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Concurrent DOS 8-16

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Adapted from the English Wikipedia article Concurrent DOS 8-16, available under CC BY-SA 4.0Updated 9 August 20254 sources
Multiuser DOS
Company typeOperating system
IndustryComputer software
PredecessorConcurrent CP/M-86, Concurrent DOS, Concurrent DOS 386
Founded1982
FounderDigital Research
FateDiscontinued by Novell in 1992; continued by VARs independently
SuccessorFlexOS family (sold to Integrated Systems, Inc. in 1994)
Headquarters
United States
ProductsMultiuser DOS operating system and variants
OwnerDigital Research (original), Novell (from 1991)

Multiuser DOS is a real-time operating system designed for IBM PC-compatible microcomputers, supporting multiple users and multitasking capabilities. It originated from earlier Digital Research products such as Concurrent CP/M-86, Concurrent DOS, and Concurrent DOS 386. Initially created by Digital Research, the system was acquired by Novell in 1991, who continued its development. 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.

Following Novell's discontinuation of Multiuser DOS in 1992, three prominent value-added resellers (VARs)—DataPac Australasia, Concurrent Controls, and Intelligent Micro Software—maintained independent enhancement efforts, marketing the software under various names including Datapac Multiuser DOS/System Manager, CCI Multiuser DOS, and IMS Multiuser DOS/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 first release of CP/M-86 1.0 (utilizing BDOS 2.x) was adapted for the IBM PC in 1982 but failed commercially against IBM’s more affordable PC DOS 1.0. Both systems did not fully exploit the capabilities offered by the new 16-bit architecture.

Shortly thereafter, MP/M-86 2.0, a multitasking variant of CP/M, enabled PCs to support multiple simultaneous users through serially connected dumb terminals. This multiuser configuration provided considerable cost efficiency compared to deploying multiple standalone PCs and supported shared-data applications such as accounting or inventory management before PC networking became common.

In late 1982, CP/M-86 1.1 (BDOS 2.2) was combined with MP/M-86 2.1 to form Concurrent CP/M-86 3.0 (BDOS 3.0), initially a single-user operating system capable of multitasking up to four CP/M-86-compatible programs concurrently. It included support for multi-processor setups and introduced "virtual screens" allowing users to switch between running programs. Subsequent releases incorporated support for dumb terminals to enable multiuser functionality. Concurrent CP/M-86 3.1 (BDOS 3.1) was launched on February 21, 1984.

Project leadership was provided by Kathryn Strutynski, with design input from Francis "Frank" R. Holsworth.

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 version of Concurrent CP/M-86 that included windowing features.

Concurrent CP/M-86/80

This version of Concurrent CP/M-86 targeted hybrid systems such as LSI-M4, LSI Octopus, and CAL PC that combined both 16-bit and 8-bit processors. Since early 16-bit PCs often ran 8-bit software more efficiently, Concurrent CP/M-86/80 enabled execution of both CP/M (8-bit) and CP/M-86 (16-bit) programs. It directed commands to the appropriate processor based on file extensions (.COM for 8-bit applications, .CMD for 16-bit), simulating a CP/M environment by translating system calls for execution on the 16-bit CPU.

Concurrent DOS

In August 1983, Bruce Skidmore, Raymond D. Pedrizetti, Dave Brown, and Gordon Edmonds created PC-MODE, an optional add-on for Concurrent CP/M-86 3.1 (BDOS 3.1) that provided fundamental compatibility with PC DOS 1.1 and MS-DOS 1.1. This technology was publicly showcased at COMDEX in December 1983 and became available to hardware manufacturers in March 1984 as Concurrent DOS 3.1 (CDOS with BDOS 3.1). It allowed the execution of simple DOS programs that did not rely on direct hardware interaction; for example, console utilities like PKZIP functioned completely and offered more capabilities compared to the native CP/M ARC archiver. Nevertheless, software requiring direct screen control, such as the DOS version of WordStar, necessitated native Concurrent CP/M or CP/M-86 editions.

Starting in 1983, Digital Research Europe’s OEM Support Group based in Newbury, UK, handled adaptations and localizations of Concurrent DOS versions ranging from 3.1 through 4.1 for original equipment manufacturers.

In 1985, Digital Research marketed Concurrent DOS 4.1 alongside the Graphics Environment Manager (GEM) as a competitor to IBM's TopView environment. Concurrent DOS 4.1 (BDOS 4.1) offered compatibility with PC DOS 2.x and was released on diverse hardware platforms; the IBM PC compatible variant was designated Concurrent PC DOS 4.1.

Concurrent PC DOS

Concurrent DOS 3.2 (featuring BDOS 3.2), launched in 1984, supported applications written for CP/M-86 1.x, Concurrent CP/M-86 3.x, and PC DOS 2.0 environments. This version was distributed across multiple hardware systems, with the IBM–PC-compatible BIOS/XIOS edition known as Concurrent PC DOS 3.2. Kathryn Strutynski held the role of product manager for this line.

Concurrent DOS 68K and FlexOS 68K

Beginning in 1984, a partnership with Motorola resulted in the development of Concurrent DOS 68K at Digital Research’s Austin, Texas facility as a successor to CP/M-68K. Written in C, this system was architected primarily by Francis "Frank" R. Holsworth. Early in 1985, Concurrent DOS 68K version 1.0 was made available for OEM evaluation; Motorola financed this project with a multimillion-dollar investment aimed at their 68000/68010 processors. Initially deployed on Motorola VME/10 platforms, it was subsequently ported to Compupro 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)

By late 1986, this product line transitioned into FlexOS 68K.

Concurrent DOS 286 and FlexOS 286

In parallel with Concurrent DOS 68K development, Digital Research unveiled Concurrent DOS 286 in collaboration with Intel in January 1985. This system was based on technologies derived from MP/M-286 and Concurrent CP/M-286 dating back to 1982 but rewritten in C with a redesigned architecture that incorporated dynamically loadable device drivers instead of fixed BIOS/XIOS components.

Francis "Frank" R. Holsworth played a major role in this project as well. Built to operate natively in Intel 80286 protected mode, it supported multi-user multitasking while maintaining an emulation layer for the older 8086 processor mode. However, issues with emulation on production-grade chips postponed its release from May until Intel produced updated processors that addressed these problems.

By August of that year, Intel’s E-1 stepping resolved documented errata although some undocumented issues remained; subsequent E-2 stepping further enhanced emulation performance through microcode updates, notably affecting instructions like LOADALL. These difficulties also influenced FlexOS 286 version 1.x—a reengineered derivative developed by Digital Research’s Flexible Automation Business Unit located in Monterey since 1986.

Later releases improved compatibility with PC DOS versions 2.x and 3.x.

Released versions included:

  • 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 created European Development Centre (EDC) located in Hungerford, UK. From version 4.11 onward, this center assumed responsibility for developing the Concurrent DOS series, along with associated products such as DOS Plus.

The development of Concurrent DOS versions 5 and 6—known as Concurrent DOS XM, with "XM" denoting Expanded Memory support—was carried out at the Hungerford facility. These editions facilitated bank switching for up to 8 MB of EEMS, allowing a real-mode platform capable of running several CP/M-86 and DOS programs simultaneously. They also supported multiuser operation with up to three users: one local and two connected via serial terminals.

In 1987, Concurrent DOS was redesigned as Concurrent DOS 386,[1] preserving the traditional XIOS and BDOS system architecture. Tailored for Intel 80386 processors and successors, it utilized hardware virtualization features to execute most DOS applications without modification, including those accessed through terminals. The operating system enabled concurrent multiuser file access, allowing applications to run as if each user had a dedicated PC networked to a server. This design permitted a single server to serve multiple users on inexpensive terminals or PCs running terminal emulation software, eliminating the need for costly workstations or network adapters. It incorporated genuine multiuser functionality with record locking mechanisms to avoid data inconsistencies.

Operating Concurrent DOS 386 demanded approximately 200K of conventional memory—considerably more than MS-DOS—and required at least 512K of RAM just for basic operation. For running substantial applications like Microsoft Word or Multiplan, around 1.6MB of memory was recommended.[2]

Concurrent DOS 6.0 also served as the foundation for the DR DOS product line, which originated from this version.

Notable 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 update 6.01
  • DR Concurrent DOS XM 6.2 (BDOS 6.2), including update 6.21
  • Versions of DR Concurrent DOS 386:
    • 1.0 (BDOS ~5.0, 1987)
    • 1.1 (BDOS ~5.2, September 1987)
    • 2.0 (BDOS 6.0, November 1987), including update 2.01
    • 3.0 (BDOS 6.2, late 1988/early 1989), including updates 3.01 and 3.02

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

Reception

In a review from 1988, BYTE magazine characterized Concurrent DOS 386 as largely compatible with MS-DOS; for instance, Microsoft Word version 3.1 functioned properly while version 4.0 did not due to reliance on undocumented DOS features. Performance diminished proportionally with the number of simultaneously running applications; however, despite its greater memory demands, video input/output operated at twice the speed compared to standard DOS systems. The publication concluded that although more sophisticated operating systems for the 386 were emerging, Concurrent DOS 386 was immediately available and ready for deployment.

Concurrent Controls, Inc. Variants

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

  • CCI Concurrent DOS 386 ranging from version 1.12 (October 1987) up to 3.10 R1 (October 2003)
  • Key releases encompassed versions 2.01 (May 1988), 3.04 (July 1991, also known as "CCI Concurrent DOS 4.0"), along with successive updates extending through 1998

Additional derivatives comprised:

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

Multiuser DOS Developments

Subsequent variants of Concurrent DOS 386 incorporated elements from Digital Research’s PC DOS-compatible DR DOS 5.0, which was rebranded as "Multiuser DOS" (MDOS) beginning with version 5.0 (BDOS 6.5) in 1991.

MDOS encountered technical limitations that restricted its ability to compete with PC DOS-based LAN environments. It demanded custom device drivers for standard hardware due to the absence of multiuser support in PC DOS drivers, and driver installation required kernel ("nucleus") relinking via SYSGEN rather than straightforward file copying or CONFIG.SYS configuration.

Moreover, MDOS initially lacked compatibility with many PC DOS enhancements such as network protocols, graphics cards, audio devices, CD-ROM drives, and mice. Although graphical terminal support eventually allowed CGA, EGA, and VGA applications to run, MDOS remained less adaptable than networks composed of individual PCs. As PC hardware became more affordable, MDOS’s benefits in terms of administration and overall cost effectiveness diminished.

The multi-user licensing model increased expenses relative to single-user systems, and unlike single-user multitasking products like Quarterdeck’s DESQview, MDOS never achieved widespread adoption for solo multitasking scenarios.

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

Documented MDOS releases 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 5.13 (BDOS 6.6, released in 1992)

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

DataPac Australasia Pty Limited

Versions attributed to DataPac Australasia Pty Limited are:

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

After being acquired by Citrix Systems, Inc. in 1997, the System Manager product was phased out shortly thereafter. In 2002, Citrix spun off its Sydney-based division into the Advanced Products Group.

Concurrent Controls Multiuser DOS

Concurrent Controls, Inc.’s known Multiuser DOS releases include:

  • Versions ranging from CCI Multiuser DOS 7.00 through to 7.22, with iterations from R1 (September 1996) up to R18 GOLD (2005), all reporting their DOS version as "IBM PC DOS" version 3.31.

The displayed DOS version string was customizable using the Multiuser DOS utility.

In 1999, Concurrent Controls rebranded itself as Applica, Inc., which later became Aplycon Technologies, Inc., in 2002.

IMS Multiuser DOS

The IMS Multiuser DOS product line comprises several versions, including:

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

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

REAL/32

In 1994, Intelligent Micro Software Ltd. (IMS), based in Thatcham, UK, obtained a license from Novell to develop Multiuser DOS further, rebranding it as REAL/32 in 1995.

IBM incorporated REAL/32 version 7.50 into their 4695 POS terminal systems under license in the same year.

The REAL/32 product line by IMS spans from version 7.50 (1995) through Integrated Solutions' distribution of version 7.95 (circa 2010). Notably, version 7.90 (1999) introduced support for LBA and FAT32 file systems.

Versions prior to 7.74 report the system as "IBM PC DOS" version 3.31, whereas versions starting at 7.80 identify as version 6.20.

After IMS declared insolvency in April 2002, Itera Ltd acquired the company but subsequently dissolved in 2006.

By around 2010, Integrated Solutions of Thatcham, UK, was the supplier of REAL/32; however, the firm eventually moved away from software development activities.


REAL/NG

REAL/NG represented IMS’s initiative to create the "Next Generation" iteration of REAL/32, marketed as "REAL/32 for the internet age." It was engineered to operate across a wide array of hardware platforms ranging from standard PCs to x86 multi-processor server architectures.

Highlighted features advertised in 2003 included:

  • Compatibility with Red Hat Linux version 7.3 and newer
  • Full backward compatibility with existing DOS and REAL/32 applications
  • Support for up to 65,535 virtual consoles, each corresponding to an individual user session
  • Administration accessible via web browser locally or remotely without requiring Linux expertise
  • Inclusion of a TCP/IP terminal emulator bundled with user license purchases
  • Native print and file sharing services along with drive mapping capabilities between Linux and REAL/NG servers
  • Enhanced support for hardware devices and multi-processor configurations
  • Performance improvements alongside significantly increased Transient Program Area (TPA)
  • Integrated firewall functionality and reduced total cost of ownership
  • Distribution on CD media containing Red Hat installation files

The most recent known release was titled "REALNG V1.60-V1.19-V1.12" as of December 2003; by 2005, realng.com redirected to IMS's primary website without any references to REAL/NG, suggesting discontinuation.


Application software

Applications designed for this environment utilized multiuser features through dedicated function calls within the platform’s API framework. REAL/NG employed pre-emptive multitasking to ensure that poorly designed programs could not monopolize CPU resources.

Compatibility persists with modern SSL/TLS libraries such as wolfSSL.

The API facilitated blocking and non-blocking message queues, mutual exclusion queues for resource synchronization, subprocess threads capable of running independently from their parent processes, and efficient pause mechanisms that avoided unnecessary CPU consumption.

Programs could execute attached to a virtual console or detach to run in the background with the option to reattach later.

Memory management adopted a shared memory architecture where code segments were separated from data sections—allowing multiple instances of the same program to share code segments while maintaining isolated data areas—thereby optimizing memory utilization.

Unlike single-tasking systems that waste CPU cycles through idle loops, Concurrent DOS provided an API call enabling processes to enter sleep states efficiently.

Subsequent kernels incorporated Idle Detection technology which monitored DOS API usage to suspend idle processes automatically and free CPU resources.

This innovation culminated in the patented DR-DOS Dynamic Idle Detection power management feature marketed under the name BatteryMAX.

See also

Further reading

  • DRTIPS.ZIP (1992) authored by Novell Desktop Systems Group, Digital Research / Novell

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

Related profiles

RREAL/32CCCPM.SYSCCCPMNET.SYSCConcurrent CP/M with Windows
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