How Do Computers Compute and Work?

About how older computers compute or functions, information about the science and process.

How Do Computers Compute?

There are 2 kinds of computers: analog and digital. Analog computers represent numbers by a physical quantity such as length, angle, or magnitude of electric voltage. The accuracy of analog computers is limited by the precision with which such physical quantities can be measured. Digital computers, on the other hand, represent numbers by separate things such as pebbles, or pulses, of electric current. A person counting on his fingers is really using the simplest of a digital computers. Thus, the accuracy of a digital computer is not limited by the accuracy of measurement, but only by the number of digit-representing elements built into the machine. Digital computers are described in the following discussion because of their great importance in the fields of science, business, and industry.

A complete understanding of how digital computers work is beyond the scope of this book, but it is possible to describe the general principles involved. When you and I do a problem in arithmetic, such as addition or multiplication, we go through several steps:

1. We write down, or store, numbers as necessary.

2. We look them up (retrieve them) as they are needed for subsequent steps.

3. We control the entire operation by performing each step in the required order.

A computer also performs the operations of storage and retrieval of information, and control of each step in the operation.

Imaging for a moment the series of operations you might follow in doing the following simple addition:




Step 1

Add the 2 units figures.


Step 2

Write the units figure of the above sum as the units figure of the answer.

--2 (answer)

A computer follows much the same routine in solving its problems, no matter how complicated they may be. The instructions are stored in the memory unit. They are "read" by the control unit, which directs the arithmetic unit to do the steps specified. The result of each step is stored in the memory unit. When a previously stored number is called for, the control unit transfers the number from the memory unit to the arithmetic unit. Thus, 3 main functions are performed: storage and retrieval of instructions and numbers, processing of numbers by arithmetic, and control of these operations by a program of instructions.

Step 3

Carry the 10s figures of the sum of step 1.

carry 1

Step 4

Add the carried figure to the two 10s figures of the problem.


Step 5

Write the units figure of the sum of step 4 as the 10s figure of the answer.

-32 (answer)

Step 6

Write the 10s figure of the sum of step 4 as the 100s figure of the answer.

132 (answer)

The required program of instructions must be available before the computation begins. When we solve a problem in arithmetic or algebra, we may not always be aware that a program has been prepared in advance. Nevertheless, stored in our memory are the rules and algorithms used to solve typical problems. If we do happen to get stuck, we merely go to a book and refresh our memory. That's where we have it over the computer. When a computer gets stuck, all it can do is seek human help!

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