Concurrent DOS 286

Digital Research
Company type	Private
Industry	Software
Founded	1970
Founder	Gary Kildall
Fate	Acquired by Novell in 1991
Successor	Novell (1991)
Headquarters	Pacific Grove, California, United States
Products	Multiuser DOS, Concurrent CP/M-86, Concurrent DOS, FlexOS

Multiuser DOS is a real-time, multi-user, multitasking operating system designed for IBM PC-compatible microcomputers. It 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 principal value-added resellers—DataPac Australasia, Concurrent Controls, and Intelligent Micro Software—continued independent development under various names including Datapac Multiuser DOS, CCI Multiuser DOS, IMS Multiuser DOS, and REAL/32.

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

Concurrent CP/M-86

The first release of CP/M-86 1.0 (BDOS 2.x) was adapted for the IBM PC in 1982 but failed commercially due to competition from IBM’s more affordable PC DOS 1.0. Both operating systems did not fully exploit the potential of the new 16-bit hardware.

In September 1981, MP/M-86 2.0 introduced multitasking and multiuser capabilities by supporting serially connected dumb terminals, allowing multiple users to share a single PC cost-effectively. This arrangement enabled applications such as accounting and inventory management before the widespread adoption of PC networking.

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). Development was led by project manager Kathryn Strutynski with designer Francis "Frank" R. Holsworth. Initially intended as a single-user system capable of multitasking up to four CP/M-86 programs simultaneously, it could be configured for multi-processor environments and featured "virtual screens" to switch among running tasks. Subsequent releases added support for dumb terminals to enable multiuser functionality. 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 released Concurrent CP/M with Windows for the IBM Personal Computer and Personal Computer XT, a variant of Concurrent CP/M-86 incorporating windowing features.

Concurrent CP/M-86/80

This version of Concurrent CP/M-86 was designed for systems such as the LSI-M4, LSI Octopus, and CAL PC computers that combined both 16-bit and 8-bit processors. Early 16-bit PCs often ran 8-bit software more efficiently due to its availability and performance benefits. Concurrent CP/M-86/80 allowed execution of both CP/M (8-bit) and CP/M-86 (16-bit) applications by directing commands to the suitable processor based on file extensions (.COM for 8-bit programs and .CMD for 16-bit). It simulated the CP/M environment by translating system calls between the two processor types.

Concurrent DOS

In August 1983, Bruce Skidmore, Raymond D. Pedrizetti, Dave Brown, and Gordon Edmonds developed PC-MODE, an optional add-on for Concurrent CP/M-86 3.1 (BDOS 3.1), which provided basic compatibility with PC DOS 1.1. This module was demonstrated at COMDEX in December 1983 and released in March 1984 as Concurrent DOS 3.1 (CDOS with BDOS 3.1) to hardware manufacturers. It enabled simple DOS applications that avoided direct hardware manipulation—such as PKZIP—to operate correctly; however, software requiring screen control like WordStar did not run unless native Concurrent CP/M versions were used.

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

Digital Research marketed Concurrent DOS 4.1 bundled with GEM as an alternative to IBM's TopView in 1985. Compatible with PC DOS version 2.x, Concurrent DOS 4.1 (BDOS 4.1) supported multiple hardware platforms; its IBM PC compatible release was named Concurrent PC DOS 4.1.

Concurrent PC DOS

Released in 1984, Concurrent DOS 3.2 (featuring BDOS 3.2) offered compatibility with CP/M-86 1.x, Concurrent CP/M-86 3.x, and PC DOS 2.0 software. It was designed to operate across various hardware platforms, with the IBM PC-compatible BIOS/XIOS variant named Concurrent PC DOS 3.2. The product management was led by Kathryn Strutynski.

Concurrent DOS 68K and FlexOS 68K

Beginning in 1984, Digital Research partnered with Motorola to develop Concurrent DOS 68K in Austin, Texas, as an advancement of CP/M-68K. This system was programmed in C, with Francis "Frank" R. Holsworth serving as a key architect. By early 1985, Concurrent DOS 68K 1.0 was made available for OEM evaluation, financially supported by Motorola with a multi-million dollar investment, targeting their 68000 and 68010 CPU families. Initially deployed on Motorola VME/10 development platforms, it was subsequently ported to additional hardware.

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)

This product line transitioned into FlexOS 68K by late 1986.

Concurrent DOS 286 and FlexOS 286

Alongside Concurrent DOS 68K, Digital Research unveiled Concurrent DOS 286 in collaboration with Intel in January 1985. Constructed as a full rewrite in C based on MP/M-286 and Concurrent CP/M-286 foundations, it introduced dynamically loadable device drivers replacing the fixed BIOS/XIOS model. Francis "Frank" R. Holsworth also played a leading role in its architecture.

Tailored for native Intel 80286 protected mode operation, it enabled multi-user multitasking while supporting 8086 emulation. Early prototype processors allowed this functionality; however, production chip issues postponed its release from May until Intel resolved these shortcomings through updated revisions incorporating microcode enhancements that improved emulation efficiency.

These challenges similarly impacted the initial versions of FlexOS 286, developed by Digital Research’s Flexible Automation Business Unit in Monterey starting in 1986.

Subsequent releases enhanced compatibility with PC DOS versions 2.x and 3.x.

Notable versions 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 European Development Centre (EDC) located in Hungerford, UK, which assumed responsibility for developing the Concurrent DOS series beginning with version 4.11 and related products such as DOS Plus.

Developed at Hungerford, versions labeled as Concurrent DOS XM (with XM denoting support for Expanded Memory)—specifically versions five and six—offered bank switching capabilities up to 8 MB of EEMS. This configuration enabled a real-mode environment capable of simultaneously running multiple CP/M-86 and DOS applications for up to three users: one local user plus two remote users via serial terminals.

By 1987, the system was reengineered into Concurrent DOS 386,^[1] which preserved the traditional XIOS & BDOS framework while utilizing the Intel 80386 processor's hardware virtualization features. This innovation permitted most DOS programs to execute without modification, supporting concurrent multiuser file access and allowing multiuser applications to function as if each user operated an individual PC connected to a network server. The system accommodated multiple terminal or low-end PC users employing terminal emulation software, reducing expenses by negating the need for costly workstations and network interface cards. It featured genuine multiuser support with record locking mechanisms ensuring database integrity.

Concurrent DOS 386 required approximately 200K of conventional memory—substantially more than MS-DOS demands. According to BYTE, a minimum of 512K RAM allowed access only to the system prompt; running three major applications such as Microsoft Word or Multiplan concurrently necessitated roughly an additional megabyte beyond the standard base memory of 640K.^[2]

Concurrent DOS version 6.0 also served as the basis for the subsequent DR DOS family derived from it.

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 version ~5.1?, January 1987)
DR Concurrent DOS XM 5.2 (BDOS version ~5.2?, September 1987)
DR Concurrent DOS XM 6.0 (BDOS version 6.0, November 1987), including update version 6.01
DR Concurrent DOS XM 6.2 (BDOS version ~6.2), including update version 6.21
DR Concurrent DOS 386 editions:
- Version 1.0 (BDOS approximately version 5.0, released in 1987)
- Version 1.1 (BDOS approximately version 5.2, September 1987)
- Version 2.0 (BDOS version 6.0, November 1987), including update version 2.01
- Version 3.0 (BDOS version 6.2, December 1988–January 1989), including updates versions 3.01 and 3.02

Concurrent PC DOS XM version 5.0 emulated IBM PC DOS 2.10,^[3] while both Concurrent DOS XM version 6.0 and Concurrent DOS 386 version 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 functioned correctly, whereas version 4.0 failed due to reliance on undocumented DOS calls. The system's performance declined in direct relation to the number of concurrently running applications. Although it demanded more memory, video input/output operations executed at twice the speed compared to standard DOS. The publication concluded that while more sophisticated 386-based operating systems were anticipated, Concurrent DOS 386 was immediately available and operational.

Adaptations

Concurrent Controls, Inc. produced multiple iterations of Concurrent DOS, including the following releases:

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 series versions 3.05 through 3.07 with various updates from 1992 until June 1998
CCI Concurrent DOS 3.10 R1 (October 2003)

Additionally, Apricot Concurrent DOS 386 version 2.01 (1987) was developed specifically for the Apricot Quad Version Level 4.3.

Multiuser DOS

Subsequent editions of Concurrent DOS 386 incorporated elements from Digital Research’s DR DOS 5.0—a single-user PC DOS clone—and were rebranded as "Multiuser DOS" (MDOS) beginning with version 5.0 (BDOS 6.5) in 1991.

Multiuser DOS encountered technical limitations that hindered its ability to compete with PC DOS-based local area networks. It necessitated unique device drivers because standard PC DOS drivers lacked support for multiuser and multitasking environments, complicating installation procedures and requiring kernel relinking through SYSGEN. Compatibility issues arose with network protocol stacks and emerging PC hardware components such as graphics cards, audio devices, CD-ROM drives, and pointing devices. Although graphical terminals eventually accommodated CGA, EGA, and VGA software, the platform proved less adaptable than networks composed of standalone PCs.

Due to its multi-user capabilities and specialized driver demands, Multiuser DOS was priced higher than single-user multitasking solutions like Quarterdeck’s DESQview but failed to gain traction as a single-user multitasking system.

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

Documented versions include:

DR Multiuser DOS 5.00 (1991), followed by 5.01
Novell DR Multiuser DOS 5.10 (April 1992), then version 5.11
Novell DR Multiuser DOS 5.13 (BDOS 6.6, released in 1992)

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

DataPac Australasia

The following releases are attributed to DataPac Australasia Pty Limited:

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

After Citrix Systems, Inc. acquired DataPac 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. (CCI) launched several editions of CCI Multiuser DOS, spanning from version 7.00 through 7.22, including releases from R1 in September 1996 up to R18 GOLD in 2005. These editions were also labeled as "IBM PC DOS" version 3.31, with the version number configurable using the DOSVER utility.

In 1999, CCI underwent a rebranding and became Applica, Inc., which later evolved into Aplycon Technologies, Inc. in 2002.

IMS Multiuser DOS

The IMS Multiuser DOS product line included:

Enhanced Release 5.1 (1992) with subsequent updates 5.11 and 5.14
Versions 7.0 and 7.1 released in 1994, based on BDOS 6.7

All these releases identified themselves as "IBM PC DOS" version 3.31.

REAL/32

In 1994, Intelligent Micro Software Ltd., based in Thatcham, UK, obtained a license from Novell for Multiuser DOS and subsequently rebranded it as REAL/32 the following year.

IBM incorporated REAL/32 version 7.50 in 1995 to power its 4695 point-of-sale terminals.

Significant IMS REAL/32 releases include:

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

Versions up to 7.74 reported as "IBM PC DOS" version 3.31, whereas versions from 7.80 onward identified as version 6.20.

Support for Logical Block Addressing (LBA) and FAT32 was introduced starting with REAL/32 version 7.90 in 1999.

Following IMS’s insolvency filing on April 19, 2002, Itera Ltd.—a prominent client—acquired the company; Itera itself was dissolved in March 2006.

By around 2010, Integrated Solutions Ltd., located in Thatcham, UK, was distributing REAL/32; however, the company eventually transitioned its focus from software products to construction services.

REAL/NG

REAL/NG represented IMS’s effort to create the "Next Generation" iteration of REAL/32, marketed as "REAL/32 for the internet age." This platform aimed to support an extensive range of hardware, from personal computers to multi-processor x86 servers.

Prominent features highlighted in the product’s 2003 promotion included:

Compatibility with Red Hat Linux version 7.3 or newer
Backward compatibility with both DOS and REAL/32 environments
Capacity for up to 65,535 virtual consoles, each functioning as an independent user session
Administration accessible via web browser interfaces locally or remotely without requiring Linux expertise
Bundled TCP/IP terminal emulator for licensed users operating on Linux or Windows platforms
Integrated services such as printing and file sharing along with drive mapping between Linux and REAL/NG servers
Enhanced support for user hardware and multi-processor configurations
Built-in firewall protection alongside improved performance and Terminal Protocol Adapter (TPA) capabilities
Competitive cost per user seat and overall ownership expenses
Distributed on CD media alongside Red Hat installation packages

By December of 2003, IMS had issued versions labeled "REALNG V1.60-V1.19-V1.12," which appear to be the last known updates released publicly; by approximately 2005, the realng.com website redirected visitors to IMS’s primary domain without referencing REAL/NG and instead concentrated exclusively on REAL/32.

Application software

Software created for this platform utilized capabilities specifically designed for multiuser environments and preemptive multitasking, ensuring that poorly optimized programs could not dominate CPU resources. Multiuser DOS maintains compatibility with contemporary SSL/TLS libraries such as wolfSSL.

The application programming interface provided both blocking and non-blocking message queues, mutual exclusion queues, subprocess threads that operated independently from their parent processes, and efficient pause mechanisms that conserved CPU cycles compared to the busy-wait loops typical in single-user operating systems. Programs could either run attached to virtual consoles or detach to execute as background tasks, with the ability to reattach when necessary.

A distinctive feature was its shared memory architecture, which separated code segments from data areas, allowing executable code to be shared among multiple instances of the same program, thereby minimizing memory usage.

Multitasking applications avoided resource-intensive infinite loops by invoking API functions to enter sleep states during idle periods. Subsequent kernel versions introduced Idle Detection to suspend dormant processes, enhancing CPU efficiency. This advancement culminated in the patented Dynamic Idle Detection technology branded as BatteryMAX for power management.

External links

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