Motorola 6809

Significant enhancements to the 6809 processor included two 8-bit batteries (A and B, which could be combined into one 16-bit register D), two 16-bit index registers (X and Y), and two 16-bit stack pointers (U and S). Index registers and stack pointer registers allowed the use of a variety of addressing methods .

The 6809 processor was backward compatible with the 6800 at the source code level. However, the number of teams was reduced from 78 to 59. Some teams were replaced by more general ones; to ensure compatibility, the assembler replaced the old teams with their new counterparts during the broadcast. The sets of instructions and registers were more orthogonal, which made writing programs for the processor easier than with the 6800 or 6502 processors.

Among other features, there was one of the first hardware implementations of the multiplication instruction, full 16-bit arithmetic, and a fast interrupt system. The processor was highly optimized and ran up to five times faster than the 6800 series processors. The processor retained the undocumented instruction for testing the address bus that appeared in the 6800 processor. It allowed to bypass the limitations of some memory controllers and was called Halt and Catch Fire (HCF) .

Unlike many processors of that time, the processor did not use microcode . Its finite state machine and control logic were mainly implemented in large PLC and asynchronous arbitrary logic (a common feature of early designs, as well as RISC processors ). Therefore, one machine cycle was performed in just one cycle, which was an advantage over, for example, the Zilog Z80 processor (one of the main competitors 6809). For example, the ADDA 63 command was executed in three cycles on 6809, while the execution of a similar ADD A,63 command of the Z80 processor required seven cycles. Theoretically, to achieve the same performance in this operation, the Z80 required a minimum of twice the frequency. However, the more synchronous design of the Z80 made it possible to use a higher (3-5 times) clock speed compared to 6809, 6800 or 6502, without the need for a faster memory (which was often the main limitation).

Several processor versions have been released, designed for different clock speeds - 6809 (1 MHz), 68A09 (1.5 MHz) and 68B09 (2 MHz). Regular versions of the processor had a built-in clock generator, requiring only an external quartz resonator. There were also versions with the letter E at the end of the designation (6809E, 68A09E, 68B09E) requiring the use of an external clock generator. Subsequently, third-party manufacturers, in particular Hitachi , released faster versions of the processor.

The 6809 is sometimes called the conceptual predecessor of the Motorola 68000 processor family, which is a misconception. Both projects were developed in parallel and were completed in 1979. There is a certain similarity between them - in particular, the flexibility of addressing methods and great orthogonality. There are also some similarities between mnemonics and assembler syntax . But the processor 6809 developed the ideas of the processor 6800, while the processor 68000 was a completely new development. Version 68000 with an eight-bit data bus ( Motorola 68008 ) was intended for use in future 8-bit systems, so the 6809 quickly became an evolutionary dead end.

The processor development team believed that future processor users might need ready-made ROMs to solve common tasks, such as floating point computing. This could allow processor-based systems developers not to waste time developing their own code. Since the developers of the processor could not know in advance where the ROM code will be located in the address space of the system, the processor device was focused on the ability to execute cost-effective routines located anywhere in the address space. This feature has not been used in practice for these purposes. The only code ROM ROM released by Motorola was the MC6839 chip, which contains floating point code. However, such a processor device made it possible to create such advanced operating systems as OS-9 and UniFlex , which used the ability to execute reentrant code that was not tied to specific addresses in memory.

The 6809 processor was used in the Commodore SuperPET dual-processor computer and in the unique Vectrex game console (68A09). Modification 6809E was used in home computers TRS-80 Color Computer (CoCo) , Acorn System 2, 3 and 4 (as an optional alternative to 6502 ), Fujitsu FM-7 (two 6809), Dragon 32/64 (CoCo clones), as well as SWTPC , Gimix , Smoke Signal Broadcasting and other SS-50 systems, including several systems for the development of Motorola's EXORmacs series. In France, Thomson SA has released a large series of computers based on the 6809E processor - TO7, TO7 / 70, TO8, TO8D, TO9, TO9Plus, MO5 , MO6, MO5E and MO5NR.

The processor was also used in a number of arcade slot machines of the early 1980s. In particular, it was often used by Williams Electronics , for example, in such arcade hits as Defender , Joust , Sinistar and Robotron: 2084 . This company also used the processor in many of its pinball machines, subsequently creating a specialized Williams Pinball Controller board based on it.

Microware developed a special OS-9 operating system (not related to Mac OS 9 ) for the 6809 processor. It was subsequently ported to Motorola 68000 , Intel 80386, and PowerPC processors .

Hitachi has released an improved processor version, Hitachi 6309 . It was distinguished by the presence of additional registers and commands, including block copy commands, additional multiplication commands, and division commands with hardware implementation. This processor was used to unofficially expand CoCo 3 computers. A special version of OS-9 called NitrOS-9 was developed that supports additional processor capabilities.

Hitachi also produced its own computers with a 6809 processor, designed for the domestic Japanese market - MB6890 and later S1. These computers have also been exported to Australia . There, the MB6890 was nicknamed “Peach” (peach), probably as an ironic reference to the popularity of Apple II computers. Computer S1 was distinguished by the presence of equipment expanding the address space of the processor (64 KB) to 1 megabyte, presented in the form of 4 kilobyte pages. Similar equipment was present in the computers of the companies SWTPC , Gimix and some others. Technical Systems Consultants has developed a Unix- like operating system, uniFlex , which only runs on such computers. OS-9 Level II also took advantage of this memory management method. Most of the other computers of that time used the page switching technique to increase the amount of addressable memory when part of the available memory was included in most of the address space (for example, 16 KB).

Motorola and Hitachi do not currently produce 6809 processors or their variants. There are VHDL processor implementations designed for embedded applications using programmable logic and capable of operating at frequencies up to 40 MHz. Some 6809 instructions are also present on Freescale processors.

• Clock frequency (MHz): from 0.89 to 4, originally produced with a frequency of 1, 1.5 and 2 MHz.
• Bit depth: 8/16 bit
• Data Bus Width : 8 bits
• Address Bus Width : 16 bits
• The amount of addressable memory: 64 KB
• I / O Address Space: None
• Number of Transistors: 9,000
• The manufacturing process (nm): 6000 (6 microns) (according to other sources, 5 microns)
• Crystal Area (sq. Mm): 21
• Maximum heat dissipation: 1 W
• Supply voltage: +5 V
• Connector: no (the chip was soldered to the board)
• Case: 40-pin ceramic or plastic DIP , later - 44-pin PLCC

• Leventhal, Lance (1981). 6809 Assembly Language Programming . Osborne / McGraw-Hill. ISBN 0-931988-35-7 .
• Warren, Carl D (1980). The MC6809 Cookbook . TAB Books, Inc. ISBN 0-8306-9683-0 .

• 6809 Emulation Page - Documentation and Utilities
• Articles from BYTE Magazine (January-February 1979) ( PDF ) - written by Terry Ritter and Joel Boney, who participated in the development of the 6809 processor (Eng.)
• Usenet 6809 Processor
• 6809 emulator based on the SWTPC 6809 system
• Processor instruction set 6809/6309 (PDF )
• An JavaScript emulation of Motorola 6809 CPU