna", start

[Einar Saukas]

It seems you are compiling the decompressor at one address but executing it at another.

[Bedazzle]

It seems you are compiling the decompressor at one address but executing it at another.

Arrgh, silly me
Thank you!

P.S.
but wait... it does correctly extract to screen with same code (besides destination) !

[Einar Saukas]

P.S.
but wait... it does correctly extract to screen with same code (besides destination) !

When you moved the compiled code, the CALL instruction was still invoking a sub-routine at the original address. I'm guessing you did not override the old address when decompressing directly to screen, so it still worked (by accident).

However you erased the old sub-routine when you decompressed over it.

[Bedazzle]

When you moved the compiled code, the CALL instruction was still invoking a sub-routine at the original address.

Yes, you are right. Thank you again. I rewrote part of code with disp/ent, and it works now.

If anybody interested:

Code: Select all

	device ZXSPECTRUM48

MEM_END  	EQU 65535
move_size	EQU file_end - file_start

	org 50000
start:
	ld sp, 24999

	; fill screen with pattern
	ld hl, 16384
	ld de, 16385
	ld bc, 6911
	ld (hl), e
	ldir

	; move packed data and unpacker to end of memory
	ld hl, move_end-1
	ld de, MEM_END
	ld bc, move_size
	lddr
	jp unpacker

	; ---------------------------
	; data and code from this point will be moved on start
move_start:
	; compile part of program to this ORG address
	DISP 65535 - (move_size) +1
file_start:
	incbin "Andy Green - Flashback (2018).scr_norm.zx0"
	include	"dzx0_standard.asm"

unpacker:
	ld hl, file_start
	;ld de, 16384
	ld de, 49152	; <=== crash
	
	push de
	call dzx0_standard
	pop hl

	; copy unpacked picture to screen
	ld de, 16384
	ld bc, 6912
	ldir

	; dead end to indicate all went ok
	xor a
loop:
	xor %111
	out(254),a
	ld b, a
pause:
	djnz pause
	jr loop

file_end:
	ENT
move_end:
	; ---------------------------

	savesna "test.sna", start

[Einar Saukas]

Awesome! You are welcome

[Alone Coder]

calgary - 98571 (lost to Exomizer3)
canterbury - 27939
graphics - 163173 (better than others)
music - 63721
misc - 254202

RAR headers included.
Decompressor size is 342 bytes (uses 1566 bytes for temporary buffer).

TR-DOS version of ZXRar optionally supports compression with 2-byte blocks (helpful for code), with 386-byte decompressor. It has to be tested too.

Temporarily added 2-byte blocks in NedoOS version:
calgary - 98525
canterbury - 27911
graphics - 163943
music - 62740
misc - 250341

ZXRar with 2-byte blocks TOTAL = 603460, including file headers (lost to exomizer and ZX0)

mRIP archiver for PC by Eugene77 (2003 format, depacker size is 218 bytes, fits in 1-sector TR-DOS BASIC loader, working area of #5C2 bytes):
calgary - 91734 (better than exomizer etc)
canterbury - 25800 (better than exomizer etc)
graphics - 155697 (better than exomizer etc)
music - 60160 (lost to exomizer and ZX0)
misc - 245702 (lost to exomizer and ZX0)
TOTAL - 579093 (better than exomizer etc)

RIP archiver for PC by Eugene77 (2000 format, depacker size is 228 bytes, working area of #5C2 bytes):
calgary - 90794 (the best)
canterbury - 24658 (the best)
graphics - 150981 (the best)
music - 57276 (the best)
misc - 240765 (the best)
TOTAL - 565474 (the best)

So, the old native formats were not bad!

[TMD2003]

I admit I have never had a reason to try ZX0 before. But now, I do: a relatively simple screen, with lots of empty space, which has absolutely no need to take up 6912 bytes. It requires compression.

I can't run it on my regular PC - it's incompatible with 64-bit Windows.
I tried it with DOSBox and it crashed - the window sits there doing nothing and can't be interrupted (and the .zx0 file never appears).
My 15-year-old laptop running Windows 7 returned the message "Program is too big to fit in memory", even though it's only 130-odd KB.
My 23-year-old laptop running Windows 2000 is flatly refusing to start up... even in Safe Mode.

Any suggestions?

I've already tried the screen compressor in BMP2SCR, and I've disassembled the initial machine code routine. For some reason that is several light years beyond my pay grade, the screen only displays if I load it from tape and RANDOMIZE USR 50000. If I copy the code into an .ASM file, along with lines and lines and lines of DEFBs to hold the screen data and assemble it with Spin, all I get is a screen full of flashing shash. The first lines are:
CALL 6034 (which is just a RET in the ROM and nothing more)
DEC SP
DEC SP
There is no reference whatsoever to the first line of the screen data - the compressor loads the code at 50000, and the screen data starts at 50114. Is it performing some mad trick with whatever the stack pointer points to at the end of a load?
I tried replacing these first lines with LD SP,50114 but it was to no available. As soon as any SP instructions are thrown my way I'm completely in the dark.

I've also taken to analysing the 6912 bytes, piece by piece, writing a load of LD HL,***** then LD (HL),*** and the odd LDIR routine to draw it effectively byte by byte, leaving out all the huge chunks of zeroes. Effectively I'm compressing the screen by hand, which is lengthy and tedious.

[Einar Saukas]

I can't run it on my regular PC - it's incompatible with 64-bit Windows.

Check if you downloaded it correctly. You are supposed to click on the "Download" button.

Direct link:

https://github.com/einar-saukas/ZX0/raw ... in/zx0.exe

[TMD2003]

...and that's working, on my main PC that outright rejected what I had yesterday. I was using the version found here:
https://github.com/einar-saukas/ZX0/tree/main/win

"Updated executables" seemed like the right place to go...

1,923 bytes! About 600 less than BMP2SCR required. I knew this would be the right option. Cheers!

[Joefish]

I've also taken to analysing the 6912 bytes, piece by piece, writing a load of LD HL,***** then LD (HL),*** and the odd LDIR routine to draw it effectively byte by byte, leaving out all the huge chunks of zeroes. Effectively I'm compressing the screen by hand, which is lengthy and tedious.

An easily understood (though not hugely efficient) way to compress a screen that's got lots of empty character cells is to start with the attributes, and exploit the fact that no-one in their right mind would ever use the FLASH bit. So you scan your screen for empty character cells, and if you find one, set the FLASH bit of that attribute. Then you store your data as one byte of attribute, followed by eight bytes of pixel data, but you only store the pixel data if the FLASH bit is not set. Otherwise, you move straight on to the attribute for the next character cell.

When you come to unpack that stream of data, you first take a byte, check the top bit, and if it's set, you erase that bit, write the byte to the attribute, then write 0s into the pixel bytes for that character cell. If the top bit isn't set, then copy it up as an attribute, then copy the next eight bytes into the pixel bytes of that character cell.

Of course your screen data can't get any lower than 768 bytes (768 empty characters), but it's a start.

And then there are far more complex approaches you can take once you get the hang of it! But however elaborate the mechanism, decoding is the same. Read some signal bits or bytes from the compressed data, and depending on what you just read, either copy some more raw data from the stream or do something special like pad with blanks or copy something from earlier, or from a prepared 'dictionary' of common byte sequences.

[Lethargeek]

An easily understood (though not hugely efficient) way to compress a screen that's got lots of empty character cells is to start with the attributes, and exploit the fact that no-one in their right mind would ever use the FLASH bit. So you scan your screen for empty character cells, and if you find one, set the FLASH bit of that attribute. Then you store your data as one byte of attribute, followed by eight bytes of pixel data, but you only store the pixel data if the FLASH bit is not set. Otherwise, you move straight on to the attribute for the next character cell.

When you come to unpack that stream of data, you first take a byte, check the top bit, and if it's set, you erase that bit, write the byte to the attribute, then write 0s into the pixel bytes for that character cell. If the top bit isn't set, then copy it up as an attribute, then copy the next eight bytes into the pixel bytes of that character cell.

Of course your screen data can't get any lower than 768 bytes (768 empty characters), but it's a start.

And then there are far more complex approaches you can take once you get the hang of it! But however elaborate the mechanism, decoding is the same. Read some signal bits or bytes from the compressed data, and depending on what you just read, either copy some more raw data from the stream or do something special like pad with blanks or copy something from earlier, or from a prepared 'dictionary' of common byte sequences.

...aaand again the example is that screen compressor of mine, LgK aka Lethargeek Kompakt
first it filters the original screen to maximize the number of empty square chunks - 8x8 then 4x4 then 2x2
then the more clean filtered picture is compressed using the variable length prefix code
the idea is that squares generally have better correlation between all their pixels than byte strips

[Einar Saukas]

I was using the version found here:
https://github.com/einar-saukas/ZX0/tree/main/win

"Updated executables" seemed like the right place to go...

In this page, clicking on a filename will take you to another page, not directly to the actual file. So if you right clicked and saved it, you actually saved a webpage, not an executable file.

To get the actual file, first you need to left click on the file you want. Then you will see a button called "View raw", that you can use to download it.

The Github interface can be quite confusing, that's not my fault!

[TMD2003]

The Github interface can be quite confusing, that's not my fault!

I've always found it unnecessarily annoying, especially since a fair few projects of this type are hosted there. Sourceforge is a lot easier to navigate.

Anyway, the actual ZX0 has done some sterling work - four blocks of code (mostly screens and attributes) compressed to about a quarter of their size, maybe less... leaving me more room for a 12K wall of text. Of course, I have to fit the actual game in there somewhere...

Also, just a thought: how much stack space does ZX0 need when decompressing a full screen? Do tell me it's not 7K, otherwise I'll be in trouble. Does it decode a few bytes at a time and write them to their destination?

[Einar Saukas]

Also, just a thought: how much stack space does ZX0 need when decompressing a full screen?

The standard version of the decompressor uses 4 bytes of stack.

The other 3 versions use 2 bytes of stack.

Does it decode a few bytes at a time and write them to their destination?

Each decompressed byte is directly written to the destination.

[TMD2003]

No problems there, then. I can afford four bytes, come what may.

[Prodatron]

Hi all, I am new in this forum, not (yet) a ZX Spectrum, but at least a Z80 guy, active on some other platforms like MSX, Enterprise and Amstrad CPC (my childhood system). Some weeks ago I discovered this fantastic ZX0 compressor and was so amazed, that I decided to fully integrate it into my project called "SymbOS", an operating system for Z80 systems (unfortunately currently not for the Spectrum).

Thanks so much to Einar Saukas for this fantastic piece of software!!

The next SymbOS version supports loading of ZX0 compressed executables (EXE, COM etc.) but also provides functions for applications for decompressing data. You can load compressed data from an opened file in a transparent way like if it would be uncompressed, which makes it very easy to update even existing applications for handling compressed data.

I added a little header to all kind of ZX0 compressed data...
- 1W: length of the compressed data
- 4B: last 4 bytes of uncompressed data
- 1W: number of bytes at the beginning, which should be skipped/shouldn't be compressed

The first word is necessary for the loading routine, which only knows the uncompressed size from the application.

The next 4 bytes are the last 4 bytes of the uncompressed data, which won't be included in the compressed ones. This makes it possible to load the compressed data exactly at the end of the area, where it will be decompressed. I never saw a delta>3, so to be safe I choosed to save 4 bytes, so there will never be a problem with the delta.

The last word is necessary for files, which have some meta-data at the beginning. These shouldn't be compressed, as you may want to load it separately without the need to decompress the whole thing. Therefore the prefix feature in ZX0 is a great thing for still using this part as dictionary.

Now all EXE (GUI executables) and COM (command line executables) files can be compressed, as well as some special executables "*.WDG" (desktop widgets) and "*.SAV" (screen saver).
Picture files (SGX) can be compressed, too, including desktop background pictures, as well as help files (HLP).
Last but not least I decided to add a header for compressed music files, which are supported in the SymAmp music player. These are PT3, ST2 (Amstrad Soundtrakker 128), SKM (Amstrad Starkos Tracker) and SA2 (Adlib Tracker 2), which can all be ZX0-compressed now.

The compression ratio is just fantastic, in most cases it's better than ZIP! Now it's possible again to place the whole operating system with all system apps, background picture and additional stuff which is loaded during booting on one Amstrad standard disc side (178K).
I am using the Turbo decompressor, which is still small and has a great speed. It's directly integrated into the SymbOS kernel, which makes it possible to decompress data with the full linear size of up to 63K!

I hope to be able to release the next SymbOS version later this year. I know that currently all this is not interesting for you Spectrum guys, but I just wanted to share this info, as I am using this awesome software now. Maybe/hopefully there will be a Spectrum Next version in the future.

Again a lot of thank you and respect to Einar Saukas (for the whole thing) and Introspec (for the Turbo decompressor)!

[Einar Saukas]

That's awesome! Thank you!!!

Now here's an idea. Your current format is as follows:

- 1W: total length of data after decompression
- last 4 bytes of uncompressed data
- 1W: number of uncompressed bytes at the beginning
- N uncompressed bytes at the beginning
- remaining compressed bytes

My suggestion is to use a simpler header like this:

- 1W: total length of data after decompression
- 1W: number of uncompressed bytes at the beginning (at least 4)
- N uncompressed bytes at the beginning (N>=4)
- remaining bytes, compressed backwards

This way, you will only need N uncompressed bytes total, instead of N+4.

To decompress, you copy into memory exactly N-4 uncompressed bytes, immediatelly followed by the compressed block. Afterwards decompress it backwards from the top of memory area. Finally copy the remaining 4 uncompressed bytes.

Compressing backwards may sound awkward, but you should get a slightly better compression in almost every file...

[Prodatron]

Hi Einar, thanks a lot for your answer and your ideas!

Using backwards compression to have the "4 bytes" (for avoiding the delta problem) at the beginning in the uncompressed area is a good idea!

Compressing backwards may sound awkward, but you should get a slightly better compression in almost every file...

Is it really like this?
I made some quick and dirty tests with everal SymbOS EXE files, and the ratio with backward compression was always a little bit worse compared to normal compression. Then I remembered, that EXE files have the relocator table at the end, which may be a bad start for the dictionary. So I did the same with bitmap files, but here there is the same result, all backward compressed files are a little bit larger than normal compressed ones.
I don't really get this, as backward or forward is just swapping LDIR and LDDR, and why should the dictionary be somehow more optimal, when you start from the beginning or from the end.
Anyway I will make some more tests, especially I like to find out, if your suggestion will save some code on OS side, which is always a very good thing

[Einar Saukas]

Is it really like this?

Ops, I should have explained what I meant!

The compression itself shouldn't make much difference. Some files work slightly better with forward compression, others with backward compression.

However the new format I suggested (using backward compression) will allow compressing 4 more bytes per file.