An Introduction to Video Game Emulation with Rust

Developing a video game emulator is becoming an increasingly popular hobby project for developers. It requires knowledge of low-level hardware, modern programming languages, and graphics systems to successfully create one. It is an excellent learning project; not only does it have clear goals, but it also very rewarding to successfully play games on an emulator you've written yourself. I am still a relatively new emulation developer, but I wouldn't have been able to reach the place I am now if it weren't for excellent guides and tutorials online. To that end, I wanted to give back to the community by writing a guide with some of the tricks I picked up, in hopes it is useful for someone else.

Intro to Chip-8

Our target system is the Chip-8. The Chip-8 has become the "Hello World" for emulation development of a sort. While you might be tempted to begin with something more exciting like the NES or Game Boy, these are a level of complexity higher than the Chip-8. The Chip-8 has a 1-bit monochrome display, a simple 1-channel single tone audio, and only 35 instructions (compared to ~500 for the Game Boy), but more on that later. This guide will cover the technical specifics of the Chip-8, what hardware systems need to be emulated and how, and how to interact with the user. This guide will focus on the original Chip-8 specification, and will not implement any of the many proposed extensions that have been created, such as the Super Chip-8, Chip-16, or XO-Chip; these were created independently of each other, and thus add contradictory features.

Chip-8 Technical Specifications

• A 64x32 monochrome display, drawn to via sprites that are always 8 pixels wide and between 1 and 16 pixels tall
• Sixteen 8-bit general purpose registers, referred to as V0 thru VF. VF also doubles as the flag register for overflow operations
• 16-bit program counter
• Single 16-bit register used as a pointer for memory access, called the I Register
• An unstandardised amount of RAM, however most emulators allocate 4 KB
• 16-bit stack used for calling and returning from subroutines
• 16-key keyboard input
• Two special registers which decrease each frame and trigger upon reaching zero:
• Delay timer: Used for time-based game events
• Sound timer: Used to trigger the audio beep

Intro to Rust

Emulators can be written in nearly any programming language. This guide uses the Rust programming language, although the steps outlined here could be applied to any language. Rust offers a number of great advantages; it is a compiled language with targets for major platforms and it has an active community of external libraries to utilize for our project. Rust also supports building for WebAssembly, allowing us to recompile our code to work in a browser with a minimal amount of tweaking. This guide assumes you understand the basics of the Rust language and programming as a whole. I will explain the code as we go along, but as Rust has a notoriously high learning curve, I would recommend reading and referencing the excellent official Rust book on any concepts that are unfamiliar to you as the guide progresses. This guide also assumes that you have Rust installed and working correctly. Please consult the installation instructions for your platform if needed.

What you will need

Before you begin, please ensure you have access to or have installed the following items.

Text Editor

Any text editor can be used for the project, but there are two I recommend which offer features for Rust like syntax highlighting, code suggestions, and debugger support.

• Visual Studio Code is the editor I prefer for Rust, in combination with the rust-analyzer extension.
• While JetBrains does not offer a dedicated Rust IDE, there is a Rust extension for many of its other products. The extension for CLion has additional functionality that the others do not, such as integrated debugger support. Keep in mind that CLion is a paid product, although it offers a 30 day trial as well as extended free periods for students.

If you do not care for any of these, Rust syntax and autocompletion plugins exist for many other editors, and it can be debugged fairly easily with many other debuggers, such as gdb.

Test ROMs

An emulator isn't much use if you have nothing to run! Included with the source code for this book are a number of commonly distributed Chip-8 programs, which can also be found here. Some of these games will be shown as an example throughout this guide.

Misc.

Other items that may be helpful as we progress:

• Please refresh yourself with hexadecimal notation if you do not feel comfortable with the concept. It will be used extensively throughout this project.
• Chip-8 games are in a binary format, it is often helpful to be able to view the raw hex values as you debug. Standard text editors typically don't have support for viewing files in hexadecimal, instead a specialized hex editor is required. Many offer similar features, but I personally prefer Reverse Engineer's Hex Editor.

Let's begin!