Motorola 6809

Motorola 6809

General information
Launched	1978(1978)
Common manufacturer	Motorola
Performance
Data width	8
Address width	16
Architecture and classification
Instruction set	6809
Number of instructions	59
Physical specifications
Transistors	9,000
Package	40-pinDIP

TheMotorola 6809( "sixty-eight-oh-nine") is an8-bitmicroprocessorwith some16-bitfeatures. It was designed byMotorola's Terry Ritter and Joel Boney and introduced in 1978. Although source compatible with the earlierMotorola 6800,the 6809 offered significant improvements over it and 8-bit contemporaries like theMOS Technology 6502,including a hardware multiplication instruction, 16-bit arithmetic, system and userstack registersallowing re-entrant code, improvedinterrupts,position-independent code, and anorthogonal instruction setarchitecture with a comprehensive set of addressing modes.

The 6809 was among the most powerful (and most expensive) 8-bit processors of its era. In 1981 a 6809 in single-unit quantities was$37compared to$9for aZilog Z80and$6for a 6502.It was launched when a new generation of 16-bit processors were coming to market, like theIntel 8086,and 32-bit designs were on the horizon, including Motorola's own68000.It was not feature competitive with newer designs and not price competitive with older ones.

Usage

The 6809 is used in theTRS-80 Color Computer,Dragon 32/64,SuperPET,ENER 1000,Fujitsu FM-7,theCybernex LC3,andThomson MO/TOhome computers, theVectrexgame console, and early 1980s arcade video games includingStar Wars,Defender,Robotron: 2084,Joust,andGyruss.1990sWilliamspinball machines are equipped withWPC-series controller boards based on 68B09.Series II of theFairlight CMIdigital audio workstation and Konami'sTime Pilot '84arcade game each use dual 6809 processors.Hitachiwas a major user of the 6809 and later produced an updated version as theHitachi 6309.

History

Dieof Motorola 6809

6800 and 6502

TheMotorola 6800was 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 theIntel 8080.It was initially fabricated using earlyNMOS logic,which normally required several differentpower supplyvoltages. A key feature was an on-chipvoltage doublerthat allowed it to run on a single+5 Vsupply, a major advantage over its competitors like theIntel 8080which required−5 V,+5 V,−12 Vandground.

The 6800 was initially fabricated using the then-currentcontact lithographyprocess. In this process, thephotomaskis placed in direct contact with thewafer,exposed, and then lifted off. 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. For complex multi-patterned designs like a CPU, this led to about 90% of the chips failing when tested. To make a profit on the small number of chips that did work, the prices for the working models had to be fairly high, on the order of hundreds of dollars in small quantities.

Some of the 6800's designers were convinced that a lower-cost system would be key to widespread acceptance. Notable among them wasChuck 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 some other members of the 6800 team left Motorola forMOS Technologyand introduced this design in 1975 as theMOS Technology 6502.

The 6800 was initially sold at$360in single-unit quantities,but had been lowered to$295.The 6502 was introduced at$25,and Motorola immediately reduced the 6800 to $125. It remained uncompetitive and sales prospects dimmed. The introduction of theMicralignto 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.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,Intelwas experiencing the same problem when the upstartZilog Z80began to steal sales from theIntel 8080.Both Motorola and Intel began new design cycles to leapfrog those designs. Intel responded by quickly introducing a small but practical upgrade of the 8080 as the8085,which made it less expensive to use and more competitive with the Z80. They also began to design a series of 16-bit processors, which emerged as theIntel 8086in 1978. Motorola also began the design of a similar high-end design, in the MACSS project,but did not initially consider an improved 8-bit design. But when they polled their existing 6800 customers, they found that many were not willing to pay for a 16-bit design for their simple needs. This led to the decision to produce a greatly improved but compatible 8-bit design that became the 6809.

Analysis of 6800 code demonstrated that loads and stores were the vast majority of all the time in CPU terms, accounting for 39% of all the operations in the code they examined. In contrast, mathematical operations were relatively rare, only 2.8% of the code. 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. Similarly, increments and decrements accounted for only 6.1% of the code, but these almost always occurred within loops where each one was performed many times. This led to the addition of post-incrementing and pre-decrementing modes using theindex registers.

The main goal for the new design was to supportposition-independent code.Motorola's market was mostlyembedded systemsand similar single-purpose systems, which often ran programs that were very similar to those on other platforms. Development for these systems often took the form of collecting a series of pre-rolledsubroutinesand combining them together. However, asassembly languageis generally written starting at abase address,combining pre-written modules normally required a lengthy process of changingconstants(or equates) that pointed to key locations in the code.

Motorola's idea was to eliminate this task and make the building-block concept much more practical. System integrators would simply combine off-the-shelf code in ROMs to handle common tasks. Libraries of common routines likefloating pointarithmetic, graphics primitives,Lempel-Ziv compression,and so forth would be available to license, combine together along with custom code, and burn to ROM.

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.

As an example, the 6800 includes a directaddressing modeused to make code smaller and faster; instead of a memory address having 16-bits and thus requiring two bytes to store, direct addresses are only 8-bits long. The downside is that it can only refer to memory within a 256-byte window, thedirect page,which is normally at the bottom of memory - the 6502 refers to this aszero page addressing.The 6809 added a new 8-bit DP register, for direct page. Code that formerly had to be in thezero pagecan now be moved anywhere in memory as long as the DP is changed to point to its new location.

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, thestackwas 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.

To aid this type of access, the 6809 renamed the SP to U for "user", and added a second stack pointer, S, for "system".The idea was user programs would use U while the CPU itself would use S to store data duringsubroutine 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.

Another reason for the expanded stack access was to supportreentrantcode, 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 ofoperating systemsmuch 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 servicinginterruptsmuch easier for the same reason.The 6809 adds a fastinterrupt request(FIRQ) interrupt that saves only the program counter and condition code register before calling the interrupt code, whereas the IRQ interrupt saves all registers, taking additional cycles, then more tounwind the stackon exit.

The 6809 includes one of the earliest dedicated hardware multipliers.It takes 8-bit numbers in the A and B accumulators and produces a result in A:B, known collectively as D.

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 usingautomatic relocating linkers and loaders,which is the solution used today.However, the decisions made by the design team enabled multi-user, multitasking operating systems likeOS-9andUniFlex.

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 cheaper 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.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 theIntel 8086,released the same year, and its lower-cost version, theIntel 8088of 1979. A feeling for the problem can be seen in theByte Sieveassembly language results against other common designs from the era (taken from 1981and 1983):

Byte Sieve comparison

Processor	Seconds	Total clocks (millions)
6502 1 MHz	13.9	13.9
Z80 4 MHz	6.8	27.2
6809 2 MHz	5.1	10.2
8086 8 MHz	1.9	15.2
68000 8 MHz	0.49	3.92

Although the 6809 did offer a performance improvement over the likes of the 6502and 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 anorder 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 theIBM PCproject, when MACSS appeared as theMotorola 68000in 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 theTRS-80 Color Computer,which happened largely by accident. Motorola had been asked to design a color-capablecomputer terminalfor 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.Tandy then re-worked the design to produce ahome computer,which became one of the 6809's most notable design wins.

SuperPET SP9000

Looking for a low-cost programming platform forcomputer sciencestudents, theUniversity of Waterloodeveloped a system that combined a 6809-based computer-on-a-card with an existingCommodore PET,including a number of programming languages and program editors in ROM. The result was later picked up by Commodore, who sold it as theSuperPET,or MicroMainframe in Europe. These were relatively popular in the mid-1980s before the introduction of thePC clonemarket took over the programming role for most users.

Other popular home computer uses include theFujitsu FM-7,Canon CX-1,Dragon 32/64,and theThomson TO7series. It was also available as an option on theAcorn System 2,3 and 4 computers. MostSS-50 busdesigns that had been built around the 6800 also had options for the 6809 or switched to it exclusively. Examples include machines fromSWTPC,Gimix,Smoke Signal Broadcasting,etc. Motorola also build a series ofEXORmacsand EXORset development systems.

Hitachiproduced its own 6809-based machines, the MB-6890 and later the S1. These were primarily for the Japanese market, but some were exported to and sold inAustralia,where the MB-6890 was dubbed the "Peach", probably in reference to theApple II.The S1 was notable in that it containedpaginghardware extending the 6809's native 64kilobyteaddressing range to a full 1megabytein 4 KB pages. It was similar in this to machines produced by SWTPC, Gimix, and several other suppliers. TSC produced a Unix-like operating systemuniFlexwhich ran only on such machines.OS-9Level 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 tobank switchingwhere 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 MC6829MMUdesign mapping 2 megabytes in 2 KB pages.

Vectrex home video game console

The 6809 also saw use in variousvideogamesystems. Notable among these, in the 68A09 incarnation, was the uniquevector graphics-basedVectrexhome videogame machine. It was also used in the Milton Bradley Expansion (MBX) system (an arcade console for theTI-99/4Ahome computer), and a series ofarcade gamesreleased during the early to mid-1980s.Williams Electronicswas a prolific user of the processor, which was deployed inDefender,Stargate,Joust,Robotron: 2084,Sinistar,and other games. The 6809 CPU forms the core of the successfulWilliams Pinball Controller.The KONAMI-1 is a modified 6809 used byKonamiinRoc'n Rope,Gyruss,andThe Simpsons.

Series II of theFairlight CMI(computer musical instrument) used dual 6809 CPUs runningOS-9,and also used one 6809 CPU per voice card. The 6809 was often employed in music synthesizers from other manufacturers such asOberheim(Xpander,Matrix 6/12/1000), PPG (Wave 2/2.2/2.3, Waveterm A), andEnsoniq(Miragesampler, SDP-1,ESQ-1,SQ-80). 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 ESQ-1 and SQ-80 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 themicrocontrollerfield. It was used intraffic signalcontrollers made in the 1980s by several different manufacturers,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.

The 6809 was used by Mitel as the main processor in its SX20 Office Telephone System.

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 (the68008). A microcontroller version with a slightly modified instruction set, the6811,was discontinued as late as the second decade of the 21st century.

TheHitachi 6309is 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 was subsequently acquired byNXP.

Neither Motorola nor Hitachi produce 6809 processors or derivatives anymore. 6809 cores are available inVHDLand can be programmed into anFPGAand used as an embedded processor with speed ratings up to 40 MHz.Some 6809 opcodes also live on in theFreescaleembedded processors. In 2015, Freescale authorizedRochester Electronicsto start manufacturing the MC6809 once again as a drop-in replacement and copy of the original NMOS device. Freescale supplied Rochester the originalGDSIIphysical 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).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.Roger Taylor's Matchbox CoCo runs at 7.16 MHz. Dave Philipsen's CoCoDEV runs at 25 MHz.

Description

The 6809's internal design is closer to simpler, non-microcodedCPU designs. Like most 8-bit microprocessors, the 6809 implementation is aregister-transfer levelmachine, using a centralPLAto implement much of the instruction decoding as well as parts of the sequencing.

Like the 6800 and 6502, the 6809 uses atwo-phase clockto gate the latches. This two-phase clock cycle is used as a fullmachine cyclein 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 divided the external clock frequency by four to create the system clock; so a 1 MHz 6809 would have a 4 MHz crystal or clock signal. Typically, on an instruction-for-instruction basis, they ran roughly twice as fast, when comparing the external clocks to other microprocessors.

The advantage to the 680x style access was thatdynamic RAMchips 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 adirect memory accesscontroller, or more commonly, agraphics 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 theprocessor registers

The original 6800 included two8-bitaccumulators,A and B, a single16-bitindex register,X, a 16-bitprogram counter,PC, a 16-bitstack pointer,SP, and an 8-bitstatus 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-compatiblewith earlier code.

Another significant addition was program-counter-relative addressing for all data manipulation instructions. This was a key addition forposition-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 newassembler 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.

Theinstruction setand register complement are highlyorthogonal,making the 6809 easier to program than contemporaries. Like the 6800, the 6809 includes an undocumented address bus test instruction which came to be nicknamedHalt and Catch Fire (HCF).

References

Bibliography

• "Ancrona is better"(PDF).Radio-Electronics(Advertising). Vol. 54, no. 4. April 1981. p. 111.

Further reading

Datasheets and manuals

• MC6809 Datasheet;Motorola; 36 pages; 1983.
• MC6809E Datasheet;Motorola; 34 pages.
• Motorola 8-bit Microprocessors Data Book;Motorola; 1182 pages; 1981.

Books

• 6809 Assembly Language Programming;1st Ed; Lance Leventhal; 579 pages; 1981;ISBN0-931988-35-7.(archive)
• The MC6809 Cookbook;1st Ed; Carl Warren; 180 pages; 1980;ISBN0-8306-9683-0.(archive)
• Advanced 8-bit Microprocessor: MC6809: Its Software, Hardware, Architecture and Interfacing Techniques;1st Ed; Robert Simpson; 274 pages; 1998;ISBN978-9813083097
• Programming the 6809;1st Ed;Rodnay Zaksand William Labiak; 362 pages; 1982;ISBN978-0895880789

Magazines

• A Microprocessor for the Revolution: The 6809;Terry Ritter & Joel Boney (co-designers of 6809); BYTE magazine; Jan-Feb 1979.(archive)Archived2010-02-15 at theWayback Machine
• MC6809 microprocessor;Ian Powers; Microprocessors, Volume 2, Issue 3; July 1978; page 162;ISSN0308-5953,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)

External links

Simulators / Emulators

• 6809 Emulation Page– collection of 6809 instructions, emulators, tools, debuggers, disassemblers, assemblers
• 6809 Emulator based on the SWTPC 6809 system

Boards

• Grant's 6-chip 6809 computer
• 6809 microprocessor training board

FPGA

• System09 6809 CPU core - VHDL source code- OpenCores -project website