The disc unit refers to the physical hardware device to address.  For
example, an HP7914 (and HP7912) disc drive actually contain two different
devices; (1) the hard (winchester) disc, and (2) a cartridge tape.

Typically, the disc drive is labelled "unit 0" and the cartridge tape drive
is labelled "unit 1".

For micro-floppies, usually the left drive is "unit 0" and the right drive
is "unit 1".
