A class of device which performs mathematical operations and carries out logical comparisons like a normal human brain is called a computer. Earlier computers were mainly mechanical ones whereas the latest versions are completely electronic devices.

In this section we shall discuss a brief history of development of computer architecture (now it is referred to as hardware) from nineteenth century to present times.

Depending upon the mechanism of operation the computers are divided in to two categories;

i) Analog Computer

ii) Digital Computer

### Analog Computer:

The computers which solve different problems by representing different quantities as continuously varying physical quantities (such as electrical potential, fluid pressure, or mechanical motion) are called as Analog Computers. An analog system is set up according to initial conditions of a specific problem and then allowed to change freely. Answers to the problem are obtained by measuring the final variables in the analog model.

The earliest analog computers were special-purpose machines, such as the tide predictor developed in 1873 by William Thomson and a harmonic analyzer built by A.A. Michelson and S.W. Stratton in 1898.

These machines were capable of generating a sinusoidal motion. One can multiply constant factors by adjustment of fulcrums on levers. The components were added by means of springs to produce a resultant.

Another milestone in the development of the modern analog computer was the invention of the so-called differential analyzer in the early 1930s by Vinegar Bush, an American electrical engineer, and his colleagues. This machine was the first practical and reliable device, using mechanical integrators (gears of variable speed) to solve differential equations.

Present-day electronic analog computers operate by manipulating electric potential differences (voltages). Their basic component is an operational amplifier, a device whose output current is proportional to its input potential difference.

By causing this output current to flow through appropriate components, further potential differences are obtained, and a wide variety of mathematical operations, including inversion, summation, differentiation, and integration, can be carried out on them.

Analog computers are specially suitable for simulation of dynamic systems. Simulation is a process of carrying out an entire dynamic process without its real components. Such simulations may be conducted in real time or at greatly accelerated rates, thereby allowing experimentation by repeated runs with altered variables.