Computer Dictionary/Motorola 6809
The 6809 was a major advance over both its predecessor, the Motorola 6800 and also over the 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 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 around 59 (including a SEX instruction). Some instructions were replaced by more general ones which the assembler would translate, and some were even replaced by addressing modes.
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.
See also SEX.