Wikipedia
TRSDOS (which stood for the Tandy Radio Shack Disk Operating System) was the operating system for the Tandy TRS-80 line of 8-bit Zilog Z80 microcomputers that were sold through Radio Shack through the late 1970s and early 1980s. Tandy's manuals recommended that it be pronounced triss-doss. TRSDOS should not be confused with Tandy DOS, a version of MS-DOS licensed from Microsoft for Tandy's x86 line of personal computers (PCs).
TRSDOS was primarily a way of extending the MBASIC ( BASIC in ROM) with additional I/O (input/output) commands that worked with disk files rather than the cassette tapes that were used by most other TRS-80 systems.
TRSDOS supported up to four floppy (mini-diskette) drives which used 5ΒΌ-inch diskettes with a capacity of 89KB each (later 160KB). The drives were numbered 0 through 3 and the system diskettes (which contained the TRSDOS code and utilities) had to be in drive 0.