Color monitors have three beams for red green and blue which are modulated

Color monitors have three beams for red green and

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intensity of the electron beam, determining whether a given pixel will be light or dark. Color monitors have three beams, for red, green, and blue, which are modulated independently. A flat panel display works very differently internally, but a CRT-compatible flat-panel display accepts the same synchronization and video signals as a CRT and uses these to control a liquid crystal element at each pixel position. A simple monochrome display might fit each character in a box 9 pixels wide by 14 pixels high (including the space between characters), and have 25 lines of 80 characters. The display would then have 350 scan lines of 720 pixels each. Each of these frames is redrawn 45 to 70 times a second. The video controller could be designed to fetch the first 80 characters from the video RAM, generate 14 scan lines, fetch the next 80 characters from the video RAM, generate the following 14 scan lines, and so on. In fact, most fetch each character once per scan line to eliminate the need for buffering in the controller. The 9-by-14 bit patterns for the characters are kept in a ROM used by the video controller. (RAM may also be used to support custom fonts.) The ROM is addressed by a 12-bit address, 8 bits from the character code and 4 bits to specify a scan line. The 8 bits in each byte of the ROM control 8 pixels; the 9th pixel between characters is always blank. Thus 14 x 80 = 1120 memory references to the video RAM are needed per line of text on the screen. The same number of references are made to the character generator ROM. [Page 305] The original IBM PC had several modes for the screen. In the simplest one, it used a character-mapped display for the console. In Fig. 3-26(a) we see a portion of the video RAM. Each character on the screen of Fig. 3-26(b) occupied two characters in the RAM. The low-order character was the ASCII code for the character to be displayed. The high- order character was the attribute byte, which was used to specify the color, reverse video, blinking, and so on. The full screen of 25 by 80 characters required 4000 bytes of video RAM in this mode. All modern displays still support this mode of operation. Figure 3-26. (a) A video RAM image for the IBM monochrome display. The xs are attribute bytes. (b) The corresponding screen.
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Contemporary bitmap displays use the same principle, except that each pixel on the screen is individually controlled. In the simplest configuration, for a monochrome display, each pixel has a corresponding bit in the video RAM. At the other extreme, each pixel is represented by a 24-bit number, with 8 bits each for red, green, and blue. A 768 x 1024 color display with 24 bits per pixel requires 2 MB of RAM to hold the image. With a memory-mapped display, the keyboard is completely decoupled from the screen. It may be interfaced via a serial or parallel port. On every key action the CPU is interrupted, and the keyboard driver extracts the character typed by reading an I/O port.
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