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.
- Wine, which runs some Microsoft Windows binaries on Unix-like systems using a program loader and the Windows API implemented in DLLs
- Windows's application compatibility layers (Application Compatibility Toolkit) to attempt to run poorly written applications, or those written for earlier versions of the platform.
- Lina, which runs some Linux binaries with native look and feel on Windows, Mac OS X and Unix-like systems.
- KernelEx, which runs some Windows 2000/XP programs on Windows 98/Me.
- Executor, which runs 68k-based "classic" Mac OS programs in Windows, Mac OS X, and Linux.
- Columbia Cycada, which runs Apple iOS applications on Android systems
- Hybris, library that translates Bionic into glibc calls.
- Darling, a translation layer that attempts to run Mac OS X and Darwin binaries onto Linux.
Compatibility layer in kernel:
- FreeBSD's Linux compatibility layer, which enables binaries built specifically for Linux to run on FreeBSD with the same way as the native FreeBSD API layer. FreeBSD also has some Unix-like system emulations includes NDIS, NetBSD, PECoff, SVR4 and different CPU version of FreeBSD.
- NetBSD has several Unix-like system emulations.
- Windows Subsystem for Linux provides a Linux-compatible kernel interface developed by Microsoft and containing no Linux code, with Ubuntu user-mode binaries running on top of it.
- 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 may or may not perform better than the same applications on Red Hat Linux, and 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. Thus, for example, a Microsoft Windows compatibility layer is not possible on PowerPC hardware, since Windows requires an x86 CPU; in that case, full emulation is needed.
Hardware compatibility layers involve tools that allow hardware emulation. Some hardware compatibility layers involve breakout boxes, since breakout boxes can provide compatibility for certain computer buses that are otherwise incompatible with the machine.
- Charlie Russel, Microsoft MVP for Windows Server and Tablet PC (2002-02-18). "Application Compatibility in Windows XP".
- "Linux emulation in FreeBSD".
- "Emulation". freebsd.org. Retrieved 2014-03-16.
- "A look inside..." freebsd.org. Retrieved 2014-03-16.
- "NetBSD Binary Emulation". netbsd.org. Retrieved 2014-03-16.
- BASH Running in Ubuntu on Windows - MSDN
- BenchMark-0.9.5 - The Official Wine Wiki
- Emmanuel Dreyfus (August 8, 2002). "IRIX Binary Compatibility, Part 1". onlamp.com. Retrieved 2014-03-16.
- Windows XP Application Compatibility Technologies (Dave Morehouse and Todd Phillips, Microsoft Corporation, 1 June 2001)