Motorola 6809

The Motorola 6809 is an 8-bit* microprocessor from Motorola, introduced circa 1979. The 6809 was a major advance over both its predecessors, the in-house Motorola 6800 and the latter's near clone the MOS Technology 6502. (* some would say 8/16-bit — see Description, below)

Description

Among the significant enhancements the 6809 introduced over its predecessors was the employment of two 8-bit accumulators (rather than one in the 6502), which could be combined into a single 16-bit register. There were also two 16-bit index registers (vs 6502 two 8-bit "index" registers getting added to 16bit pointers located in memory) and two stack pointers (vs one 8bit in the 6502), allowing very advanced addressing modes.

The 6809 was source-compatible with the 6800, even though the 6800 had 78 instructions and the 6809 only had 59. Some instructions were replaced by more general ones which the assembler would translate, and some were even replaced by addressing modes. The instruction set and register complement were highly orthogonal, making assembly language programming quite pleasant, not the special case nightmare of most other microcomputer CPUs of the time.

Other features were one of the first hardware-implemented multiplication instructions of the time, full 16-bit arithmetic and an especially fast interrupt system. But the 6809 was also highly optimised, gaining up to five times the speed of the 6800 series CPU. Like the 6800, it included the undocumented Halt and Catch Fire (HCF) bus test instruction.

The optimisation of the 6809 processor meant that, unlike many processors of the day, the instructions were mostly hardwired into the processor (a common trait of the RISC CPUs not prevalent until the 1990s), rather than written using microcode (a typical CISC trait). This meant that it took significantly fewer CPU clock cycles to process instructions. As an example, the instruction "ADDA 63" took 3 clock cycles (2 for the instruction fetch, and one for the operation to take place). On the 6502 immediate ops took 2 clock cycles.

On the Zilog Z80, which was probably the main competitor to the 6809, "ADD A,63" took 7 clock cycles. This meant that the Z80 needed to have a clock speed of at least twice that of the 6809 to match its performance. In addition, the Motorola 8-bit CPUs used one clock cycle per memory access, not the internal state clock of most other microcomputers of the time. A single memory read operation on a Z-80, for instance, needed several clock cycles.

The 6809 had an internal clock generator (needing only an external crystal). The 6809E needed an external clock generator. There were variants such as the 68A09(E) and 68B09(E) where the middle letter indicated the clock speed that the processor was rated at.

History

The Motorola 6809 was originally produced in 1MHz and 2MHz versions, but faster versions were produced later. It is considered to be the 'moral precursor' to the Motorola 68000 family of processors, though 68K design actually overlapped the 6809 project in time.

The 6809 was used in Commodore's dual-CPU SuperPET computer, and, in its 68A09 incarnation, in the unique vector graphics based Vectrex home video game console with built-in screen display. The 6809E was used in the TRS-80 Color Computer (CoCo), Acorn System 2, 3 and 4 computers and in the CoCo's UK clones, the Welsh-made Dragon 32/64 home computers.

Software development company Microware developed the original OS-9 operating system (not to be confused with Mac OS 9) for the 6809, later porting it to the 68000 series of microprocessors.

The Hitachi 6309 was an enhanced version of the 6809 with extra registers and additional instructions, including block move, additional multiply instructions, and hardware-implemented division. It was used in 'unofficially upgraded' CoCo 3's, and a version of OS-9 were written to take advantages of the 6309's extra features.

Unfortunately neither Motorola nor Hitachi produce 6809 processors or derivatives anymore, despite the fact that it must be said to have been one of the most powerful general-purpose 8-bit CPU ever designed. Somewhat ahead of its time, it had many innovative features several of which were copied and used in other designs later.

References

• Leventhal, Lance (1981). 6809 Assembly Language Programming. Berkeley, California: Osborne/McGraw-Hill. ISBN 0-931988-35-7.

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