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An aspect of maintaining backward compatibility with an older system is that such systems' client programs often do not only depend on their specified interfaces, but also on bugs and unintended behaviour. This must also be preserved by the newer replacement. Besides the significantly higher complexity that needs to be maintained during the natural evolution of the code or interface, this can sometimes cause performance or security issues, and the inconsistencies in the behaviour of interfaces can sometimes lead to new bugs in the software using it, creating difficult to resolve multi-directional cross dependencies between various pieces of code.
Examples of this can be found in MS-DOS / PC DOS, where the executable loader contains code specially designed to detect and fix a number of widespread bugs in applications and runtime libraries by patching the loaded image before executing it. Over the course of development, DR-DOS also had to be modified to not only emulate many undocumented peculiarities of MS-DOS / PC DOS, but also actual bugs in the kernel and several drivers in order to make certain other drivers and applications run on DR-DOS, when they were tested on specific versions of MS-DOS only.
Windows, which has traditionally emulated many old system bugs in order to allow older low-level programs to run, is another example. As a result, Wine, which makes it possible to run many Windows applications on other platforms, also needs to maintain bug compatibility with Windows.
During development of its PC compatible, Compaq engineers found that Microsoft Flight Simulator would not run because of what subLOGIC's Bruce Artwick described as "a bug in one of Intel's chips", forcing them to make their computer bug compatible with the IBM PC. Another hardware example is found in the design of the IBM AT A20 address line to emulate the behaviour in older processors.
- "Bug-compatible - www.jargon.net". Retrieved 2010-02-03.
- Matthias Paul (2002-02-20). "Need DOS 6.22 (Not OEM)". alt.msdos.programmer. Retrieved 2006-10-14.
- "WineFeatures - The Official Wine Wiki". Retrieved 2010-02-03.
- Yakal, Kathy (1985-01). "Bruce Artwick / The Designer Behind Flight Simulator II". Compute!'s Gazette. p. 32. Retrieved 6 July 2014. Check date values in:
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