Although source compatible with the earlier Motorola 6800, the 6809 offered significant improvements over it and 8-bit contemporaries like the MOS Technology 6502, including a hardware multiplication instruction, 16-bit arithmetic, system and user stack registers allowing re-entrant code, improved interrupts, position-independent code, and an orthogonal instruction set architecture with a comprehensive set of addressing modes.

[1] It was launched when a new generation of 16-bit processors were coming to market, like the Intel 8086, and 32-bit designs were on the horizon, including Motorola's own 68000.

In the 1990s, Williams introduced the WPC platform (based on the 68B09) for the pinball machines the company was producing at the time.

[3] Series II of the Fairlight CMI digital audio workstation and Konami's Time Pilot '84 arcade game each use dual 6809 processors.

A key feature was an on-chip voltage doubler that allowed it to run on a single +5 V supply, a major advantage over its competitors like the Intel 8080 which required −5 V, +5 V, and +12 V.[5] The 6800 was initially fabricated using the then-current contact lithography process.

There was a small chance that some of the etching material would be left on the wafer when it was lifted, causing future chips patterned with the mask to fail.

Notable among them was Chuck Peddle, who was sent on sales trips and saw prospective customers repeatedly reject the design as being too expensive for their intended uses.

He began a project to produce a much less costly design, but Motorola's management proved uninterested and eventually told him to stop working on it.

The introduction of the Micralign to Motorola's lines allowed further reductions and by 1981 the price of the then-current 6800P was slightly less than the equivalent 6502, at least in single-unit quantities.

However, a careful examination of the loads and stores noted that many of these were being combined with adds and subtracts, revealing that a significant amount of those math operations were being performed on 16-bit values.

This led to the decision to include basic 16-bit mathematics in the new design: load, store, add, and subtract.

However, as assembly language is generally written starting at a base address, combining pre-written modules normally required a lengthy process of changing constants (or equates) that pointed to key locations in the code.

Libraries of common routines like floating point arithmetic, graphics primitives, Lempel-Ziv compression, and so forth would be available to license, combine together along with custom code, and burn to ROM.

To solve the problem of easily referring to such data while remaining position independent, the 6809 added a variety of new addressing modes.

[11]: 36  It takes 8-bit numbers in the A and B accumulators and produces a 16-bit result in A:B, known collectively as D.[12]: 1.1 Much of the design had been based around the market concept of building-block code.

The industry as a whole solved the problem of integrating code modules from separate sources by using automatic relocating linkers and loaders, which is the solution used today.

[13] However, the decisions made by the design team enabled multi-user, multitasking operating systems like OS-9 and UniFlex.

Although too late to be chosen for the IBM PC project, when MACSS appeared as the Motorola 68000 in 1979 it took any remaining interest in the 6809.

Motorola had been asked to design a color-capable computer terminal for an online farm-aid project, a system known as "AgVision".

Tandy (Radio Shack) was brought in as a retail partner and sold them under the name "VideoTex", but the project was ultimately canceled shortly after its introduction in 1980.

These were primarily for the Japanese market, but some were exported to and sold in Australia, where the MB-6890 was dubbed the "Peach", probably in reference to the Apple II.

Williams Electronics was a prolific user of the processor, which was deployed in Defender, Stargate, Joust, Robotron: 2084, Sinistar, and other games.

[19] Series II of the Fairlight CMI (computer musical instrument) used dual 6809 CPUs running OS-9, and also used one 6809 CPU per voice card.

The 6809 was often employed in music synthesizers from other manufacturers such as Oberheim (Xpander, Matrix 6/12/1000), PPG (Wave 2/2.2/2.3, Waveterm A), and Ensoniq (Mirage sampler, SDP-1, ESQ-1, SQ-80).

In 2015, Freescale authorized Rochester Electronics to start manufacturing the MC6809 once again as a drop-in replacement and copy of the original NMOS device.

Australian developer John Kent has synthesized the Motorola 6809 CPU in hardware description language (HDL).

Depending on version and speed grade, approximately 40–60% of a single clock cycle is typically available for memory access in a 6800, 6502, or 6809.

These new modes had the same opcodes as the previously separate instruction, so these changes were only visible to the programmer working on new code.

[12]: 1.2 The instruction set and register complement are highly orthogonal, making the 6809 easier to program than contemporaries.

Like the 6800, the 6809 includes an undocumented address bus test instruction which came to be nicknamed Halt and Catch Fire (HCF).