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, 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.

Apart from avoiding the complexity of full hardware emulation, a compatibility layer also avoids the speed penalty of full emulation. Some may even be faster than the original, e.g. some programs running on some versions of the FreeBSD Linux emulation compared to then-current versions of 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.

External links

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