Code_ The Hidden Language of Computer Hardware and Software - Charles Petzold [158]
The faster a computer can respond to the keyboard and alter the video display, the tighter the potential interaction between user and computer. Most of the software written in the first decade of the IBM Personal Computer (through the 1980s) wrote directly to video display memory. Because IBM set a hardware standard that other computer manufacturers adhered to, software manufacturers could bypass the operating system and use the hardware directly without fear that their programs wouldn't run right (or at all) on some machines. If all the PC clones had different hardware interfaces to their video displays, it would have been too difficult for software manufacturers to accommodate all the different designs.
For the most part, early applications for the IBM PC used only text output and not graphics. The use of text output also helped the applications run as fast as possible. When a video display is designed like the one described in Chapter 21, a program can display a particular character on the screen by simply writing the character's ASCII code into memory. A program using a graphical video display usually needs to write 8 or more bytes into memory to draw the image of the text character.
The move from character displays to graphics was, however, an extremely important step in the evolution of computers. Yet the development of computer hardware and software that work with graphical images rather than just text and numbers evolved very slowly. As early as 1945, John von Neumann envisioned an oscilloscope-like display that could graph pictorial information. But it wasn't until the early 1950s that computer graphics were ready to become a reality when MIT (with help from IBM) set up the Lincoln Laboratory to develop computers for the Air Force's air defense system. This project was known as SAGE (Semi-Automatic Ground Environment) and included graphics display screens to help the operators analyze large amounts of data.
The early video displays used in systems such as SAGE weren't like those we use today on personal computers. Today's common PC displays are known as raster displays. Much like a TV, the total image is composed of a series of horizontal raster lines drawn by an electron gun shooting a beam that moves very rapidly back and forth across the screen. The screen can be visualized as a large rectangular array of dots called pixels (picture elements). Within the computer, a block of memory is devoted to the video display and contains 1 or more bits for each pixel on the screen. The values of these bits determine whether pixels are illuminated and what color they are.
For example, most computer displays nowadays have a resolution of at least 640 pixels horizontally and 480 pixels vertically. The total number of pixels is the product of these two numbers: 307,200. If only 1 bit of memory is devoted to each pixel, each pixel is limited to just two colors, usually black and white. A 0 pixel could be black and a 1 pixel could be white, for example. Such a video display requires 307,200 bits of memory, or 38,400 bytes.
Increasing the number of possible colors necessitates more bits per pixel and increases the memory requirements of the display adapter. For example, a byte could be used for each pixel to encode gray shades. In such an arrangement, the byte 00h is black, FFh is white, and the values in between are shades of gray.
Color on a CRT is achieved by means of three electron guns, one for each of the three additive primary colors, red, green, and blue. (You can examine a television or color computer screen with a magnifying glass to convince yourself that this is true. Printing uses a different set of primaries.) The combination of red and green is yellow, the combination of red and blue is magenta, the combination