The introduction to this article has already listed a few of the reasons why application compatibility technologies are necessary: a change in data formats from one version to another of the operating system, or different locations or formats for user information. There are still other reasons. The application may refuse to run, for example, when Windows reports new, later version numbers. Often in cases like these, the application will work well on the new version of Windows if the user can get past this block in the application. Or, the application may call older versions of Microsoft Win32® API functions that return unexpected values on computers with large amounts of resources, such as free disk space.
Further, those who upgrade their computers specifically from Windows 95, Windows 98, or Windows Me may encounter compatibility problems, because applications written exclusively for that platform may use programmatic methods of hardware access that are not permitted on Windows XP. Direct hardware access can greatly reduce operating system stability, and because of its Windows NT heritage, Windows XP requires that hardware access be handled through the correct channels.
Applications originally written for Windows NT or Windows 2000, however, are unlikely to generate serious compatibility problems on Windows XP, given the common code base of these operating systems. Regardless, some applications may have been written to run on a single version of Windows NT, and may therefore require a compatibility fix to run properly on Windows XP.
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