Motorola 6845 дисплей контроллері және оның түрлері

Display controller

The Motorola 6845, or MC6845, is a display controller that was widely used in 8 bit computers during the 1980s. Originally intended for designs based on the Motorola 6800 CPU and given a related part number, it was more widely used alongside various other processors, and was most commonly found in machines based on the Zilog Z80 and MOS 6502. The 6845 is not an entire display solution on its own; the chip's main function is to properly time access to the display memory, and to calculate the memory address of the next portion to be drawn. Other circuitry in the machine then uses the address provided by the 6845 to fetch the pattern and then draw it. The implementation of that hardware is entirely up to the designer and varied widely among machines. The 6845 is intended for character displays, but could also be used for pixel based graphics, with some clever programming. Among its better known uses is the BBC Micro, Amstrad CPC, and Videx VideoTerm display cards for the Apple II. Its functionality was duplicated and extended by custom circuits in the EGA and VGA PC video adapters.

Originally designed by Hitachi as the HD46505, Hitachi built versions are in a wide variety of Japanese computers, from Sony, Sharp, Panasonic, and Casio. Cloned later as MB89321A by Fujitsu. It is also known as the 6845 CRTC or the CRTC6845, meaning "cathode ray tube controller". This version was used on the Apricot PC and Victor 9000 to provide a 800x400 resolution monochrome display. A common clone of this CRT controller is the United Microelectronics Corporation (UMC) UM6845E CRT controller. During the time of cold war technology embargoes, the 6845 was cloned in Bulgaria under the designation CM607. The 6845 was very similar and related to the later 6545 manufactured by MOS Technology (Commodore Semiconductor Group) and Rockwell (in two versions).

Using the full address range RA0 RA4:CA0 CA13 the 6845 can address words of memory. A word may be any number of bits chosen by the system designer as the memory width: though the number of unique addresses that the 6845 can address is limited to 524,288, the amount of memory that the 6845 can address may be significantly larger than might be assumed because the 6845 imposes no limit on the size of each memory location that it addresses. If the word size is one byte, as is often the case, the 6845 can address 512 KiB. If the word size is 32 bits, e. g. for 32 bit color graphics with one pixel per word, then the 6845 can address 2048 kiB; for 64 bit words, it can address twice as much. These limits arise from the combination of the 6845 and the design of the external memory connected to it, not from the 6845 alone.

As described before, the 6845 cannot ordinarily provide large linear framebuffers. One design could only use 14 bit character addressing and set the number of lines per character to 1 but would be restricted to 128 lines and 16 KB of addressable memory.

Although overwhelmingly compatible, a number of small variations exist between the 6845 and 6545. The biggest difference is that the 6545 may be configured so that it has sole access to the address bus for video memory. Two additional registers are included for setting any address the CPU wishes to read and the chip alternates between outputting addresses for display generation and the display set for CPU access. Smaller changes are that the MOS Technology and one variation of the Rockwell 6545 lack interlaced output support and all 6545s include an optional address skew, which delays display enable for one character cycle if set. This second feature was incorporated into later variations of the Motorola 6845. The 6545 may be set to work in linear 14 bit mode using a status bit. On the 6845 the same thing requires adjustment of the character height.

The 6845 reads the start address for its display once per frame. However, if the internal timing values on the chip are altered at the correct time it can be made to prepare for a new frame without ending the current one creating a non continuous break in generated addresses midway through the display. This is commonly used by demos and much more rarely games to provide one moving area of the display (usually the play field) and one static (usually a status display). Vertical scrolling appears constrained because only the character start address can be set and the row address is always zeroed at frame start, but by adjusting border times it is possible to shift the position the framebuffer is shown on the raster display for increments in between whole characters. With drawing of blank pixels at the screen edges, this can be made invisible to the user creating just the illusion of a smooth vertical scroll.