Chip-8 interpreter

Chip-8 is an interpreted language developed in the 70's, intended to ease video game programming. Initially used on the COSMAC VIP and Telmac 1800, chip-8 interpreters have been implemented on various platforms.

Specification

Chip-8's design determines:

• Memory in size of 4KB, with addresses from 0x000 to 0xFFF. Addresses from 0x0 to 0x200 are reserved for the interpreter, usually it holds the default font data.

• Stack in size of 16 16-bit values, allowing for up to 16 subroutines (chip-8 interpreter should allow for at least 12 subroutine calls).

• 16 general purpose 8-bit registers from V0 to VF (hexadecimal digits), special 16-bit I-register, 2 8-bit DT and ST registers for delay and sound timers, 16-bit PC register for a program counter and an 8-bit SP register for a stack pointer.

• Keyboard with 16-key hexadecimal valued keys.

• 64x32 pixel monochrome display.

• 35 instructions, each 2-byte long.

• A set of predefined sprites representing hexadecimal digits from 0x0 to 0xF. Each sprite is 5 bytes long. In this interpreter, the sprites are stored in the reserved memory, beginning from the address 0x0.

More information about the chip-8's design can be found at the References section.

How to build

You'll need maven to build the project, though you can also compile it with javac.

Clone the repository with:

git clone https://github.com/skni-kod/chip-8.git

Then enter the repository and build the jar package using:

mvn clean package

The output package can be found in the target folder.

How to use

java -jar chip8.jar (ROM PATH) (PARAMETERS)

Keyboard mapping

Keyboard that was used by computers the chip-8 was initially designed for consists of 16 keys. Each key has its individual hexadecimal value. To fit the keyboard onto the modern ones, a key mapping is a necessity. This interpreter uses the following key mapping:

Original	Interpreter
1 2 3 C	1 2 3 4
4 5 6 D	Q W E R
7 8 9 E	A S D F
A 0 B F	Z X C V

Parameters

Parametrs allow you to configure the way the interpreter works. Changing the launch parameters is necessary for some games to run flawlessly, as described in Quirks/Compatibility section. If you run the interpreter without any of these parameters explicitly set, the default value will be set.

Parameter	Value	Purpose	Default value	Example
-freq	Integer	Sets chip-8's CPU frequency (in Hz).	500	-freq 300
-regGUI	None	Shows the window with real time view on registers and executed instructions. Use the parameter to turn the GUI on.	Turned off	-regGUI
-overlap	Boolean	Sets the screen overlapping mode - True to overlap sprites over the screen edges, False to turn the overlapping off.	True	-overlap false
-loadq	Boolean	Turns on the load-store quirk - True to turn on, False to turn off.	False	-loadq true
-shiftq	Boolean	Turns on the shift quirk - True to turn on, False to turn off.	True	-shiftq false

More information on the quirks can be found in the Quirks/Compatibility section.

Quirks/Compatibility

Chip-8's references aren't consistent on some specific behaviours, such as whether sprites should overlap to the other side of the screen, when the screen's border is met or which register should be shifted when using 8xyE or 8xy7 shift instructions. What's more, some games are written to make use of these quirks, so to run properly, each game may require different launch parameters. Example of such game is BLITZ by David Winter, which requires us to turn screen overlapping off for the game to run flawlessly.

If the game you're trying to run behaves strange, try to experiment with the launch parameters and set the quirks to values different from the default ones.

Load - store quirk

The load-store quirk concerns instructions Fx55 and Fx65 - instructions that store and load register values to and from the memory, beginning from register V0 to register Vx. Cowgod's reference doesn't determine whether the I register should be modified when these instructions are executed. Mattmikolay's reference on the other hand states that the I register should be set to I + X + 1 when the instructions are executed.

When the load-store quirk is turned on, the I register is left unmodified.

When the load-store quirk is turned off, the I register is incremented with each stored/loaded registers (set to I + X + 1).

Shift quirk

The shift quirk concerns instructions 8xy6 and 8xyE - instructions that shift value stored in the registers one bit right or left. Cowgod's reference describes shifting instructions as shifting Vx rather than Vy, so the Vy register is not used at all. Mattmikolay's reference states that the register Vyis shifted and then stored in the Vx register.

When the shift quirk is turned on, register Vx is the shifted one.

When the shift quirk is turned off, register Vy is the shifted one.

Screen overlapping

Cowgod's reference states, that when a sprite is drawn on the screen and a part of the sprite is positioned partially outside the screen border, it should wrap around to the other side of the screen. Mattmikolay's reference states, that in this situation sprite's sticking out part should just be clipped.

When the overlapping mode is turned on, sprites positioned partially outside the screen border will be wrapped around and drawn on the other side.

When the overlapping mode is turned off, sprites positioned partially outside the screen border will be clipped.

Compatible games

Here are some games I tested and found working properly with certain launch parameters. Tuning the launch parameters to your own preferences is always a good idea! It also seems that older games tend to not use the shift quirk.

Name	Author	Overlapping mode	Load-store quirk	Shift quirk	CPU frequency
Default values	---	True	False	True	500
Space Invaders	David Winter	True	False	True	400
Blitz	David Winter	False	False	True	300-400
Connect 4	David Winter	True	True	True	100
Hidden	David Winter	True	True	True	200
Lunar lander	Udo Pernisz	True	False	True	300
UFO	---	True	False	True	400-500
Missile	David Winter	True	False	True	200
Most Dangerous Game	Peter Maruhnic	True	False	False	500

Games I found working properly with the default settings:

• Brix (Andreas Gustafsson),
• Kaleidoscope (Joseph Weisbecker),
• Merlin (David Winter),
• Pong (Paul Vervalin),
• Tetris (Fran Dachille),
• Rush Hour (hap),
• Animal Race (Brian Astle).

Screenshots

Ideas for future improvements

• Add the ability to change screen colours.
• Add the ability to pause, save and load the interpreter's state.
• Add the GUI for ROM loading and quirk setting.

References

Materials used while creating the interpreter:

• https://en.wikipedia.org/wiki/CHIP-8
• http://devernay.free.fr/hacks/chip8/C8TECH10.HTM
• https://github.com/mattmikolay/chip-8/wiki/CHIP%E2%80%908-Technical-Reference