In software engineering, a compatibility layer is an interface that allows binaries for a legacy or foreign system to run on a host system. This translates system calls for the foreign system into native system calls for the host system. With some libraries for the foreign system, this will often be sufficient to run foreign binaries on the host system. A hardware compatibility layer consists of tools that allow hardware emulation.
Compatibility layer in kernel:
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 compatibility layer may perform better than the same applications on Red Hat Linux. Benchmarks are occasionally run on Wine to compare it to Windows NT-based operating systems.
Even on similar systems, the details of implementing a compatibility layer can be quite intricate and troublesome; a good example is the IRIX binary compatibility layer in the MIPS architecture version of NetBSD.
A compatibility layer requires the host system's CPU to be (upwardly) compatible to that of the foreign system. For example, a Microsoft Windows compatibility layer is not possible on PowerPC hardware because Windows requires an x86 CPU. In this case full emulation is needed.
Hardware compatibility layers involve tools that allow hardware emulation. Some hardware compatibility layers involve breakout boxes because breakout boxes can provide compatibility for certain computer buses that are otherwise incompatible with the machine.