CAPABILITIES OF COMMERCIAL REAL-TIME OPERATING SYSTEMS: This section describes several real-time operating systems that run on common processors and have sizable user bases. They are LynxOS [Bunn], pSOSystem [Moto], QNX [QNX], VRTX [Ment] and VxWorks [Wind]. In many respects, these operating systems are similar. Below is a summary of their commonalities; with few exceptions, you can replace the noun “the operating system” in the summary by the name of any of them.
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Conformance to Standards: The operating system is compliant or partially compliant to the Real-Time POSIX API standard, so you have preemptive fixed-priority scheduling, standard synchronization primitives, and so on, but the operating system may support only threads or processes (i.e., not both) and may implement only a subset of the standard functions. The operating system also has its own set of API functions, which may differ significantly from the corresponding POSIX functions and, in the cases where they are similar, may have more features. Some of the operating systems also provide AT&T System V and BSD system call interfaces (e.g., LynxOS) and Willows Win32 Library (e.g., QNX).
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Modularity and Scalability: The kernel is small, and the operating system configurable. In particular, the operating system can be scaled down to fit with the application in ROM in small embedded systems. By adding optional components to the kernel, the operating system can be configured to provide I/O, file, and networking services.
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Speed and Efficiency: Most of these operating systems are microkernel systems. Unlike microkernel systems of old, which usually have higher run-time overhead than monolithic operating systems, these microkernel operating systems have low overhead. In some, sending a message to a system service provider incurs no context switch. Important timing figures such as context-switch time, interrupt latency, semaphore get/release latency, and so on, are typically small (i.e., one to a few microseconds).
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System Calls: Nonpreemptable portions of kernel functions necessary for mutual exclusion are highly optimized and made as short and deterministic as possible.
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Split Interrupt Handling: Similarly, the nonpreemptable portion of interrupt handling and the execution times of immediate interrupt handling routines are kept small. The bulk of the work done to handle each interrupt is scheduled and executed at an appropriate priority.
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Scheduling: All real-time operating systems offer at least 32 priority levels, which is the minimum number required to be Real-Time POSIX compliant; most offer 128 or 256 (or 255). They all offer the choice between FIFO or round-robin policies for scheduling equal-priority threads. They allow you to change thread priorities at run time, but none provides adequate support for EDF scheduling and good hook for user-level bandwidthpreserving servers and slack stealers.
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Priority Inversion Control: The operating system provides priority inheritance but may allow you to disable it to save the overhead of this mechanism if you choose to use a resource access-control scheme that does not need priority inheritance. The system may also provide ceiling-priority protocol.
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Clock and Timer Resolution: The operating system may provide a nominal timer resolution down to nanoseconds. However, even on the fastest processor today, the operating system takes about a microsecond to process a timer interrupt, so you should not expect to be able to release a thread periodically with a period down to microseconds or precision and granularity in time down to tens and hundreds of nanoseconds.
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Memory Management: The operating system may provide virtual-to-physical address mapping but does not do paging. The operating system may not offer memory protection. Even when it does, it provides the user with the choice among multiple levels of memory protection, ranging from no protection to private virtual memory.
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Networking: The operating system can be configured to support TCP/IP, streams, and so on.
The remainder of the section highlights some distinguishing features of the operating systems listed above, especially those features not mentioned in the above summary. The fact that we say that an operating system has a feature but do not mention the same feature in our discussion on another operating system does not imply that the other operating system does not have the same or similar features! The section refrains from giving you a table of features and performance data on the operating systems described here. Such information will most likely to be out-of-date by the time this book reaches you. The best source of up-to-date information on any operating system is the home page provided by the vendor. There you are also likely to find a good tutorial on basic operating system primitives in general.
