Plan 9 from Bell Labs is a distributed operating system, developed during the late 1980s and early 1990s by the same research centre that developed Unix in the early 1970s. Plan 9 is not based on Unix; all the source code is new. The Plan 9 release includes the full source code. It supports multiprocessors in both kernel and user programs. Many programs are structured for simplicity as communicating processes. That can be done directly using a few system calls, but for more CSP-style programming there is a threads library. The novel programming environment Acme and the window system rio are concurrent programs.
All Plan 9 software, including the kernel, is highly portable. Machine-dependent code is carefully isolated and kept to a minimum. As distributed, Plan 9 currently supports the MIPS, Sparc, 680x0 and 386/486/Pentium architectures. Porting the system requires a Plan 9 C compiler suite (compiler and linker, and assembler) for the target architecture, an object code support library, low-level graphics support if the port is to include bitmap graphics terminals, a port of the CPU/terminal kernel, and for completeness, a port of the file server kernel. User-level software, including the window system, does not need to be `ported', just compiled with the compiler. Note that to obtain a `cross compiler' for a given architecture one simply compiles it with the compiler for another architecture.