EXEPACK2
EXEPACK2, or EXEPACK:2, is the (informal?) name of a compressed data format used in some OS/2 executable files (see Linear Executable), and certain other OS/2 formats. It was introduced in OS/2 Warp (OS/2 3.0).
Such executables are usually created by using the "/EXEPACK:2" option of the LINK386 utility, or by the special REPACK utility.
The decompressor is built into the OS/2 loader, so a compressed executable does not contain the code for decompressing itself.
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See also
Format details
The compression scheme is essentially a derivative of LZ77/LZSS, with a 4K history buffer. It also incorporates run-length encoding.
Assuming that OS/2 Boot Logo files use the same EXEPACK2 compression as executables, there is a small but significant error in the Wynn/Wu paper. For a "Long String token", it says the length field is biased by 3, when in fact it is not biased.
Other clarifications:
- When a token specifies both a set of uncompressed bytes, and a copy-from-history instruction, it is processed as if it were two separate instructions, in that order.
- The "distance back" values are encoded such that the most recently decompressed byte is 1, not 0 as it is in most formats.
Specifications
- 2000 Publications (Jie Wu) → A. C. Wynn and J. Wu, "The Effect of Compression on Performance in a Demand Paging System," Journal of Systems and Software 50 (2000), pp. 151-170 - §3.2. Exepack2 page-based compression.
Software
- [1] → repack
- lxLite - possibly related
- See also OS/2 Boot Logo#Software, notably lgoproc.
