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OS-9® is the name of a family of soft real-time, multitasking, multi-user Unix-like operating systems developed by Microware Systems Corporation starting in about 1979/1980. They have been popular in both interactive general purpose computing and in embedded systems.

The first version ("level one") was written in assembly language for the Motorola 6809 CPU, and provides a single 64KB address space in which all processes ran. A later 6809 version ("level two") takes advantage of memory mapping hardware, supported up to 1MB of memory (ca 1980!) in most implementations, included a GUI on some platforms, and so on.

Later, OS-9/6809 was ported to Motorola 68000 assembly language and extended (called OS-9/68K); and a still later version was rewritten mostly in C for further portability. (The portable version was initially called OS-9000.) These later versions lack the memory mapping facilities of OS-9/6809 Level Two, nor did they need them. They used a single flat address space that all processes share; memory mapping hardware, if present, is mostly used to ensure that processes access only that memory they have the right to access. The 680x0 and 80386 (and later) MPUs all directly support far more than 64KB of memory, in any case.

OS-9's notion of processes and I/O paths is quite similar to Unix' in nearly all respects, but there are some significant differences:

OS-9/6809 ran on Motorola EXORbus systems using the Motorola 6809, SS-50 and SS-50C bus systems from companies such as SWTPC, Tano, Gimix, and Smoke Signal Broadcasting, STD-bus 6809 systems, personal computers such as the Fujitsu FM-7 and FM-77, and many others, but the best known hardware was the Radio Shack/Tandy Color Computer and its clones such as the British Dragon series. Even on the Color Computer, a mnimalist hardware platform, it was possible, under OS-9/6809 Level One, to have more than one user running concurrently (eg, one on the console keyboard and another via a serial connection). On a computer like an SS-50 Gimix, which had more memory and I/O controllers that did not load the CPU as much as did the CoCo, multiple users were common, even with 64KB of RAM. With hardware supporting memory management circuits (address translation) and OS-9 Level 2, GUI use was successfully routine. This was several years prior to successful GUIs on the 16-bit IBM PC class machines.
Actually, OS-9/6809 still lives, 20+ years on; the Color Computer (and clones) still has users and an annual conference, the Nth Annual "Last" Chicago CoCoFest, where N == 11 in 2002. A group of Canadian programmers rewrote OS-9/6809 Level II for the CoCo 3 (w/ address translation circuitry) for efficiency, and to take advantage of the native mode of the Hitachi 6309. Today's serious CoCo users now typically have replaced the 68B09E in the CoCo 3 with an Hitachi 63B09E and run the rewrite, called "NitrOS9." The combination is fast. Very fast. Especially considering it runs on an 8-bit CPU!

The various versions of OS-9/68K run on a wide variety of 68000 family platforms, including the Sharp X68000 in Japan, some personal computers intended by their designers as upgrades from the Color Computer (e.g. the 68070 or 68340-based MM/1, and on other computers from Frank Hogg Laboratories and Delmar Co).

OS-9/68K was also found in some embedded applications, including the Quanta Delta television broadcast character generator, still in production by ScanLine Technologies in Utah. While the user-level interface code on this system started at boot time, there was a hidden, undocumented keyboard sequence that would provide a user (who had sufficient social engineering skills :-) with a root shell prompt, in a scroll window on the device's edit-channel monitor. As you might expect, this could be both useful, and dangerous.

OS-9000/80x86 can be run on machines built around the Intel x86 CPUs. OS-9000 has also been ported to the PowerPC, MIPS, some versions of Advanced RISC Machines' ARM processor, and some of the Hitachi SH family of processors.

In addition to the embedded market, where OS-9 has found application in such devices as the Fairlight CMI synthesizers, robotics, and Philips' Compact Disc-Interactive industry standard, OS-9's multiuser capabilities make it usable for a general-purpose interactive computer system. There have been many third-party interactive applications written for it, such as the Dynacalc spreadsheet, the VED text formatter, and the Stylograph and Screditor-3 WYSIWYG word processors.

In 1999, nineteen years after the first release of OS-9, Apple Computer named a version of the operating system for the Macintosh "Mac OS9". Microware sued Apple that year for trademark infringement, but a judge rejected the claim, dismissing the suit in the following year. The judge said that there is little chance for confusion, but one still periodically finds postings to the comp.os.os9 newsgroup from Macintosh users who are at the very least confused about the purpose of that group.

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