Motorola 6809

The Motorola 6809 ("sixty-eight-oh-nine") is an 8-bit microprocessor CPU with some 16-bit features. It was designed by Motorola's Terry Ritter and Joel Boney and introduced in 1978. Although source compatible with the earlier Motorola 6800, the 6809 offered a great number of improvements over it and contemporary 8-bit designs like the 6502, including a hardware multiplication, 16-bit math, user and system stack registers for fast operating system context switches, improved interrupts, and position-independant code to make memory management simpler.

Motorola 6809
General information
Launched1978 (1978)
Common manufacturer(s)
Performance
Data width8
Address width16
Architecture and classification
Instruction set6809
Instructions59
Physical specifications
Transistors
  • 9,000
Package(s)

Although among the most powerful 8-bit processors of its era, it was also much more expensive; in 1980 a 6809 in single-unit quantities was $37 compared to $9 for a Zilog Z80 and $6 for a 6502.[1] Additionally, it was launched at the same time as a new generation of 16-bit designs like the Intel 8086, and 32-bit designs were on the horizon, including Motorola's own 68000. It was not feature competitive with newer designs and not price competitive with older ones, and saw relatively little use in the market as a result.

It found use in the TRS-80 Color Computer, Dragon 32/64, and Thomson MO/TO home computers, the Vectrex game system, and early 1980s arcade machines including Star Wars, Defender, Robotron: 2084, Joust, and Gyruss. Series II of the Fairlight CMI digital audio workstation and Konami's Time Pilot '84 arcade game each use dual 6809 processors.[2]

Hitachi was a major user of the 6809 and later produced a further updated version as the Hitachi 6309.

History
Die of Motorola 6809

6800 and 6502

The Motorola 6800 was designed beginning in 1971 and released in 1974. In overall design terms, it has a strong resemblance to other CPUs that were designed from the start as 8-bit designs, like the Intel 8080.[lower-alpha 1] It was initially fabricated using early NMOS logic, which normally required several different power supply voltages. A key feature was an on-chip voltage doubler allowed it to run on a single +5V supply, a major advantage over its competitors like the Intel 8080 which required -5, +5, -12 and ground.[3] It was fabricated using the then-current contact lithography process, which led to the vast majority of the resulting chips being unusable and leading to high prices.[4]

A number of the 6800's designers were convinced that a lower-cost system would be key to widespread acceptance. 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. Peddle and a number of other members of the 6800 team left Motorola for MOS Technology and introduced this design in 1975 as the MOS Technology 6502.[5] In single-unit quantities, the 6502 sold for $25,[6] whereas 6800 was initially aimed at $360,[7] but had been lowered to $295 by this point.

There were three reasons for the 6502's low cost. One was that the designers stripped out any feature that wasn't absolutely required. This led to the removal of one accumulator and the use of smaller 8-bit index registers, both resulting in less internal wiring. Another change was the move to depletion-load NMOS logic, a new technique that required only +5V; the 6800 had only a single +5V pin externally but had multiple voltages inside the chip that all require separate power rails. These two changes allowed the 6502 to be 16.6 mm2,[8] as opposed to the 6800's 29.0 mm2,[9] meaning twice as many chips could be produced from a single wafer. Finally, MOS was using the new Micralign lithography system that improved average yield from perhaps 10% to 70%.[4]

With the introduction of the 6502, Motorola immediately lowered the price of the 6800 to $125, but it remained uncompetitive and sales prospects dimmed. The introduction of the Micralign to Motorola's lines allowed further reductions and by 1981 the price of the then-current 6800P was slighly less than the equivalent 6502, at least in single-unit quantities.[1] By that point, however, the 6502 had sold tens of millions of units and the 6800 had been largely forgotten.

6809

While the 6502 began to take over the 6800's market, Intel was experiencing the same problem when the upstart Zilog Z80 began to steal sales from the Intel 8080. Both Motorola and Intel began new design cycles to leapfrog those designs. This process led Intel to begin the design of a series of 16-bit processors, which emerged as the Intel 8086 in 1978. Motorola also began the design of a similar high-end design, in the MACSS project,[10] but also decided to produce a greatly improved but compatible 8-bit designs that became the 6809.

One of the major goals for the new design was to support position-independent code. Motorola believed that system integrators would look to off-the-shelf code in ROMs to handle common tasks. 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.[lower-alpha 2] A larger example is found in Motorola's 6809 programming manual, which contains the full listing of assist09, a so-called monitor, a miniature operating system intended to be burned in ROM.[11]

In previous processor designs, including the 6800, there was a mix of ways to refer to memory locations. Some of these were relative to the current location in memory or to a value in an index register, while others were absolute, a 16-bit value that referred to a physical location in memory. The former style allows code to be moved because the address it references will move along with the code. The absolute locations do not; code that uses this style of addressing will have to be recompiled if it moves. To address this, the 6809 filled out its instruction opcodes so that there were more instances of relative addressing where possible.[12]

As an example, the 6800 included a special "direct" addressing mode that was used to make code smaller and faster; instead of a memory address having 16-bits and thus requiring two bytes to store, direct addresses were only 8-bits long. The downside was that it could only refer to memory within a 256-byte window, the "direct page", which was normally at the bottom of memory - the 6502 referred to this as "zero page addressing" instead. The 6809 added a new 8-bit DP register, for "direct page". Code that formerly had to be in the zero page could now be moved anywhere in memory as long as the DP was changed to point to its new location.[12]

Using DP solved the problem of referring to addresses within the code, but data is generally located some distance from the code, outside ROM. To solve the problem of easily referring to data while remaining position independent, the 6809 added a variety of new addressing modes. Among these was program-counter-relative addressing which allowed any memory location to be referred to by its location relative to the instruction. Additionally, the stack was more widely used, so that a program in ROM could set aside a block of memory in RAM, set the SP to be the base of the block, and then refer to data within it using relative values.[13]

To further aid this type of access, the 6809 renamed the SP to U for "user", and added a second stack pointer, S, for "system".[14] The idea was user programs would use U while the CPU itself would use S to store data during subroutine calls. This allowed system code to be easily called by changing S without affecting any other running program. For instance, a program calling a floating-point routine in ROM would place its data on the U stack and then call the routine, which could then perform the calculations using data on its own private stack pointed to by S, and then return, leaving the U stack untouched.[14]

Another major reason for the expanded stack access was to support reentrant code, code that can be called from various different programs concurrently without concern for coordination between them, or that can recursively call itself. This makes the construction of operating systems much easier; the operating system had its own stack, and the processor could quickly switch between a user application and the operating system simply by changing which stack pointer it was using. This also makes servicing interrupts much easier for the same reason.[15]

Another major addition for the 6809 was one of the earliest dedicated hardware multipliers, which took 8-bit numbers in the A and B accumulators and produced a result in A+B, known collectively as D.[16] It also added a new fast interrupt style, which automatically saved only the program counter and condition code register before calling the interrupt code, whereas the original style in the 6800, now referred to as "normal", saved out all of the registers and thus took additional cycles before the interrupt code was called, and more to unwind the stack on exit.[17]

Market acceptance

Much of the design had been based around the market concept of building-block code. But the market for pre-rolled ROM modules never materialized: Motorola's only released example was the MC6839 floating-point ROM. The industry as a whole solved the problem of integrating code modules from separate sources by using automatic relocating linkers and loaders–which is still the solution used today. However, the decisions made by the design team enabled multi-user, multitasking operating systems like OS-9 and UniFlex.

The added features of the 6809 were costly; the CPU had approximately 9,000 transistors compared to the 6800's 4,100 or the 6502's 3,500. While process improvements meant it could be fabricated for less cost than the original 6800, those same improvements were being applied to the other designs and so the relative cost remained the same. Such was the case in practice; in 1981 the 6809 sold in single-unit quantities for roughly six times the price of a 6502.[1] For those systems that needed some of its special features, like the hardware multiplier, the system could justify its price, but in most roles, it was overlooked.

Another factor in its low use was the presence of newer designs with significantly higher performance. Among these was the Intel 8086, released the same year, and its lower-cost version, the Intel 8088 of 1979. A feeling for the problem can be seen in the Byte Sieve assembly language results against other common designs from the era (taken from 1981[18] and 1983[19]):

Byte Sieve comparison
ProcessorSeconds
6502 1 MHz13.9
Z80 4 MHz6.8
6809 2 MHz5.1
8086 8 MHz1.9
68000 8 MHz0.49

Although the 6809 did offer a performance improvement over the likes of the 6502[lower-alpha 3] and Z80, the improvement was not in line with the increase in price. For those where price was not the primary concern, but outright performance was, the new designs outperformed it by as much as an order of magnitude.

Even before the 6809 was released, in 1976 Motorola had launched its own advanced CPU project, then known as Motorola Advanced Computer System on Silicon project, or MACSS. 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 soon announced that their future 8-bit systems would be powered by cut-down versions of the 68000 rather than further improved versions of the 6809.

Major uses
TRS-80 Color Computer

Its first major use was in the TRS-80 Color Computer, which happened largely by accident. Motorola had been asked to design a color-capable computer terminal for a 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.[20] Tandy then re-worked the design to produce a home computer, which became one of the 6809's most notable design wins.[21]

SuperPET SP9000

Looking for a low-cost programming platform for computer science students, the University of Waterloo developed a system that combined a 6809-based computer-on-a-card with an existing Commodore PET, including a number of programming languages and program editors in ROM. The result was later picked up by Commodore, who sold it as the SuperPET, or MicroMainframe in Europe. These were relatively popular in the mid-1980s before the introduction of the PC clone market took over the programming role for most users.

Other popular home computer uses include the Fujitsu FM-7, Canon CX-1, Dragon 32/64, and the Thomson TO7 series. It was also available as an option on the Acorn System 2, 3 and 4 computers. Most SS-50 bus designs that had been built around the 6800 also had options for the 6809 or switched to it exclusively. Examples include machines from SWTPC, Gimix, Smoke Signal Broadcasting, etc. Motorola also build a series of EXORmacs and EXORset development systems.

Hitachi produced its own 6809-based machines, the MB6890 and later the S1. These were primarily for the Japanese market, but some were exported to and sold in Australia, where the MB6890 was dubbed the "Peach", probably in reference to the Apple II. The S1 was notable in that it contained paging hardware extending the 6809's native 64 kilobyte (64×210 byte) addressing range to a full 1 mebibyte (1×220 byte) in 4 KB pages. It was similar in this to machines produced by SWTPC, Gimix, and several other suppliers. TSC produced a Unix-like operating system uniFlex which ran only on such machines. OS-9 Level II, also took advantage of such memory management facilities. Most other computers of the time with more than 64 KB of memory addressing were limited to bank switching where much if not all the 64 KB was simply swapped for another section of memory, although in the case of the 6809, Motorola offered their own MC6829 MMU design mapping 2 mebibytes (2×220 byte) in 2 KB pages.[22]

Vectrex home video game console

The 6809 also saw some use in various videogame systems. Notable among these, in its 68A09 incarnation, in the unique vector graphics based Vectrex home videogame machine. It was also used in the Milton Bradley Expansion (MBX) system (an arcade console for use with the Texas Instruments TI-99/4A home computer, and a series of arcade games, released during the early to mid-1980s. Williams Electronics was a prolific user of the processor, which was deployed in Defender, Stargate, Joust, Robotron: 2084, Sinistar, and other games. The 6809 CPU forms the core of the successful Williams Pinball Controller. The KONAMI-1 is a modified 6809 used by Konami in Roc'n Rope, Gyruss, and The Simpsons.[23]

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, ESQ1, SQ80). The latter used the 6809E as their main CPU. The (E) version was used in order to synchronize the microprocessor's clock to the sound chip (Ensoniq 5503 DOC) in those machines; in the ESQ1 and SQ80 the 68B09E was used, requiring a dedicated arbiter logic in order to ensure 1 MHz bus timing when accessing the DOC chip.

In contrast to earlier Motorola products, the 6809 did not see widespread use in the microcontroller field. It was used in traffic signal controllers made in the 1980s by several different manufacturers,[24] as well as Motorola's SMARTNET and SMARTZONE Trunked Central Controllers (so dubbed the "6809 Controller"). These controllers were used as the central processors in many of Motorola's trunked two-way radio communications systems.[25]

Versions

The Motorola 6809 was originally produced in 1 MHz, 1.5 MHz (68A09) and 2 MHz (68B09) speed ratings. Faster versions were produced later by Hitachi. With little to improve, the 6809 marks the end of the evolution of Motorola's 8-bit processors; Motorola intended that future 8-bit products would be based on an 8-bit data bus version of the 68000 (the 68008). A micro-controller version with a slightly modified instruction set, the 6811, was discontinued as late as the second decade of the 21st century.

The Hitachi 6309 is an enhanced version of the 6809 with extra registers and additional instructions, including block move, additional multiply instructions, and division.

Legacy

Motorola spun off its microprocessor division in 2004. The division changed its name to Freescale and has subsequently been acquired by NXP.

Neither Motorola nor Hitachi produce 6809 processors or derivatives anymore. 6809 cores are available in VHDL and can be programmed into an FPGA and used as an embedded processor with speed ratings up to 40 MHz. Some 6809 opcodes also live on in the Freescale embedded processors. 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. Freescale supplied Rochester the original GDSII physical design database. At the end of 2016, Rochester's MC6809 (including the MC68A09, and MC68B09) is fully qualified and available in production.

Australian developer John Kent has synthesized the Motorola 6809 CPU in hardware description language (HDL).[26] This has made possible the use of the 6809 core at much higher clock speeds than were available with the original 6809. Gary Becker's CoCo3FPGA runs the Kent 6809 core at 25 MHz.[27] Roger Taylor's Matchbox CoCo runs at 7.16 MHz. Dave Philipsen's CoCoDEV runs at 25 MHz.[28]

Description

General design

The 6809's internal design is closer to simpler, non-microcoded CPU designs. Like most 8-bit microprocessors, the 6809 implementation is a register-transfer level machine, using a central PLA to implement much of the instruction decoding as well as parts of the sequencing.

Like the 6800 and 6502, the 6809 uses a two-phase clock to gate the latches. This two-phase clock cycle is used as a full machine cycle in these processors. Simple instructions can execute in as little as two or three such cycles. The 6809 has an internal two-phase clock generator (needing only an external crystal) whereas the 6809E needs an external clock generator. There are variants such as the 68A09(E) and 68B09(E); the internal letter indicates the processor's rated clock speed.

The 6800, 6502, the 6809's clock system differs from other processors of the era. For instance, the Z80 uses a single external clock and the internal steps of the instruction process continue on each transition. This means that the external clock generally runs much faster; 680x designs generally ran at 1 or 2 MHz while the Z80 generally ran at 2 or 4. Internally, the 680x's converted the slower external clock into a higher frequency internal schedule, so on an instruction-for-instruction basis, they ran roughly twice as fast when comparing the external clocks.

The advantage to the 680x style access was that dynamic RAM chips of the era generally ran at 2 MHz. Due to the cycle timing, there were periods of the internal clock where the memory bus was guaranteed to be free. This allowed the computer designer to interleave access to memory between the CPU and an external device, say a direct memory access controller, or more commonly, a graphics display chip. By running both chips at 1 MHz and stepping them one after the other, they could share access to the memory without any additional complexity or circuitry. 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.

Registers and instructions
6809 programming model, showing the processor registers.

The original 6800 included two 8-bit accumulators, A and B, a single 16-bit index register, X, a 16-bit program counter, PC, a 16-bit stack pointer, SP, and an 8-bit status register. The 6809 added a second index register, Y, a second stack pointer, U (while renaming the original S), and allowed the A and B registers to be treated as a single 16-bit accumulator, D. It also added another 8-bit register, DP, to set the base address of the direct page. These additions were invisible to 6800 code, and the 6809 was 100% source-compatible with earlier code.[16]

Another significant addition was program-counter-relative addressing for all data manipulation instructions. This was a key addition for position-independent code, as it allows data to be referred to relative to the instruction, and as long as the resulting memory location exists then the instructions can be moved in memory freely. The system retained its previous addressing modes as well, although in the new assembler language, what were previously separate instructions were now considered to be different addressing modes on other instructions. This reduced the number of instructions from the 6800's 78 instructions to the 6809's 59. 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]

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).[29]

Notes
  1. In contrast to other early CPU designs that attempted to emulate minicomputer instruction sets and were much more complex.
  2. Other examples are matrix arithmetic, Huffman encoding/decoding, statistical functions, string searching (e.g. by the Boyer-Moore algorithm) and tree structure management.
  3. Which by this time was also widely available in 2 MHz versions, as used in the Atari 8-bit family.

References

Citations
  1. Ancrona.
  2. "Konami Dual 6809 Based Hardware". System 16.
  3. "8080A microprocessor - DIP 40 package". CPU World.
  4. "Moore's Law Milestones". IEEE.
  5. Peddle, Chuck (12 June 2014). "Oral History of Chuck Peddle" (Interview). Interviewed by Doug Fairbairn and Stephen Diamond.
  6. "MOS 6502 the second of a low cost high performance microprocessor family". Computer. IEEE Computer Society. 8 (9): 38–39. September 1975. doi:10.1109/C-M.1975.219074.
  7. "Motorola microprocessor set is 1 MHz n-MOS". Control Engineering. 21 (11): 11. November 1974. MC6800 microprocessor price was $360. The MC6850 asynchronous communications interface adaptor (ACIA) was slated for first quarter 1975 introduction.
  8. Cushman, Robert H. (September 20, 1975). "2-1/2 Generation μP's -$10 Parts That Perform Like Low-End Mini's" (PDF). EDN. Boston: Cahners Publishing. 20 (17): 36–42. Archived from the original (PDF) on 24 April 2016.
  9. Motorola 6800 Oral History (2008), p. 10.
  10. "Motorola 68000". The Vintage Technology Association. 17 May 2007.
  11. Manual 1983, p. B.1.
  12. Manual 1983, p. 1.2.
  13. Manual 1983, pp. 2.1-2.4.
  14. Manual 1983, p. 1.3.
  15. Manual 1983, 4.
  16. Manual 1983, p. 1.1.
  17. Manual 1983, p. 1.9.
  18. Gilbreath, Jim (September 1981). "A High-Level Language Benchmark". Byte. pp. 180–198.
  19. Gilbreath, Jim; Gilbreath, Gary (January 1983). "Eratosthenes Revisited: Once More through the Sieve". Byte. pp. 283–325.
  20. "AgVision Videotex terminal". Vintage Computer. 2016-12-22. Retrieved 16 April 2018.
  21. G. Pitre, Boisy; Loguidice, Bill (2013). CoCo: The Colorful History of Tandy's Underdog Computer. Europe: CR Press. p. 19. ISBN 9781466592483. Retrieved 16 February 2018.
  22. "1981 Motorola Microprocessors Data Manual". archive.org. 1981. Retrieved 2 April 2018.
  23. Nicola Salmoria and the MAME Team. "konami: Portable Konami cpu emulator". Archived from the original on 2012-10-04. Retrieved 2012-10-21.
  24. Robert J. Simpson; Raveendran Paramesran (1998). Advanced 8-bit Microprocessor, MC6809: Its Software, Hardware, Architecture & Interfacing Techniques. Springer. ISBN 978-981-3083-09-7.
  25. "Motorola Type II SmartZone". The RadioReference Wiki.
  26. "John Kent's Home Web Page".
  27. http://www.davebiz.com/wiki/CoCo3FPGA
  28. http://www.davebiz.com/wiki/CoCoDEV
  29. John Bayko. "Great Microprocessors of the Present and the Past". Archived from the original on 2013-07-01. Retrieved 2013-07-01.

Bibliography

Further reading
Datasheets and manuals
Books
  • 6809 Assembly Language Programming; 1st Ed; Lance Leventhal; 579 pages; 1981; ISBN 0-931988-35-7. (archive)
  • The MC6809 Cookbook; 1st Ed; Carl Warren; 180 pages; 1980; ISBN 0-8306-9683-0. (archive)
  • Advanced 8-bit Microprocessor: MC6809: Its Software, Hardware, Architecture and Interfacing Techniques; 1st Ed; Robert Simpson; 274 pages; 1998; ISBN 978-9813083097
Magazines
  • A Microprocessor for the Revolution: The 6809; Terry Ritter & Joel Boney (co-designers of 6809); BYTE magazine; Jan-Feb 1979. (archive)
  • MC6809 microprocessor; Ian Powers; Microprocessors, Volume 2, Issue 3; July 1978; page 162; ISSN 0308-5953, doi:10.1016/0308-5953(78)90010-7.
Reference cards
  • MC6809 Reference Card; Motorola; 16 pages; 1981. (archive)
  • 6809/6309 Reference Card; Chris Lomont; 10 pages; 2007. (archive)
Simulators / Emulators
Boards
FPGA
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