Editing Motorola 6809 (section)

===Registers and instructions===
[[File:6809 Internal Registers.svg|thumb|300px|6809 programming model, showing the [[processor register]]s ]]
The original 6800 included two [[8-bit computing|8-bit]] [[accumulator (computing)|accumulator]]s, A and B, a single [[16-bit computing|16-bit]] [[index register]], X, a 16-bit [[program counter]], PC, a 16-bit [[stack pointer]], SP, and an 8-bit [[status register]]. The 6809 added a second index register, Y, a second stack pointer, U (while renaming the original S), and allowed the A and B registers to be treated as a single 16-bit accumulator, D. It also added another 8-bit register, DP, to set the base address of the direct page. These additions were invisible to 6800 code, and the 6809 was 100% [[source-code compatibility|source-compatible]] with earlier code.<ref name="motorola_mc6809_programming_manual"/>{{rp|pages=1.1}}

Another significant addition was program-counter-relative addressing for all data manipulation instructions. This was a key addition for [[position-independent code]], as it allows data to be referred to relative to the instruction, and as long as the resulting memory location exists then the instructions can be moved in memory freely. The system retained its previous addressing modes as well, although in the new [[assembler language]], what were previously separate instructions were now considered to be different addressing modes on other instructions. This reduced the number of instructions from the 6800's 78 instructions to the 6809's 59. These new modes had the same opcodes as the previously separate instruction, so these changes were only visible to the programmer working on new code.<ref name="motorola_mc6809_programming_manual"/>{{rp|pages=1.2}}

The [[instruction set]] and register complement are highly [[Orthogonal (computing)|orthogonal]], making the 6809 easier to program than contemporaries. Like the 6800, the 6809 includes an undocumented address bus test instruction which came to be nicknamed [[Halt and Catch Fire (computing)|Halt and Catch Fire (HCF)]].<ref>{{cite web |url=https://github.com/hoglet67/6809Decoder/wiki/Undocumented-6809-Behaviours |title=Undocumented 6809 Behaviours |author=David Banks |access-date=2023-01-22 |archive-date=2023-01-22 |archive-url=https://web.archive.org/web/20230122183942/https://github.com/hoglet67/6809Decoder/wiki/Undocumented-6809-Behaviours |url-status=live }}</ref><ref>{{cite web |url=https://student.brighton.ac.uk/burks/pcinfo/hardware/cpu.htm |title=Great Microprocessors of the Present and the Past|author=John Bayko|access-date=2013-07-01 |url-status=dead |archive-url=https://archive.today/20130701061311/https://student.brighton.ac.uk/burks/pcinfo/hardware/cpu.htm |archive-date=2013-07-01 }}</ref>