What is the "The display system of Computer" and how it works?

Every computer needs a display system .IBM designed one for its personal computer with experience won in the office  and in making computer terminals.it had to have sharp easy to read characters and no annoying flicker.experience with mainframe and the typical implementation of CP/M computers might have lead IBM to use so called teletype video where the computer sends characters to a terminal and the terminal then displays them.The terminal circuitry does most of  the worrying where text will appear on the screen.
apple however used a different techinque called character mapping a character mapped display is divided into a matrix of character position and each character position is assigned a memory location.The display system matches the character designation in the map with the dot-pattern of the character that is stored elsewhere in a font library(which can be either RAM or ROM) independently of the host micro processor the video system send the appropriate dot information from the font library to the screen.
The advantage of this arrangement is that it is fast and efficient from the microprocessor and memory point of view.it  needs to deal with only efficient character codes.only two kilobytes of data need to be moved or stored for a full 80-coloumn by 25 row screen of next.it also eliminates much of the cost of a terminal.making it work requries only a display essentially a picture tube and its control circuity -instead of a complete terminal.
IBM added an extra byte for each character of the display to hold at tribute information-whether the character was dim  highlighted under lined,or shown in reverse video on the screen.IBM also chose to put the character and attribute bytes  for each on screen cell adjacent to one another,using a total of four kilobytes of memory area.