Display Hardware

UMBC CMSC 211

Display Hardware

Your monitor, or CRT, works like a TV set. There is an electron gun shooting an electron beam at a phosphorescent-coated screen that causes the material on the screen to glow when struck by the beam. There are electromagnets that are used to aim the beam at each pixel. Each pixel is three in one: red, green, and blue (RGB). These three colors can be combined to form any other color.

The screen is rows of pixels and the beam traces across the row, then snaps back (horizontal retrace). The trace does the rows and then when it reaches the bottom, it goes back to the top (vertical retrace). The adaptor has a character generator that turns a 8-bit ASCII character into a 8 x 8 pixel pattern. It stores the image that is on the screen in display memory that is used to update the screen approximately 30 times per second.

MOst Personal computers make the display memory a part of the computer's memory, so that it can be updated at the computer's speed. Now it takes less than one screen trace (1/30th of a second) to update the screen, whereas before it could be at least 1/2 of a second to update (more for color).Some display adaptors have several screens (pages) of data and can quickly change the screen by changing where the information is coming from.

In display programming of any sophication, DOS calls are virtually worthless. The alternatives are:

INT 10h calls have a variety of display fucntions that are reasonable save.
Moving information directly into, and out of, display memory. It is very fast, but puts the burden on the programmer.
Acutal hardware I/O instructions.