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Concurrent PC DOS

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Multiuser DOS
Company typeOperating system
IndustryComputer software
PredecessorConcurrent CP/M-86, Concurrent DOS, Concurrent DOS 386
Founded1980s
FounderDigital Research
FateDiscontinued by Novell in 1992; continued by VARs
SuccessorDatapac Multiuser DOS and System Manager, CCI Multiuser DOS, IMS Multiuser DOS, REAL/32
Headquarters
United States
Area served
IBM PC-compatible microcomputers
OwnerDigital Research (original), Novell (from 1991)

Multiuser DOS is a real-time operating system designed for IBM PC-compatible microcomputers, offering multi-user and multitasking capabilities.

This system originated from earlier Digital Research products such as Concurrent CP/M-86, Concurrent DOS, and Concurrent DOS 386. Initially created by Digital Research and later acquired by Novell in 1991, its lineage extends back to the 8-bit CP/M and MP/M systems as well as the 16-bit single-tasking CP/M-86.

Following Novell's discontinuation of Multiuser DOS in 1992, three main value-added resellers (VARs)—DataPac Australasia, Concurrent Controls, and Intelligent Micro Software—continued independent development efforts. These resulted in versions like Datapac Multiuser DOS and System Manager, CCI Multiuser DOS, and IMS Multiuser DOS along with REAL/32.

The related FlexOS family, which evolved from Concurrent DOS 286 and Concurrent DOS 68K, was sold to Integrated Systems, Inc. (ISI) in July 1994.

Concurrent CP/M-86

The original CP/M-86 1.0 (BDOS 2.x) was adapted for the IBM PC in 1982 but failed commercially against IBM's PC DOS 1.0 due to its pricing strategy and limited utilization of the 16-bit architecture.

In September 1981, MP/M-86 2.0 introduced multitasking functionality that allowed several users to operate simultaneously via dumb terminals connected through serial ports. This multi-user setup reduced costs and supported shared-data applications such as accounting or inventory management at a time when PC networking was both expensive and uncommon.

By late 1982, CP/M-86 1.1 (BDOS 2.2) and MP/M-86 2.1 were combined into Concurrent CP/M-86 3.0 (BDOS 3.0). The project was led by Kathryn Strutynski with design contributions from Francis "Frank" R. Holsworth. Initially intended as a single-user operating system supporting multitasking of up to four CP/M-86 programs, it incorporated multi-processor capabilities and "virtual screens" for switching between running applications. Subsequent releases added support for dumb terminals to enable true multi-user operation. Concurrent CP/M-86 3.1 (BDOS 3.1) was launched on February 21, 1984.

Concurrent CP/M-86 with Windows

In February 1984, Digital Research introduced Concurrent CP/M with Windows for the IBM Personal Computer and Personal Computer XT, a variant of Concurrent CP/M-86 that included windowing features.

Concurrent CP/M-86/80

This version targeted computers like the LSI-M4, LSI Octopus, and CAL PC, which combined both 16-bit and 8-bit processors. Concurrent CP/M-86/80 enabled execution of both CP/M (8-bit) and CP/M-86 (16-bit) applications by selecting the processor based on file extensions (.COM for 8-bit programs, .CMD for 16-bit). It simulated an 8-bit CP/M environment by converting system calls into their CP/M-86 equivalents processed by the 16-bit CPU.

Concurrent DOS

In August 1983, Bruce Skidmore, Raymond D. Pedrizetti, Dave Brown, and Gordon Edmonds developed PC-MODE, an optional component for Concurrent CP/M-86 3.1 (BDOS 3.1), introducing basic compatibility with PC DOS 1.1 (and MS-DOS 1.1). First demonstrated at COMDEX in December 1983, it was released to hardware manufacturers in March 1984 as Concurrent DOS 3.1 (also known as CDOS with BDOS 3.1). This enabled simple DOS applications that did not require direct hardware access to run—console-based programs like PKZIP operated effectively, offering enhanced functionality compared to the native CP/M ARC archiver. However, software relying on direct screen manipulation, such as the WordStar word processor for DOS, was incompatible unless native Concurrent CP/M or CP/M-86 versions existed.

While Concurrent DOS releases up to version 4.1 were developed in the United States, OEM customizations and localization were managed by Digital Research Europe’s OEM Support Group based in Newbury, UK starting in 1983.

Digital Research positioned Concurrent DOS 4.1 alongside the Graphics Environment Manager (GEM) as a competitor to IBM's TopView in 1985. Concurrent DOS 4.1 (BDOS 4.1) supported PC DOS 2.x compatibility across multiple hardware platforms; its IBM PC-compatible edition was marketed as Concurrent PC DOS 4.1.

Concurrent PC DOS

Released in 1984, Concurrent DOS 3.2 (incorporating BDOS 3.2) provided support for applications originally designed for CP/M-86 1.x, Concurrent CP/M-86 3.x, and PC DOS 2.0. It was distributed across multiple hardware platforms, with the IBM–PC-compatible BIOS/XIOS variant known as Concurrent PC DOS 3.2. Kathryn Strutynski held the role of product manager during this period.

Concurrent DOS 68K and FlexOS 68K

Beginning in 1984, Digital Research partnered with Motorola to develop Concurrent DOS 68K in Austin, Texas, as a successor to CP/M-68K. This operating system was written in C and spearheaded by lead architect Francis "Frank" R. Holsworth. By early 1985, Concurrent DOS 68K version 1.0 was available for OEM evaluation, supported by substantial Motorola funding amounting to several million dollars. The system targeted Motorola 68000/68010 processors and initially ran on development platforms such as the Motorola VME/10 and Compupro systems.

The release timeline for Concurrent DOS 68K included:

  • Concurrent DOS 68K 1.0 (1985)
  • Concurrent DOS 68K 1.1
  • Concurrent DOS 68K 1.20 (April 1986)
  • Concurrent DOS 68K 1.21 (1986)

By late 1986, this line had evolved into what became known as FlexOS 68K.

Concurrent DOS 286 and FlexOS 286

In parallel with Concurrent DOS 68K, Digital Research unveiled Concurrent DOS 286 in January 1985 through collaboration with Intel. This OS was based on MP/M-286 and Concurrent CP/M-286 but was rewritten in C to incorporate a new architecture featuring dynamically loadable device drivers instead of a fixed BIOS or XIOS structure. Francis R. Holsworth played a key role in this development.

Concurrent DOS 286 operated exclusively in native 80286 mode, offering protected-mode multi-user multitasking capabilities along with support for 8086 emulation mode. Although prototype chips initially supported this functionality, production releases were delayed due to issues that Intel addressed by fixing chip errata and enhancing microcode for improved emulation performance.

These hardware challenges also impacted FlexOS 286, a reengineered variant developed by Digital Research's Flexible Automation Business Unit starting in 1986 at Monterey, California.

Subsequent iterations incorporated compatibility with PC DOS versions 2.x and 3.x.

Versions documented include:

  • Concurrent DOS 286 1.0 (1985)
  • Concurrent DOS 286 1.1 (January 1986)
  • Concurrent DOS 286 1.2 (1986)
  • FlexOS 286 1.3 (November 1986)
  • FlexOS 286 1.31 (May 1987)

Concurrent DOS XM and Concurrent DOS 386

In 1986, Digital Research shifted its OEM Support Group to the European Development Centre (EDC) located in Hungerford, UK, which assumed responsibility for developing the Concurrent DOS series from version 4.11 onward, including associated products such as DOS Plus.

Concurrent DOS versions numbered 5 and 6—known as Concurrent DOS XM (with XM indicating support for Expanded Memory)—were developed at Hungerford and featured bank switching capabilities up to 8 MB of EEMS. This allowed a real-mode environment capable of running multiple CP/M-86 and DOS applications simultaneously for up to three users: one local user plus two via serial terminals.

In 1987, the operating system was rewritten as Concurrent DOS 386 while retaining the traditional XIOS & BDOS framework.[1] Designed for Intel's 80386 and subsequent processors, it utilized hardware virtualization technology to run most DOS programs without modification—even on terminal devices. It supported true concurrent multiuser file access and applications functioning independently as if each user had a dedicated PC connected over a network server. This architecture enabled a single server to accommodate multiple users on either dumb terminals or low-performance PCs using terminal emulation software, thus reducing expenses by eliminating the need for costly workstations and network interface cards. The OS also provided genuine multiuser functionality with record locking mechanisms suitable for shared databases.

Concurrent DOS 386 required approximately 200 KB of conventional memory—significantly more than MS-DOS—and recommended at least 1.5 MB of RAM to efficiently operate major software such as Microsoft Word or Multiplan.[2]

Concurrent DOS version 6.0 represented the foundation from which the DR DOS family originated.

Significant releases include:

  • DR Concurrent PC DOS XM 5.0 (BDOS version 5.0)
  • DR Concurrent DOS XM 5.0 (BDOS version ~5.0, October 1986)
  • DR Concurrent DOS XM 5.1 (BDOS approximately version ~5.1, January 1987)
  • DR Concurrent DOS XM 5.2 (BDOS roughly version ~5.2, September 1987)
  • DR Concurrent DOS XM 6.0 (BDOS version 6.0, November 1987), with subsequent updates to versions like 6.01 and later the series culminating in version 6.2
  • DR Concurrent DOS versions for the Intel platform from version 1.0 (1987) through to version 3.02 (1989), progressing BDOS from approximately version ~5.0 to version ~6.2

The release DR Concurrent PC DOS XM version 5.0 emulated IBM PC DOS 2.10,[3] whereas both Concurrent DOS XM 6.0 and Concurrent DOS 386 2.0 maintained compatibility with IBM PC DOS 3.30.[4]

Reception

In 1988, BYTE magazine characterized Concurrent DOS 386 as largely compatible with MS-DOS; for instance, Microsoft Word version 3.1 operated correctly, whereas version 4.0 failed due to reliance on undocumented DOS features. The system's performance declined in direct relation to the number of concurrently running applications. Although it demanded more RAM, video input/output speeds were twice as fast compared to standard DOS environments. The publication concluded that while more sophisticated 386 operating systems were anticipated, Concurrent DOS 386 was already available and fully operational.

Adaptations

Concurrent Controls, Inc. produced multiple variants of Concurrent DOS, including:

  • CCI Concurrent DOS 386 1.12 (October 1987)
  • CCI Concurrent DOS 386 2.01 (May 1988)
  • CCI Concurrent DOS 386 3.01 (March 1989)
  • CCI Concurrent DOS 386 3.02 (April 1990)
  • CCI Concurrent DOS 386 3.03 (March 1991)
  • CCI Concurrent DOS 386 3.04 (July 1991), also known as "CCI Concurrent DOS 4.0"
  • CCI Concurrent DOS versions 3.05 R1 through R11 (1992–August 1993)
  • CCI Concurrent DOS versions 3.06 R1 through R7 (December 1993–July 1994)
  • CCI Concurrent DOS versions 3.07 R1 through R7 (March 1995–June 1998)
  • CCI Concurrent DOS 3.08
  • CCI Concurrent DOS 3.10 R1 (October 2003)

Additional adaptations encompassed Apricot Concurrent DOS 386 version 2.01 (1987), designed for the Apricot Quad Version Level 4.3 platform.

Multiuser DOS

Subsequent releases of Concurrent DOS 386 integrated advanced capabilities derived from Digital Research's DR DOS 5.0—a single-user PC DOS alternative—resulting in its rebranding as "Multiuser DOS" (MDOS) beginning with version 5.0 (BDOS 6.5) in 1991.

Technical constraints limited Multiuser DOS’s ability to compete effectively with PC DOS-based local area networks. The system required custom device drivers for most hardware components because standard PC DOS drivers lacked support for multiuser and multitasking environments, making installation more complex by necessitating kernel relinking through SYSGEN.

Moreover, Multiuser DOS was incompatible with many PC DOS extensions such as network protocol stacks and offered only limited support for emerging PC-compatible peripherals including graphics cards, audio devices, CD-ROM drives, and mice. Although graphical terminals capable of running CGA, EGA, and VGA software were developed, Multiuser DOS remained less adaptable than networks composed of individual PCs—which grew increasingly cost-effective over time. Despite higher expenses and the need for specialized drivers compared to single-user multitasking solutions like Quarterdeck's DESQview, Multiuser DOS provided superior management capabilities and reduced overall ownership costs.

Unlike MP/M, Multiuser DOS never achieved widespread adoption as a single-user multitasking operating system.

Following Novell’s acquisition of Digital Research in 1991 and the subsequent discontinuation of Multiuser DOS in 1992, three Master Value-Added Resellers—DataPac Australasia, Concurrent Controls, and Intelligent Micro Software—secured source code licenses to continue independent development starting in 1994.

Recorded versions include:

  • DR Multiuser DOS versions 5.00 (1991) and 5.01
  • Novell DR Multiuser DOS versions 5.10 (April 1992) and 5.11
  • Novell DR Multiuser DOS version 5.13 (BDOS 6.6, released in 1992)

All MDOS editions from Digital Research and Novell identified themselves as "IBM PC DOS" version 3.31 to maintain compatibility standards.

DataPac Australasia

DataPac Australasia Pty Limited produced several known iterations, including:

  • Datapac Multiuser DOS 5.0
  • Datapac Multiuser DOS 5.1 (BDOS 6.6)
  • Datapac System Manager 7.0 (released on 1996-08-22)

Following its acquisition by Citrix Systems, Inc. in 1997, the System Manager product line was phased out soon thereafter. In 2002, the Sydney-based division was separated into Citrix's Advanced Products Group.

Concurrent Controls

Concurrent Controls, Inc. (CCI) developed multiple versions of CCI Multiuser DOS, including:

  • CCI Multiuser DOS 7.00
  • CCI Multiuser DOS 7.10
  • CCI Multiuser DOS 7.21
  • CCI Multiuser DOS 7.22 R1 (September 1996) through R18 GOLD (2005-04-21)

Each of these versions identifies itself as "IBM PC DOS" version 3.31, with the version string adjustable through the DOSVER utility.

In 1999, Concurrent Controls rebranded as Applica, Inc., and subsequently in 2002, Applica Technology became Aplycon Technologies, Inc.

IMS Multiuser DOS

The IMS Multiuser DOS product line includes these known releases:

  • IMS Multiuser DOS Enhanced Release 5.1 (1992)
  • IMS Multiuser DOS 5.11
  • IMS Multiuser DOS 5.14
  • IMS Multiuser DOS 7.0
  • IMS Multiuser DOS 7.1 (BDOS 6.7, released in 1994)

All editions report their version as "IBM PC DOS" 3.31.

REAL/32

In 1994, Intelligent Micro Software Ltd., based in Thatcham, UK, obtained licensing rights from Novell to continue developing Multiuser DOS and subsequently rebranded it as REAL/32 in 1995.

IBM licensed REAL/32 version 7.50 in the same year for integration with its 4695 POS terminal systems.

The REAL/32 version history comprises:

  • Versions 7.50 through 7.54 (1995–1996)
  • Versions 7.60 through 7.63 (1997)
  • Versions 7.70 through 7.74 (1998)
  • Versions 7.80 through 7.83 (1999)
  • Versions 7.90 through 7.92 (1999)
  • ITERA IMS REAL/32 versions 7.93 (2002) and 7.94 (2003)
  • Integrated Solutions IMS REAL/32 version 7.95 (2010 or earlier)

Up to version 7.74, all releases identify as "IBM PC DOS" version 3.31; starting with version 7.80, they report as version 6.20.

Version 7.90 (released in 1999) introduced support for Logical Block Addressing (LBA) and FAT32 file systems.

In April of 2002, Intelligent Micro Software Ltd entered insolvency proceedings and was acquired by Itera Ltd., which itself dissolved in March of 2006.

By around 2010, REAL/32 was distributed by Integrated Solutions located in Thatcham, UK; however, this company later shifted its primary focus away from software development.


REAL/NG

REAL/NG represented IMS' effort to create a "Next Generation" iteration of REAL/32, marketed as "REAL/32 for the internet age," designed to operate across a wide spectrum of hardware platforms ranging from PCs to x86 multiprocessor servers.

Features promoted in the year 2003 included:

  • Compatibility with Red Hat Linux versions starting at release 7.3
  • Full backward compatibility with existing DOS and REAL/32 applications
  • Capacity to support up to 65,535 virtual consoles, each corresponding to an individual user session
  • Administration and configuration accessible via web browser interfaces both locally and remotely
  • An integrated TCP/IP terminal emulator available for Linux and Windows clients
  • Built-in capabilities for printing and file sharing along with drive mapping between Linux systems and REAL/NG servers
  • Enhanced support for hardware configurations including multi-processor setups
  • Embedded firewall functionality for improved security
  • Economical cost per user license alongside reduced total cost of ownership
  • Distribution provided on CD media bundled with Red Hat Linux installation packages

The latest known release by December of 2003 was REALNG V1 series: V1.60-V1.19-V1.12.

By the year 2005, the domain realng.com redirected visitors to IMS’s main website without any mention of REAL/NG products, suggesting that development or support for this product line had ceased entirely.


Application software

This platform's software utilized multiuser functionality through designated function calls. It incorporated pre-emptive multitasking to ensure that inefficient applications could not dominate CPU resources. Multiuser DOS maintains compatibility with contemporary SSL/TLS libraries such as wolfSSL.

The API provided support for both blocking and non-blocking message queues, mutual exclusion queues, subprocess threads that operated independently from their parent processes, and effective pause mechanisms designed to minimize processor usage. Applications were capable of running attached to virtual consoles or detaching to function as background services.

A key aspect of memory management was the shared memory architecture: separating code and data segments enabled multiple instances of the same program to share code sections, thereby decreasing total memory consumption.

Multitasking applications avoided CPU wastage from idle loops by invoking API calls to enter sleep states when inactive. Subsequent kernel versions introduced Idle Detection, which suspended idle processes to enhance system efficiency. This advancement culminated in the patented Dynamic Idle Detection power management solution marketed under the name BatteryMAX.

See also

Further reading

External links

  1. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named IW_1987_MDOS386
  2. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named lane198807
  3. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named 1986_CDOSXM
  4. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named DRI_1988_CDOS
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