Multiuser DOS

Multiuser DOS
Multiuser DOS
Company / developer Digital Research, Inc.
OS family CP/M
Working state Current
Source model Closed source
Latest stable release V7.22 R18 / 2004
Kernel type Monolithic kernel
Default user interface Command line interface
License Proprietary
Official website Various (see notes)

Multiuser DOS is a soft real-time multi-user multi-tasking operating system for IBM PC-compatible microcomputers.

An evolution of the older Concurrent CP/M-86 and Concurrent DOS operating systems, it was originally developed by Digital Research and later further developed by Novell. Its ancestry lies in the earlier Digital Research operating systems CP/M and MP/M.


Concurrent CP/M-86

The initial version of CP/M for the IBM PC, CP/M-86, was commercially unsuccessful, as Microsoft's MS-DOS offered much the same facilities for a considerably lower price. Like MS-DOS 1.0, CP/M-86 did not fully exploit the power and capabilities of the new 16-bit machine.

It was soon superseded by an implementation of CP/M's multitasking 'big brother', MP/M-86. This turned a PC into a multiuser machine capable of supporting multiple concurrent users using dumb terminals attached by serial ports. The environment presented to each user made it seem as if they had the entire computer to themselves. Since terminals cost a fraction of the then-substantial price of a complete PC, this offered considerable cost savings, as well as facilitating multi-user applications such as accounts or stock control in a time when PC networks were rare, very expensive and difficult to implement.

CP/M-86 and MP/M-86 were later merged to create Concurrent CP/M-86 (also known as CCP/M-86 with BDOS 3.0), which offered more complete CP/M-86 compatibility in addition to MP/M-86's multiuser capabilities. Concurrent CP/M-86 3.1 (BDOS 3.1) shipped on 21 February 1984.[1]


Concurrent DOS

In August 1983, Bruce Skidmore, Ray Pedrizetti, Dave Brown and Gordon Edmonds teamed up to create PC-MODE,[2] an optional module for Concurrent CP/M-86 3.1[1] (with BDOS 3.1) to provide basic PC-DOS compatibility. This was shown publicly in December 1983[2] and shipped in March 1984 as Concurrent DOS 3.1 (aka CDOS with BDOS 3.1) to hardware vendors.[3] Simple DOS applications, which did not directly access the screen or other hardware, could be run. For example, although a console program such as PKZIP worked perfectly and offered more facilities than the CP/M-native ARC archiver, more complex applications which performed screen manipulations, such as WordStar for DOS, would not, and thus native Concurrent CP/M versions were required.

Concurrent DOS 3.2 (with BDOS 3.2) in 1984 was compatible with PC DOS 1.x and was available for many different hardware platforms, the version with an IBM PC compatible BIOS/XIOS was named Concurrent PC DOS 3.2.

In January 1985, Digital Research previewed Concurrent DOS 286 in cooperation with Intel. The product would function strictly as an 80286 native mode operating system, allowing users to take full advantage of the protected mode to perform multi-user, multitasking operations while running 8086 emulation.[4] While this worked on the B-1 step of prototype chip samples, Digital Research, with evaluation copies of their operating system already shipping in April, discovered problems with the emulation on the production level C-1 step of the processor in May, which would not allow Concurrent DOS 286 to run 8086 software in protected mode. The release of Concurrent DOS 286 was scheduled for late May, but was delayed until Intel would develop a new version of the chip.[4] In August, after extensive testing E-1 step samples of the 80286, Digital Research acknowledged that Intel corrected all documented 286 errata, but said there were still undocumented chip performance problems with the prerelease version of Concurrent DOS 286 running on the E-1 step. Intel said, the approach, Digital Research wished to take in emulating 8086 software in protected mode, differed from the original specifications, nevertheless they implemented minor changes in the microcode that would allow Digital Research to run emulation mode much faster, incorporated into the E-2 step.[5] These same limitations also affected FlexOS 286 version 1.0, a derivation of Concurrent DOS 286.

Later versions added compatibility with PC DOS 2.x and 3.x.

Versions 5 and 6 (Concurrent DOS XM) could bank switch multiple programs using EMS.

In 1987, Concurrent DOS was rewritten as Concurrent DOS 386. This ran on machines equipped with the Intel 80386 processor (and later processors), using the 386's hardware facilities for virtualizing the hardware, allowing most DOS applications to run unmodified under Concurrent DOS 386, even on terminals. The OS supported concurrent multiuser file access, allowing multiuser DOS applications to run as if they were on individual PCs attached to a network server. Thus Concurrent DOS 386 was not only a cheaper alternative to individual PCs but allowed a single server to support a number of users on dumb terminals or low-specification PCs running terminal emulation software, without the expense of individual workstations and network cards. Appropriate software allowed several users to use a single database without mutual interference: it was a true multiuser system.


Multiuser DOS

Later versions of Concurrent DOS incorporated some of the enhanced functionality of DR's later single-user PC DOS clone DR-DOS, after which the product was renamed to the more explanatory Multiuser DOS (aka MDOS).

Multiuser DOS suffered from several technical limitations that restricted its ability to compete with LANs based on PC DOS. It required its own special device drivers for much common hardware, as PC DOS drivers were not multiuser or multi-tasking aware. Driver installation was more complex than the simple PC DOS method of copying the files onto the boot disk and modifying CONFIG.SYS appropriately - it was necessary to relink the Multiuser DOS kernel (known as a nucleus) using the SYSGEN command.

Multiuser DOS was also unable to use many common PC DOS additions such as network stacks, and it was limited in its ability to support later developments in the PC-compatible world, such as graphics adaptors, sound cards, CD-ROM drives and mice. Although many of these were soon rectified—for example, graphical terminals were developed, allowing users to use CGA, EGA and VGA software—it was less flexible in this regard than a network of individual PCs, and as the prices of these fell, it became less and less competitive, although it still offered benefits in terms of management and lower total cost of ownership. Unlike MP/M, it never became popular as a single-user but multitasking OS, partly because of license costs and partly because of the requirement for special device drivers—unlike multitasking, but single user, DOS additions such as Quarterdeck's DESQview.


Datapac Multiuser DOS and System Manager

Concurrent Controls Multiuser DOS

Intelligent Micro Software Multiuser DOS, REAL/32 and REAL/NG

Application software

While the various releases of this operating system had increasing ability to run MS-DOS programs, software written for the platform could take advantage of its features by using function calls specifically suitable for multiuser operation.

The API provided support for blocking and non-blocking message queues, mutual-exclusion queues, the ability to create sub-process threads which executed independently from the parent. Applications were started as "attached" to a console. However, if an application didn't need user interaction it could "detach" from the console and run as a background process, later reattaching to a console if needed.

Another key feature was that the memory management supported a "shared" memory model for processes (in addition to the usual models available to normal DOS programs). In the shared memory model the "code" and "data" sections of a program were isolated from each other. Because the "code" section was pure "code" those sections could be shared among as many processes that were running the same application code.

Programs written, or adapted, for the platform (or any multitasking platform) needed to avoid the technique used by single-tasking systems of going into endless loops until interrupted when, for example, waiting for a user to press a key; this wasted processor time that could be used by other processes. Instead, Concurrent DOS provided an API call which a process could call to "sleep" for a period of time. Later versions of the Concurrent DOS kernel included Idle Detection, an innovative feature which monitored MS-DOS API calls to determine whether the application was doing useful work or in fact idle, in which case the process was suspended allowing other processes to run. Idle Detection was the catalyst for the patented DR-DOS Dynamic Idle Detection power management feature invented in 1989 by Roger Gross and John Constant and marketed as BatteryMax (US Patent 5,355501)

See also

  • CP/M
  • MP/M
  • FlexOS
  • DR DOS
  • PC-DOS - IBM's OEM version of MS-DOS
  • PC-MOS/386 - unrelated, but similarly named multitasking DOS clone


  1. ^ a b Digital Research (1984): Concurrent CP/M ships early in response to team effort. Digital Dialogue - Employee Newsletter of Digital Research Inc., Volume 3, Number 1, p. 1 ([1]).
  2. ^ a b Digital Research (1984): PC-Mode bridges CP/M and PC-DOS. Digital Dialogue - Employee Newsletter of Digital Research Inc., Volume 3, Number 1, p. 3 ([2]).
  3. ^ Digital Research: Concurrent™ DOS bridges PC-DOS, CP/M. Digital Research News - For Digital Research Users Everywhere, Volume 4, No. 2, p. 3, May 1984 ([3]): '"Concurrent DOS Release 3.1 is rapidly gaining momentum and support from a wide range of microcomputer manufacturers," Wandryk said. "Some 60 hardware companies have licensed the product since it was released in early March."'
  4. ^ a b Edward Foster: Super DOS awaits new 80286 - Concurrent DOS 286 - delayed until Intel upgrades chip - offers Xenix's power and IBM PC compatibility. InfoWorld Media Group, InfoWorld, Volume 7, No. 19, 1985-05-13, ISSN 0199-6649, pages 17-18, ([4]).
  5. ^ Edward Foster: Intel shows new 80286 chip - Future of DRI's Concurrent DOS 286 still unclear after processor fixed. InfoWorld Media Group, InfoWorld, Volume 7, No. 34, 1985-08-26, ISSN 0199-6649, page 21 ([5]).

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