Jan's Computer Basics:4-2 Digital Data

Computer Basics

4 - Processing: Digital Data

Modern computers are digital, that is, all info is stored as a string of zeros or ones - off or on. All the thinking in the computer is done by manipulating these digits. The concept is simple, but working it all out gets complicated.

1 bit = one on or off position
1 byte = 8 bits

So 1 byte can be one of 256 possible combinations of 0 and 1.
Numbers written with just 0 and 1, are called binary numbers.

Each 1 is a power of 2 so that the digits in the figure represent the number:

= 2⁷+ 0 + 2⁵ + 0 + 2³+ 2² + 0 +0

= 128 +0 +32 + 0 + 8 + 4 + 0 + 0

= 172

Every command and every input is converted into digital data, a string of 0's and 1's.

For more information on binary numbers, see Base Arithmetic.

Where you are: JegsWorks > Lessons > Computer Basics
	1. Computer Types
	2. Applications
	3. Input
	4. Processing Intro Digital Data CPU: framed no frames Machine Cycle Memory Addresses Processor Speed Physical Components Motherboard: framed no frames Quiz
	5. Output
	6. Storage
	7. Computer to Computer
	8. System Software
	9. Programming

	10. What You See
	11. Hands On!
	12. On Your Own

	Search Glossary Appendix

Digital Codes

All letters, numbers, and symbols are assigned code values of 1's and 0's. A number of different digital coding schemes are used by digital devices.

Three common code sets are:

ASCII         (used in UNIX and DOS/Windows-based computers)
EBCDIC     (for IBM System 390 main frames)
Unicode    (for Windows NT and recent browsers)

The ASCII code set uses 7 bits per character, allowing 128 different characters. This is enough for the alphabet in upper case and lower case, the symbols on a regular English typewriter, and some combinations reserved for internal use. An extended ASCII code set uses 8 bits per character, which adds another 128 possible characters. This larger code set allows for foreign languages symbols and several graphical symbols.

ASCII has been superceded by other coding schemes in modern computing. It is still used for transferring plain text data between different programs or computers that use different coding schemes.

ASCII and EBCDIC codes

If you're curious to see the table of ASCII and EBCDIC codes, see Character Codes.

Unicode uses 16 bits per character, so it takes twice the storage space that ASCII coding, for example, would take for the same characters. But Unicode can handle many more characters. The goal of Unicode is to represent every element used in every script for writing every language on the planet. Whew! Quite a task!

Version 3 of Unicode has 49,194 characters instead of the wimpy few hundred for ASCII and EBCDIC. All of the current major languages in the world can be written with Unicode, including their special punctuation and symbols for math and geometry.

At the Unicode site you can view sections of the Unicode code charts. The complete list is too long to put on one page!

Parity

With all these 0's and 1's, it would be easy for the computer to make a mistake! Parity is a clever way to check for errors that might occur during processing.

In an even parity system an extra bit (making a total of 9 bits) is assigned to be on or off so as to make the number of on bits even. So in our example above 10101100 there are 4 on bits (the four 1's). So the 9th bit, the parity bit, will be 0 since we already have an even number of on bits.

In an odd parity system the number of on bits would have to be odd. For our example number 10101100, there are 4 on bits (the 1's), so the parity bit is set to on, that is 1, to make a total of 5 on bits, an odd number.

If the number of on bits is wrong, an error has occurred. You won't know which digit or digits are wrong, but the computer will at least know that a mistake occurred.

Memory chips that store your data can be parity chips or non-parity chips. Mixing them together can cause odd failures that are hard to track down.

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Last updated: 22 Jan 2008