Motorola 6809
The finest 8-bit CPU
The 1978 Motorola processor widely considered the best 8-bit CPU ever designed, featuring clean orthogonal architecture, 16-bit operations, and position-independent code support.
Overview
The Motorola 6809 is widely considered the finest 8-bit CPU ever designed. Released in 1978, it featured clean orthogonal architecture, native 16-bit operations via the D register, two stack pointers, and support for position-independent code—revolutionary features that made it feel like a 16-bit processor.
Fast Facts
- Released: 1978
- Clock: 1-2 MHz typical
- Architecture: 8-bit with 16-bit operations
- Registers: A, B, D (A+B), X, Y, U, S, DP, CC
- Addressing: Orthogonal, PC-relative supported
Register Set
| Register | Size | Purpose |
|---|---|---|
| A, B | 8-bit | Accumulators |
| D | 16-bit | Combined A:B |
| X, Y | 16-bit | Index registers |
| U, S | 16-bit | User/System stacks |
| DP | 8-bit | Direct page |
| CC | 8-bit | Condition codes |
Key Features
| Feature | Significance |
|---|---|
| D register | Native 16-bit arithmetic |
| Two stacks | Elegant subroutine handling |
| PC-relative | Position-independent code |
| Orthogonal | All modes work everywhere |
vs Other 8-bit CPUs
| Aspect | 6502 | Z80 | 6809 |
|---|---|---|---|
| 16-bit ops | Limited | Some | Extensive |
| Stack | 8-bit, one | 16-bit, one | 16-bit, two |
| Design | Quirky | Complex | Clean |
Systems Using 6809
- Dragon 32/64
- TRS-80 Color Computer
- Vectrex
- Williams arcade (Defender, Robotron)
- Konami arcade
Legacy
The 6809 was technically superior to contemporary 8-bit CPUs but lost on economics—it cost more than the 6502 and arrived after those platforms dominated. Its clean design influenced later processors and remains beloved by assembly programmers.