Microsoft's Raymond Chen describes the string-matching heuristic behind the OS's system-file recovery.
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The full match list ran to six terms: setup, install, inst, imposta, ayarla, and felrak. Chen wrote that "install" was redundant, because any name containing it already contains "inst," and speculated that the shorter entry was added later to catch installers named along the lines of "blahinst" without anyone deleting the original. A program whose own name produced no match got a second test, with Windows 95 checking whether the word "Setup" appeared anywhere in the path to the executable. A separate live check ran after any multimedia driver was installed through an INF file, added because those drivers frequently overwrote system DLLs.
This heuristic gated a recovery mechanism Chen described in March . Installers of the period overwrote system files without checking versions, disregarding Microsoft's rule that a file should only ever be replaced by a newer one. An installer carrying Windows 3.1 copies of shared DLLs, for example, would bury the newer Windows 95 versions underneath them, and every program that relied on the current files would break.
Windows 95 kept backup copies of commonly clobbered files in a hidden C:\Windows\SYSBCKUP directory. It would then let each installer finish, check its work, and restore the correct versions where the installer had downgraded them. This safety net depended entirely on correctly guessing that an installer had run, so a setup routine with an unusual name slipped past it, while an ordinary program named something like instant.exe tripped it for nothing.
The file check was often deferred until the next boot rather than run immediately, with Chen explaining that some installers, unable to replace a file already in use, would drop back to MS-DOS, run a batch file to swap the file, and restart Windows, so the cleanup pass had to wait for the reboot to catch anything the batch file had altered.
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Reference reading
- https://www.tomshardware.com/software/windows/SPONSORED_LINK_URL
- https://www.tomshardware.com/software/windows/windows-95-detected-installers-by-scanning-program-names-for-the-word-setup#main
- https://www.tomshardware.com/subscription
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