In: Root » Computers and technology » Microsoft OS family
Windows marked the transition of the primary operating mode of PC display systems. From character-based displays, Windows ushered in the age of the bit-mapped display. Bit-mapped graphics improve the poor quality of block graphics by making the blocks smaller. The smaller the blocks making an image, the finer grain it can show and the more detail. Physical aspects of the display system impose a distinct and unbreakable limit on how small each block can be-the size of the individual dots that make up the image on the video screen. The sharpest and highest quality image that could be shown by any display system would individually control every dot on the screen. These dots are often called pixels, a contraction of the descriptive term picture element. Like atomic elements, pixels are the smallest building blocks from which known reality can be readily constructed. The terms dot and pixel are often used as synonyms but their strict definitions are somewhat different. When a system operates at its limits, putting as many dots on the screen as it is physically capable of handling, the number of dots and the number of pixels are the same. Often, however, systems operate with somewhat less sharpness than they are capable of, with the result that one pixel may be made from several onscreen dots. The most straightforward way of handling the information to be displayed on such a screen is to assign some part of memory to each pixel, just as two bytes are given over to each character of a character mapped display. In the PC scheme of things, because the data controlling each pixel is stored as one or more memory bits, this kind of display system is often called bit-mapped graphics. Alternately, because each pixel or point on the video screen can be separately addressed through memory, this method of controlling the video display is often called all points addressable graphics or an APA display. In the bit-mapped graphics system, display memory stores an exact electronic representation of the onscreen image. It's actually a time slice of what you see-the software running on your PC is constantly sending new data into display memory to update the screen image. The memory temporarily stores or buffers the changes' frame until they are read out as a complete image frame dozens of times per second. Because of this function, graphic display memory is often called the frame buffer. As with character mapped memory, programmers have their choice of methods to write bit-mapped graphics to your monitor screen. The PC BIOS provides basic support that allows any bit on the screen to be altered by programs. Using the BIOS to alter individual pixels is, however, a slow and painstaking process. Most applications write directly to the frame buffer to achieve satisfactory performance. Bit-mapped graphics hold the potential for being much sharper than block graphics. More pixels means more detail. The number of dots on a screen and the ultimate number of pixels are many times the number of characters that are displayed on that same screen, from 64 to 126 times greater. However, bit-mapped graphics imposes its own, interrelated penalties-memory usage and speed. The amount of memory required by a graphic display system depends on two factors: the sharpness of the display image and the number of colors (or gray levels) that are to be displayed. Each increase in sharpness and number of colors means that your PC is putting more detail (more information)on its screen and storing more information in its display buffer. As a result, it must move around more information (more bytes) and that means more work. And the harder your PC works, the longer it takes to complete its job.
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