Compatibility layer

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In software engineering, a compatibility layer allows binaries for an emulated system to run on a host system. This translates system calls for the emulated system into system calls for the host system. With some libraries for the emulated system, this will often be sufficient to run binaries for the host system.

Examples include:

the Linux compatibility layer on FreeBSD, NetBSD, OpenBSD and DragonFly BSD, which emulates Red Hat Linux using a system call translator and Red Hat libraries;
Wine, which runs some Microsoft Windows binaries on Unix-like systems using a program loader and the Windows API implemented in DLLs;
Windows XP's use of compatibility layers to attempt to better run Windows 98 and MS-DOS applications.
Mac OS X has several compatibility layers: Classic for running older programs, X11.app for running X Windows programs.

A compatibility layer avoids both the complexity and the speed penalty of full hardware emulation. Some programs may even run faster than the original, e.g. some Linux applications running on FreeBSD's Linux emulation were often held by advocates to perform better than the same applications on Red Hat Linux.

A compatibility layer requires the host system's CPU to be (upwardly) compatible to that of the emulated system. Thus, for example, a MS Windows compatibility layer is not possible on PowerPC hardware, since MS Windows requires an x86 CPU; in that case, full emulation is needed.

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References

Linux Binary Compatibility (FreeBSD Handbook)
Application Compatibility Tools in Windows XP (Paul McFedries, Microsoft Expert Zone) (link broken)
IRIX Binary Compatibility (Emmanuel Dreyfus, ONLamp) — detailed description of practical issues in compatibility layer implementation

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