Compatibility Layer - Software

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'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
• 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 some Unix-like system emulations.
  • Mach and Darwin binary compatibility 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 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, 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.