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The Motorola 6845 (commonly MC6845) is a video address generator first introduced by Motorola and used among others in the Videx VideoTerm display cards for the Apple II computers, in the MDA, HGC and CGA video adapters for the IBM PC, and in the Amstrad CPC and BBC Micro. Its functionality was duplicated and extended by custom circuits in the EGA and VGA PC video adapters. It is related to the later 6545 manufactured by MOS Technology (Commodore Semiconductor Group) and Rockwell (in two variations) and was cloned as the Hitachi HD46505 which was used in many Japanese computers from Sony, Sharp, Panasonic and Casio, and later was used in Videx's UltraTerm card). It is also known as the 6845 CRTC or the CRTC6845, meaning "cathode ray tube controller". Although intended for designs based on the Motorola 6800 CPU and given a related part number, it was more commonly used alongside various other processors. ==Overview== The chip generates the signals necessary to interface with a raster display but does not generate the actual pixels, though it does contribute cursor and video-blanking information to the pixel video (intensity) signals. It is used to produce correctly timed horizontal and vertical sync and provide the address in memory from which the next pixel or set of pixels should be read. The process of reading that value, converting it into pixels, and sending it to a CRT is left to other circuits. Because of this, systems using the 6845 may have very different numbers and values of colors, or may not support color at all. Interlaced and non-interlaced output modes are supported, as is a hardware text cursor. The sync generation includes generation of horizontal and vertical video blanking signals, which are used to condition the external pixel generation circuits. Also, an internal latch is provided which when triggered will duplicate and retain a copy of the video address so that it can later be read back by the CPU. This is useful for light pens and light guns which can function by sending a pulse to the 6845 when the electron beam passes, allowing a running program to read back the location that was pointed at. (Because of this feature, most computer video adapters using a 6845 included a light pen interface, though it was usually an internal connector on the board itself, not on the outside of the computer, and it was usually undocumented in the user manual.) Because all aspects of video timing are programmable, a single machine can switch between NTSC and PAL timings in software. The 6845 can be used to drive monitors or any other raster display. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Motorola 6845」の詳細全文を読む スポンサード リンク
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