Most people using UNIX—or computers in general—back when it was new didn't use a graphical display. They connected via a dumb terminal, a simple device that pretended to be a teletype, with a text-only display and a keyboard.

Early graphical workstations appeared a decade or so later. The graphics hardware in these machines was simple: a frame buffer. Every pixel on the screen had a corresponding memory address. Because memory was so expensive, these often used a palette rather than true color. With a true-color display, each memory address stores a color value. Typically, this requires 3 bytes per pixel (24-bit color), but some modern hardware uses more. With a palletized display, each pixel value stores an index in a lookup table, which stores the real color value. If you have a 16-color display, then each pixel needs just 4 bits of memory.

The display hardware would read the pixel values, in turn, and would feed these values (or the color values calculated by looking up the index in the palette) to a digital-to-analog converter which drove the CRT. A lot of tricks were used to let you write to the memory in the display while it was being sent to the screen. Later, when memory became a bit cheaper, it was common for graphics cards to support double-buffering, where you drew to one frame buffer while another was drawn, then swapped them around.

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