AMSDOS is a disk operating system for the 8-bit Amstrad CPC Computer (and various clones).
AMSDOS was provided built into ROM (either supplied with the external disk drive or in the machine ROM, depending on model) and was accessible through the built-in Locomotive BASIC as well as through firmware routines.
Its main function was to map the cassette access routines (which were built into every CPC model) through to a disk drive.
This enabled the majority of cassette-based programs to work with a disk drive with no modification.
AMDOS extends the AMSTRAD BASIC by the addition of a number of external commands which are identified by a preceding ¦ (bar) symbol.