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RxDOS kernel compression (Developers)

posted by cm(R) Homepage E-mail, Düsseldorf, Germany, 08.09.2018, 23:45

> As algorithm, I used Joergen Ibsen's BriefLZ, which is very simple and
> available under the zlib usage conditions. It still manages to compress the
> kernel to about 50% of its size, which is plenty for me. I haven't compared
> the compression ratio against that of UPX (as used by FreeDOS's and
> EDR-DOS's kernel compression), [...]

I tested this now: On a file made from the first 65_000 bytes of some kernel image, UPX compresses to 26_325 bytes (40.50%, with depacker), while BriefLZ compresses to 32_115 bytes (49%, without depacker).

The actual full kernel image is 86_352 bytes, which UPX doesn't accept as a .COM format file, so cannot be tested that way with UPX. BriefLZ compresses it to 43_316 bytes (50%, without depacker).

The depacker INIT0 and INIT1 code is less than 2 KiB large. Additionally, the iniload stage is 2 KiB. So, add 4 KiB of uncompressed handling.

Note: To compress a flat binary as .COM file with UPX, the filename extension must be .com to allow its detection.



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