MINIX
MINIX is a free Unix clone distributed with full source code. Its small footprint, microkernel-based design, and ample documentation served mostly hobbyists (before Linux) wanting to run a Unix-like OS on personal computers and study its operation. MINIX played an important role in the genesis of Linux, having inspired Linus Torvalds to build his first version of Linux:

Hello everybody out there using minix - I'm doing a (free)
operating system?(just a hobby, won't be big and
professional?like gnu) for 386(486) AT clones.

The MINIX project page lives on at www.cs.vu.nl/~ast/minix.html with additional information and download posted at www.minix.org.

OpenSolaris
Starting on June 14, 2005, Sun Microsystems opened a large portion of the source code for its Solaris operating system, a version of proprietary SVR4 Unix. OpenSolaris provides the core kernel, libraries, and commands that are currently distributed with the Solaris OS. Some industry pundits and members of the press have voiced concern that OpenSolaris will draw development resources away from Linux, but this new addition to our list of open source OSes has yet to prove a major distraction. Most developers interested in Open Solaris were already writing software for Solaris in its 100% proprietary incarnations, and had been urging Sun to open its code base for the sake of documentation, debugging, and to accelerate the introduction of new features.

Sun licenses OpenSolaris under the Common Development and Distribution License (CDDL), a modified Mozilla-type open source license. Sun was able to accomplish the relicensing of Solaris SVR4 code (theoretically owned by SCO) through an earlier license buy-out, and by thorough scrubbing of other encumbered code and IP in a regular Solaris code base. To learn more about OpenSolaris, visit www.opensolaris.org/os.

Plan 9
Plan 9 is an open source operating system project developed at Bell Labs in the mid-1990s. Plan 9 is obviously Unix-like, but definitely not a Unix family OS. Rather, it's a distributed system, built from terminals, CPU servers, and file servers. Plan 9 observes three very strong architectural principles. First, resources are named and accessed like files in a hierarchical file system. Second, a standard protocol, called 9P, allows users and programs to access these resources. Third, otherwise disjointed hierarchies offered by different services are conjoined into a single private hierarchical file name space. Plan 9 code uses the Lucent Public License (an OSI-recognized license). You can find the Plan 9 project home page at www.cs.bell-labs.com/plan9dist.

RTEMS
The Real-Time Executive for Multiprocessor Systems offers deeply embedded applications and a fairly full-featured embedded OS. The RTEMS project aims to provide a free, deterministic real-time operating system that is also competitive with closed source RTOS products. RTEMS supports over one dozen CPU families, including 8/16-bit CPUs such as Hitachi H8/300, DSPs like the TI C3/C4 families, OpenRISC CPUs, and also mainstream 32-bit processors such as ARM7, MIPS, and M68K. RTEMS is maintained by the OAR Corporation and a diverse developer community, and is licensed under the GNU GPL; to learn more, visit www.rtems.org.

RTLinuxFree
RTLinuxFree provides a real-time execution environment that virtualizes the GNU/Linux interrupt chain to enable hard real-time responsiveness with standard Linux 2.4 and 2.6 kernels. RTLinuxFree is maintained by FSMLabs (which markets commercial RTLinux) and by the Spanish integrator OS3 as part of OCERA (Open Components for Embedded Real-time Applications). RTLinux is licensed under the GNU GPL and the FSMLabs Open Patent License. More information is available at www.rtlinuxfree.org.

RTAI
The "Real-time Application Interface" originated as a derivative of the RTLinux project, but today appears to have been extensively rewritten as an independent piece of software. Like RTLinux, RTAI supplies real-time execution capabilities to standard Linux by virtualizing parts of the hardware control interface. RTAI is maintained by the Aeronautics Department of the University of Milano, Italy, and is licensed under the GNU GPL and LGPL. To learn more, visit www.rtai.org.

Other Free and Open Source Embedded OSes
At one time, the embedded software industry boasted more than 350 distinct RTOSes, kernels, and executives. Most of them were proprietary and/or commercial, but many were free or open software. The majority of these prodigious pieces of software went the way of the dinosaurs, their source code lost, their binary legacy fossilized deep inside surplus gadgets.

Among those embedded OSes surviving today are proprietary/commercial products like Nucleus, QNX, and VxWorks, and open source projects such as Evolution (the RTOS, not the mail client), FreeOSEK (for automotive applications), FreeRTOS, pico]OS, uCOS, and XMK. Another trend has been the dual (re)licensing of commercial RTOSes, with non-commercial use covered by the GPL or another open license, and all other uses governed by a proprietary, royalty-bearing scheme. Vita Nova Inferno and, of course, RTLinux belong to this category.

Not Quite Open Source
While the Microsoft Shared Source Program and License surely don't conform to OSI Open Source Principles, Redmond's shared source does seem to offer many of the benefits of "real" open source to Windows developers. In particular, the shared source program lets developers examine actual Windows source code, and gain insight into how Windows interfaces and OS mechanisms function. Shared source serves as extended documentation for Windows APIs. Restrictions on reuse and relicensing, and the narrowest of "two-way streets" between the code creators and a community, clearly separate shared source and similar programs from the "real" open source projects familiar to most readers of LinuxWorld Magazine. While this distinction is very clear for savvy developers, its importance does not always impress IT and corporate management.

Microsoft was not the first or only commercial/proprietary software supplier to attempt to bridge the gaps between open and closed (or to attempt to co-opt the advantages of openness). In the last decade, OSes whose source was first supplied sparingly, at premium prices, today either ship with "read-only" source code or provide a short path for binary licensees to access it. Examples of this kind of read-only licensing include RTOS vendors QNX (Neutrino) and Wind River (VxWorks). Many other vendors have always included source code for their OSes as a matter of course, for documentation and porting purposes. The most vocal of these players was Accelerated Technology (makers of Nucleus, used in cell phones - now a division of Mentor Graphics). ATI used to market their source code program by claiming "It just makes sense."

Open and Shut
The open source OSes described above owe a large share of their technical advances and the scope of their deployment to the choice of an open license, and the communities engendered by that licensing choice. But, don't be fooled by imitations - "now you see it, now you don't" source licensing is not open source.

Real open source does indeed make sense. This simple truth still applies best to Linux, whose unprecedented community and ubiquity build on the freedoms guaranteed by the GPL.