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 5¼-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.


Some typical TRSDOS utilities:

TRSDOS commands and counterparts in other operating systems
Command DOS, OS/2, Windows Unix, Unix-like
APPEND type file1 >> file2 cat file >> file2
ATTRIB attrib chmod
AUTO AUTOEXEC.BAT ~/.profile or ~/.login or /etc/rc*
BACKUP diskcopy tar, cpio, pax, (many others)
CLOCK prompt $t * in some shells: PS1="...\t..." *
COPY copy cp
DIR dir ls
FORMAT format mkfs
FREE chkdsk df
KILL del rm
LIST type cat
LOAD program (no equivalent) (no equivalent)
PRINT type file >> prn lpr
PROT attrib chmod
RENAME ren or rename mv
  • Since TRSDOS did not have the notion of redirection as UNIX/Linux and MS-DOS do, the APPEND command is somewhat different in concept than the UNIX or MS-DOS notion of appending via output redirection.
  • The CLOCK command displays a real-time clock in the upper corner of the display, almost like a DOS TSR (Terminate and Stay Resident); no exactly corresponding feature exists in MS-DOS or UNIX, though many programs provided similar features for DOS and the common UNIX shells could embed the time into their user-defined "prompt string".
  • Program invocation under TRSDOS, DOS and UNIX is done by filename; no explicit LOAD command is required for normal binary executables nor for text command files (batch files in DOS and shell scripts in UNIX/Linux). The LOAD command under TRSDOS would load a binary program into memory, but would not execute it; neither DOS nor UNIX has an equivalent.
  • Under DOS and UNIX printing a file can be done with redirection; under UNIX it is normally done by spooling the file to the "line printer" (using the lpr command) because UNIX is conventionally a multi-user system.
  • ATTRIB, PROT, and the chmod UNIX command are all somewhat different in their semantics. UNIX/Linux is multi-user and each user can control read, write, and execute permissions on his or her own files and directories. MS-DOS is single user and the file attributes for "read-only," "hidden," and "system" are advisory in nature. TRSDOS was single user but supported some sort of on-disk password protection for specific files.
  • The AUTO command set an automatic command to be executed on TRSDOS boot; under MS-DOS the special, reserved file named AUTOEXEC.BAT contained a list of such commands. On UNIX a set of one or more rc files under /etc/ are a set of boot time "run commands" and special "dot files" in a user's home directory are run for each time that a given user logs into the system. UNIX supports many other "dotfiles" for many of its commands which are akin to the Macintosh "preferences" folder contents.
  • TRSDOS (version II) was notable for the inclusion of noise words, similar to the 1959 COBOL specification. These made commands more English-like. For example, the following commands functioned identically:
    • COPY filea fileb
    • COPY filea TO fileb
  • Many versions supported a simple password security for files and programs, with separate Read/Execute and full access capabilities. ex: filename/ext.password:drive#

Although MS-DOS owes its heritage most closely to CP/M and thence to TOPS-10, many of the file manipulation commands are very similar to those of TRSDOS. By comparison the CP/M command for copying files was called pip (both a pun on the Pip printers, a chain of copy centers in that era, and an acronym standing for "Peripheral Interchange Program").


  • May 8, 1979 - Radio Shack releases TRSDOS 2.3
  • May 1, 1981 - Radio Shack releases Model III TRSDOS 1.3


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