Halcyon Tracker is a tool for creating chiptunes / "8bit" music.
It is built on top of the Halcyon library that provides gameboy-like audio output capabilities via writing to "registers" (a.k.a. a global variable).
Instruments in Halcyon Tracker are code - functions that can modify these registers at 60hz. Currently they are native (compiled C) code, but, before public release this will likely be changed to a custom interpreted language so that songs can be distributed as simple text files.
Many parts of the tracker are not finished yet, but there are examples of music created with the in-development version here: https://i.abaines.me.uk/tracker/