Blog entry by AMELIA SAHIRA RAHMA 5116201024

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Real Time Operating Systems

The execution platform for most application systems is an operating system that manages shared resources and provides features such as a file system, run-time process management, etc. However, the extensive functionality in a conventional operating system takes up a great deal of space and slows down the operation of programs. Furthermore, the process management features in the system may not be designed to allow fine-grain control over the scheduling of processes.

For these reasons, standard operating systems, such as Linux and Windows, are not normally used as the execution platform for real-time systems. Very simple embedded systems may be implemented as ‘bare metal’ systems. The systems themselves include system startup and shutdown, process and resource management, and process scheduling. More commonly, however, embedded applications are built on top of a real time operating system (RTOS), which is an efficient operating system that offers the features needed by real-time systems. Examples of RTOS are Windows/CE, Vxworks, and RTLinux.



Figure 1


A real-time operating system manages processes and resource allocation for a realtime system. It starts and stops processes so that stimuli can be handled and allocates memory and processor resources. The components of an RTOS (Figure 1) depend on the size and complexity of the real-time system being developed. For all except the simplest systems, they usually include :

  1. A real-time clock, which provides the information required to schedule processes periodically.
  2. An interrupt handler, which manages aperiodic requests for service.
  3. A scheduler, which is responsible for examining the processes that can be executed and choosing one of these for execution.
  4. A resource manager, which allocates appropriate memory and processorresources to processes that have been scheduled for execution.
  5. A dispatcher, which is responsible for starting the execution of processes.

Real-time operating systems for large systems, such as process control or telecommunication systems, may have additional facilities, namely disk storage management, fault management facilities that detect and report system faults, and a configuration manager that supports the dynamic reconfiguration of real-time applications.


Process Management

Real-time systems have to handle external events quickly and, in some cases, meet deadlines for processing these events. This means that the event-handling processes must be scheduled for execution in time to detect the event. They must also be allocated sufficient processor resources to meet their deadline. The process manager in an RTOS is responsible for choosing processes for execution, allocating processor and memory resources, and starting and stopping process execution on a processor.

The process manager has to manage processes with different priorities. For some stimuli, such as those associated with certain exceptional events, it is essential that their processing should be completed within the specified time limits. Other processes may be safely delayed if a more critical process requires service. Consequently, the RTOS has to be able to manage at least two priority levels for system processes :

  1. Interrupt level This is the highest priority level. It is allocated to processes that need a very fast response. One of these processes will be the real-time clock process.
  2. Clock level This level of priority is allocated to periodic processes.

There may be a further priority level allocated to background processes (such as a self checking process) that do not need to meet real-time deadlines. These processes are scheduled for execution when processor capacity is available.

Within each of these priority levels, different classes of process may be allocated different priorities. For example, there may be several interrupt lines. An interrupt from a very fast device may have to pre-empt processing of an interrupt from a slower device to avoid information loss. The allocation of process priorities so that all processes are serviced in time usually requires extensive analysis and simulation.

Periodic processes are processes that must be executed at specified time intervals for data acquisition and actuator control. In most real-time systems, there will be several types of periodic process. Using the timing requirements specified in the application program, the RTOS arranges the execution of periodic processes so that they can all meet their deadlines.



Figure 2


The actions taken by the operating system for periodic process management are shown in Figure 2. The scheduler examines the list of periodic processes and selects a process to be executed. The choice depends on the process priority, the process periods, the expected execution times, and the deadlines of the ready processes. Sometimes, two processes with different deadlines should be executed at the same clock tick. In such a situation, one process must be delayed. Normally, the system will choose to delay the process with the longest deadline.

Processes that have to respond quickly to asynchronous events may be interruptdriven. The computer’s interrupt mechanism causes control to transfer to a predetermined memory location. This location contains an instruction to jump to a simple and fast interrupt service routine. The service routine disables further interrupts to avoid being interrupted itself. It then discovers the cause of the interrupt and initiates, with a high priority, a process to handle the stimulus causing the interrupt. In some high-speed data acquisition systems, the interrupt handler saves the data that the interrupt signaled was available in a buffer for later processing. Interrupts are then enabled again and control is returned to the operating system.

At any one time, there may be several processes, all with different priorities, that could be executed. The process scheduler implements system-scheduling policies that determine the order of process execution. There are two commonly used scheduling strategies :

  1. Non-pre-emptive scheduling Once a process has been scheduled for execution it runs to completion or until it is blocked for some reason, such as waiting for input. This can cause problems, however, when there are processes with different priorities and a high-priority process has to wait for a low-priority process to finish.
  2. Pre-emptive scheduling The execution of an executing process may be stopped if a higher-priority process requires service. The higher-priority process preempts the execution of the lower-priority process and is allocated to a processor.

Within these strategies, different scheduling algorithms have been developed. These include round-robin scheduling, where each process is executed in turn, rate monotonic scheduling, where the process with the shortest period (highest frequency) is given priority; and shortest deadline first scheduling, where the process in the queue with the shortest deadline is scheduled (Burns and Wellings, 2009).

Information about the process to be executed is passed to the resource manager. The resource manager allocates memory and, in a multiprocessor system, also adds a processor to this process. The process is then placed on the ‘ready list’, a list of processes that are ready for execution. When a processor finishes executing a process and becomes available, the dispatcher is invoked. It scans the ready list to find a process that can be executed on the available processor and starts its execution.

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