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Computers

Useful Data

By Sarah E. Hutchinson and Stacey C. Sawyer

How Data and Programs Are Represented in the Computer
Before we study the inner workings of the processor, we need to expand on an earlier discussion of data representation in the computer—how the processor "understands" data. We started with a simple fact: electricity can be either on or off.
Other kinds of technology also use this two-state on/off arrangement. An electrical circuit may be open or closed. The magnetic pulses on a disk or tape may be present or absent. Current may be high voltage or low voltage. A punched card or tape may have a hole or not have a hole. This two-state situation allows computers to use the binary system to represent data and programs.
The decimal system that we are accustomed to has 10 digits (0, 1, 2, 3, 4, 5, 6, 7, 8, and 9). By contrast, the binary system has only two digits: 0 and 1. (Bi- means "two.") Thus, in the computer the 0 can be represented by the electrical current being off (or at low voltage) and the 1 by the current being on (or at high voltage). All data and programs that go into the computer are represented in terms of these numbers. For example, the letter H is a translation of the electronic signal 01001000, or off-on-off-off-on-off-off-off. When you press the key for H on the computer keyboard, the character is automatically converted into the series of electronic impulses that the computer recognizes.
Binary Coding Schemes
All the amazing things that computers do are based on binary numbers made up of 0s and 1s. Fortunately, we don't have to enter data into the computer using groupings of 0s and 1s. Rather, data is encoded, or arranged, by means of binary, or digital, coding schemes to represent letters, numbers, and special characters.
There are many coding schemes. Two common ones are EBCDIC and ASCII. Both use 7 or 8 bits to form each byte, providing up to 256 combinations with which to form letters, numbers, and special characters, such...