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EGA Facts |
The information in this document applies to:
WordPerfect® 5.1 for DOS
Problem
Solutions: The following information was taken from PC Magazine Vol 7 No. 12 EGA is designed to operate in two different modes: character (or text) mode, and graphics mode. In character mode the display is divided into distinct rows and columns, somewhat like a checkerboard. The intersection of each row and column is called a character box and can hold a single, complete character. When a PC with a typical EGA system is booted, the BIOS initializes the hardware to character mode 3: 80 columns wide and 25 rows high. Thus, a maximum of 4,000 (80 * 25) characters can be displayed on the screen at once, and only in the boxes formed by the intersections of the rows and columns. Each character on the screen is really formed by an arrangement of smaller dots of light. On the EGA, the default character box for mode 3 is 8 dots wide and 14 dots high. These dots are called picture elements, or pixels. (IBM refers to pixels as PELs.) With a little multiplication, we find that in mode 3 the EGA is really displaying 640 pixels across the screen and 350 pixels down. (In its highest resolution graphics mode, each pixel is under direct control and can be turned on and off individually.) Displaying More Characters The EGA is designed so that the height of a character is under software control can thus can be changed. A new set of characters, called a font, can be loaded into memory. Even more important, the height of the new characters can be different from the default 14 pixels. If a font that is 8 pixels high is loaded, for example, the EGA can be used to display 43 lines of text (43 * 8 = 344 pixels high). Unlike the height, however, the width of the character box cannot be changed. The IBM EGA design requires that the character box always be 8 pixels wide. The monochrome text mode of the EGA is also restricted to a character box that is 9 pixels wide. Thus, in order to display more characters across the screen, more pixels must be generated on each line. To increase the number of 8-pixel-wide characters across the width of the display to 132, for example, a total of 132 * 8 = 1,056 pixels must be displayed. This is done by increasing the frequency of the dot clock and reprogramming the CRT controller. IBM EGA, as it comes from the box, contains only the standard dot clock and so cannot display more than 80 characters on a line. Other EGA makers, as they improved on IBM's design, added more features, including dot clocks of different frequencies. Because these features go beyond the support of the BIOS, DOS, and the MODE command, special software, usually supplied by the manufacturer, must be used to take advantage of these enhanced modes. The GENOA SuperEGA, for example, comes with a control program that lets the user select from over 40 display modes, including displaying text as 80 by 25, 94 by 29, and 132 by 44. Until EGA came along, all displays were assumed to be 80 columns wide and 25 rows high. Because of this, many programs were written with these numbers hard coded as absolute values. For a program to work with something besides the standard 80 by 25 screen, it had to have been designed to accommodate different screen sizes. While some application programs, such as WordPerfect, added support for extended display modes, many other popular programs, such as Lotus 1-2-3, did not. The EGA Border If properly adjusted, the CGA border completely fills the area between the edge of the text and the edge of the physical display. When set to the same color as the background, this gives a very pleasing effect. In order to achieve its higher resolution, however, the EGA must spend more of its time drawing in the character region of the display. Very simply, this leaves the EGA less time than the CGA to draw a border and results in a smaller border. |
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