Real-time Software Design: A Guide for Microprocessor Systems.

Computers these days spend a fairly low fraction of their time computing.

In fact, the very word "computer" has become something of a misnomer.

In the American History museum of the Smithsonian Institute in Wash- ington, D.C., there is an exhibit of early computers.

Three features of these machines are striking. First, they are enormous, especially in com- parison to their capabilities.

The museum visitor who has just come from the Natural History building next door may be reminded of fossilized di- nosaur bones.

Second, they don't look at all like modern computing ma- chines.

The cases are made of crude metal or beautifully worked wood, recalling an approach to the design of scientific apparatus which belongs to a previous generation.

Lastly, the function of these machines is mainly to compute-to perform rapid arithmetic.

The computer of today bears little resemblance in size, form, or function to its ancestors.

It is, most obviously, smaller by several orders of mag- nitude.

Its form has changed from the carefully crafted one-of-a-kind in- strument to the mass-produced microchip.

But the change in its function is perhaps the most dramatic of all.

Instead of being a computing engine, it is a machine for the processing of information.

The word "processor" has come into common usage.

A processor used to be a central processing unit-a set of wires and vacuum tubes, or later a set of printed circuit boards-which was nestled deep within the computer.

Today a processor is an off-the-shelf component.