Motorola 6809

The 6809 was a major advance over both its predecessor, the Motorola 6800 and also over the MOS Technologies 6502. The 6809 had two 8-bit accumulators, rather than one in the 6502, and could combine them into a single 16-bit register. It also featured two 16-bit index registers and two stack pointers, which allowed for some 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 orthagonal, 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 multiplication instructions of the time, 16-bit arithmetic and a special fast interrupt. But it was also highly optimised, gaining up to five times the speed of the 6800 series CPU. Like the 6800, it included the undocumented HCF (Halt and Catch Fire) 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, rather than written using microcode. This meant that it took many fewer CPU clock cycles to process instructions. For instance 'ADDA 63' took 3 clock cycles (2 for the instruction fetch, and one for the operation to take place). 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 vs the 6800 / 6809 single clock cycle. Different clocks!

The 6809 was originally produced in 1MHz and 2MHz versions, but faster versions were produced later.

The 6809E was used in the UK Dragon 32 and Dragon 64 home computers. It is considered to be the precursor to the Motorola 68000 family of processors, though 68K design overlapped the 6809 project.

The 6809E was used in the Tandy Color Computer sold by Radio Shack[?] 1 (http://www.cs.unc.edu/~yakowenk/coco/text/history).

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 and 68B09 where the letter indicated the clock speed that the processor was rated at.

Microware developed the original OS 9 (not the later MacOS 9) on the 6809, later porting it to the 68000 series of microprocessors.

The Hitachi 6309 is a version with extra registers and additional instructions, including block move, additional multiply instructions, and a divide.

Unfortunately both Motorola and Hitachi no longer produce the 6809 processor even though it was almost certainly the most powerful general purpose 8 bit CPU ever designed. It had many innovative features, several of which were copied and used elsewhere later.