Computers always follow blind logic. Executing the same program always gives the same results. That’s what the computer books say. But is it true? My computer can appear to behave differently on different occasions. Why do you think that this might be so?

Question

What will be an ideal response?

Answer 1

In principle, you would think that computers are entirely predictable and you always get the same response to the same actions. In practice, clicking on an icon may sometimes work and sometimes cause a crash. Why?

Of course, it is true that basic computer operations always yield the same results using the same data; for example, if you perform the operation a = f(b,c) you will always get the same value of a for data b and c and operation f.

However, computers are complex systems and are not entirely synchronous; for example, the state of a signal from an external device may be sampled (read) and it may be read as a 1 or 0, depending at which instant it is sampled. In a sophisticated system with memory management, data may sometimes be in memory and sometimes on disk. If data is read from cache it may be available in 1 clock cycle, getting it from main store may take 50 cycles before it is available. If the data is on disk, it may take more than 20 million clock cycles to retrieve it. Whether data is immediately available or requires a very significant wait is dependent on the current job load. The interaction between individual jobs, asynchronous events such as interrupts and data transmission, means that it is difficult to predict the operation of a computer in many circumstances.

Note that, under certain circumstances, the sampling of digital signals can lead to transitory, random errors called glitches.

The study of systems that require guaranteed behavior (industrial process controls, fly?by?wire aircraft, and nuclear reactors) is a branch of computing called real?time systems. In real?time systems the hardware and software are constructed to take account of the effect of the problems we have highlighted and to minimize them. In the 1980s a processor called the Viper was designed in the UK (sponsored by the UK Ministry of Defence) for applications that required predictability.