“A real-time system is a computer system in which the correctness of the system behavior depends not only on the logical results of the computation, but also on. Filesystems/Stable storage. We will focus on concurrence and real-time issues. First, a little history. Batch Operating Systems. ▫. Original computers ran in batch . Real-Time Operating Systems (RTOS) Real-Time System Characteristics. • A real-time system is a computer system which is required by its specification to.
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understand the basic features of such operating systems. In this chapter we present the fundamental concepts of a real-time operating system in a generic. Realtime Operating Systems. Concepts and Implementation of Microkernels for Embedded Systems. Dr. Jürgen Sauermann, Melanie Thelen. 1. Today's topic: RTOS. 1. Why OS? ▫. To run a single program is easy. ▫. What to do when Real-time. ▫ synchronization. ▫. User control over OS policies. ▫.
Such mechanisms involve system calls, and usually invoke the OS's dispatcher code on exit, so they typically take hundreds of CPU instructions to execute, while masking interrupts may take as few as one instruction on some processors. A non-recursive mutex is either locked or unlocked. When a task has locked the mutex, all other tasks must wait for the mutex to be unlocked by its owner - the original thread.
A task may set a timeout on its wait for a mutex. There are several well-known problems with mutex based designs such as priority inversion and deadlocks.
In priority inversion a high priority task waits because a low priority task has a mutex, but the lower priority task is not given CPU time to finish its work. A typical solution is to have the task that owns a mutex at, or 'inherit,' the priority of the highest waiting task.
But this simple approach gets more complex when there are multiple levels of waiting: task A waits for a mutex locked by task B, which waits for a mutex locked by task C.
Handling multiple levels of inheritance causes other code to run in high priority context and thus can cause starvation of medium-priority threads. In a deadlock , two or more tasks lock mutex without timeouts and then wait forever for the other task's mutex, creating a cyclic dependency.
The simplest deadlock scenario occurs when two tasks alternately lock two mutex, but in the opposite order. Deadlock is prevented by careful design. Message passing[ edit ] The other approach to resource sharing is for tasks to send messages in an organized message passing scheme. In this paradigm, the resource is managed directly by only one task.
When another task wants to interrogate or manipulate the resource, it sends a message to the managing task. Although their real-time behavior is less crisp than semaphore systems, simple message-based systems avoid most protocol deadlock hazards, and are generally better-behaved than semaphore systems.
However, problems like those of semaphores are possible. Priority inversion can occur when a task is working on a low-priority message and ignores a higher-priority message or a message originating indirectly from a high priority task in its incoming message queue. Protocol deadlocks can occur when two or more tasks wait for each other to send response messages. Interrupt handlers and the scheduler[ edit ] Since an interrupt handler blocks the highest priority task from running, and since real-time operating systems are designed to keep thread latency to a minimum, interrupt handlers are typically kept as short as possible.
The interrupt handler defers all interaction with the hardware if possible; typically all that is necessary is to acknowledge or disable the interrupt so that it won't occur again when the interrupt handler returns and notify a task that work needs to be done. This can be done by unblocking a driver task through releasing a semaphore, setting a flag or sending a message.
A scheduler often provides the ability to unblock a task from interrupt handler context. An OS maintains catalogues of objects it manages such as threads, mutexes, memory, and so on. Updates to this catalogue must be strictly controlled. For this reason it can be problematic when an interrupt handler calls an OS function while the application is in the act of also doing so. The OS function called from an interrupt handler could find the object database to be in an inconsistent state because of the application's update.
There are two major approaches to deal with this problem: the unified architecture and the segmented architecture. To find all necessary details for message forming there is a set of totally application oriented chained tables.
If the dedicated RTOS is to be applied to another chemical process i.
One another exception handling may depend on application - handling of power failure. In some cases after power failure it is necessary to start the system from the very beginning. In computer control systems it is usually advisable to continue its work - i. In the RTOS under discussion the latter version is used. Other Means of Dedication Control system software is divided into two, core residential and disc residential, parts.
Core residential part is loaded using bootstrap loader.
Basic Concepts of Real Time Operating Systems
In addition to programs some more residential data and astronomical time are needed, activation of programs in proper order is also important for systems start up. To support these functions the loading system is modified. Considering specific and fixed nature of control system's data base fixed and small number of files, known structure of files, etc.
This is working well, but the authors are convinced of advantages of using general purpose file handling system whenever possible. It works on a two-processor computer of the two computer complex. The hardware configuration of the data logger consists of one CPU, 64 kB core memory, one fully dedicated processor with in-wired programs, one fix-head disc kB , display, two plotters, line printer, special process interface, and another fix-head disc and magnetic tapes in DMA channel.
Separate design is one reason for not using an application oriented scheduler. The other reason is that functions of the data logger are more concentrated and multitasking is not so intensive as in computer control system, consequently the need for a scheduler is also less. Inside of the RTOS the only dedication is in the exception handling system.
Exception Handling ConSidering the working conditions of the data logger and also user demands, the RTOS uses nonstandard handling of trap conditions compare with 3. When trap occurs the running process is not aborted.
Exception handling system registers the available information about trap condition and system continues its work. So far this procedure has been useful. Centralized system of forming exception messages for user programs had been implemented already when the RTOS was designed.
Because of that it was not necessary to include that system into RTOS.
Power failure handling system is the same as in the previous RTOS. Since it was impossible to use bootstrap loader for direct loading of application software, the loading system of the previous RTOS see 3. The result works as follows: after loading the dedicated RTOS by bootstrap loader, the user has a choice to automatically load and activate application software of the data logger or start any other work on the computer.
When activating Dedicated real-time operating systems the data logger, the RTOS also asks astronomical time and other initial data. Each computer system has a dialogue with its user. Quite often the volume and intensity of the dialogue is not so high as to provide a special terminal.
In such cases the authors propose to include the application bounded dialogue into the dialogue system of the RTOS.
Experiences with Dedicated Real-Time Operating Systems
The latter can be organized in a tree structure with core resident root and disc resident branches. Each branch implements one topic. To extend the dialogue it is necessary to add a new branch - core usage is not increasing, the response time is also fairly good.
It is important to design the whole complex at the same time in order to get better balance between different parts. A decision in the case of general-purpose versus dedicated RTOS must be made each time when designing a new application.
From the authors point of view, dedicated RTOS has the following advantages: - it provides better support for certain specific features of the application - since it usually supports the structure of application software the implementation of application part of the system becomes simpler - its use shortens implementation and testing time of the application system and facilitates the use of top-down technique - remarkable simplifications may be achieved in start up and restart procedures - maintenance of the real-time system is usually simpler.
Our experiences clearly show that most of the additional investments and extra work is needed for creation of the bank of RTOS modules and for obtaining the knowhow. Besides, a good dedicated RTOS is applicable to the class of applications, not to one single application.
As main body of this report shows there are not too many ways to dedicate the kernel. The opinion of the authors is that whenever possible, let the kernel remain unchanged, as it is specified in TC-8, or in the following reports. Although both of the described dedicated RTOSes were implemented on a minicomputer R , the authors have the feeling that dedicated RTOSes will be even more useful in microcomputer systems.
The authors wish to thank the operating system's team of Control System Department for valuable ideas and selfless work. Kaaramees, K. MBtus, V. Nurmela A real-time executive for a chemical process control system. MBtus, L. Hanson Multi-core architectures need special techniques for process management, memory management, and synchronization. RTOSs implementing the unified architecture solve the problem by simply disabling interrupts while the internal catalogue is updated.
Operating Systems. Springer-Verlag Telos, Santa Clara, In this paradigm, the resource is managed directly by only one task.
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