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Real-time operating systems

Real-time processing
When batch processing was discussed, you saw that one of the features of batch processing was that the output wasn't immediate. You may not get the output for a number of hours. In real-time processing, time is critical! The length of time a program takes is important, as is the way that the CPU deals with instructions. Real time is the most important thing (real-time and rapid-response processing are used interchangeably). Making sure that the CPU is used as efficiently as possible as with batch processing is not as important. Most real time applications can be found in booking systems or in control applications.

Introduction to real-time systems
With batch processing you don't get the output straight away. With real-time processing, an input is received and then processed. Any output generated is processed quickly enough to influence the system before the next input is received! You get the output 'immediately'.

Some examples of real-time systems
For example, in an air traffic control system, planes' details are displayed on a screen. When a plane has moved to a different position, the screen needs to be updated. If this didn't happen quickly enough, the screen would show a situation that didn't reflect the actual situation. The air traffic controller might then give the pilot instructions based on the inaccurate information displayed on the screen. This may result in an accident. When a missile is going towards a pre-programmed destination, it is constantly readjusting its aim because of wind conditions. It is feeding back into its processor information about its direction from input sensors, comparing that information with where it should be going and then making any necessary adjustments.

Booking systems
Real-time systems don't have to react with the speed of light, however! For example, if you buy a ticket for the theatre, the booking system will receive an input to say that you have booked a particular seat. It has to process the input and update the file that stores which seats are still available before the next ticket is sold. If the system weren't updated quickly enough then the same seat might be sold again. This is an example of a real-time system, even though the process of booking a ticket may take a few minutes. The reason it is a real-time system is that the master file (the file which holds details of which seats have been sold and which ones are free) is updated before the next seat is booked. Each booking affects the next one. Just to confuse things, the above example is also known as a 'transaction process' as well as a real-time process. This is because:

• • The operator enters some data to be processed from a transaction with a customer.
  • The system takes the transaction's data and processes it.
  • Whilst processing that transaction, all others are 'frozen' until that one is finished.
  • The master file of seats available is updated before the next transaction is processed.

It is a little harder to argue that the air traffic control or the missile guidance systems are examples of 'transaction processing', however, because there isn't any interaction between a customer and an operator taking place! Transaction processing can also be called 'interactive processing'. This is because there is an exchange of data (an interaction) between a customer and an operator.

To summarise, computer systems can be set up to process data in different ways or modes. Some systems can be designed to use batch processing. Other systems can be designed to use real-time processing, or transaction processing. The designer of any proposed system must look at the application and make a judgement about what to set up.

A process control system
A process control system is another example of a real-time system. It is a computer system that automatically monitors and reacts to changes in a process. There are many types of process that could be used as an example, from a greenhouse temperature and humidity control system, to a chemical reaction monitoring system, to an air-conditioning system that maintains a nice cool temperature in a hot computer room. Any processing system will typically have a number of common characteristics. We can summarise them as follows:

• • Inputs signals from sensors and transducers.
  • Some processing software, a program.
  • Some outputs, possibly using some actuators. (An actuator is simply any device that causes movement and can be controlled by a computer. Examples include computer-controlled motors and pumps).
  • Some feedback, to allow a comparison of the current situation with a target situation.

1. 1. When the system is started, the desired temperature is inputted into the system, perhaps using a keypad.
   2. A signal is sent to the heater and the tank heats up.
   3. The thermistor (a temperature measuring sensor) constantly sends a signal back to the computer. This is known as a 'feedback signal' because it is used to compare the current temperature with a target one.
   4. The process control program in the computer checks the current temperature against the desired temperature.
   5. If the desired temperature hasn't been reached yet, the heater is kept on. This process is continued until the desired temperature is reached or exceeded.
   6. When it is reached or exceeded, the heater is switched off. If the tank ever becomes too hot, then a pump (an actuator) is switched on. This pumps cool water around the tank.
   7. It is switched off when the temperature returns to an acceptable level.

Speed mismatch between humans and devices
We have come across the term 'speed mismatch' several times before in previous chapters when talking about slow peripherals working with fast CPUs. We can also talk about it here because there is a speed mismatch concern between a CPU and the users of a computer system. Computers work very quickly. Humans work very slowly by comparison.

Consider any control system. A computer may be able to read in information from sensors, process it and then take appropriate actions in fractions of a second. A human, on the other hand, may take seconds to read information displayed on a screen, seconds to think about it (process it) and then may take more time taking appropriate action, giving the equipment instructions. For some situations, the speed mismatch between the computer and the human may be a serious problem. For example, a pilot may put the safety of passengers at risk if the on-board anti-collision systems of their plane were a manual system. They would first of all have to realise there was a problem then decide what to do about it and finally take appropriate action. It would be far safer (quicker) to rely on the computer making the necessary adjustments to the flight path immediately a problem was detected. Manufacturing lines need adjustments! Temperatures, pressures, flows of liquids and so on all need to be constantly adjusted to take into account a constantly changing set of circumstances on the production lines. If these changes were all left to operators to do, there may well be a lot of products that don't pass quality control! It would take operators time to realise a change was required to the temperature in the first place, for example. They would then spend time deciding what to do about it. Finally, they would need time to make the necessary adjustments. The time spent making adjustments would reduce the number of quality products produced. If control of all the variables were left to the computer, adjustments could be done automatically with minimal time delay and maximum production.

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