Real-Time Opeating Systems

The key to identifying a real-time operating system is that it must be interruptable and "deterministic." That is, you must be able to pre-determine the maximum amount of time that will elapse between receiving an interrupt (usually hardware but can be software) and the response time for the interrupt-handler to execute. This includes saving the state of the executing thread (process), completeing the context switch, and responding to the event that caused the interrupt. Real-time OS's are primarily used in factory automation-type environments where NOT responding within a specified length of time might cause catastrophic results. An extreme example would be in monitoring the temperature of a Nuclear reactor. The flow of water to coolthe reactor cannot be held up waiting for the OS because some printer is out of paper and the printer thread deadlocks the OS. No windows product is currently a real-time OS. Especially not Win2K. In WinNT 4.0, Microsoft developed the concept of an"Interrupt Queue" (isn't this an Oxymoron?) where almost all interrupts just get added to the queue and wait their turn to be handled. FYI, the traditional Unix is also not a real-time OS.
Real-time OS's come in various levels based on guaranteed response time. One of the original RTOS's is "iRMX" developed by Intel. "LynxOS" is generally considered one of the top-end RTOS'. "QNX" has been around for a long time and has a good rep but is not as robust. One of the up and coming contendersis "OS-9" from Microware. Free RTOS's are available from Green Hills software called "INTEGRITY" and "ThreadX" but I am not personally familiar with either of those.