PosixStandard 1003.1 defines "real-time" for operating systems as: 

"Realtime in operating systems: the ability of the operating system to provide a required level of service in a bounded response time"

See RealTime and OperatingSystem. Also see AdeOs, EcOs, InfernoOs, MenuetosOperatingSystem, PlanNineFromBellLabs (some interesting papers about Plan 9's real time implementation: http://doc.cat-v.org/plan_9/real_time/ ), QnxNeutrino, RtLinux and VxWorks.
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I have found a site worth its weight in gold when it comes shopping for an RTOS. I haven't found any other site that has done as much conclusive testing on almost every RTOS. The WinCE 4.2 testing and a comparison to QnxOs and VxWorks is there as well.
** http://www.realtime-info.be/encyc/
-- Andrew Gardso, RTUnet Pty Ltd

Thanks, Andrew. There aren't enough hardcore embedded technoweenies on this Wiki to form much of a group, so we have to share as best we can. Nice to see somebody has a handle on comparing RTOSs toe to toe. -- MartySchrader
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IronyWarning: Don't forget WinCe -- officially called a "RealTime system" by its manufacturer.

No, really. We have a project that is grounded in this concept. Stop laughing! It's RealTime, I tell you!

Of course, this has required plenty of fiddling down in the BSP to make anything work that has a resolution of tighter than 100ms. Glad we're not flying missiles.

From Microsoft, 10 Jun 10:

''"Windows Embedded Compact 7 is the next generation of Windows CE. Use the Compact 7 PCTP to unlock your vision for embedded devices with high performance, powerful user interfaces and a seamless connected experience to media Windows and productivity applications."''

You betcha.
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CategoryRealTime, CategoryOperatingSystem, CategoryTime