# Motorola 6845

> display controller

**Wikidata**: [Q2708459](https://www.wikidata.org/wiki/Q2708459)  
**Wikipedia**: [English](https://en.wikipedia.org/wiki/Motorola_6845)  
**Source**: https://4ort.xyz/entity/motorola-6845

## Summary
The Motorola 6845 is a display controller integrated circuit used in early personal computers and terminals to generate video timing and address video memory. It was widely adopted in systems like the IBM PC, BBC Micro, and Amstrad CPC. The chip handles tasks such as character generation, cursor control, and screen refresh.

## Key Facts
- The Motorola 6845 was introduced in the late 1970s as a CRT controller for video display systems.
- It is classified as both an integrated circuit and a graphics processing unit.
- The chip has over 10 known aliases including CRTC6845, MC6845, and MOS Technology 6545.
- It was used in iconic systems like the IBM PC, BBC Micro, Amstrad CPC, and early Unix workstations.
- The 6845 supports multiple screen resolutions and character sets through programmable registers.
- It was manufactured by Motorola and later licensed to MOS Technology as the 6545 variant.
- The chip operates by generating horizontal and vertical sync signals for CRT monitors.
- It has a Wikipedia presence in 10 languages and a Commons category with images.
- The freebase ID for the 6845 is /m/046tsb, established in 2013.

## FAQs
### Q: What is the Motorola 6845 used for?
A: The Motorola 6845 is a display controller chip used to manage video output in early computers and terminals. It generates timing signals, addresses video memory, and controls character display on CRT monitors.

### Q: Which computers used the Motorola 6845?
A: The Motorola 6845 was used in the IBM PC, BBC Micro, Amstrad CPC, early Tandy computers, and various terminals and workstations from the 1980s.

### Q: Is the Motorola 6845 still used today?
A: No, the Motorola 6845 is obsolete and no longer used in modern computers. It was replaced by more advanced graphics controllers and GPUs as display technology evolved.

## Why It Matters
The Motorola 6845 was a foundational component in the development of personal computing and video display technology. By providing a standardized way to control CRT monitors and generate video timing, it enabled manufacturers to build affordable and reliable computer displays without designing custom video hardware. Its adoption in landmark systems like the IBM PC helped establish industry standards for video output, influencing the design of future graphics controllers. The 6845's programmable nature allowed for flexibility in screen resolution and character sets, making it adaptable to a wide range of computer architectures. Its legacy lives on in the principles of video timing and memory addressing that underpin modern display controllers, even as the technology has advanced far beyond CRT-based systems.

## Notable For
- Being the display controller of choice for the original IBM PC, setting a standard for PC graphics.
- Enabling the BBC Micro to deliver high-quality graphics and text in an educational computer.
- Providing the video timing foundation for the Amstrad CPC's advanced color graphics.
- Serving as the basis for the MOS Technology 6545, a cost-reduced variant used in other systems.
- Influencing the design of subsequent CRT controllers and video interface chips in the 1980s and beyond.

## Body
### Technical Overview
The Motorola 6845 is a cathode ray tube controller (CRTC) that manages the interface between a computer's video memory and its display. It generates the horizontal and vertical synchronization signals required to refresh a CRT monitor, as well as the addresses needed to read video data from memory. The chip contains 16 internal registers that can be programmed to control screen resolution, character size, cursor position, and other display parameters.

### Architecture and Operation
The 6845 operates by counting horizontal and vertical lines to determine when to send sync pulses and when to increment the video memory address. It supports a range of resolutions, typically from 40 to 132 characters per line and 25 to 50 lines per screen. The chip can address up to 16 KB of video memory, which was sufficient for the text and low-resolution graphics modes of its era. It also includes hardware cursor support and light pen input for interactive applications.

### Variants and Derivatives
The most notable variant is the MOS Technology 6545, which is a second-source version of the 6845 with minor electrical differences. Some manufacturers produced slightly modified versions to suit their specific hardware requirements, but the core functionality remained consistent. The 6845's design influenced later CRT controllers and video interface chips, many of which retained compatibility with its programming model.

### Historical Impact
The widespread adoption of the Motorola 6845 in the early 1980s helped standardize video timing and memory addressing in personal computers. Its use in the IBM PC, in particular, meant that software developers could write graphics and text display routines that would work across a range of compatible systems. This standardization was a key factor in the rapid growth of the PC industry and the development of graphical user interfaces. Although the 6845 itself is now obsolete, its influence can still be seen in the way modern graphics hardware handles video timing and memory access.

## References

1. Freebase Data Dumps. 2013