MC6809

Introduction

A library written in C++ that emulates the MC6809 cpu. The enclosed CMakeLists.txt file (standard cmake procedure) will build the library and a small test application. To use this library in your project, copy the five source files from ./src/ into your source tree.

At this very moment, the following is not yet implemented:

• CWAI opcode
• illegal opcode exceptions (vector at $fff0)

API

Constructor

mc6809::mc6809()

Read and Write to Memory

Abstract class, read8 and write8 must be implemented in your subclass.

Make sure the connected memory has a functioning ROM and vector table from $fff0 to $ffff. Please note that an extra vector at $fff0 (originally reserved by Motorola) has been added that enables handling of illegal opcodes (a feature borrowed from the Hitachi 6309).

NMI / FIRQ / IRQ

When nothing is assigned by the hosting software, the pin states will default to high (1 or true) internally, effectively meaning no exceptions of the above three types will happen. It is up to the programmer to supply proper connections:

void mc6809::assign_nmi_line(bool *line)

void mc6809::assign_firq_line(bool *line)

void mc6809::assign_irq_line(bool *line)

These functions take a pointer to a boolean value (the actual line/value that represent interrupt states from the connected devices). If more devices are to be connected to one line, this must be separately programmed (see E64 source code for an example, exceptions_ic class).

Reset

void mc6809::reset()

Execute

uint16_t mc6809::execute()

Main execute function. Runs only one instruction, and returns the number of cycles consumed. Checking for breakpoints must be done with the mc6809::breakpoint() member function inbetween calls to the mc6809::execute() function. In the broadest sense, one instruction also means starting an exception (be it nmi/firq/irq).

Running more that one instruction or the ability to run N cycles has been considered but is not implemented. In most use cases we want to do nmi/firq/irq things immediately after each instruction anyway. Also checking for breakpoints becomes simpler this way.

Links

• E64 - A virtual computer system inspired by the Commodore 64 using an MC6809 cpu and implementing some Amiga 500 and Atari ST technology.
• lib65ce02 - CSG65CE02 emulator written in C.
• Moira - Motorola 68000 cpu emulator written in C++ by Dirk W. Hoffmann.
• vAmiga - An Amiga 500, 1000, or 2000 on your Apple Macintosh by Dirk W. Hoffmann.
• vasm - A portable and retargetable assembler.
• VICE - The Versatile Commodore Emulator.
• VirtualC64 - A Commodore 64 on your Apple Macintosh by Dirk W. Hoffmann.
• vlink - A portable linker for multiple file formats.

References

Leventhal, Lance A. 1981. 6809 ASSEMBLY LANGUAGE PROGRAMMING. OSBORNE/McGraw-Hill.

Motorola. 1981. MC6809-MC6809E 8-BIT Microprocessor Programming Manual. Motorola Inc.

Motorola Semiconductors. 1983. MC6809 Datasheet.

Osborne, Adam. 1976. An introduction to microcomputers - Volume I Basic Concepts. SYBEX.

Zaks, Rodnay and William Labiak. 1982. Programming the 6809. SYBEX.