Mint

MiNT is a free software alternative operating system kernel for the Atari ST series. It offers a partially preemptive, multi-tasking alternative to TOS and MagiC, with some memory protection. Together with the free system components fVDI device drivers, XaAES graphical user interface widgets, and TeraDesk file manager, MiNT provides a free TOS compatible replacement OS that can multitask a subset... (Wikipedia)