Motorola 6845

Among its better-known uses are the BBC Micro, Amstrad CPC, and Videx VideoTerm display cards for the Apple II.

[3] Its functionality was duplicated and extended by custom circuits in the EGA and VGA PC video adapters.

Originally designed by Hitachi as the HD46505,[4] Hitachi-built versions are in a wide variety of Japanese computers, from Sony, Sharp, Panasonic, and Casio.

The 6845 was very similar and related to the later 6545 manufactured by MOS Technology (Commodore Semiconductor Group) and Rockwell (in two versions).

The process of reading that value, converting it into pixels, and sending it to a CRT is left to other 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.

However, the 6845 left to the designer the freedom to route the bits of the memory and row addresses to video RAM as they saw fit.

Different routing of the bits, omitting the character ROM, could be used to emulate a frame buffer.

For example the IBM CGA graphics mode uses a word size of one byte, and each word represents four or eight pixels (in the medium- or high-resolution graphics modes respectively); the 6845 is configured for a 2-pixel character height and the row address bit RA0 is used for bit 13 of the frame buffer address.

On the other hand, the 6845 put the onus on its users to provide enough memory bandwidth to support rereading data on every line.

A different video display controller that buffers one whole line of character data internally can avoid this repeated reading of each line of characters from the display buffer RAM, reducing the required memory bandwidth and allowing either slower, less expensive memory chips to be used, more time for a system CPU to access the memory, or a combination of both.

The biggest difference is that the 6545 may be configured so that it has sole access to the address bus for video memory.

[8] 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.

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.