Data Representation

Data Representation refers to the methods used internally to represent information stored in a computer. Computers store lots of different types of information:
  • numbers
  • text
  • graphics of many varieties (stills, video, animation)
  • sound
  • At least, these all seem different to us. However, ALL types of information stored in a computer are stored internally in the same simple format: a sequence of 0's and 1's. How can a sequence of 0's and 1's represent things as diverse as your photograph, your favorite song, a recent movie, and your term paper?
    It all depends on how we interpret the information. Computers use numeric codes to represent all the information they store. These codes are similar to those you may have used as a child to encrypt secret notes: let 1 stand for A, 2 stand for B, etc. With this code, any written message can be represented numerically. The codes used by computers are a bit more sophisticated, and they are based on the binary number system (base two) instead of the more familiar (for the moment, at least!) decimal system. Computers use a variety of different codes. Some are used for numbers, others for text, and still others for sound and graphics.
    Memory Structure in Computer
    • Memory consists of bits (0 or 1)
      • a single bit can represent two pieces of information
    • bytes (=8 bits)
      • a single byte can represent 256 = 2x2x2x2x2x2x2x2 = 28 pieces of information
    • words (=2,4, or 8 bytes)
      • a 2 byte word can represent 2562 pieces of information (approximately 65 thousand).
    • Byte addressable - each byte has its own address.
    • Binary  Numbers
    Normally we write numbers using digits 0 to 9. This is called base 10. However, any positive integer (whole number) can be easily represented by a sequence of 0's and 1's. Numbers in this form are said to be in base 2 and they are called binary numbers. Base 10 numbers use a positional system based on powers of 10 to indicate their value. The number 123 is really 1 hundred + 2 tens + 3 ones. The value of each position is determined by ever-higher powers of 10, read from left to right. Base 2 works the same way, just with different powers. The number 101 in base 2 is really 1 four + 0 twos + 1 one (which equals 5 in base 10). 
    Text can be represented easily by assigning a unique numeric value for each symbol used in the text. For example, the widely used ASCII code (American Standard Code for Information Interchange) defines 128 different symbols (all the characters found on a standard keyboard, plus a few extra), and assigns to each a unique numeric code between 0 and 127. In ASCII, an "A" is 65," B" is 66, "a" is 97, "b" is 98, and so forth. When you save a file as "plain text", it is stored using ASCII. ASCII format uses 1 byte per character 1 byte gives only 256 (128 standard and 128 non-standard) possible characters The code value for any character can be converted to base 2, so any written message made up of ASCII characters can be converted to a string of 0's and 1's.
    Graphics that are displayed on a computer screen consist of pixels: the tiny "dots" of color that collectively "paint" a graphic image on a computer screen. The pixels are organized into many rows on the screen. In one common configuration, each row is 640 pixels long, and there are 480 such rows. Another configuration (and the one used on the screens in the lab) is 800 pixels per row with 600 rows, which is referred to as a "resolution of 800x600." Each pixel has two properties: its location on the screen and its color.
    A graphic image can be represented by a list of pixels. Imagine all the rows of pixels on the screen laid out end to end in one long row. This gives the pixel list, and a pixel's location in the list corresponds to its position on the screen. A pixel's color is represented by a binary code, and consists of a certain number of bits. In a monochrome (black and white) image, only 1 bit is needed per pixel: 0 for black, 1 for white, for example. A 16 color image requires 4 bits per pixel. Modern display hardware allows for 24 bits per pixel, which provides an astounding array of 16.7 million possible colors for each pixel!

    Files today are so information-rich that they have become very large. This is particularly true of graphics files. With so many pixels in the list, and so many bits per pixel, a graphic file can easily take up over a megabyte of storage. Files containing large software applications can require 50 megabytes or more! This causes two problems: it becomes costly to store the files (requires many floppy disks or excessive room on a hard drive), and it becomes costly to transmit these files over networks and phone lines because the transmission takes a long time. In addition to studying how various types of data are represented, you will have the opportunity today to look at a technique known as data compression. The basic idea of compression is to make a file shorter by removing redundancies (repeated patterns of bits) from it. This shortened file must of course be de-compressed - have its redundancies put back in - in order to be used. However, it can be stored or transmitted in its shorter compressed form, saving both time and money.
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