Xinu

XINU Is Not Unix (XINU, a recursive acronym), is an operating system for embedded systems, originally developed by Douglas Comer for educational use at Purdue University in the 1980s. The name is both recursive, and is Unix spelled backwards. It has been ported to many hardware platforms, including the DEC PDP-11 and VAX systems, Motorola 68k (Sun-2 and Sun-3 workstations, AT&T UNIX PC, MECB), Intel x86, PowerPC G3, MIPS, ARM architecture and AVR (atmega328p/Arduino). Xinu was also used for some models of Lexmark printers.

Despite its name suggesting some similarity to Unix, Xinu is a different type of operating system, written with no knowledge of the Unix source code, or compatibility goals. It uses different abstractions, and system calls, some with names matching those of Unix, but different semantics.