SMS2 ("Small Multitasking System 2") was a computer operating system developed for the Atari ST by Tony Tebby, the designer of the Qdos operating system for the Sinclair QL personal computer. SMS2 evolved from Qdos but was not intended to provide full Qdos compatibility.
Later, Tebby developed SMSQ and SMSQ/E, which added Qdos (and SuperBASIC) compatibility and ran on various QL-emulating and Atari platforms.