[cmds] Add decomp - ELKS executable decompressor by ghaerr · Pull Request #1866 · ghaerr/elks (original) (raw)

@toncho11, kernel decompression has been implemented for probably 3+ years now, it works well.

I added the feature years ago and one person tested it, saying it was slow on an 8088. The current elkscmd/Applications mechanism currently allows only for all or no programs to be compressed, so I stopped with the idea of fully compressed disk images. Now, with decomp, if a user wants to decompress ls, running decomp ls would uncompress it permanently on the distribution. Of course, there may be some commands that should not be compressed, the individual selection feature likely needs to be added before we should move to higher density distributions, and user testing should occur on real hardware. On QEMU, there is no discernible difference when running fully compressed distributions other than noticing that tons more space is available (~450k on 1440k floppies).

The idea would be to have a mechanism that allows for specifically setting which programs would be compressed for which disk image sizes. That has not been built yet.

Don't understimate the utility of having more space... the current executable compression facility typically reduces file sizes by an average of around 30%. This makes a huge difference for 360k and all other floppy types. We're talking about 20+ more programs per disk image, at least, rather than having the lower density disk images having very few usable programs. This is only made worse with the recent games being added - only one is available on 360k.

If something go wrong with the decompression it will effectively disable a bunch of commands/programs.

The feature is entirely transparent. If the disk has a bad sector, the program wouldn't run regardless of being compressed or not. All compressed executables will always decompress properly otherwise. The algorithm is actually extremely cool - it decompresses in place without using any extra memory, so it will also always work even on systems with very little memory. The time it takes to read the program from floppy is also decreased, since the executable is smaller. The only real issue is that very slow 8088 systems take possibly as much time to decompress as the time saved from reading from the floppy, except when the program is read from the kernel system buffers rather than disk.

A 360k compressed disk image can be created now by setting CONFIG_APPS_COMPRESS=y in .config or make menuconfig. Then the entire floppy will be compressed executables and should have ~100k free on 360k, rather than the 0k free now.