SYBSC IT SEM IV EMBEDDED SYSTEMS UNIT V Real Time Operating System (RTOS)
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The document compares general-purpose operating systems (GPOS) and real-time operating systems (RTOS), highlighting that RTOS are time-critical and must meet strict timing requirements. It explains task states, task scheduling, context switching, and task synchronization as foundational concepts in operating systems. Additionally, it outlines characteristics of RTOS, including determinism and interrupt latency, and discusses considerations for selecting an appropriate operating system based on project needs.
SYBSC IT SEM IV EMBEDDED SYSTEMS UNIT V Real Time Operating System (RTOS)
- 1. Real Time Operating System (RTOS) UNIT V
- 2. TYPES OF OS • GPOS (Normal General Purpose Operating System) • RTOS (Real Time Operating System)
- 3. Difference between a GPOS (Normal General Purpose Operating System) and an RTOS (Real Time Operating System) • The basic difference of using a GPOS or an RTOS lies in the nature of the system – i.e whether the system is “time critical” or not! • A system can be of a single purpose or multiple purpose. • Example of a “time critical system” is – Automated Teller Machines (ATM). • Here an ATM card user is supposed to get his money from the teller machine within 4 or 5 seconds from the moment he press the confirmation button. The card user will not wait 5 minutes at the ATM after he pressed the confirm button. • So an ATM is a time critical system. • Where as a personal computer (PC) is not a time critical system. • The purpose of a PC is multiple. A user can run many applications at the same time. After pressing the SAVE button of a finished document, there is no particular time limit that the doc should be saved within 5 seconds. It may take several minutes (in some cases) depending upon the number of tasks and processes running in parallel.
- 4. BASICS: Tasks • Task is a piece of code or program that is separate from another task and can be executed independently of the other tasks. • In embedded systems, the operating system has to deal with a limited number of tasks depending on the functionality to be implemented in the embedded system. • Multiple tasks are not executed at the same time instead they are executed in pseudo parallel i.e. the tasks execute in turns as the use the processor.
- 5. BASICS: Task States • Depending upon execution or not a task may be classified into the following three states: • Running state - Only one task can actually be using the processor at a given time that task is said to be the “running” task and its state is “running state”. No other task can be in that same state at the same time • Ready state - Tasks that are are not currently using the processor but are ready to run are in the “ready” state. There may be a queue of tasks in the ready state. • Waiting state - Tasks that are neither in running nor ready state but that are waiting for some event external to themselves to occur before the can go for execution on are in the “waiting” state.
- 6. BASICS: SCHEDULER • The heart and soul of any operating system is its scheduler. • This is the piece of the operating system that decides which of the ready tasks has the right to use the processor at a given time. • It simple checks to see if the running task is the highest priority ready task. • Some of the more common scheduling algorithms: – First-in-first-out – First-in-first-out (FIFO) scheduling describes an operating system which is not a multitasking operating system. – Each task runs until it is finished, and only after that is the next task started on a first come first served basis. – Shortest job first – Shortest job first scheduling uses algorithms that will select always select a task that will require the least amount of processor time to complete. – Round robin. – Round robin – scheduling uses algorithms that allow every task to execute for a fixed amount to time. – A running task is interrupted an put to a waiting state if its execution time expires.
- 7. Basics:CONTEXT SWITCH • The actual process of changing from one task to another is called Context Switch. • Since contexts are processor-specific, so is the code that implements the context switches, hence, it must always be written in assembly language.
- 8. Basics: TASK SYNCHRONIZATION • All the tasks in the multitasking operating systems work together to solve a larger problem and to synchronize their activities, they occasionally communicate with one another. • For example, in the printer sharing device the printer task doesn’t have any work to do until new data is supplied to it by one of the computer tasks. • So the printer and the computer tasks must communicate with one another to coordinate their access to common data buffers.
- 9. REAL TIME CHARACTERISTIC • An Operating system is called “Real-Time Operating System” (RTOS) only if it has following characteristics: • Deterministic • An OS is said to be deterministic if the worst case execution time of each of the system calls is calculable. • The data sheet of an OS should publish the real-time behavior of its RTOS provides average, minimum and maximum number of clock cycles required by each system call. • Interrupt Latency • Interrupt Latency is the total length of time from an interrupt signal’s arrival at the processor to the start of the associated interrupt service routine. • Context Switch • Context Switch is important because it represents overhead across your entire system.
- 10. SELECTION PROCESS • The process of selecting the best commercial operating system that best fits the needs of one’s project depends on various factors. • Commercial operating systems form a continuum of functionality, performance and price. • Operating Systems that offer only a basic scheduler and a few other system calls are inexpensive and come with the source code that one can modify and do not require payment of royalties. • While on the other hand operating systems that include a lot of useful functionality beyond just the scheduler are quite expensive and royalties due on every copy shipped in ROM and they might also make a stronger guarantees about real-time performance. • Two important points to be considered while selecting an operating system :- – Put your processor, real time performance and budgetary requirements first. – Contact all of the vendors of the remaining operating systems for more detailed technical information.
- 13. THANK YOU!