嵌入式操作系统设计1课件.pptVIP

Embedded Operating System design Embedded Systems Requirements and Features Multitasking Scalability Real time Reliability Safety In real-time systems, correctness implies not only logical correctness of the results, but the time at which the results are produced is important. Hard real-time tasks Firm real-time tasks Soft real-time tasks A hard real-time task is one that is constrained to produce its results within certain predefined time bounds. The system is considered to have failed whenever any of its hard real-time tasks does not produce its required results before the specified time bound. Every firm real-time task is associated with some predefined deadline before which it is required to produce its results. However, unlike a hard real-time task, even when a firm real-time task does not complete within its deadline, the system does not fail. The late results are merely discarded. Soft real-time tasks also have time bounds associated with them, However, unlike hard and firm real-time tasks, the timing constrains on soft real-time tasks are not expressed as absolute values. Instead, the constrains are expressed either in terms of the average response times required. Real-time operating system must be fast and predictable. Interrupt latency: Interrupt latency is the time difference between a device raising an interrupt and the corresponding handler being called. ISR duration: the time needed by an interrupt handler to execute. Scheduler latency: Scheduler latency is the amount of time that elapses between the interrupt service routine completing and the scheduling function being run. Scheduler duration: This is the time taken by the scheduler function to select the next task to run and context switch to it. Fixed-priority scheduling with real-time scheduling classes Memory locking POSIX message queues POSIX shared memory Real-time signals POSIX semaphores POSIX clocks and timers Asynchronous I/O (AIO) Monolithic Dual-kernel Microkernel