UPX

UPX (Ultimate Packer for Executables) is a free and open source executable packer supporting a number of file formats from different operating systems.

Compression

UPX uses a data compression algorithm called UCL, which is an open source implementation of portions of the proprietary NRV (Not Really Vanished) algorithm.

UCL has been designed to be simple enough that a decompressor can be implemented in just a few hundred bytes of code. UCL requires no additional memory to be allocated for decompression, a considerable advantage that means that a UPX packed executable usually requires no additional memory.

UPX (since 2.90 beta) can use LZMA on most platforms; however, this is disabled by default for 16-bit due to slow decompression speed on older computers (use --lzma to force it on).

Starting with version 3.91, UPX also supports 64-Bit (x64) executable files on the Windows platform. This feature is currently declared as experimental.

Decompression

UPX supports two mechanisms for decompression: an in-place technique and extraction to temporary file.