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

2. Administration
 Examination schedule

3. 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

4. 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

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

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

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

8. 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

9. 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

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

11. 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

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

13. 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

14. Example: CD Controller4 3 1
Input block 5 2
Output block 6 7
Circular buffer

15. 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

16. 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

17. 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

18. 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!

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