amigaos ancient unix systems application programming interface beos bell labs berkeley software distribution cpu modes certification mark comparison of operating systems computer multitasking computer network context switch cygwin de facto dennis ritchie device driver dragonfly bsd euphemism exokernel freebsd free software gnu gnu project general protection fault genericized trademark graphical user interface hp-ux hardware abstraction layer history of operating systems hybrid kernel irix interrupt kassel linux linux distribution list of linux distributions list of operating systems loadable kernel module lynxos minix ms-dos mac os x mac os x server memory protection memory segmentation microkernel microsoft windows monolithic kernel netbsd openbsd opensolaris open source operating system operating system development posix paging preboot execution environment proprietary software protected mode qnx reactos research unix sco openserver segmentation fault single unix specification software license standard sunos the open group timeline of operating systems trademark trademark trial and appeal board ultrix uniflex unix user space virtual file system virtual tape library vxworks wildcard character xenix