Fantasm stands for "FAmicom NinTendo ASseMbler." It is a new assembler for making NES games. It uses familiar rules of other assembly languages.
I wrote the assembler in C++. As of writing, it required around 3,000 lines of C++ source code.
The software is free to download. It is intended for Unix/Linux systems. Simply run make to build it for your development machine. It requires the g++ compiler and c++11. I don't have any Windows binaries, but you should be able to build it in Visual Studio, for example, by creating a new solution and project and importing the source files.
I wanted an assembler that was more efficient and more stable than alternative assemblers today, so I wrote my own. Fantasm will be well-maintained and well-supported.
I intend to release a stable version (1.0) quite soon. I have built a number of large projects with it. It will be available for download within the next few weeks.
I will include a short example game soon.
I may move the project to github or a similar public repository. For now, I'll post releases here.