The Art of CHIP-8

CHIP-8 is a programming language that was developed for the COSMAC VIP kit computer in 1977. Its programs are made up of a series of two-byte instructions that resemble machine code for a simple virtual instruction-set architecture. Because of this, CHIP-8 interpreters are often referred to as "emulators." Writing a CHIP-8 interpreter is a great way to learn the principles behind emulators for antique computers and game consoles.

As a result, there are numerous CHIP-8 runtimes available for various platforms. However, this abundance of implementations has led to confusion due to a long history of misinterpretation, resulting in a wide variety of divergent behaviors in interpreters. To address this, Octo, a high-level CHIP-8 assembler, was developed, helping to popularize and standardize "quirks flags" that capture common divergences between different CHIP-8 flavors.

Test suites have been created for CHIP-8 interpreters and their variants, ensuring that modern interpreters follow the correct details. Despite this, the actual implementation of CHIP-8 varies between interpreters, particularly for obscure platforms. This article will examine CHIP-8 as an instruction set and its practical implications for writing new programs, focusing on approaches that are portable across most existing CHIP-8 interpreters.

The code examples will use Octo's notation, and the document is not intended as a comprehensive manual for Octo assembly language but will explain new ideas as they arise.