Chapter 19: Real-Time SystemsSlide 2ObjectivesOverview of Real-Time SystemsDefinitionsSystem CharacteristicsCharacteristics - 1Characteristics - 2Bus-Oriented SystemCharacteristics - 3Features of Real-Time KernelsMemory Mapping SchemesAddress TranslationImplementing Real-Time Operating Systems1. Priority-Based Scheduling2. Preemptive Kernels2. Preemptive Kernels - 23. Minimizing LatencyMinimizing Latency – continuedInterrupt LatencyDispatch LatencyReal-Time CPU SchedulingReal-Time CPU Scheduling - moreSlide 24Preface to Scheduling AlgorithmsScheduling of tasks when P2 has a higher priority than P1Rate Monotonic SchedulingEarliest Deadline First SchedulingSlide 29End of Chapter 19Chapter 19: Real-Time SystemsChapter 19: Real-Time Systems19.2Silberschatz, Galvin and Gagne ©2005Operating System ConceptsChapter 19: Real-Time SystemsChapter 19: Real-Time SystemsOverview and IntroductionSystem CharacteristicsFeatures of Real-Time SystemsImplementing Real-Time Operating SystemsReal-Time CPU Scheduling19.3Silberschatz, Galvin and Gagne ©2005Operating System ConceptsObjectivesObjectivesTo explain the timing requirements of real-time systemsTo distinguish between hard and soft real-time systemsTo discuss the defining characteristics of real-time systemsTo describe scheduling algorithms for hard real-time systems19.4Silberschatz, Galvin and Gagne ©2005Operating System ConceptsOverview of Real-Time SystemsOverview of Real-Time SystemsThe differences between real time computing systems and general purpose computing systems are very profound.We will examine many of these differences in the upcoming slides. A real time computing system is one that requires that correct results be produced within specified deadline periods. (Deadline is a key word)Results produced after a specific time period has elapsed may well be of absolutely no value and may mean loss of life or an aircraft crash; yet other failures might not be quite as disastrous if real time system is not quite as responsive. . Real time: Run on wide range of computer hardware; Used in many different kinds of applications. Some real time systems are embedded in aircraft instrumentation:your microwave, cell phone, cruise control, and hosts of other applications.Are often part of a larger system;Oftentimes their presence is not obvious to a user.19.5Silberschatz, Galvin and Gagne ©2005Operating System ConceptsDefinitionsDefinitionsA real-time system requires results produced within a specified deadline period.A hard real time system has stringent requirements, guaranteeing that critical real-time tasks be completed within their deadlines.E.g. Safety-critical systems; health systems; etc. A soft real-time system is less restrictive and guarantees simply that a critical real-time task will receive priority over other tasks;Further, these tasks retain priority over other tasks until the tasks complete.An embedded system is a computing device that is part of a larger system (I.e. automobile, airliner.)A safety-critical system is a real-time system with catastrophic results in case of failure.Again, a hard real-time system guarantees that real-time tasks be completed within their required deadlines, while a soft real-time system provides priority of real-time tasks over non real-time tasks.19.6Silberschatz, Galvin and Gagne ©2005Operating System ConceptsSystem CharacteristicsSystem CharacteristicsWill look at both soft and hard real-time operating systems..Real time systems typically exhibit the following characteristics:Single purposeSmall sizeInexpensively mass-producedSpecific timing requirementsAnd, so many other rather unique features spring from these characteristics.These are the big four defining characteristics…Let’s look at those…19.7Silberschatz, Galvin and Gagne ©2005Operating System ConceptsCharacteristics - 1Characteristics - 1Single Purpose: Single purpose is typical: controlling anti-lock brakes; toaster, cell phone.This makes the operating system simple too, as many characteristics integral to general – purpose operating systems are not available or needed. Size:Often found in severely cramped space – but sufficient for operations.Examples: wrist watches, cell phones, toysThus, CPU processing power is minimalAmount of primary memory is also minimal.Architectures: Compare 32 / 64-bit architectures with 8/16 bit processors.Architecture: Compare several gigs of memory with < 1 MB memory.19.8Silberschatz, Galvin and Gagne ©2005Operating System ConceptsCharacteristics - 2Characteristics - 2CostTypically mass produced: as in microwave ovens, thermostats.Thus, real time microprocessors are often inexpensive Organization of real time systems are designed to minimize cost:To eliminate bus architectures, the physical organization for embedded controllers are often organized as a system-on-a-chip, which has all necessary interconnections.Chip includes memory, cache, a MMU (for possible address translation), any peripheral ports necessary - all in a single integrated circuit.Such organization is typically much less expensive than typical bus-oriented architectures.19.9Silberschatz, Galvin and Gagne ©2005Operating System ConceptsBus-Oriented SystemBus-Oriented System19.10Silberschatz, Galvin and Gagne ©2005Operating System ConceptsCharacteristics - 3Characteristics - 3Timing: This is the feature that impacts almost everything else and makes real time systems what they really are!Both ‘hard’ and ‘soft’ real time systems have timing requirements.So, we will need to develop real time scheduling algorithms that provide priority to the highest scheduled processes.Schedulers absolutely must ensure that the priority of real time tasks does not degrade over time.So, we must minimize the response time to interrupts – recognize the interrupt, save context, transfer control to the handler, etc… as one can easily imagine.19.11Silberschatz, Galvin and Gagne ©2005Operating System ConceptsFeatures of Real-Time KernelsFeatures of Real-Time KernelsMost real-time systems do not provide features found in desktop systems.Simply not needed.Do not need (in general) support forA variety of peripheral devices – graphical displays, CD, DVD drives…Protection and security mechanismSupport for multiple