SYMON - A 6502 System Simulator

NOTE: THIS SOFTWARE IS UNDER ACTIVE DEVELOPMENT. Feedback is welcome!

Version: 0.9.1.0

Last Updated: 29 December, 2013

Copyright (c) 2013 Seth J. Morabito <web@loomcom.com>

See the file COPYING for license.

![Symon Simulator in Action] (https://github.com/sethm/symon/raw/master/screenshots/full.jpg)

1.0 About

Symon is a general purpose simulator for systems based on the MOS Technologies 6502 microprocessor and compatibles. Symon is implemented in Java. Its core goals are accuracy, ease of development, clear documentation, and extensive test suites for validating correctness.

Symon simulates a complete system with a 1 MHz NMOS 6502, 32KB of RAM, 16KB of ROM, a 6551 ACIA, a 6522 VIA, and a 6545 CRTC.

Symon has extensive unit tests to verify correctness, and fully passes Klaus Dormann's 6502 Functional Test Suite as of version 0.8.2 (See [this thread on the 6502.org Forums] (http://forum.6502.org/viewtopic.php?f=2&t=2241) for more information about this functional test suite).

2.0 Requirements

• Java 1.5 or higher
• Maven 2.0.x or higher (for building from source)
• JUnit 4 or higher (for testing)

3.0 Features

3.1 Memory Map

• $0000--$7FFF: 32KB RAM
• $8000--$800F: 6522 VIA
• $8800--$8803: 6551 ACIA (Serial Console)
• $9000--$9001: 6545 CRTC
• $C000--$FFFF: 16KB ROM

The CRT Controller uses memory address $7000 as the start of Video memory.

3.2 Serial Console and CPU Status

![Serial Console] (https://github.com/sethm/symon/raw/master/screenshots/console.png)

The main window of the simulator acts as the primary Input/Output system through a virtual serial terminal. The terminal is attached to a simulated MOS 6551 ACIA, including a programmable baud rate generator that tries to approximate the correct "feel" of the programmed baud rate. (The sample Enhanced BASIC ROM image is programmed for 9600 baud)

It also provides CPU status. Contents of the accumulator, index registers, processor status flags, disassembly of the instruction register, and stack pointer are all displayed.

![Font Selection] (https://github.com/sethm/symon/raw/master/screenshots/font_selection.png)

The console supports font sizes from 10 to 20 points.

3.3 16 KB ROM Loading

![ROM Loading] (https://github.com/sethm/symon/raw/master/screenshots/load_rom.png)

Symon can load any 16 KB (16384 bytes) ROM image from the "File" menu. The selected ROM will be placed in memory from locations $C000 to $FFFF

3.4 Memory Window

![Memory Window] (https://github.com/sethm/symon/raw/master/screenshots/memory_window.png)

Memory contents can be viewed (and edited) one page at a time through the Memory Window.

3.5 Trace Log

![Trace Log] (https://github.com/sethm/symon/raw/master/screenshots/trace_log.png)

The last 20,000 execution steps are disassembled and logged to the Trace Log Window.

3.6 NEW - Experimental 6545 CRTC Video

![Composite Video] (https://github.com/sethm/symon/raw/master/screenshots/video_window.png)

This feature is highly experimental. It's possible to open a video window from the "View" menu. This window simulates the output of a MOS 6545 CRT Controller located at address $9000 and $9001.

By default, the 40 x 25 character display uses video memory located at base address $7000. This means that the memory from address $7000 (28672 decimal) to $73E8 (29672 decimal) is directly mapped to video.

• Address Register (at address $9000)
• R1: Horizontal Displayed Columns
• R6: Vertical Displayed Rows
• R9: Scan Lines per Row
• R10: Cursor Start Scan Line and Cursor Control Mode
• R11: Cursor End Scan Line
• R12: Display Start Address (High Byte)
• R13: Display Start Address (Low Byte)
• R14: Cursor Position (High Byte)
• R15: Cursor Position (Low Byte)

Although the simulation is pretty good, there are a few key differences between the simulated 6545 and a real 6545:

• The simulated 6545 supports only the straight binary addressing mode of the real 6545, and not the Row/Column addressing mode.
• The simulated 6545 has full 16 bit addressing, where the real 6545 has only a 14-bit address bus.
• The simulation is done at a whole-frame level, meaning that lots of 6545 programming tricks that were achieved by updating the frame address during vertical and horizontal sync times are not achievable. There is no way (for example) to change the Display Start Address (R12 and R13) while a frame is being drawn.

For more information on the 6545 CRTC and its programming model, please see the following resources

• [CRTC 6545/6845 Information (André Fachat)] (http://6502.org/users/andre/hwinfo/crtc/index.html)
• [CRTC Operation (André Fachat)] (http://www.6502.org/users/andre/hwinfo/crtc/crtc.html)
• [MOS 6545 Datasheet (PDF)] (http://www.6502.org/users/andre/hwinfo/crtc/crtc.html)

3.6.1 Example BASIC Program to test Video

This program will fill the video screen with all printable characters.

10 J = 0
20 FOR I = 28672 TO 29672
30 POKE I,J
40 IF J < 255 THEN J = J + 1 ELSE J = 0
50 NEXT I
60 END

4.0 Usage

4.1 Building

To build Symon with Apache Maven, just type:

$ mvn package

Maven will build Symon, run unit tests, and produce a jar file in the target directory containing the compiled simulator.

Symon is meant to be invoked directly from the jar file. To run with Java 1.5 or greater, just type:

$ java -jar symon-0.9.1.0.jar

When Symon is running, you should be presented with a simple graphical interface.

4.2 ROM images

The simulator requires a 16KB ROM image loaded at address $C000 to $FFFF to work properly. Without a ROM in memory, the simulator will not be able to reset, since the reset vector for the 6502 is located in this address space.

By default, any 16KB file named rom.bin that exists in the same directory where Symon is launched will be loaded as a ROM image. ROM images can also be swapped out at run-time with the "Load ROM Image..." in the File menu.

The "samples" directory contains a ROM image named 'ehbasic.rom', containing Lee Davison's Enhanced 6502 BASIC. This serves as a good starting point for exploration.

4.3 Loading A Program

In addition to ROM images, programs in the form of raw binary object files can be loaded directly into memory from "Load Program..." in the File menu.

Programs are loaded starting at addres $0300. After loading the program, the simulated CPU's reset vector is loaded with the values $00, $03, and the CPU is reset.

There are two very simple sample program in the "samples" directory, for testing.

• 'echo.prg' will echo back anything typed at the console.

• 'hello.prg' will continuously print "Hello, 6502 World!" to the console.

4.4 Running

After loading a program or ROM image, clicking "Run" will start the simulator running.

5.0 Revision History

• 0.9.1: 26 January, 2014 - Support for IRQ and NMI handling.

• 0.9.0: 29 December, 2013 - First pass at a 6545 CRTC simulation.

• 0.8.5: 30 March, 2013 - ASCII display for memory window. Allows user to select a step count from a drop-down box.

• 0.8.4: 4 March, 2013 - Fix for ZPX, ZPY display in the trace log (change contributed by jsissom)

• 0.8.3: 12 January, 2013 - Added tool-tip text. Memory is no longer cleared when resetting. Fixed swapped register labels.

• 0.8.2: 01 January, 2013 - Fully passes Klaus Dormann's 6502 Functional Test suite!

• 0.8.1: 30 December, 2012

• 0.8.0: 29 December, 2012

• 0.7.0: 9 December, 2012

• 0.6: 5 November, 2012

• 0.5: 21 October, 2012 - Able to run Enhanced BASIC for the first time.

• 0.3: 14 October, 2012

• 0.2: 22 April, 2012

• 0.1: 20 January, 2010

6.0 To Do

• Feedback (in the form of dialogs, status bar, etc).

• Better debugging tools from the UI, including breakpoints and disassembly.

• More accurate timing.

• UI needs a ton more polish.

• More extensive testing.

• Clean up JavaDoc.

• Implement CMOS 65C02 instructions and NMOS / CMOS mode flag.

• Allow displaying ACIA status and dumping ACIA buffers, for debugging.

7.0 Acknowledgements

This project would not have been possible without the following resources:

• [Graham Edgecombe's JTerminal project] (https://github.com/grahamedgecombe/jterminal), which forms the core of Symon's console.

• [Andrew Jacobs' 6502 Pages] (http://www.obelisk.demon.co.uk/6502/), for wonderfully detailed information about the 6502

• [Neil Parker's "The 6502/65C02/65C816 Instruction Set Decoded"] (http://www.llx.com/~nparker/a2/opcodes.html), for information about how instructions are coded

8.0 Licensing

Symon is free software. It is distributed under the MIT License. Please see the file 'COPYING' for full details of the license.