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Motorola 6845The MC6845 Cathode Ray Tube Controller (CRTC) was an early graphics chip developed by Motorola for use in computer terminals in the mid-1970s. The 6845 contained counters that counted down at a speed determined by an external clock; the outputs of these counters were used to generate sync signals for an attached video monitor, and also timing and addressing signals for an attached frame buffer. The 6845 also had rudimentary hardware cursor support for text modes, as well as logic to support a light pen. The 6845 had built-in colour support, as well as support for interlacing. Since the frame buffer's other attributes (such as text mode versus bitmap, highlighting and character set) were determined by other hardware, designs using the 6845 vary widely in operation and complexity. Most machines that originally used the 6845 did not stray far from Motorola's original intent; Commodore's CBM series and the original IBM PC used it to display monochrome text on a relatively small screen. Later on, though, the 6845's basic design was integrated into the Color Graphics Adapter, Enhanced Graphics Adapter and Video Graphics Array, making its interface one of the most common in computing while expanding its capabilities greatly.
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