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Text and Graphical Modes
With the exception of the very earliest cards used on old PCs in the early to mid 80s, all video cards are able to display information in either text or graphical modes. In a text mode, video information is stored as characters in a character set; usually on PCs this is the ASCII character set. A typical PC text screen has 25 rows and 80 columns. The video card has built into it a definition of what the dot shape is for each character, which it uses to display the contents of the screen. You cannot access the individual dots that make up the letter "M" on the screen. This is similar to how fonts work in a dot matrix printer; when you type the letter "M", the letter as stored as one or two bytes in the file (the extra byte is often for attribute information such as color, underlining etc.). When you go to print the "M" it is translated to a pattern of dots by the printer.
Graphical modes are of course totally different; here the dots on the screen are manipulated directly, so both text and images are possible. The conversion of letters, numbers etc. to visible images is done by software. This is the concept behind fonts; open the same file and display it under a different font and the appearance is totally different. Graphical modes allow for much more flexibility in terms of what is displayed on the screen, but at a cost: they require much more information to be manipulated, and also much more memory to hold the screen image. The increase is significant: typically a factor of up to 100 times or more! This has led almost directly to the need for increased hardware power in newer PCs.
Most PCs use both text and graphical modes, and can be switched between them under software control. While most computing is now done in a graphics mode, DOS is still text-based. PCs also generally boot up in a text or text-emulated mode.