Yes this OS was called SMS2 & came on a cartridge.
"SMS2 was designed with a fundamental understanding modern
hardware. It was built to multitask as a first principle. It is
tiny, efficient, reliable and easy to use. Advanced computing
concepts have been built in at a fundamental level.
In order that the average QDOS user can have access to SMS2 it
has been implemented on the Atari range of computers with a user
interface that provides a stepping stone to the future by
supporting the most advanced QDOS programs."
Here's a link for some info:
