Chapter 19 Real Time Systems Chapter 19 Real Time Systems Overview and Introduction System Characteristics Features of Real Time Systems Implementing Real Time Operating Systems Real Time CPU Scheduling Operating System Concepts 19 2 Silberschatz Galvin and Gagne 2005 Objectives 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 systems Operating System Concepts 19 3 Silberschatz Galvin and Gagne 2005 Overview 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 Operating System Concepts 19 4 Silberschatz Galvin and Gagne 2005 Definitions 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 Operating System Concepts 19 5 Silberschatz Galvin and Gagne 2005 System 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 Operating System Concepts 19 6 Silberschatz Galvin and Gagne 2005 Characteristics 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 Operating System Concepts 19 7 Silberschatz Galvin and Gagne 2005 Characteristics 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 busoriented architectures Operating System Concepts 19 8 Silberschatz Galvin and Gagne 2005 Bus Oriented System Operating System Concepts 19 9 Silberschatz Galvin and Gagne 2005 Characteristics 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 Operating System Concepts 19 10 Silberschatz Galvin and Gagne 2005 Features 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 users Note Windows XP has 40 000 000 lines of code a typical real time operating system usually is written in thousands of source code lines Reasons include Real time systems are typically single purpose Real time systems often do not require interfacing with a user General purpose features often require more substantial hardware than that found in a RT system Operating System Concepts 19 11 Silberschatz Galvin and Gagne 2005 Memory Mapping Schemes 1 Real addressing mode where programs generate actual addresses But there is no memory protection between processes We might also need to specify exactly where in memory the program is to be loaded But the speed is very difficult to beat No time spent in address translations Very commonly found in real time systems having hard real time constraints Some real time operating systems running on microprocessors containing a MMU actually disable the MMU to gain performance benefit in referencing physical addresses directly 2 Relocation register mode In this scheme we have a relocation register is set to the process load point Physical addresses are added to contents of the relation register formed by adding logical L to R relocatable to form P Physical addresses But here again there is no protection between processes 3 Implementing full virtual