written by Matthew Reed

MULTIDOS advertisement from 80 Microcomputing

MULTIDOS advertisement from the
November 1981 issue of
80 Microcomputing

MULTIDOS was one of the major TRS‑80 operating systems, described in advertisements as “the most compatible, user friendly operating system on the market.” It was written by Vernon Hester, the author of ULTRADOS, and was in some ways a continuation of that operating system. MULTIDOS was the least expensive TRS‑80 operating system and also the one with the most recent update (MULTIDOS 5.1 in 2005). It was also the only TRS‑80 operating system to offer software compatible versions for the TRS‑80 Model I, Model III, Model 4, and the Lobo MAX‑80.


ULTRADOS was an earlier TRS‑80 operating system for the Model I that was sold by Level IV Products. After Vernon Hester parted ways with Level IV Products in 1981, he began writing a completely new operating system. This operating system, which was briefly advertised as ULTRA-II, soon became known as MULTIDOS. Vernon Hester originally sold MULTIDOS through his company, Cosmopolitan Electronics Corporation. The Model I version of MULTIDOS was released in late 1981 and the Model III version was released in January 1982. The price started out at $79.95 but soon increased to $99.95. Later on, MAX‑80 MULTIDOS was released in late 1983 and Model 4 MULTIDOS (originally known as MULTIDOS 80/64) in 1985.


The name MULTIDOS derived from its unique ability to read all Model I TRS‑80 disk formats. As the manual stated:

One of the purposes of MULTIDOS is a link to all the Model I DOS’s on the market. MULTIDOS will permit you to read/write/copy to/from any Model I single/double density operating system configured as purchased. (NEWDOS80/2.0 double density data diskettes can only be read.) MULTIDOS is capable of reading/writing/copying to/from several Model III operating systems diskettes. (NEWDOS/2.0 and TRSDOS are excluded.)

MULTIDOS 80/64advertisement from 80 Micro

Portion of AlphaBit Communications
advertisement from the August 1985
issue of 80 Micro

MULTIDOS was often described as a “programmer’s” operating system. One of the reasons was its speed (boot-up time was less than a second) and another was its excellent BASIC. MULTIDOS included an improved version of SUPERBASIC, the enhanced BASIC from ULTRADOS that had impressed so many people. As stated in one advertisement:

MULTIDOS has an excellent extended disk basic, SUPERBASIC, which has a super fast string sort and leaves the user with more than 40K (48K RAM) of free memory. What was sacrificed in obtaining the 40K—NOTHING! In fact, SUPERBASIC has several innovative and useful functions which the programmer/user will find invaluable.

Another useful included program was VFU, the Versatile File Utility, which allowed “multiple file copying, purging of files, printing a disk directory, and menu based execution of all programs on a disk.”

Later versions of MULTIDOS offered other advanced features, such as multiple commands on the same line (separated by commas) and flexible wildcards (such as */CMD or FILE?/*).

Unlike all other Model 4 operating systems, the Model 4 version of MULTIDOS was software compatible with Model III MULTIDOS. Even though it supported Model 4 features such as extra memory and the 80 by 24 screen, it was still software compatible because of the Model III ROM replacement included as part of MULTIDOS. In my experience, this meant that almost all Model III BASIC programs and most machine language programs (even games) would run under Model 4 MULTIDOS. MAX‑80 MULTIDOS also offered compatibility with Model III software, although not to the same level due to MAX‑80 hardware differences. Both Model 4 and MAX‑80 MULTIDOS offered greatly enhanced versions of SUPERBASIC, with new features that used the increased memory available.


Because of its long development life, there were probably more releases of MULTIDOS than any other TRS‑80 operating system. In addition to Z’DOS, a $39.95 stripped down version of MULTIDOS created for marketing purposes, there were versions of MULTIDOS for the Model I, Model III, Model 4, and Lobo MAX‑80. Vernon Hester continued to make improvements to MULTIDOS and SUPERBASIC and release updated versions.

AlphaBit Communications became the primary distributor of MULTIDOS in 1984. They continued selling MULTIDOS until shutting down around 1994. After that, Computer News 80 sold MULTIDOS from 1996 to 1998.

The last version of MULTIDOS that Vernon Hester released was MULTIDOS 5.1 in 2005.