Documentation

js65 is a ca65 compatible assembler, with an emphasis on making it easier to patch existing files.

Why js65?

ca65 has a very long history and is a popular assembler in the NES community, but it doesn’t work as well when you are using it to patch files. It can patch files, but it’s hard to do it well, and wouldn’t it be nice if you could just have your familiar ca65 cake but also get to enjoy a patching-friendly syntax? js65 fills the gap by simplifying segment definitions, placing code blocks into free space automatically, and many more nice-to-have features, all while having an almost perfect recreation of everything else ca65 already does well.

Additionally, js65 was designed from the start to allow it to be used as a library in other applications, perfect for randomizers or other projects that want to customize a game with various builds. The js65 library gives you full control over the assembly steps, through a fluent-api builder pattern that you can control programmatically. Gone are the days of patching game code by writing giant chunks of raw hex bytes at raw file offsets!

What other ASM features does js65 offer?

For a more detailed look at the many additions, check out the ca65 comparison page. But for a quick overview of some of the powerful features offered by js65

• No linker script needed! Build a rom in one go with segments defined in the code itself.
• Free space tracking, let js65 place code patches wherever there is room.
• Custom Pattern matching based .define macro support.
• Multipass assembly, values known only at link time can be used in assembler conditionals.
• Run Javascript code inline as a preprocessor to handle asset conversions and other source generating scripts

Where do I start?

At the quick start guide! Whether you have a homebrew project, or a romhack, or a randomizer project, there’s something useful that js65 brings to the table.