The Motorola 6845 (commonly MC6845) is a video address generator first introduced by Motorola and used in the CGA and EGA video adapters, Amstrad CPC and BBC Micro. It's functionality was duplicated and extended by custom circuits in the VGA. It is related to the later 6545 manufactured by Commodore and Rockwell (in two variations) and was cloned by the Hitachi 46505. Motorola (NYSE: MOT) (TYO: 6686) is an electronics company based in Schaumburg, Illinois, a Chicago suburb. ... The Color Graphics Adapter (CGA), introduced in 1981, was IBMs first color graphics card, and the first color computer display standard for the IBM PC. The standard IBM CGA graphics card was equipped with 16 kilobytes of video memory. ... The Enhanced Graphics Adapter (EGA) is the IBM PC computer display standard specification located between CGA and VGA in terms of graphics performance (that is, colour and space resolution). ... Amstrad CPC 464, with CTM644 colour monitor The Amstrad CPC was an 8-bit home computer produced by Amstrad in the 1980s. ... Top view of the BBC Micro The BBC Micro, affectionately known as the Beeb, was an early home computer. ... Video Graphics Array (VGA) is a computer display standard first marketed in 1987 by IBM. VGA belongs to a family of earlier IBM video standards and largely remains backward compatible with them. ... Commodore has several meanings: Commodore International is a computer company Commodore 64 and Amiga were home computers Commodore (rank) is a naval rank Commodore (yacht club) is the senior officer of a yacht club The Holden Commodore is a type of car The Opel Commodore is a type of car... Rockwell can refer to: Rockwell International - a defense company in the United States Rockwell Automation - an industrial automation company that descended from Rockwell International Rockwell Collins - a communications and aviation electronics company that also descended from Rockwell International Willard Rockwell - businessman who helped shape and name what became Rockwell International... This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ...

It is also known as the CRTC6845, meaning Cathode Ray Tube Controller. The cathode ray tube or CRT, invented by Karl Ferdinand Braun, is the display device used in most computer displays, video monitors, televisions and oscilloscopes. ...

Although intended for designs based on the MOS Technology 6502 CPU and given a related part number, it has been used alongside various other processors. MOS Technology, Inc. ... The MOS Technology 6502 is an 8-bit microprocessor designed by MOS Technology in 1975. ...

Overview

The 6845 generates the signals necessary to interface with a raster display but does not display any pixels. 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. This article needs to be cleaned up to conform to a higher standard of quality. ... The cathode ray tube or CRT, invented by Karl Ferdinand Braun, is the display device used in most computer displays, video monitors, televisions and oscilloscopes. ...

Interlaced and non-interlaced output modes are supported, as is a hardware text cursor. 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 pulsing the 6845 when the electron beam passes, allowing a running program to read back the location pointed at. Interlacing is a method of displaying images on a raster-scanned display device, such as a cathode ray tube (CRT). ... A lightpen is a device similar to a touch screen, but is facilitated by use of a special light sensitive pen instead of the finger. ... Super Scope, Nintendo´s light gun for the Super Nintendo A light gun is a pointing device for computers and a control device for arcade and video games. ...

Because all aspects of video timing are programmable, a single machine can switch between NTSC and PAL timings in software. The chip can also be used to drive monitors or any other raster display. NTSC is the analog television system in use in the United States and some other places, mostly the Americas (see map). ... For other meanings of PAL see PAL (disambiguation). ... Nineteen inch (48 cm) CRT computer monitor A computer display, monitor or screen is a computer peripheral device capable of showing still or moving images generated by a computer and processed by a graphics card. ...

Internals

The chip has a toal of 18 8-bit registers controlling all aspects of video timings. Only two addresses are exposed to external components - one to select which internal register is to be read or written to and another to access that register. 8-bit refers to the number of bits used in the data bus of a computer. ... In computing, a hardware register is a storage area for hardware I/O (input/output) of different kinds. ...

The 6845 is intended for character based displays. Every address it generates is composed of two parts - a 14 bit character address and a 5 bit row address.

The character address increases linearly. When the chip signals horizontal sync it increases the row address. If the row address does not equal the programmatically set number of rows per character then the character address is reset to have the same value as it did at the beginning of the current scanline. Otherwise the row address is reset to zero.

If the character address is used to look up a character reference in RAM and the row address to index a table of character graphics in ROM an ordinary text mode display is constructed. This article is about the animal, sheep; for other meanings of Sheep, see Sheep (disambiguation). ... ROM, Rom, or rom may refer to: Rom, a member of the Roma people (or Gypsy). ... A text mode program communicates with the user by only displaying text and possibly a limited set of predefined semi-graphical characters, which allow to draw rudimentary boxes around portions of text, either to highlight the content or to simulate widget or control interface objects found in GUI programs. ...

Linear Framebuffers

As described above, the 6845 is not ordinarily able to provide a linear framebuffers. A design could use only the 14 bit character address and set the number of rows per character to 1 but it would be constrained to 16KiB of addressable memory. A kibibyte (a contraction of kilo binary byte) is a unit of information or computer storage, commonly abbreviated KiB (never kiB). 1 kibibyte = 210 bytes = 1 024 bytes The kibibyte is closely related to the kilobyte, which can be used either as a synonym for kibibyte or to refer to...

A compromise solution is found in the Amstrad CPC which combines the row address and character address together in such a way as to provide linear scanlines within a non-linear buffer. This has the advantages of easier programming for non-character display and easy smooth horizontal scrolling but severely impedes vertical scrolling. Amstrad CPC 464, with CTM644 colour monitor The Amstrad CPC was an 8-bit home computer produced by Amstrad in the 1980s. ...

Differences with the 6545

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 Commodore 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. Commodore has several meanings: Commodore International is a computer company Commodore 64 and Amiga were home computers Commodore (rank) is a naval rank Commodore (yacht club) is the senior officer of a yacht club The Holden Commodore is a type of car The Opel Commodore is a type of car... Rockwell can refer to: Rockwell International - a defense company in the United States Rockwell Automation - an industrial automation company that descended from Rockwell International Rockwell Collins - a communications and aviation electronics company that also descended from Rockwell International Willard Rockwell - businessman who helped shape and name what became Rockwell International... Motorola (NYSE: MOT) (TYO: 6686) is an electronics company based in Schaumburg, Illinois, a Chicago suburb. ...

The 6545 may be set to work in linear 14bit mode using a status bit. On the 6845 the same thing requires adjustment of the character height.

Tricks

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 games to provide one moving area of the display (usually the play field) and one static (usually a status display). In film, video production, animation, and related fields, a frame is one of the many still images which compose the complete moving picture. ... Games, like most other forms of media, may be categorized into genres based on gameplay, atmosphere, and various other factors. ...

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 inbetween 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.

References

• HTML documentation of the 6845 registers
• Zipped TIFs of the 6845 data sheets
• Differences of CRTC models

External links

• VHDL implementation of the 6845