Second, OS-9 does not have a Unix-style fork () system call — instead it has a system call which creates a process running a specified program, performing much the same function as a fork-exec or a spawn.

OS-9 has had a modular design from the beginning, influenced by notions of the designers of the 6809 and how they expected software would be distributed in the future ( see the three-part series of articles in Jan-Mar 1979 Byte by Terry Ritter, et al.

* In the embedded market, where OS-9 has found application in such devices as the Fairlight CMI synthesizers, robotics, in-car navigation systems, and Philips ' Compact Disc-Interactive ( CD-i ) industry standard.

In microcomputers, SWTPC's 6800 and 6809 machines used TSC's FLEX disk operating system, Radio Shack's TRS-80 machines used TRS-DOS, their Color Computer used OS-9, and most of the Intel 8080 based machines from IMSAI, MITS ( makers of the legendary Altair 8800 ), Cromemco, North Star, etc., used the CP / M-80 disk operating system.

OS-9 / 6809 ran on Motorola EXORbus systems using the Motorola 6809, SS-50 Bus and SS-50C bus systems from companies such as SWTPC, Tano, Gimix, Midwest Scientific, and Smoke Signal Broadcasting, STD-bus 6809 systems from several suppliers, personal computers such as the Fujitsu FM-11, FM-8, FM-7 and FM-77, Hitachi MB-S1, and many others.

OS-9 ( especially the 68K version and thereafter ) clearly distinguishes itself from the prior generation of embedded operating systems in many aspects.

* Versions of OS-9 / 68K ran 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 and 68340-based MM / 1, and on other computers from Frank Hogg Laboratories, PEP Modular Computers, and Delmar Co .) It was also ported to the Atari ST by Recc-o-ware in the early 1990s, and was distributed by Cumana in Europe.

* Microware and Microware OS-9, from the RadiSys website

* Archive of OS-9 information and software, including an OS-9 FAQ, from Real-time Services Inc.

It was paired with the OS-9 Operating system, also from Microware and took advantage of several OS-9 features ( e. g., shared memory, module loading and unloading, etc.

The OS-9 family was popular for general-purpose computing and remains in use in commercial embedded systems and amongst hobbyists.

This was especially true when OS-9 was available for popular 6809-based computers such as the FM-7, FM-77, and the Tandy TRS-80 Color Computer and its near-clone, the Dragon.

With hardware supporting memory management circuits ( that is, address translation ) and OS-9 Level 2, GUI use was successfully routine, even on the minimal resourced CoCo.

Additionally, OS-9 processes keep track of two " current directories " rather than just one ; the " current execution directory " is where it will by default look first to load programs to run ( which is of course similar to the use of PATH environment variable under UNIX ).

:* When one types a command to the OS-9 shell, it will look first in the current module directory for a module of the specified name and will use it ( and increase its link count ) if found, or it will look on disk for an appropriately named file if not.

As of 2012, OS-9 is still supported by RadiSys and Microware.

In 1983, OS-9 / 6809 was ported to Motorola 68000 assembly language and extended ( called OS-9 / 68K ); and a still later ( 1989 ) version was rewritten mostly in C for further portability.

* OS-9 / 68K is also found in some embedded applications, including the Quanta Delta television broadcast character generator, still in production by ScanLine Technologies in Utah.

As a consequence of early pervasive design decisions taking advantage of the easily used reentrant object code capabilities of the 6809 processor, programs intended for OS-9 are required to be reentrant ; compilers produce reentrant code automatically and assemblers for OS-9 offer considerable support for it.

This acquisition infused Microware with capital and allowed Microware to continue OS-9 development and support.

* Older versions of OS-9 do not support POSIX threads while all OS-9 supported processors support POSIX threads.

* No SMP support for multiple sockets, cores, or hardware threads in the same OS-9 instance ( can run as a RTOS on one of the cores of dual core processors like Core Duo and Core 2 Duo, when Linux is running on the other core doing general purpose tasks )

Memory-expanded and MMU-equipped Dragons are able to run OS-9 Level 2.

This allows OS-9 to run on older systems which do not include an MMU.

All OS-9 programs run as processes containing at least one lightweight process ( thread ) but may contain an effectively unlimited number of threads.

* CD-RTOS ( based on Microware's OS-9 )

Even on the CoCo, a quite minimalist hardware platform, it was possible under OS-9 / 6809 Level One to have more than one interactive user running concurrently ( for example, one on the console keyboard, another in the background, and perhaps a third interactively via a serial connection ) as well as several other non-interactive processes.

The now defunct Pesa Electronica used OS-9 on their CGs such as CG 4722 and CG4733.

:* Modules are not only used to hold programs, but can also be created on the fly to hold data, and are the way in which OS-9 supports shared memory.

* Repository for command line tools for manipulating 6809 OS-9 disk images, on SourceForge

* Execution environment for OS-9 user-mode programs on Mac / Win / linux host OS

OS-9 went on to versions for the 68000 family of processors and, rewritten mostly in C, to the Intel 80x86, PowerPC, ARM, MIPS, and some of the Hitachi SuperH ( SH ) series processors.

Over time, Microware concentrated on industrial customers and neglected the hobbyist base that was porting a great many Unix packages and utilities to OS-9.

Modular operating systems such as OS-9 and most modern monolithic operating systems such as OpenVMS, Linux, BSD, and UNIX variants such as SunOS, and AIX, in addition to MULTICS, can dynamically load ( and unload ) executable modules at runtime.

The modern boards can be delivered with a range of operating systems, amongst which is OS-9.

* Kernel is entirely written in assembly language ( OS-9 / 68K version only ) as well as C ( portable version to other architectures )

OS-9 Level II, also took advantage of such memory management facilities.

Microware later started calling all of its operating systems — including what had been originally called OS-9000 — simply OS-9, and started shifting its business interest towards portable consumer device markets such as cellphones, car navigation, and multimedia.

Initially, in the days of the SS-50 and SS-50C, bus systems such as SWTPC, Gimix, and Smoke Signal Broadcasting, OS-9 was used more as a general purpose microcomputer operating system, and had a large, active hobbyist user population, and industrial and embedded system users.

The decreasing cost of display equipment and processors made it practical to provide graphical user interfaces for many operating systems, such as the generic X Window System that is provided with many Unix systems, or other graphical systems such as Microsoft Windows, the RadioShack Color Computer's OS-9 Level II / MultiVue, Commodore's AmigaOS, Apple's Mac OS, or even IBM's OS / 2.

The company collaborates with its customer's in-house engineering groups, providing expertise in computer architecture, systems integration, embedded operating systems such as OS-9, ASIC design, and middleware.