Computer operations and the structure of information.

Abstract

The first step in understanding computer operations is to establish an accurate image of what a computer is and what it does. Physically a computer is some boxes of electronics connected by wires; they consume cards and paper tapes, draw, print, punch, wind magnetic tapes, spin drums and disks, flash lights, and make a certain amount of noise. However, this image may not be very useful in terms of what a computer does. In more abstract and general terms the computer is an automatic logic device which processes information at unimaginable speeds and with remarkable precision. Its capabilities are very flexible, particular jobs being specified as programs of any necessary complexity, and provided that the information which it processes has been reduced to the form of discrete symbols, which means letters, numbers, punctuation marks, or binary digits, it can in principle do with data anything which can be done with data. This of course is an awe-inspiring claim liable to daunt the irresolute so this general image ofcomputer functions may not be more useful than the physical image. Therefore we may prefer a more homely alternative and describe the capabilities of the various boxes in terms of the three Rs; most of their operations can indeed be considered to be aspects of reading, writing, and arithmetic. However, a little care is needed in the use of each term. First our 'arithmetic' must include some logic, in particular the evaluation of whether a statement is true or false, so that it may serve as the basis of decisions. Second we should realize that reading and writing, in computer terms, are simply aspects of copying from one place to another and that in fact nothing is ever written without being read, or read without being written. Conventionally, the programmer imagines himself to be sitting in the box known as the 'processor' and 'reading' is the term applied to incoming copying and 'writing' the term applied to outgoing copying. This is different from the conventional imagery of using a desk calculator where we do not normally think of ourselves as reading with our fingers. 52 These are minor points unlikely in fact to confuse, but a more fundamental difference between the desk calculator and the computer is that, while the first executes each operation as it is specified (by pressing the button), the computer accepts and stores the whole sequence of instructions given to it, and it does not obey the first one until the last one has been accepted. Computers are sometimes described specifically as 'stored-program' machines. However, once it does begin to obey them, it obeys them in order without pause, except when one of the instructions says that it should jump onwards and skip a bit of program, or jump backwards and do some of it again. The data are accepted by the machine after the program has been started, and the order in which items of data are read, calculations performed, and storage, or print, operations carried out is a matter for the programmer. He specifies these instructions in his program and also the format in which the data are to be expected and the results laid out.