Compatibility layer

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A compatibility layer is a term that refers to components that allow for non-native support of components.

In software engineering, a compatibility layer allows binaries for a 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.

Hardware compatibility layers involve tools that allow hardware emulation.

1 Software
2 Hardware
3 See also
4 References
5 External links

Software

Examples include:

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^[1]
Lina, which runs some Linux binaries with native look and feel on Windows, Mac OS X and Unix-like systems.
Alky Project, DirectX10 compatible runtime for Windows XP (incomplete). It also runs on OSX and Linux.
Compatibility layer in Kernel
- Linuxulator, the Linux compatibility layer on the FreeBSD, which enables binaries built specifically for Linux to run on FreeBSD^[2] with the same way as the native FreeBSD API layer^[3]. FreeBSD also has some Unix-like system emulations includes NDIS, NetBSD, PECoff, SVR4 and different CPU version of FreeBSD^[4].
- NetBSD has some Unix-like system emulations^[5].
- Mach and Darwin binary compatiblity for NetBSD/powerpc and NetBSD/i386.
- The PEACE Project(aka COMPAT_PECOFF) has Win32 compatible layer for NetBSD. The project is now inactive.

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 were often held by advocates to perform better than the same applications on Red Hat Linux^{[citation needed]}, and benchmarks are occasionally run on Wine to compare it to Windows NT-based operating systems[1].

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.^[6]

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

Hardware

Hardware compatibility layers involve tools that allow hardware emulation. Also, the term can refer to tape adaptors for tape players.^{[citation needed]} Other hardware compatibility layers involve breakout boxes, since breakout boxes can provide compatibility for certain computer buses that are otherwise incompatible with the machine.