Windows 95 let installers trash its files then fixed the mess behind their backs
I'll just clear up that up, shall I?
Microsoft veteran Raymond Chen has shared another nugget of Windows lore – what Windows 95 did when installers stomped on its system files.
The problem traces back to the days of 16-bit Windows, when many system components were redistributable. Installers could include copies of those components and place them on a target computer. The setup program was expected to compare the version numbers of what was on the system and what was being copied, only overwriting if the file being installed had a higher version number.
Simpler days compared to the pain of creating install sets now and worrying about wholesale registry destruction, or a dependency not being what a developer thought it was.
"This rule relies on the fact that Windows maintains backward compatibility, so the newer version still works even if used by an older program," Chen wrote.
In practice, nothing stopped an installer from ignoring that rule entirely. It was not uncommon for setup programs to simply overwrite whatever was in their way, replacing Windows 95 components with their Windows 3.1 counterparts.
"You can imagine how much of a disaster this caused to the rest of the system," said Chen.
At this point, even though it is many decades since the era of Windows 95, a reader would be forgiven for wondering why the operating system allowed this at all. The answer is backward compatibility: blocking file operations risked breaking the installer itself.
"This ended up creating more problems," explained Chen. "Some installers declared the installation to be a failure and gave up. [Others] displayed an error message to the user and asked the user what to do next. (Like the user knows what to do next.)"
"You even had installers that took even more extreme measures and said, 'Okay, fine, I can't overwrite the file, so I'm going to reboot the system and then overwrite the file from a batch file, see if you can stop me.'"
Microsoft's solution was a hidden directory (c:\windows\sysbckup) of commonly overwritten files. After setup finished, Windows quietly checked the file version numbers on the disk. Higher version numbers would be copied into the hidden directory, or the file in the hidden directory would replace a lower version number.
"Basically," wrote Chen, "Windows 95 waited for each installer to finish, and then went back and checked its work, fixing any mistakes that the installer made."
The belt-and-braces approach depended on the operating system being aware of a setup program rummaging through its internals.
Today - based on recent performance - users should worry less about installers and more about what Microsoft itself is doing to Windows' system files. ®