Motorola 6809 microprocessor family

Motorola 6809 (MC6809) is an enhanced version of the 6800 microprocessor. The 6809 is not object-code compatible with the 6800, but it is source-code compatible. The 6809 processor added many new features:

• New instructions for loading effective address into registers and exchanging the contents of registers.
• New stack handling instructions, including instructions for saving and retrieving the contents of one or more registers to/from the stack. In addition to the system stack pointer the 6809 includes additional user stack pointer.
• New Direct Page register was added to the MC6809 processor. This register allowed the CPU to use any memory page for direct page addressing.
• Indexed addressing of the 6809 was expanded to provide many new addressing modes: indexed addressing using one of 4 registers (2 stack points, X index register or new Y index register) with 5-bit, 8-bit or 16-bit offset, indexed with accumulator offset, indexed with auto-increment or auto-decrement by 1 or 2, and relative to program counter addressing.
• Limited support for 16-bit data processing.
• 16-bit relative branches could be used to branch anywhere in memory. These branches, Direct Page register and big number of indexed addressing modes greatly simplified process of writing of position-independent code.

The Motorola 6809 was produced in two versions - with on-chip clock (6809) and with external clock input (6809E).