How Computers Store Data

'— CONCEPT: All data that is stored in a computer is converted to sequences of 0s and Is.

A computer's memory is divided into tiny storage locations known as bytes. One byte is only enough memory to store a letter of the alphabet or a small number. In order to do anything meaningful, a computer has to have lots of bytes. Most computers today have millions, or even billions, of bytes of memory.

Each byte is divided into eight smaller storage locations known as bits. The term bit stands for binary digit. Computer scientists usually think of bits as tiny switches that can be either on or off. Bits aren't actual "switches," however, at least not in the conventional sense. In most computer systems, bits are tiny electrical components that can hold either a positive or a negative charge. Computer scientists think of a positive charge as a switch in the on position, and a negative charge as a switch in the off position. Figure 1-7 shows the way that a computer scientist might think of a byte of memory: as a collection of switches that are each flipped to either the on or off position.

Figure 1-7 Think of a byte as eight switches

Figure 1-7 Think of a byte as eight switches

OFF " ' OFF OFF OFF

When a piece of data is stored in a byte, the computer sets the eight bits to an on/off pattern that represents the data. For example, the pattern shown on the left in Figure 1-8 shows how the number 77 would be stored in a byte, and the pattern on the right shows how the letter A would be stored in a byte. We explain below how these patterns are determined.

Figure 1 -8 Bit patterns for the number 77 and the letter A

Figure 1 -8 Bit patterns for the number 77 and the letter A

The number 77 stored in a byte. The letter A stored in a byte.

Storing Numbers

A bit can be used in a very limited way to represent numbers. Depending on whether the bit is turned on or off, it can represent one of two different values. In computer systems, a bit that is turned off represents the number 0 and a bit that is turned on represents the number 1. This corresponds perfectly to the binary numbering system. In the binary numbering system (or binary, as it is usually called) all numeric values are written as sequences of Os and Is. Here is an example of a number that is written in binary:

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