Windows XP, released in 2001, was designed with a hidden compatibility database to support legacy software. This database, located in the C:WINDOWSAppPatch folder, contained approximately 200 compatibility fixes at launch and utilized a sophisticated matching system based on file size, checksum, version, and date. Windows XP could modify its behavior for specific applications through the use of shims, which acted as intermediaries to adjust application calls to the Windows API. Notable shims included Win98VersionLie, which faked the Windows version for compatibility, and EmulateHeap, which replicated the Windows 95 memory management style. Microsoft recognized that blocking applications relying on undocumented features could hinder user upgrades, as many organizations depended on legacy software. The compatibility fixes were managed through DLLs in the AppPatch folder, allowing the core operating system files to remain unchanged. Microsoft continued to enhance XP's compatibility with updates even years after its release.