VDOS
From CPCWiki - THE Amstrad CPC encyclopedia!
The disc operating system for use with the Vortex Disc Drives from Vortex.
VDOS was supplied on rom, and came in two versions: VDOS 1.0 and VDOS 2.0
Manual
Vortex Vdos 2.0 Manual (pdf) (also describes how to upgrade from VDOS 1.0 to 2.0)