How does the computer work ? Structure of the Computer :
PREFACE :
The field of use of computers is very vast and their possibilities are extraordinary : they can indeed see, read, hear, speak, direct missiles, play chess, order machine tools, learn and compose music. ...
All this can, in a certain aspect, frighten us ; even the funny stories that are told about computers reveal in general a fear of the unknown, a state of inferiority to distant intelligence and perhaps superior to ours, the fear of competition and to be replaced in his own work by the microcomputer.
The fear of some electronics technicians is to be overwhelmed by a technology they do not fully understand. In fact, we often hear the term "brain" instead of computer and boast about it.
If we talk about the brain, it makes sense to think that it is intelligent ; but an electronic brain similar to ours does not exist.
A computer is not a brain and does not have intelligence.
As you move through the lessons, you will see that a computer is only doing things for which it has been programmed. It can, however, increase, facilitate and speed up human capabilities.
It is a tool that man can use to improve his comfort and save time if he knows how to use it.
The power of computers goes far beyond that of other devices because they run very complex programs created by humans.
If one examines a computer in detail, one finds that it recognizes little instructions ; it can perform in particular the following tasks :
mathematical calculations,
storage and selection of large amounts of information,
evaluation of the best solution of a problem.
The set of instructions constitutes the program ; it can be changed easily and quickly, but it can take a long time to prepare.
The computer is a programmable machine capable of performing successive or simultaneous tasks. Moreover, being composed of different parts, it can be extended or modified so that it can be adapted to different tasks.
It is very rare that computers make mistakes and some are designed to detect this possibility.
Often, the errors are due to the fact that the program contains some erroneous instructions.
Ultimately, the computer is only a programmable electronic data processing system.
If we look inside a computer, we realize that it consists of electronic elements that are now familiar to us : printed circuits, connectors, integrated circuits, resistors, capacitors, wires, switches, LEDs. ...
But within these integrated circuits, there are tens of thousands of transistors and the whole works at a very high speed : hundreds of thousands of elementary operations per second. Computers can produce a large amount of work in a much shorter time than is necessary to perform the same tasks using any other means.
The power of the computer lies mainly in its speed of processing and execution of data and that is why the man uses it.
We saw in the previous lesson the general structure of a computer and that the operation of it is based on a program. This last point is the fundamental characteristic of all computers. (See the 1st theoretical lesson).
VON NEUMANN realized that the problem was to make sure that the machine knew, only, which operation to execute after finishing the previous one. Thus, he had got the calculations done in automatic succession, according to a pre-established program.
VON NEUMANN machine consists of a fast executor of elementary operations and an organ which has the sequence according to which the operations must be executed ; it keeps in memory the values of the numbers which enter the operations and their results.
This machine was a marvelous invention so much so that even today, computers are made on the same principle.
They work in a so-called sequential manner by the fact that the calculations are executed in sequence. But in this respect, they are already showing their limits because the masses of data they process require processing speeds that are greater and greater than electronic circuits can provide.
At the moment, we are trying to build computer networks, each of which performs a part of the work at the same time.
This new system is known as a parallel process system. It is already adopted for scientific calculations and it will take years for it to be used for ordinary applications but when that happens, the computing systems will have achieved a huge increase in capacity.
In the previous lesson, the architecture of a microcomputer was examined ; let us now see in detail its internal composition.
In Figure 1, is reproduced the block diagram of a computer. This diagram is still very abstract but then, details will enrich it until highlighting the circuits of a microprocessor.
We have already talked about the input and output units that we will now examine in more detail.
2. 1. - UNITÉ D'ENTRÉE (INPUT UNIT)
It consists of devices capable of introducing data or instructions into the computer and can be considered as consisting of two parts (Figure 2).
The first part is represented by a keyboard or by other transducer devices that transform a movement or an external state (temperature, pressure, position ...) into a binary electrical signal.
The second part is generally composed of sequential and combinatorial circuits whose function is :
to temporarily store the data coming from outside,
to manage the control signals for the external organs,
to memorize the conditions in which the external device is located (for example if it is powered, if it has correctly received an order ...).
The input unit performs two main tasks :
introduce data and programs to predispose the computer,
enable the computer to communicate with devices that are not generally compatible with it. In this case, the input unit takes the interface name.
On individual computers, programs and data are introduced by means of keyboards practically similar to those of typewriters or, in the simplest cases, to those of pocket calculators. Each key activates a momentary closing switch ; other types of two-position switches may give non-momentary commands.
In some types of computers with interfaces, signal transducers are used. For example, the results obtained by the computer can be displayed on a screen. Often, some interfaces are already part of the computer while others are options.
2. 2. - UNITÉ DE SORTIE (OUTPUT UNIT)
It is used to convey data (information) outside the computer. It too, like the input unit, can be imagined composed of two parts (Figure 3).
The most common way for the computer to communicate with the user is the video screen (or video monitor) on which numbers, images, and graphics appear. This is a screen similar to that of a television set ; sometimes even using appropriate interfaces, a television can be used as a video monitor.
In case of lack of video screen, the computer can be equipped with a display (display) generally LED.
When the computer is called to communicate with other machines, for example to control certain mechanical work, it is equipped with actuators.
The actuator is a device capable of performing certain maneuvers in the place of the man. Counters, solenoid valves and auxiliary motors for executing orders are typical actuators.
2. 3. - LA MÉMOIRE (MEMORY)
The memory serves to preserve the programs and the data as well as the results of the operations carried out by the computer. In an elaborate system, there are usually two types of memories :
a main memory, internal to the computer, generally consisting of semiconductor components,
a secondary or mass memory, external to the computer, represented by magnetic disks or tapes.
The memories are characterized by the access to information mode :
Random Access Memories (RAM) in which information can be accessed at any time ; semiconductor memories are of this type.
sequential access memories (Sequential Access Memories) in which the desired information is accessible only after having reviewed all those preceding it ; the magnetic stripe memories are of this category.
The central memory of the computer is constituted by a set of locations in which the binary information (bits or words) is stored.
The information can be a datum, an instruction or a part of it, as will be seen below.
Each piece of information is formed by a group of bits of fixed length.
We have the following possibilities :
from 1 to 4 bits : word used by systems for numerical control,
8 bits (byte) : word used by a large part of the microcomputers,
16 bits: word used by high end microcomputers,
32 or 64 bits: word used by large professional computers.
Note:
The American word Byte, which is often referred to as a byte (8 bits), actually means "word" regardless of the number of bits of this word.
The memory is defined by an address (number representing the location in the memory space) and data which is the content of the information at the address in question.
L'Unité
Centrale de
calcul (UC),
Central
Processing
Unit en anglais
(CPU) est le cœur de
l'ordinateur. Elle est composée essentiellement de deux parties : l'Unité
Arithmétique et
Logique
(UAL,
en anglais Arithmetic and
Logic
Unit ou
ALU) et l'Unité de
Commande (Figure 5).
The CPU of microcomputers and personal computers is integrated into a single device : the microprocessor.
Further on, we will see how the UC works and how it is formed in turn by other elements (registers and counters).
Outside the CPU is the clock (CLOCK) : it is the pulse generator that gives the rhythm to all the circuits of the computer.
The UAL performs arithmetic operations (sums, subtractions) and logical operations (AND, NAND, OR, EX-OR and so on).
The cadence of the clock is determined in such a way that the UAL works to the maximum of its possibilities, that is to say in order to make the best use of its speed.
The speed of the UAL qualifies the performance of the machine ; for example, a multiplication is executed in a time of between one and ten thousandths of a second by a microcomputer, in a millionth of a second by a very powerful mini-computer and in a shorter time by the scientific computers of professional type.
The control unit manages the operation of the UAL as well as the exchange of data and instructions with the memory.
2. 5. - POWER SUPPLY VOLTAGE
In the diagram of Figure 1, this module does not appear but all computers need a power supply that gives them the necessary power.
Sometimes the power is embedded in the computer, sometimes it is separated.
It may also be necessary to increase the power of the power supply to increase the capacity of a computer by grafting other components such as a group of memories or interfaces.
Large computers require stabilizing circuits that compensate for fluctuations in the voltage of the network responsible for damage (destruction of programs in progress).
In some cases, an emergency power supply, using batteries, intervenes automatically in the event of a mains voltage failure ; the computer can then continue the running of a program without losing the information stored in memory.
How does the computer work ?
Click here for the next lesson or in the summary provided for this purpose.
Top of page
Next Page 