CS 501: Software Engineering Fall 1999 Lecture 25 Real Time Systems

Administration  Examination schedule

Real Time System A real time system is a software system whose correct functioning depends upon the results produced and the time at which they are produced.  A soft real time system is degraded if the results are not produced within required time constraints  A hard real time system fails if the results are not produced within required time constraints

Embedded Systems Software and hardware are combined to provide an integrated unit, usually dedicated to a specific task  Digital telephone  Automobile engine control  GPS  Scientific instruments The software may be embedded in the device in a manner that can not be altered after manufacture

Autonomous Land Vehicle GPS Steer Sonar Model Control signals Throttle Laser Controls Sensors Signal processing

Other Applications Response critical  Network router  Telephone switch  Seat bag controller Shared systems  Multi-user data processing  Time sharing

Techniques  Special purpose hardware  Multi-threading and multi-tasking  Parallel processing => digital signal processing  Interrupts => levels and priorities

Multi Threading Several similar threads operating concurrently  Re-entrant code -- separation of pure code from data for each thread  Testing -- single thread and multi thread May be real time (e.g., telephone switch) or non-time critical

Real Time Executive Schedules and dispatches tasks in a real time system  Real time clock  Interrupt handler  Scheduler  Resource manager  Dispatcher Must be extremely reliable

Timing Timing mechanisms  Synchronous (clocked) -- periodic stimuli  Asynchronous -- wait for next signal Example: Communications protocols may be synchronous or asynchronous

Hardware v. Software Design of embedded systems requires close understanding of hardware characteristics  Special purpose hardware requires special tools and expertise.  Some functions may be implemented in either hardware of software (e.g., floating point unit)  Design requires separation of functions Distinction between hardware and software may be blurred

Dartmouth Time Shared System Central processor Communications processor I/O Mulitplexor Central processor Communications processor Central processor

Software Considerations Resource considerations may dictate software design and implementation:  Low level language (e.g., C) where programmer has close link to machine  Inter-process communication may be too slow (e.g., C fork).  May implement special buffering, etc., to control timings

Example: CD Controller 4 3 1 Input block 5 2 Output block 6 7 Circular buffer

Continuous Operation Many embedded systems must operate continuously  Software update while operating  Hardware monitoring and repair  Alternative power supplies, networks, etc.  Remote operation These functions must be designed into the fundamental architecture

Routers and Other Network Computing  Interoperation with third party devices  Support for several versions of protocols  Restart after total failure  Defensive programming -- must survive => erroneous or malicious messages => extreme loads  Time outs, dropped packets, etc.  Evolution of network systems

Software Development Testing and debugging need special tools and environments  Debuggers, etc., can not be used to test real time performance  Simulation of environment may be needed to test interfaces -- e.g., adjustable clock speed  General purpose tools may not be available

Software Engineering for Real Time The special characteristics of real time computing require extra attention to good software engineering principles:  Requirements analysis and specification  Development of tools  Modular design  Exhaustive testing Heroic programming will fail!

Reading Before the examination, read Sommerville Chapters 25, 26 and 27