Concurrent DOS 386

Multiuser DOS
Company type	Operating system
Industry	Computer software
Predecessor	Concurrent CP/M-86, Concurrent DOS, Concurrent DOS 386
Founded	1982
Founder	Digital Research
Fate	Development ceased by Novell in 1992; continued by VARs
Successor	Datapac Multiuser DOS, CCI Multiuser DOS, IMS Multiuser DOS, REAL/32
Headquarters	United States / United Kingdom (development centers)
Key people	Kathryn Strutynski (project manager), Francis "Frank" R. Holsworth (designer)
Products	Multiuser DOS, Concurrent CP/M-86, Concurrent DOS series, FlexOS
Parent	Digital Research, later Novell

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

Originating from the evolution of Concurrent CP/M-86, Concurrent DOS, and Concurrent DOS 386, this system was initially created by Digital Research and later further developed by Novell following their acquisition in 1991. Its foundation lies in Digital Research’s earlier 8-bit operating systems CP/M and MP/M, as well as the 16-bit single-tasking CP/M-86.

When Novell ceased development of Multiuser DOS in 1992, three principal value-added resellers (VARs)—DataPac Australasia, Concurrent Controls, and Intelligent Micro Software—were licensed to independently continue its progression. This led to the emergence of Datapac Multiuser DOS and System Manager, CCI Multiuser DOS, along with IMS Multiuser DOS and REAL/32.

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

Concurrent CP/M-86

The first release of CP/M-86 1.0 (BDOS 2.x) was adapted for IBM PCs in 1982 but failed commercially because IBM’s PC DOS 1.0 provided comparable features at a more affordable price. Both operating systems underutilized the capabilities offered by the new 16-bit architecture.

Soon after, the multitasking variant of CP/M, MP/M-86 2.0, launched in September 1981, enhanced this environment by transforming PCs into multiuser platforms. It supported multiple simultaneous users through dumb terminals connected via serial ports. This configuration offered significant cost savings over providing individual PCs to each user and enabled shared-data applications such as accounting or inventory management during a period when PC networking was both uncommon and expensive.

In late 1982, CP/M-86 1.1 (BDOS 2.2) and MP/M-86 2.1 were consolidated into Concurrent CP/M-86 3.0 (CCP/M-86) featuring BDOS 3.0. The project was managed by Kathryn Strutynski with key design contributions from Francis "Frank" R. Holsworth. Initially designed as a single-user operating system supporting true multitasking of up to four CP/M-86 compatible programs, it also accommodated multi-processor setups and introduced "virtual screens" allowing users to switch smoothly between running applications. Subsequent releases incorporated multiuser functionality via dumb terminals connected to the CP/M-86 host machine. Concurrent CP/M-86 3.1 (BDOS 3.1) was released on February 21, 1984.

Concurrent CP/M-86 with Windows

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

Concurrent CP/M-86/80

This version of Concurrent CP/M-86 was developed for LSI-M4, LSI Octopus, and CAL PC systems that combined both 16-bit and 8-bit processors. Since early 16-bit PCs often ran existing 8-bit software more efficiently due to its availability, Concurrent CP/M-86/80 allowed execution of both CP/M (8-bit) and CP/M-86 (16-bit) programs by directing commands to the suitable processor based on file extensions (.COM or .CMD). It provided an emulated CP/M environment for 8-bit applications by converting their system calls into equivalent CP/M-86 operations handled by the 16-bit CPU.

Concurrent DOS

In August 1983, Bruce Skidmore, Raymond D. Pedrizetti, Dave Brown, and Gordon Edmonds created PC-MODE, an optional extension for Concurrent CP/M-86 3.1 (BDOS 3.1) that provided fundamental compatibility with PC DOS 1.1. This was showcased at COMDEX in December 1983 and released to hardware manufacturers in March 1984 under the name Concurrent DOS 3.1 (CDOS with BDOS 3.1). This enabled straightforward DOS programs that refrained from direct hardware interaction—such as PKZIP—to operate effectively, outperforming native CP/M archivers like ARC. Nevertheless, software requiring direct screen control (for example, WordStar for DOS) remained incompatible unless native Concurrent CP/M versions existed.

While the initial development of Concurrent DOS versions 3.1 through 4.1 took place in the United States, localization and OEM customization were managed by Digital Research Europe’s OEM Support Group located in Newbury, UK starting in 1983.

In 1985, Digital Research marketed Concurrent DOS 4.1 alongside the Graphics Environment Manager (GEM) as a competitor to IBM's TopView. Concurrent DOS 4.1 (BDOS 4.1) offered compatibility with PC DOS 2.x across multiple hardware platforms; the IBM PC-compatible BIOS/XIOS variant was branded Concurrent PC DOS 4.1.

Concurrent PC DOS

Released in 1984, Concurrent DOS 3.2 (featuring BDOS 3.2) supported applications designed for CP/M-86 1.x, Concurrent CP/M-86 3.x, and PC DOS 2.0 environments. It was compatible with various hardware configurations, with the IBM PC-compatible BIOS/XIOS edition identified as Concurrent PC DOS 3.2. Kathryn Strutynski held the role of product manager for this release.

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. The system architect Francis "Frank" R. Holsworth played a pivotal role in this project. By early 1985, Concurrent DOS 68K version 1.0 was available for OEM evaluation; funded by Motorola with several million dollars, it targeted their 68000 and 68010 processors. Initially deployed on Motorola VME/10 development platforms, it was later ported to Compupro systems. Versions 1.20 and 1.21 were released in April and later in 1986 respectively, offered to OEM customers at approximately . This platform eventually evolved into FlexOS 68K by late 1986.

Documented releases include:

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

Concurrent DOS 286 and FlexOS 286

In parallel with Concurrent DOS 68K development, Digital Research introduced Concurrent DOS 286 in collaboration with Intel in January 1985, building upon earlier work from MP/M-286 and Concurrent CP/M-286 dating back to 1982. Architect Francis "Frank" R. Holsworth led a rewrite of the system in C language, incorporating dynamically loadable device drivers instead of fixed BIOS/XIOS components.

This system was designed to operate natively on the Intel 80286 processor's protected mode, supporting multi-user multitasking while maintaining 8086 emulation. Although early prototype processors supported these features, production-level issues delayed its release beyond the initial late May target until Intel addressed errata and performance concerns through updated chip revisions featuring microcode enhancements such as 286 LOADALL. These hardware limitations also impacted FlexOS 286 version 1.x, which was developed by Digital Research's Flexible Automation Business Unit starting in 1986.

Subsequent iterations expanded support for PC DOS versions 2.x and 3.x.

Known releases 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 transferred its OEM Support Group to the newly founded European Development Centre (EDC) located in Hungerford, UK. From version 4.11 onward, this team spearheaded the advancement of the Concurrent DOS series, including associated products such as DOS Plus.

The development of Concurrent DOS versions 5 and 6—known as Concurrent DOS XM, where XM denotes Expanded Memory support—occurred in Hungerford. These editions implemented bank switching to access up to 8 MB of EEMS, facilitating a real-mode environment capable of running multiple CP/M-86 and DOS applications simultaneously. This setup supported up to three users: one local and two connected via serial terminals.

In 1987, the operating system was redesigned and released as Concurrent DOS 386,^[1] preserving the traditional XIOS and BDOS framework. Tailored for Intel 80386 processors and newer, it utilized hardware virtualization to execute most DOS programs without modification, including those run on terminals. The OS enabled simultaneous multiuser file access, allowing applications to function as if each user had a dedicated PC linked to a network server. This approach provided an economical multiuser solution by employing simple terminals or low-end PCs with terminal emulation software, eliminating the necessity for costly workstations and network adapters. It delivered genuine multiuser support with record locking mechanisms for shared databases.

Concurrent DOS 386 demanded approximately 200K of conventional memory—considerably more than MS-DOS—and at least 512K RAM was essential merely to display the command prompt. To operate major software such as Microsoft Word or Multiplan, around 1MB beyond the standard 640K base memory was recommended.^[2]

Concurrent DOS 6.0 also served as the basis for the DR DOS product line, which evolved from it.

Documented releases include:

DR Concurrent PC DOS XM 5.0 (BDOS 5.0)
DR Concurrent DOS XM 5.0 (BDOS 5.0, October 1986)
DR Concurrent DOS XM 5.1 (BDOS 5.1?, January 1987)
DR Concurrent DOS XM 5.2 (BDOS 5.2?, September 1987)
DR Concurrent DOS XM 6.0 (BDOS 6.0, November 1987), including version 6.01
DR Concurrent DOS XM 6.2 (BDOS 6.2), including version 6.21
DR Concurrent DOS 386 versions ranging from 1.0 (1987) through 3.02 (1989), with BDOS versions spanning from 5.x to 6.2

Concurrent PC DOS XM 5.0 provided emulation of IBM PC DOS 2.10,^[3] whereas Concurrent DOS XM 6.0 and Concurrent DOS 386 version 2.0 were compatible with IBM PC DOS 3.30.^[4]

Reception

In a 1988 review by BYTE, Concurrent DOS 386 was characterized as "substantially compatible" with MS-DOS; for instance, Microsoft Word version 3.1 operated successfully, while version 4.0 failed due to reliance on undocumented DOS functions. Performance declined proportionally with the number of concurrently running applications; however, despite higher memory demands, video input/output throughput was reported to be twice as fast compared to standard DOS systems. The publication concluded that although more sophisticated operating systems for the 386 processor were anticipated soon, Concurrent DOS 386 "is available now and fully operational".

Concurrent Controls Adaptations

Concurrent Controls, Inc. developed several variants of Concurrent DOS, which include:

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"
Versions 3.05 through 3.07 of CCI Concurrent DOS with numerous updates from 1992 until June 1998
CCI Concurrent DOS 3.10 R1 (October 2003)

Additional adaptations include:

Apricot Concurrent DOS 386 version 2.01 (1987) tailored for the Apricot Quad Version Level 4.3

Multiuser DOS

Subsequent releases of Concurrent DOS 386 incorporated capabilities from Digital Research’s single-user PC DOS derivative DR DOS 5.0, which was renamed "Multiuser DOS" (MDOS) starting with version 5.0 (BDOS 6.5) in 1991.

Multiuser DOS encountered technical limitations that hindered its ability to compete effectively with LAN solutions based on PC DOS. It necessitated specialized device drivers because standard PC DOS drivers lacked multitasking and multiuser functionality, making setup more complex as it required kernel relinking through the SYSGEN utility.

The platform also had difficulty supporting typical PC DOS peripherals and enhancements such as network protocol stacks, contemporary graphics adapters, audio hardware, CD-ROM drives, and mouse devices. Although graphical terminals were created to support CGA, EGA, and VGA applications, Multiuser DOS proved less adaptable compared to networks of standalone PCs, which gained popularity due to their reduced total cost of ownership and simpler administration.

With a higher price point than single-user alternatives and dependence on unique drivers—contrasting with single-user multitasking solutions like Quarterdeck's DESQview—Multiuser DOS failed to establish a significant presence in the single-user multitasking market.

After Novell acquired Digital Research in 1991 and ceased development of Multiuser DOS in 1992, key value-added resellers such as DataPac Australasia, Concurrent Controls, and Intelligent Micro Software obtained source code licenses in 1994 to pursue independent enhancements.

Versions released under this lineage 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 Digital Research and Novell Multiuser DOS iterations identified themselves as "IBM PC DOS" version 3.31.

DataPac Australasia

DataPac Australasia Pty Limited is known for releasing the following versions:

Datapac Multiuser DOS version 5.0
Datapac Multiuser DOS version 5.1 (BDOS 6.6)
Datapac System Manager version 7.0 (released August 22, 1996)

Following its acquisition by Citrix Systems, Inc. in 1997, the System Manager product line was discontinued shortly thereafter. In 2002, the Sydney-based division was reorganized into Citrix’s Advanced Products Group.

Concurrent Controls

Concurrent Controls, Inc. produced multiple versions of Multiuser DOS including:

CCI Multiuser DOS from versions 7.00 through to 7.22 with various updates spanning from Release 1 (September 1996) up to Release 18 GOLD (2005), all presenting as "IBM PC DOS" version 3.31.

In the year 1999, Concurrent Controls changed its corporate identity to Applica, Inc., and subsequently in 2002 Applica Technology was renamed Aplycon Technologies, Inc.

IMS Multiuser DOS

Variants of IMS Multiuser DOS include:

IMS Multiuser DOS Enhanced Release 5.1 (1992)
IMS Multiuser DOS versions 5.11 and 5.14
IMS Multiuser DOS versions 7.0 and 7.1 (BDOS 6.7, 1994)

All these editions identify themselves as "IBM PC DOS" version 3.31.

REAL/32

In 1994, Intelligent Micro Software Ltd., based in Thatcham, UK, acquired licensing rights from Novell to continue the development of Multiuser DOS, subsequently rebranding the product as REAL/32 in 1995.

IBM licensed version 7.50 of REAL/32 in 1995 for deployment on its 4695 POS terminal systems.

The following REAL/32 versions were released by IMS:

Releases 7.50 through 7.54 (1995–1996)
Releases 7.60 through 7.63 (1997)
Releases 7.70 through 7.74 (1998)
Releases 7.80 through 7.83 (1999)
Releases 7.90 through 7.92 (1999)
ITERA IMS REAL/32 versions up to 7.94 (2002–2003)
Integrated Solutions IMS REAL/32 version 7.95 (circa 2010)

Versions up to 7.74 identify as "IBM PC DOS" version 3.31; starting with version 7.80, they report as version 6.20.

Support for Logical Block Addressing (LBA) and the FAT32 file system was introduced in version 7.90 during 1999.

Following IMS's insolvency filing in April 2002, Itera Ltd, led by major customer Barry Quittenton, acquired the product; Itera itself was dissolved in March 2006.

By around 2010, Integrated Solutions Ltd of Thatcham, UK, had taken over distribution of REAL/32, though the company eventually shifted away from software development activities.

REAL/NG

REAL/NG represented IMS's initiative to create a "Next Generation" iteration of REAL/32, also referred to as "REAL/32 for the internet age," designed to support a wider array of hardware platforms ranging from personal computers to x86 multiprocessor servers.

Prominent features promoted in 2003 included:

Compatibility with Red Hat Linux versions 7.3 and later
Full backward compatibility with existing DOS and REAL/32 applications
Capability to manage up to 65,535 virtual consoles, each corresponding to an individual user
Administration and configuration accessible via web browser interfaces both locally and remotely
Inclusion of a TCP/IP terminal emulator for licensed users on Linux or Windows platforms
Native print and file sharing functionality along with drive mapping between Linux and REAL/NG servers
Enhanced support for user hardware and multiprocessor environments
Performance improvements coupled with a significantly expanded transient program area (TPA)
Integrated firewall protection
Economical per-seat pricing model and reduced total cost of ownership
Distribution provided on CD alongside Red Hat installation media

By December of 2003, IMS released what appears to be the final iteration titled "REALNG V1.60-V1.19-V1.12." By the year 2005, the realng.com domain mirrored IMS’s primary website but no longer referenced REAL/NG, focusing exclusively on REAL/32.

Application software

Applications designed for this platform utilized multiuser capabilities through dedicated function calls combined with pre-emptive multitasking mechanisms that prevented poorly designed programs from monopolizing CPU resources.

Multiuser DOS continued support for SSL/TLS libraries such as wolfSSL.

The provided API facilitated blocking and non-blocking message queues, mutual exclusion queues, subprocesses running independently in separate threads, and efficient pause methods that avoided unnecessary processor consumption.

Programs could either launch attached to a virtual console or detach to operate as background processes, with the option to reattach when needed.

Memory management employed a shared memory architecture where code segments were isolated from data; identical code sections could be shared across multiple processes running the same application, thereby optimizing memory usage.

Unlike single-tasking operating systems relying on idle loops that waste CPU cycles, Concurrent DOS offered an API call enabling processes to "sleep" without consuming processor time.

Subsequent kernel versions incorporated Idle Detection technology that suspended idle processes to free system resources—this innovation formed the basis of the patented Dynamic Idle Detection feature in DR-DOS marketed under the BatteryMAX power management brand name.

External links

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