Yet Another (Super) Chip 8 Emulator

An Octo compatible XO Chip, Super Chip, and Chip 8 emulator.

Table of Contents

1. What is it?
2. License
3. Compiling
4. Running
   1. Requirements
   2. Starting the Emulator
   3. Running a ROM
   4. Screen Scale
   5. Execution Delay
   6. Quirks Modes
      1. Shift Quirks
      2. Index Quirks
      3. Jump Quirks
      4. Clip Quirks
      5. Logic Quirks
   7. Memory Size
   8. Colors
5. Customization
   1. Keys
   2. Debug Keys
6. ROM Compatibility
7. Third Party Licenses and Attributions
   1. JCommander
   2. Apache Commons IO

What is it?

This project is a Chip 8 emulator written in Java. There are two other versions of the emulator written in different languages:

• Chip8Python
• Chip8C

The original goal of these projects was to learn how to code a simple emulator.

In addition to supporting Chip 8 ROMs, the emulator also supports the XO Chip and Super Chip specifications. Note that while there are no special flags that are needed to run an XO Chip, Super Chip, or normal Chip 8 ROM, there are other compatibility flags that may need to be set for the ROM to run properly. See the Quirks Modes documentation below for more information.

License

This project makes use of an MIT license. Please see the file called LICENSE for more information. Note that this project may make use of other software that has separate license terms. See the section called Third Party Licenses and Attributions below for more information on those software components.

Compiling

To compile the project, you will need a Java Development Kit (JDK) version 8 or greater installed. Recently, Oracle has changed their license agreement to make personal and developmental use of their JDK free. However, some other use cases may require a paid subscription. Oracle's version of the JDK can be downloaded here. Alternatively, if you prefer to use a JRE with an open-source license (GPL v2 with Classpath Exception), you may visit https://adoptopenjdk.net and install the latest Java Development Kit (JDK) for your system. Again, JDK version 8 or better will work correctly.

To build the project, switch to the root of the source directory, and type:

./gradlew build

On Windows, switch to the root of the source directory, and type:

gradlew.bat build

The compiled JAR file will be placed in the build/libs directory, as a file called emulator-2.0.0-all.jar.

Running

Requirements

You will need a copy of the Java Runtime Environment (JRE) version 8 or greater installed in order to run the compiled JAR file. For most systems, you can install Java 8 JRE by visiting http://java.com and installing the Oracle Java Runtime Environment for your platform. This version of the JRE is free for personal use but contains a custom binary license from Oracle. Alternatively, if you prefer to use a JRE with an open-source license (GPL v2 with Classpath Exception), you may visit https://adoptopenjdk.net and install the latest Java Development Kit (JDK) for your system, which will include an appropriate JRE.

Starting the Emulator

By default, the emulator can start up without a ROM loaded. Simply double-click the JAR file, or run it with the following command line:

java -jar emulator-2.0.0-all.jar

Running a ROM

The command-line interface currently requires a single argument, which is the full path to a Chip 8 ROM:

java -jar emulator-2.0.0-all.jar /path/to/rom/filename

This will start the emulator with the specified ROM.

Screen Scale

The --scale switch will scale the size of the window (the original size at 1x scale is 64 x 32):

java -jar emulator-2.0.0-all.jar /path/to/rom/filename --scale 10

The command above will scale the window so that it is 10 times the normal size.

Execution Delay

You may also wish to experiment with the --delay switch, which instructs the emulator to add a delay to every operation that is executed. For example,

java -jar emulator-2.0.0-all.jar /path/to/rom/filename --delay 10

The command above will add a 10 ms delay to every opcode that is executed. This is useful for very fast computers (note that it is difficult to find information regarding opcode execution times, as such, I have not attempted any fancy timing mechanisms to ensure that instructions are executed in a set amount of time).

Quirks Modes

Over time, various extensions to the Chip8 mnemonics were developed, which resulted in an interesting fragmentation of the Chip8 language specification. As discussed in Octo's Mastering SuperChip documentation, one version of the SuperChip instruction set subtly changed the meaning of a few instructions from their original Chip8 definitions. This change went mostly unnoticed for many implementations of the Chip8 language. Problems arose when people started writing programs using the updated language model - programs written for "pure" Chip8 ceased to function correctly on emulators making use of the altered specification.

To address this issue, Octo implements a number of quirks modes so that all Chip8 software can run correctly, regardless of which specification was used when developing the Chip8 program. This same approach is used here, such that there are several quirks flags that can be passed to the emulator at startup to force it to run with adjustments to the language specification.

Additional quirks and their impacts on the running Chip8 interpreter are examined in great depth at Chromatophore's HP48-Superchip repository. Many thanks for this detailed explanation of various quirks found in the wild!

Shift Quirks

The --shift_quirks flag will change the way that register shift operations work. In the original language specification two registers were required: the destination register x, and the source register y. The source register y value was shifted one bit left or right, and stored in x. For example, shift left was defined as:

Vx = Vy << 1

However, with the updated language specification, the source and destination register are assumed to always be the same, thus the y register is ignored and instead the value is sourced from x as such:

Vx = Vx << 1

Index Quirks

The --index_quirks flag controls whether post-increments are made to the index register following various register based operations. For load (Fn65) and store (Fn55) register operations, the original specification for the Chip8 language results in the index register being post-incremented by the number of registers stored. With the Super Chip8 specification, this behavior is not always adhered to. Setting --index_quirks will prevent the post-increment of the index register from occurring after either of these instructions.

Jump Quirks

The --jump_quirks controls how jumps to various addresses are made with the jump (Bnnn) instruction. In the original Chip8 language specification, the jump is made by taking the contents of register 0, and adding it to the encoded numeric value, such as:

PC = V0 + nnn

With the Super Chip8 specification, the highest 4 bits of the instruction encode the register to use (Bxnn) such. The behavior of --jump_quirks becomes:

PC = Vx + nn

Clip Quirks

The --clip_quirks controls whether sprites are allowed to wrap around the display. By default, sprits will wrap around the borders of the screen. If turned on, then sprites will not be allowed to wrap.

Logic Quirks

The --logic_quirks controls whether the F register is cleared after logic operations such as AND, OR, and XOR. By default, F is left undefined following these operations. With the flag turned on, F will always be cleared.

Memory Size

The original specification of the Chip8 language defined a 4K memory size for the interpreter. The addition of the XO Chip extensions require a 64K memory size for the interpreter. By default, the interpreter will start w ith a 64K memory size, but this behavior can be controlled with the --mem_size_4k flag, which will start the emulator with 4K.

Colors

The original Chip8 language specification called for pixels to be turned on or off. It did not specify what color the pixel states had to be. The emulator lets the user specify what colors they want to use when the emulator is running. Color values are specified by using HTML hex values such as AABBCC without the leading #. There are currently 4 color values that can be set:

• --color_0 specifies the background color. This defaults to 000000.
• --color_1 specifies bitplane 1 color. This defaults to FF33CC.
• --color_2 specifies bitplane 2 color. This defaults to 33CCFF.
• --color_3 specifies bitplane 1 and 2 overlap color. This defaults to FFFFFF.

For Chip8 and SuperChip 8 programs, only the background color color_0 (for pixels turned off) and the bitplane 1 color color_1 (for pixels turned on) are used. Only XO Chip programs will use color_2 and color_3 when the additional bitplanes are potentially used.

Customization

The file components/Keyboard.java contains several variables that can be changed to customize the operation of the emulator. The Chip 8 has 16 keys:

Keys

The original Chip 8 had a keypad with the numbered keys 0 - 9 and A - F (16 keys in total). The original key configuration was as follows:

1	2	3	C
4	5	6	D
7	8	9	E
A	0	B	F

The Chip8Java emulator maps them to the following keyboard keys by default:

1	2	3	4
Q	W	E	R
A	S	D	F
Z	X	C	V

Debug Keys

Pressing a debug key at any time will cause the emulator to enter into a different mode of operation. The debug keys are:

Keyboard Key	Effect
ESC	Quits the emulator

ROM Compatibility

Here are the list of public domain ROMs and their current status with the emulator, along with links to public domain repositories where applicable.

Chip 8 ROMs

ROM Name	Working	Flags
1D Cellular Automata	✔️
8CE Attourny - Disc 1	✔️
8CE Attourny - Disc 2	✔️
8CE Attourny - Disc 3	✔️
Bad Kaiju Ju	✔️
Br8kout	✔️
Carbon8	✔️
Cave Explorer	✔️
Chipquarium	✔️
Danm8ku	✔️
down8	✔️
Falling Ghosts	✔️
Flight Runner	✔️
Fuse	✔️
Ghost Escape	✔️
Glitch Ghost	✔️
Horse World Online	✔️
Invisible Man	✔️	clip_quirks
Knumber Knower	✔️
Masquer8	✔️
Mastermind	❌
Mini Lights Out	✔️
Octo: a Chip 8 Story	❌
Octogon Trail	❓
Octojam 1 Title	✔️
Octojam 2 Title	✔️
Octojam 3 Title	✔️
Octojam 4 Title	✔️
Octojam 5 Title	✔️
Octojam 6 Title	✔️
Octojam 7 Title	✔️
Octojam 8 Title	✔️
Octojam 9 Title	✔️
Octojam 10 Title	✔️
Octo Rancher	❓
Outlaw	✔️
Pet Dog	❓
Piper	✔️
Pumpkin "Dress" Up	✔️
RPS	❓
Slippery Slope	✔️
Snek	✔️
Space Jam	✔️
Spock Paper Scissors	✔️
Super Pong	✔️
Tank!	✔️
TOMB STON TIPP	✔️
WDL	❌

Super Chip ROMs

ROM Name	Working	Flags
Applejak	❌
Bulb	❌
Black Rainbow	✔️
Chipcross	✔️
Chipolarium	✔️
Collision Course	✔️
Dodge	❌
DVN8	✔️
Eaty the Alien	✔️
Grad School Simulator 2014	✔️
Horsey Jump	❓
Knight	❌
Mondri8	✔️
Octopeg	✔️
Octovore	✔️
Rocto	✔️
Sens8tion	✔️
Snake	✔️
Squad	❌
Sub-Terr8nia	✔️
Super Octogon	✔️
Super Square	✔️
The Binding of COSMAC	✔️
Turnover '77	✔️

XO Chip ROMs

ROM Name	Working	Flags
An Evening to Die For	✔️
Business Is Contagious	✔️
Chicken Scratch	✔️
Civiliz8n	✔️
Flutter By	✔️
Into The Garlicscape	✔️
jub8 Song 1	✔️
jub8 Song 2	✔️
Kesha Was Biird	✔️
Kesha Was Niinja	✔️
Octo paint	✔️
Octo Party Mix!	✔️
Octoma	✔️
Red October V	✔️
Skyward	✔️
Spock Paper Scissors	✔️
T8NKS	❓
Tapeworm	✔️
Truck Simul8or	✔️
SK8 H8 1988	✔️
Super NeatBoy	✔️
Wonky Pong	✔️

Third Party Licenses and Attributions

JCommander

This links to the JCommander library, which is licensed under the Apache License, Version 2.0. The license can be downloaded from http://www.apache.org/licenses/LICENSE-2.0.html. The source code for this software is available from https://github.com/cbeust/jcommander

Apache Commons IO

This links to the Apache Commons IO, which is licensed under the Apache License, Version 2.0. The license can be downloaded from http://www.apache.org/licenses/LICENSE-2.0.html. The source code for this software is available from http://commons.apache.org/io