Computer Hardware: Simple Graphics Adaptor

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Before graphics can be displayed on the screen they have to first be written to addressable RAM memory locations. These are now called graphics adapters, but more accurately described as Video RAM (VRAM). VRAM locations are then scanned and the resulting output from each cell activates a corresponding pixel on the screen display. Our example is a simple monochrome two level display, there can be no shades between black and white and there are just 150 locations. Writing and reading the VRAM contents would normally be continuous, occurring many thousands of times a second.

Monochrome displays only require one bit per pixel that is set to binary 1 or 0 ON or OFF. Monochrome displays are less common now and frequently had a green screen background with a lighter shade for the text output. By today’s standards the screen resolution was poor, graphics images had ragged outlines, as would this display if it were shown on a much larger screen size. The amount of VRAM required grows as screen resolution increases, since each screen pixel needs a corresponding cell. In practice all sorts of data compression techniques are applied to reduce the size of video RAM, mainly in an attempt to increase the refresh rate for high quality fast changing images.

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