Windows XP's Hidden Tricks for Running Old Apps
Microsoft's Windows XP was known for its legendary compatibility, allowing old software to run on it without any issues. However, a hidden database of problematic apps helped achieve this feat.
According to Microsoft engineer Raymond Chen, the Application Compatibility Database, or ACDB, contained a list of fixes that allowed old apps to work on XP. This database was stored in the C:\ vironment\AppPatch folder and used a binary format for rapid scanning.
The ACDB included roughly 200 compatibility fixes when XP launched in 2001. When an app matched one of these entries, Windows would apply what Microsoft calls shims to fix the issue. Shims could tweak how an app interacted with Windows APIs or even lie to the app about the Windows version.
In some cases, shims went as far as emulating a Windows 95 memory manager for apps that depended on it. This allowed old software to run without any issues, but Chen noted that this approach had its limitations and could introduce security risks.