Motorola 6809E

The Motorola 6809 ("sixty-eight-oh-nine") is an 8-bit microprocessor CPU with some 16-bit features from Motorola. It was 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 systems to use the 6809 are the Dragon home computers, TRS-80 Color Computer, the Vectrex home console, and early 1980s arcade machines including Defender, Robotron: 2084, Joust, and Gyruss.

The 6809 was the first microprocessor able to use fully position-independent, or reentrant, or both, code without the use of difficult programming tricks. It also contained one of the first hardware-implementations of a multiplication instruction in an MPU, full 16-bit arithmetic, and an especially fast interrupt system.

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.

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