Gopher2600 is an emulator for the Atari 2600 written in the Go language. The accuracy of the emulation is very high and the 6507, TIA and RIOT chips are well represented. The key features of the emulator:
- Support for many of the known cartridge formats
- Emulation of the ARM7TDMI as found in the
- Gameplay recording and playback
- Support for (and auto-detection of) keypad,paddle and joystick
- Network access through PlusROM emulation
- Savekey support
- Rudimentary CRT Effects
- Support for common TIA revisions
- Implementation of Movie Cart
The graphical debugger is still in development but the current features include:
- CPU and Video stepping
- Breakpoints, traps, watches on various CPU, TIA, RIOT targets
- Interactive rewinding
- Specialist windows for specific cartridge types (eg. supercharger tape)
- Script recording and playback
- Line terminal interface for harder to reach parts of the emulation
- Regression Database
The following screenshots were taken in playmode with CRT effects enabled
Games shown: Pitfall; He-Man; Krull; Ladybug; Thrust; Panda Chase; Man Goes Down; Legacy of the Beast; Chiphead; Egypt 2600BC by Genesis Project; Draconian; Zevious.
Scope of the project
Gopher2600 was started as for fun and educational purposes, as way of learning more about the
Atari 2600 and also about the Go programming language.
The original intent was to provide a tool for static analysis of a 6507 program. I soon realised that I would need to emulate more and more of the 2600 and not just the CPU. Eventually, I realised I would also need a way of see the video output from the TIA. At this point it became obvious that I was indeed writing a complete 2600 emulator.
Because of its origins, any flaws or limitations in the design should be borne in mind while the project is still in development. I am open to any suggestions on how to improve the project.
The development machine for
Gopher2600 was an i3-3225 with 16GB of RAM. Host operating system throughout the development has been a GNU/Linux system.
In playmode I can get a sustained frame rate of 60fps. However, video output is not properly vsynced and the audio buffer is not handled very well, sometimes running out of bits, resulting in pops and stutters in the sound.
In debug mode, I can get around 35fps - good enough for development work.
To get a performance rating for your installation you can run the following:
> gopher2600 performance -display -fpscap=false
Which gives the maximum frame rate with the display.
Memory usage is around 40MB of system memory in playmode and around 120MB in debug mode. This can vary on the ROM used however. It shouldn't ever be a problem on modern hardware.
statsview is also available. See the section below on the Statistics Viewer for details.
The project has most recently been tested with Go v1.15. Earlier versions may work but v1.15 is recommended due to recent performance improvements.
The project uses the Go module system and dependencies will be resolved automatically. Do note however, that you will also require the SDL development kit installed on the system. For users of UNIX like systems, installation from your package manager is the easiest option (for MacOS use the homebrew package manager https://formulae.brew.sh/formula/sdl2)
Compile with GNU Make
> make release
During development, programmers may find it more useful to use the go command directly
> go run gopher2600.go
Gopher2600 makes use of SDL2. The SDL2 go binding used by the project requires a minimum SDL2 version of
Gopher2600 is known to run on several platforms. It is known to work best however on Linux based systems, on which it is being developed.
Native compilation of a Windows executable has not yet been tried. But cross-compilation does work via the Makefile:
> make cross_windows
Or for a statically linked binary:
> make cross_windows_static
This has been tested on a Linux system with mingw installed.
Once compiled run the executable with the help flag:
> gopher2600 -help
This will list the available sub-modes. Use the -help flag to get information about a sub-mode. For example:
> gopher2600 debug -help
To run a cartridge, you don't need to specify a sub-mode. For example:
> gopher2600 roms/Pitfall.bin
Although if want to pass flags to the run mode you'll need to specify it.
> gopher2600 run -help
Joystick, paddle and keypad inputs are supported. Currently, only joysticks and paddles for the left player are available but keypad input is available for both players.
For conveniece the joystick and paddle can be controlled through a gamepad. For the joystick, use the left thumbstick or the DPad. Any of the face buttons will act as the joystick's fire button.
To control the paddle use the left and right analogue triggers. Note that you will need to 'waggle' the triggers a couple of times for the emulator to detect that you want to switch to the paddle.
The console's reset switch can be triggered with the gamepad's start button.
Without A Gamepad
As an alternative to the gamepad, the joystick can be controlled with the cursor keys of the keyboard. The spacebar will act as the joystick's fire button.
The paddle can be accessed with the computer's mouse. First, capture the mouse by right-clicking the emulator's window and then waggle the mouse a few times to tell the emulator that you want to switch to the paddle. The left mouse button is the paddle's fire button.
Once captured, press the right mouse button to release the mouse and return mouse control to your desktop,
Keypads for both player 0 and player 1 are supported.
The VCS panel is controlled through the function keys of the keyboard.
F4Player 0 Pro Toggle
F5Player 0 Pro Toggle
In playmode, the additional keys are available:
F9Show TIA Revisions window
F10Show CRT Preferences window
F12Show FPS Indicator
In the debugger, the additional keys are available:
ESCtoggle mouse capture for debugging TV screen
- ` (backtick) start/stop emulation
To run the debugger use the
> gopher2600 debug roms/Pitfall.bin
Because the debugger is still in development, full documentation is not yet available. But briefly, the features we can see in this screeshot are:
The menu-bar across the top of the screen shows the
VCS and in this instance a
Debuggermenu you can open the preferences windows the terminal and others.
VCSmenu contains options to open windows that relate to the VCS itself.
- Many of them are already open but other, less frequently required windows are also available
Cartridgemenu appears when the loaded cartridge type has additional features. For example:
- Cartridge memory and or registers
- In this case, this is the menu for the
- At the very right of the menu bar we can see the file path for the loaded cartridge
Below the menu-bar are the debugging windows. In this screenshot we can see:
TV Screen. This shows the television output from the VCS.
- The screen is 'interactive' and will show information about the pixel underneath the cursor
- Clicking the screen will move the VCS emulation to the point where the VCS is outputting that pixel. This is part of the rewind system.
- Also available are a variety of
overlays. Click the overlay button (labelled
no overlayin the screenshot and select which overlay you want.
Audiowindow shows the waveform of the recent sound output.
Controlwindow allow you to
rewindthrough the emulation.
- The toggle to right of the
Stepbutton will put the emulation into
video-cyclemode. This will allow you to step by a single color-clock at a time, rather than a single CPU instruction.
<buttons will step back by eith a CPU instruction, a scanline or an entire frame. Stepping back by video-cycle is not currently supported.
- The toggle to right of the
Timerwindow shows the current state of the RIOT Timer.
CPUwindow shows the current state of the 6507.
Disassemblywindow shows the disassembled ROM.
- If a 'DASM' generated symbol file is available then that will be used.
- If a symbols file isn't available then the standard symbols will be used. This includes symbols for special cartridge areas that might exists. For example, a hotspot address for switching banks will be indicated with
BANK2, etc. instead of the address.
- Add or remove a
PC Breakpointby clicking on the address.
RAMwindow shows the contents of the VCS RAM. Cartidge RAM if available will be show in the
Cartridge RAMwindow. Not shown but available through the cartridge menu when appropriate.
- The highlighted bytes indicate those bytes that have changed since the emulation last halted.
TIAwindow details the six graphical parts of the VCS's graphics chip.
- The state of the
TIAcan be changed manually but note that the changes will not be retained when the emulation next updates that part of the TIA.
- The state of the
Note that much of the information presented in the windows is editable. For example, the contents of the CPU's PC register can be edited via the window. As in all areas of this project, the user is encouraged to experiment.
A (old) video of the debugger in action can be found here.
As an alternative to GUI interaction the debugger can also be controlled through a terminal. This is available through the
terminal window. The rest of this section describes the operation of the terminal in detail.
Help is available with the HELP command. Help on a specific topic is available by specifying a keyword. The list below shows the currently defined keywords. The rest of the section will give a brief run down of debugger features.
[ $f000 SEI ] >> help AUDIO BALL BREAK CARTRIDGE CLEAR CONTROLLER CPU DISASSEMBLY DISPLAY DROP GREP HALT HELP INSERT KEYBOARD LAST LINT LIST LOG MEMMAP MEMUSAGE MISSILE ONHALT ONSTEP ONTRACE PANEL PATCH PEEK PLAYER PLAYFIELD PLUSROM POKE PREFS QUANTUM QUIT RAM RESET REWIND RIOT RUN SCRIPT STEP STICK SYMBOL TIA TRACE TRAP TV WATCH
The debugger allows tab-completion in most situations. For example, pressing
W followed by the Tab key on your keyboard, will autocomplete the
WATCH command. This works for command arguments too. It does not currently work for filenames, or symbols. Given a choice of completions, the Tab key will cycle through the available options.
Addresses can be specified by decimal or hexadecimal. Hexadecimal addresses can be written
$80. The debugger will echo addresses in the first format. Addresses can also be specified by symbol if one is available. The debugger understands the canonical symbol names used in VCS development. For example,
WATCH NUSIZ0 will halt execution whenever address 0x04 (or any of its mirrors) is written to.
Watches are one of the three facilities that will halt execution of the emulator. The other two are
BREAK. Both of these commands will halt execution when a "target" changes or meets some condition. An example of a target is the Programmer Counter or the Scanline value. See
HELP BREAK and
HELP TRAP for more information.
Whenever the emulation does halt, the
ONHALT command will run. For example, a previous call to
ONHALT CPU will cause the
CPU command to run whenever the emulation stops. Similarly, the
ONSTEP command applies whenever the emulation is stepped forward. By default, the
LAST command is run on every step.
The debugger can step forward either, one CPU instruction at a time, or by one video cycle at a time. We can change this mode with the
QUANTUM command. We can also conveniently use the
STEP command, for example
STEP CLOCK, performing the quantum change and stepping forward in one go. The STEP command can also be to step forward one scanline or one frame. It can also be used to STEP backwards, although in some cases the REWIND command might be more convenient.
Scripts can be recorded and played back with the
SCRIPT command. All commands are available when in script recording mode, except
RUN and further
SCRIPT RECORD command. Playing back a script while recording a new script is possible.
Gopher2600 allows emulation state to be rewound to an earlier frame, scanline or colour-clock. Rewinding by frame is best done through the
Control window of the debugger.
The slider can be scrolled to any frame between the two limits indicated. Alternatively, a single frame can be stepped back and forward using the arrow buttons.
Rewinding to a scanline/colour-clock is done by clicking the left mouse button on the debug screen, at the position required. This will change the state of the emulation accordingly. This can be done with any frame in the rewind history without damaging the rewind history.
The rewind history will be cropped and continue from the current point whenever the emulation is run or stepped.
The number of rewind states stored can be set via the preferences window (or through the terminal). In addition the snapshot frequency can also be altered. The frequency defines how many frames must pass before another snapshot is taken. This affects the number of frames that can be stored. For example, if number of states is 100 and frequency is 1 then one-hundred frames can be stored in the rewind history. On the other hand, if the number of states is 100 and frequency is 5 then five-hundred frames can be stored.
The rewind frequency does not affect the granularity of the rewind history. This means that you can rewind to any frame in the rewind history even if no no explicit snapshot has been taken.
The downside of large frequencies is that input events (from a joystick for example) may be lost if they occurred between snapshots. Future versions of
Gopher2600 will correct this.
Rewind is also not currently available in playmode. Again, future version of
Gopher2600 will allow this.
Video quantum is also not fully supported. While the rewind system will work when in video-stepping mode you can not currently interactively alter the screen position to the level of an individual colour-clock. Left-clicking on the screen, as described above, will 'quantise' to the next CPU instruction. Future versions of
Gopher2600 will correct this.
Gopher2600 supports common revisions in the TIA chip. The configuration window is accessible through the debugger and through the playmode. In the latter case by pressing
A summary of the known TIA revisions / bugs can be found at on Atari Compendium. Not all revisions / bugs are supported by
Gopher2600 but the common ones are.
Gopher2600 offers basic emulation of a CRT television. This is by no means complete and is an area of active development.
playmode enables CRT effects. You can turn them off with a commandline switch:
> gopher2600 -crt=false roms/Pitfall.bin
Individual CRT effects can be turned on and off. From the debugger select the
CRT Preferences entry in the
Debugger menu. The following window will be shown:
The preview window give an impression of the effect the CRT preferences on the current ROM. You can move the preview image by dragging it with the mouse.
From playmode, the preferences window can by opened by pressing
F10. An example of the screen shot is given below.
Gopher2600 will look for certain files in a configuration directory. The location of this directory depends on whether the executable is a release executable (built with "make release") or a development executable (made with "make build"). For development executables the configuration directory is named
.gopher2600 and is located in the current working directory.
For release executables, the directory is placed in the user's configuration directory, the location of which is dependent on the host OS. On modern Linux systems, the location is
For MacOS the directory for release executables is
For Windows, a
gopher2600 will be placed somewhere in the user's
%AppData% folder, either in the
In all instances, the directory, sub-directory and files will be created automatically as required.
Gopher2600 can load supercharger tapes from MP3 and WAV file, in addition to supercharger BIN files.
Multiload "tapes" are supported although care should be taken in how multiload files are created.
In the case of BIN files a straight concatenation of individual files should work, resulting in a file that is a multiple of 8448 bytes.
For MP3 and WAV files however, the waveform should be concatenated, not the individual MP3/WAV files themselves. A command line tool like SoX can be used for this, or a graphical program like Audacity if you prefer.
Of course, in all cases, the individual parts must be part of a multiload set for this to work.
Supercharger eulation relies on the presence of the real Supercharger BIOSt. The file must be named one of the following:
- Supercharger BIOS.bin
The file can be placed in the current working directory or in the same directory as the Supercharger file being loaded. Alternatively, it can be placed in the emulator's configuration directory.
Gopher2600 has basic support for the
SaveKey peripheral. This will be expanded on in the future.
For now, the presence of the peripheral must be specified with the
-savekey arugment. This is only available in
debug mode. The simplest invocation to load a ROM with the
> gopher2600 -savekey roms/mgd.bin
Note that the
SaveKey will always be inserted in the second player port.
Data saved to the
SaveKey will be saved in the configuration directory to the binary file named simply,
The Atari2600 Pluscart is a third-party peripheral that gives the Atari2600 internet connectivity.
Gopher2600 will automatically determine when a PlusROM enabled ROM is loaded.
The very first time you load a PlusROM cartridge you will be asked for a username. This username along with the automatically generated ID, will be used to identify you on the PlusROM server (different ROMs can have different servers.)
You can change your username through the debugger, either through the PlusROM preferences window or through the terminal with the
PlusROM cartridges are rewindable but cannot be rewound backwards past a network event 'boundary'. This to prevent the resending of already sent network data.
Gopher2600 emulates the ARM7TDMI CPU that is found in the
Harmony cartridge. The presence of this CPU allows for highly flexible coprocessing.
Although the Harmony itself executes in both ARM and Thumb modes,
Gopher2600 currently only emulates Thumb mode. It has been decided that ARM mode emulation is not required - better to reimplement the ARM driver in the emulator's host language (Go) - but it may be added in the future.
Debugging support for the ARM7TDMI is rudimentary. A
last execution window is available. This shows the disassembly of the most recent execution of the ARM program. The window is available via the
ARM7TDMI menu which will appear in the menubar if the coprocessor is present.
Goto button. This is a rewind shortcut that takes the emulation to the point where the ARM program last executed.
Estimation of ARM Execution Time
In the real Harmony hardware the ARM program runs whenever the
CALLFN register is written to. While it is running the 6507 program is stalled until the ARM program finishes. This means that ARM programs that run for too long can interfere with the normal operation of the console.
Gopher2600 tries to emulate the real behaviour of the hardware, the benefits of this being seen in the debugger. The image below shows the
ARM7TDMI overlay. The period when the ARM is active is highlighted with purple pixels.
While the operation of all ARM using cartridges is accuration (ie. DPC+ and CDF/CDFJ cartridges) the length of time for which the ARM program runs is difficult to estimate. The time indicated in the debugging overlay therefore is only an estimate and there is no substitute for testing ROMs on real hardware. None-the-less the overlay provides an indication of programs that might be problematic.
This area of the project is an ongoing area of improvement and future versions aim to make this measurement 100% accurate.
Movie Cart is a new cartridge type specifically aimed at playing full length movies on the Atari VCS. The reference code and circuit board information can be found on Github: https://github.com/lodefmode/moviecart.
Gopher2600 allows Movie Cart files to be played just like any other ROM. Files must have the '.mvc' file extension and can only be streamed from the a local filing system. Streaming over HTTP will be supported in the future.
Gopher2600 can record all user input and playback for future viewing. This is a very efficient way of recording gameplay and results in far smaller files than a video recording. It also has other uses, not least for the recording of complex tests for the regression database.
To record a gameplay session, use the
record flag. Note that we have to specify the
run mode for the flag to be recognised:
> gopher2600 run -record roms/Pitfall.bin
This will result in a recording file in your current working directory, with a name something like:
To playback a recording, simply specify the recording file instead of a ROM file:
> gopher2600 recording_Pitfall_20200201_093658
To help with the development process a regression testing system was added. This will prove useful during further development. To quickly add a ROM to the database:
> gopher2600 regress add roms/Pitfall.bin
By default, this adds a "video digest" of the first 10 frames of the named ROM. We can alter the number of frames, and also other parameters with
regress add mode flags. For example, to run for 100 frames instead of 10:
> gopher2600 regress add -frames 100 roms/Pitfall.bin
The database also supports the adding of playback files. When the test is run, the playback file is run as normal and success measured. To add a playback to the test data, simply specify the playback file instead of a rom:
> gopher2600 regress add recording_Pitfall_20200201_093658
Consult the output of
gopher2600 regress add -help for other options.
To listing all previously add tests use the "list" sub-mode:
> gopher2600 regress list > 000 [video] player_switching [AUTO] frames=10 [NUSIZ] > 001 [video] NUSIZTest [AUTO] frames=10 [NUSIZ] > 002 [video] testSize2Copies_A [AUTO] frames=10 [NUSIZ] > 003 [video] testSize2Copies_B [AUTO] frames=10 [NUSIZ] > 004 [video] player8 [AUTO] frames=10 [NUSIZ] > 005 [video] player16 [AUTO] frames=10 [NUSIZ] > 006 [video] player32 [AUTO] frames=10 [NUSIZ] > 007 [video] barber [AUTO] frames=10 [NUSIZ] > 008 [video] test1.bas [AUTO] frames=10 [TIMER] > 009 [video] test2.bas [AUTO] frames=10 [TIMER] > 010 [video] test3.bas [AUTO] frames=10 [TIMER] > 011 [video] test4.bas [AUTO] frames=10 [TIMER] > Total: 12
To run all tests, use the
> gopher2600 regress run
To run specific tests, list the test numbers (as seen in the list command result) on the command line. For example:
> gopher2600 regress run 1 3 5
An interrupt signal (ctrl-c) will skip the current test. Two interrupt signals within a quarter of a second will stop the regression run completely.
Delete tests with the
delete sub-mode. For example:
> gopher2600 regress delete 3
The setup system is currently available only to those willing to edit the "database" system by hand. The database is called
setupDB and is located in the project's configuration directory. The format of the database is described in the setup package. Here is the direct link to the source level documentation: https://godoc.org/github.com/JetSetIlly/Gopher2600/setup
This area of the emulation will be expanded upon in the future.
Supported Cartridge Formats
Gopher2600 currently supports the following formats:
- Atari 2k/4k/16/32k
- all of the above with the
- CBS (FA)
- Tigervision (3F)
- Parker Bros (E0)
- M-Network (E7)
In also supports the Supercharger format in both the
.bin format and is also able to load from an
MP3 recording of the supercharger tape.
Modern formats supported:
- CDF (including CDFJ)
The last two formats often make use of the
ARM7TDMI coprocessor as found in the
Harmony cartridge and are fully supported by
- X07. This was only ever used as far as I know, with
Stella's Stockingwhich has never been released (ROM dumped).
Playmode and debug mode can both be launched with a statistics viewer available locally on your machine
> gopher2600 -statsview <rom> > gopher2600 debug -statsview <rom>
The screen below shows an example of the featured statistics. In this instance, this is the debugger running a 4k Atari cartridge (specifically, Pitfall).
For people who really want to dig deep into the running program,
localhost:12600/debug/pprof/ gives more raw, but still useful information.
Note that this feature requires you run a suitably compiled executable. The easiest way to do this is to use the Makefile.
> make release_statsview
See the https://github.com/JetSetIlly/Gopher2600-Utils/ repository for examples of tools that use
The Stella project (https://stella-emu.github.io/) was used as a reference for video output. In the absence of VCS hardware (which I don't have) Stella was the only option I had for checking video accuracy.
No reference to the Stella source was made at all except for the following:
During the development of the CDF cartridge formats. These recent formats don't seem to be documented anywhere accept in the Stella source.
The audio sub-system to convince myself that I was going about the problem correctly. Primarily however,
Gopher2600'saudio implementation references Ron Fries' original TIASound.c file.
ParkerBros fingerprint taken from Stella. I can't remember why I did this but a comment in the fingerprint.go file says I did.
Many notes and clues from the AtariAge message boards. Most significantly the following threads proved very useful indeed:
- "Cosmic Ark Star Field Revisited"
- "Properly model NUSIZ during player decode and draw"
- "Requesting help in improving TIA emulation in Stella"
- "3F Bankswitching"
And from and old mailing list:
- "Games that do bad things to HMOVE..." https://www.biglist.com/lists/stella/archives/199804/msg00198.html
These mailing lists and forums have supplied me with many useful test ROMs. I will package these up and distribute them sometime in the future (assuming I can get the required permissions).
Extensive references have been made to Andrew Towers' "Atari 2600 TIA Hardware Notes v1.0"
Cartridge format information was found in Kevin Horton's "Cart Information v6.0" file (sometimes named bankswitch_sizes.txt)
The "Stella Programmer's Guide" by Steve Wright is of course a key document, used frequently throughout development.
The 6507 information was taken from Leventhal's "6502 Assembly Language Programming" and the text file "64doc.txt" v1.0, by John West and Marko Makela.
US Patent Number 4,644,495 was referenced for the implementation of the DPC cartridge format (the format used in Pitfall 2)
DPC+ format implemented according to notes provided by Spiceware https://atariage.com/forums/topic/163495-harmony-dpc-programming and https://atariage.com/forums/blogs/entry/11811-dpcarm-part-6-dpc-cartridge-layout/
DPC+ARM information on Spiceware's blog https://atariage.com/forums/blogs/entry/11712-dpc-arm-development/?tab=comments#comment-27116
The "Mostly Inclusive Atari 2600 Mapper / Selected Hardware Document" (dated 03/04/12) by Kevin Horton
Supercharger information from the Kevin Horton document above and also the
Reference for the ARM7TDMI thumb instruction set, as used by the Harmony cartridge formats:
Further information from the ARM Architecture Reference Manual:
In addition to this readme, more information can be found with the command line
-help system. Many modes and sub-modes will accept operational flags. Specifying the
-help flag will print a brief summary of available options.
Help on debugger commands is available with the
HELP command at the debugger command line.
More information is available in the Go source files and can be viewed with the Go documentation system. With
> GOMOD=$(pwd) godoc -http=localhost:1234 -index >/dev/null &
Alternatively, the most current version of the docs available on github can be viewed at https://godoc.org/github.com/JetSetIlly/Gopher2600
Finally, development and maintenance documentation is beginning to be stored in its own Github repository: https://github.com/JetSetIlly/Gopher2600-Dev-Docs
There are some design decisions that would perhaps be made differently if I had known where the program was going. For instance, because the project was a way of learning a new programming language I chose to implement my own "database" to store regression test information. A more natural choice would be to use SQlite but actually the current solution works quite well.
A couple of packages may well be useful in other projects. The
prefs package is quite versatile. With a bit of work it could be generalised and put to use in other projects. I think though, this package is a natural candidate to be rewritten with type parameters. Not yet available in Go but scheduled for release in 2022.
I would also replace the
commandline package. It works quite nicely but as you would expect from a home-baked solution there are limitations to the parser. It should be rewritten with
Other Software / Libraries
The following projects are used in the
Statsview provided by:
For testing instrumentation:
Some ideas for the fragment shader taken from: