ATLAST (originally Autodesk Threaded Language Application System Toolkit) is a FORTH-like language implemented in standard C which can be integrated into all kinds of programs to make them user-programmable and extensible. For additional details, see the ATLAST home page and ATLAST User Guide.
ATLAST source code has been available from Fourmilab since 1995, and a 64-bit native version since 2014. As of today, 2022-02-16, the master ATLAST source code archive is available on GitHub in the ATLAST repository. The repository includes separate source trees for the 32-bit and 64-bit versions of ATLAST. All subsequent updates to ATLAST will be posted to this repository, improving visibility of changes.