M6809

The Motorola 6809 ("sixty-eight-oh-nine") is an 8-bit (with some 16-bit features) microprocessor CPU from Motorola, designed by Terry Ritter and Joel Boney and introduced in 1978. It was a major advance over both its predecessor, the Motorola 6800, and the related MOS Technology 6502.

Among the significant enhancements introduced in the 6809 were the use of two 8-bit accumulators (A and B, which could be combined into a single 16-bit register, D), two 16-bit index registers (X, Y) and two 16-bit stack pointers. The index and stack registers allowed advanced addressing modes. Program counter relative addressing allowed for the easy creation of position-independent code, while a user stack pointer (U) facilitated the creation of reentrant code. The 6809 was the first microprocessor able to use fully position-independent, or reentrant, or both, code without the use of difficult programming tricks.

The 6809 was assembler source-compatible with the 6800, though the 6800 had 78 instructions to the 6809's 59. Some instructions were replaced by more general ones which the assembler translated into equivalent operations and some were even replaced by addressing modes. The instruction set and register complement were highly orthogonal, making the 6809 easier to program than the 6800 or 6502.

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