All computers that use raster scanned video contain the basic elements shown in Fig. 2-2A, namely a microprocessor, a bus, ROM and RAM memory, keyboard i/o, mass storage i/o, and, most importantly, a video i/o section. The video circuit is expanded in the figure to show its basic components. We will study these later in more detail.


(A) Hardware of typical memory-mapped video circuit (board). Fig. 2-2. Memory-mapped video. (B) This example of a memory-mapped video display has 40 lines and 86 characters per line.

All graphics computers start with some way to get text information such as letters and numbers on the screen—the circuits for graphics then grow around this. A popular technique for getting letters and text (and later graphics) on the television screen is called memory-mapped video. The screen output of a memory-mapped video display board that plugs into any S-100 computer and outputs to a high-performance monitor is shown in Fig. 2-2B. Note that the various dashed areas are special “windows,” where each receives the output from a specific source.


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