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Multiuser DOS

From Alfawiki
Digital Research
Company typePrivate
IndustrySoftware
Founded1974; 52 years ago (1974)
FounderGary Kildall
FateAcquired by Novell in 1991
SuccessorNovell (for Multiuser DOS development)
Headquarters
Pacific Grove, California
,
United States
ProductsMultiuser DOS, Concurrent CP/M-86, Concurrent DOS, FlexOS

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

Originating from Concurrent CP/M-86, Concurrent DOS, and Concurrent DOS 386, this OS was initially created by Digital Research before being acquired and further developed by Novell in 1991. Its lineage extends back to Digital Research's earlier 8-bit platforms CP/M and MP/M, as well as the 16-bit single-tasking system CP/M-86.

Following Novell's decision to cease Multiuser DOS development in 1992, three principal value-added resellers (VARs)—DataPac Australasia, Concurrent Controls, and Intelligent Micro Software—were licensed to continue independent enhancements. These entities released Datapac Multiuser DOS with System Manager, CCI Multiuser DOS, and IMS Multiuser DOS alongside REAL/32, respectively.

The FlexOS series, 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 iteration of CP/M-86 1.0 (utilizing BDOS 2.x) was adapted for the IBM PC platform in 1982 but failed commercially due to stiff competition from IBM’s more affordable PC DOS 1.0. Both operating systems faced challenges in fully exploiting the new 16-bit architecture.

This situation prompted the launch of multitasking with MP/M-86 2.0 in September 1981, which converted PCs into multiuser environments supporting several simultaneous users via dumb terminals connected through serial interfaces. This setup provided significant cost efficiencies compared to deploying multiple individual PCs and facilitated shared-data applications such as accounting or inventory management before networking became common on PCs.

The features of CP/M-86 1.1 (BDOS 2.2) and MP/M-86 2.1 were consolidated into Concurrent CP/M-86 3.0 (also known as CCP/M-86) with BDOS 3.0 by late 1982. Development was overseen by project manager Kathryn Strutynski alongside designer Francis "Frank" R. Holsworth. Initially designed as a single-user OS capable of multitasking up to four CP/M-86 programs simultaneously, it also supported multi-processor setups and introduced "virtual screens" for switching between active programs. Subsequent releases added support for multiple users via dumb terminals. The release of Concurrent CP/M-86 3.1 (BDOS 3.1) occurred on 21 February 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 incorporated windowing functionality.

Concurrent CP/M-86/80

This variant targeted hybrid systems such as the LSI-M4, LSI Octopus, and CAL PC computers, which combined both 16-bit and 8-bit processors. Since 8-bit software was more prevalent and often performed better at the time, Concurrent CP/M-86/80 allowed users to run both CP/M (8-bit) and CP/M-86 (16-bit) applications seamlessly. The operating system executed programs on the appropriate processor depending on the file extension (.COM for 8-bit programs, .CMD for 16-bit), simulating a CP/M environment by translating system calls from CP/M into those compatible with CP/M-86.

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), which provided fundamental compatibility with PC DOS 1.1. This module was showcased at COMDEX in December 1983 and subsequently released as Concurrent DOS 3.1 (CDOS) in March 1984, targeting hardware manufacturers. It enabled straightforward DOS programs—those that did not interact directly with hardware—to operate; for example, console utilities like PKZIP functioned properly, whereas screen-oriented applications such as WordStar required versions native to Concurrent CP/M.

While the US-based teams developed Concurrent DOS versions from 3.1 through 4.1, the OEM customizations and language localizations were handled by Digital Research Europe’s OEM Support Group located in Newbury, UK starting in 1983.

In 1985, Digital Research marketed Concurrent DOS 4.1 combined with GEM as an alternative to IBM’s TopView environment. Concurrent DOS 4.1 (BDOS 4.1) offered PC DOS 2.x compatibility across various hardware platforms; the IBM PC-compatible BIOS/XIOS variant was marketed under the name Concurrent PC DOS 4.1.

Concurrent PC DOS

Released in 1984, Concurrent DOS 3.2 (featuring BDOS 3.2) supported software designed for CP/M-86 1.x, Concurrent CP/M-86 3.x, and PC DOS 2.0 environments. The system was distributed across multiple hardware platforms, with the IBM–PC-compatible BIOS/XIOS version designated as Concurrent PC DOS 3.2. Product management was overseen by Kathryn Strutynski.

Concurrent DOS 68K and FlexOS 68K

Beginning in 1984, a partnership with Motorola led to the development of Concurrent DOS 68K in Austin, Texas, as a successor to CP/M-68K. Written in C, the project was spearheaded by Francis "Frank" R. Holsworth. The initial release, Concurrent DOS 68K 1.0, was made available for OEM evaluation in early 1985 and was funded by Motorola with a multi-million dollar investment aimed at their 68000/68010 microprocessors. Initially deployed on Motorola's VME/10 development platform, it was subsequently ported to additional hardware configurations.

The product line included Concurrent DOS 68K versions 1.0 (1985), 1.1, and 1.20/1.21 (1986), offered at prices around . This operating system eventually transformed into FlexOS 68K by late 1986.

Concurrent DOS 286 and FlexOS 286

Alongside the development of the 68K variant, Digital Research introduced Concurrent DOS 286 in collaboration with Intel in January 1985. Derived from MP/M-286 and Concurrent CP/M-286 (under development since 1982), it was rewritten in C and featured a redesigned architecture that employed dynamically loadable device drivers instead of fixed BIOS/XIOS components. Frank R. Holsworth played a leading role in its design.

Targeting native 80286 mode operation, this system supported protected-mode multi-user multitasking combined with an emulation layer for the 8086 processor. Although prototype chips initially supported these features, production releases were delayed due to hardware errata and performance challenges that Intel resolved through updated microcode (E-2 stepping), which enhanced emulation efficiency.

These same constraints impacted FlexOS 286, an extensively reengineered offshoot developed by Digital Research's Flexible Automation Business Unit starting in 1986 at Monterey, California.

Subsequent releases extended support to PC DOS versions 2.x and 3.x.

Notable releases include:

Concurrent DOS XM and Concurrent DOS 386

In 1986, Digital Research transferred its OEM Support Group to the newly established European Development Centre (EDC) located in Hungerford, UK. From version 4.11 onward, this center assumed responsibility for advancing the Concurrent DOS series and associated software such as DOS Plus.

Versions 5 and 6, developed at Hungerford under the name Concurrent DOS XM (with XM denoting Expanded Memory support), introduced bank switching capabilities for up to 8 MB of EEMS. This allowed a real-mode environment where multiple CP/M-86 and DOS applications could run simultaneously, supporting up to three users—one local and two connected via serial terminals.

By 1987, Concurrent DOS 86 was redesigned into Concurrent DOS 386,[1] preserving the traditional XIOS and BDOS framework. Tailored for Intel 80386 processors and beyond, it utilized hardware virtualization to execute most DOS programs without modification, including those accessed through terminals. The operating system facilitated multiuser file access concurrently, enabling several users to work on a single database with record locking mechanisms—functioning similarly to a network server but without the need for expensive workstations or network interface cards. This genuine multiuser platform supported both simple terminals and low-performance PCs equipped with terminal emulation software.

Concurrent DOS 386 demanded approximately 200K of conventional memory—exceeding MS-DOS requirements—and needed at least 512K of RAM just to present the command prompt; around 1.6MB RAM was necessary to operate three significant applications like Microsoft Word or Multiplan.[2]

The release of Concurrent DOS 6.0 also marked the foundation of the DR DOS line, which originated from this version.

Notable releases comprised:

  • 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 approximately 5.1, January 1987)
  • DR Concurrent DOS XM 5.2 (BDOS approximately 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 editions:
    • Version 1.0 (BDOS roughly 5.0, 1987)
    • Version 1.1 (BDOS roughly 5.2, September 1987)
    • Version 2.0 (BDOS 6.0, November 1987), including version 2.01
    • Version 3.0 (BDOS 6.2, December 1988–January 1989), including versions 3.01 and 3.02

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

Reception

According to a BYTE magazine review in 1988, Concurrent DOS 386 demonstrated "substantial compatibility" with MS-DOS; for example, Microsoft Word version 3.1 operated correctly while version 4.0 did not due to undocumented system calls in DOS. Although performance declined as more applications ran concurrently, video input/output was twice as fast compared to DOS despite higher memory consumption. The article concluded that while more sophisticated operating systems for the Intel386 were on the horizon, Concurrent DOS 386 "is available now and fully functional."

Adaptations

Concurrent Controls, Inc., produced various adaptations of CCI Concurrent DOS:

  • CCI Concurrent DOS 386 version 1.12 (possibly BDOS 5.0, October 1987)
  • CCI Concurrent DOS 386 version 2.01 (possibly BDOS 6.0, May 1988)
  • CCI Concurrent DOS 386 version 3.01 (possibly BDOS 6.2, March 1989)
  • CCI Concurrent DOS 386 version 3.02 (April 1990)
  • CCI Concurrent DOS 386 version 3.03 (March 1991)
  • CCI Concurrent DOS 386 version 3.04 (July 1991), also known as "CCI Concurrent DOS 4.0"
  • CCI Concurrent DOS versions from 3.05 R1 through R11 (1992–1993)
  • CCI Concurrent DOS versions from 3.06 R1 through R7 (1993–1994)
  • CCI Concurrent DOS versions from 3.07 R1 through R7 (1995–1998)
  • CCI Concurrent DOS version 3.08
  • CCI Concurrent DOS version 3.10 R1 (October 2003)

Additional variants include:

  • Apricot Concurrent DOS 386 version 2.01 (1987), designed for Apricot Quad Version Level 4.3

Multiuser DOS

Subsequent editions of Concurrent DOS 386 incorporated elements from Digital Research's PC DOS-compatible DR DOS 5.0, which was renamed "Multiuser DOS" (MDOS) beginning with release 5.0 (BDOS 6.5) in 1991.

Multiuser DOS encountered technical limitations that hindered its ability to compete effectively against LANs built around PC DOS. It demanded the use of specialized device drivers because standard PC DOS drivers lacked support for multiuser and multitasking environments, making installation more complex than merely copying files and adjusting configuration settings; kernel relinking via the SYSGEN command was also required.

Additionally, it had difficulties accommodating typical PC DOS enhancements such as network protocol stacks and newer peripherals including graphics adapters, sound cards, CD-ROM drives, and mice—although graphical terminals eventually allowed software designed for CGA, EGA, and VGA to run. Despite these advancements, Multiuser DOS was less adaptable compared to networks of standalone PCs, whose declining costs diminished its market appeal despite advantages in management and expense control.

As an operating system supporting multiple users, it carried a higher cost than single-user alternatives and necessitated proprietary drivers, unlike single-user multitasking solutions like Quarterdeck's DESQview. Unlike MP/M, it did not achieve widespread adoption as a single-user multitasking platform.

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

Recognized releases include:

  • DR Multiuser DOS 5.00 (1991), 5.01
  • Novell DR Multiuser DOS 5.10 (April 1992), 5.11
  • Novell DR Multiuser DOS 5.13 (BDOS 6.6, 1992)

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

DataPac Australasia

Versions produced by DataPac Australasia Pty Limited comprise:

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

In 1997, Citrix Systems, Inc. acquired DataPac, leading to the discontinuation of System Manager shortly thereafter. By 2002, the Sydney-based division was reorganized into Citrix's Advanced Products Group.

Concurrent Controls

Concurrent Controls, Inc. (CCI) developed the following CCI Multiuser DOS versions:

  • 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 (April 21, 2005)

All CCI Multiuser DOS releases identify as "IBM PC DOS" version 3.31, which can be modified using the Multiuser DOS utility.

CCI rebranded as Applica, Inc. in 1999; later in 2002, Applica Technology transitioned to Aplycon Technologies, Inc.

IMS Multiuser DOS

Documented IMS Multiuser DOS variants 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, released in 1994)

Each version identifies itself as "IBM PC DOS" version 3.31.


REAL/32

Intelligent Micro Software Ltd. (IMS), based in Thatcham, UK, acquired the rights to Multiuser DOS from Novell in 1994 and subsequently rebranded it as REAL/32 in 1995.

In 1995, IBM licensed REAL/32 version 7.50 for integration with their 4695 point-of-sale terminals.

The release timeline of REAL/32 versions is as follows:

  • 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, the system identified itself as "IBM PC DOS" version 3.31; starting with version 7.80, it reported as version 6.20.

The introduction of Logical Block Addressing and FAT32 file system support came with the release of REAL/32 version 7.90 in 1999.

Following IMS’s insolvency filing in April 2002, Itera Ltd., a principal client led by Barry Quittenton, assumed control of operations; Itera was dissolved in March 2006.

By approximately 2010, REAL/32 distribution was managed by Integrated Solutions of Thatcham, UK, though the company eventually transitioned away from software development towards building services.


REAL/NG

REAL/NG represented IMS’s initiative to create a "Next Generation" version of REAL/32, marketed as "REAL/32 for the internet age," designed to operate across an expanded hardware spectrum from personal computers to x86 multiprocessor servers.

Prominent features promoted in 2003 included:

  • Compatibility with Red Hat Linux version 7.3 and later
  • Full backward compatibility with both DOS and REAL/32 applications
  • Support for as many as 65,535 virtual consoles, each corresponding to an individual user session
  • Administration accessible via web browser without requiring Linux expertise, supporting both local and remote management
  • A TCP/IP terminal emulator included for licensed users on Linux and Windows platforms
  • Integrated printing and file sharing capabilities along with drive mapping between Linux and REAL/NG servers
  • Enhanced hardware support for users and multiprocessor architectures
  • Performance improvements accompanied by a significantly expanded transient program area (TPA)
  • Built-in firewall functionality
  • Competitive per-seat pricing coupled with low overall cost of ownership
  • Distribution via CD bundled with Red Hat installation media

The final update appears to be version V1.60-V1.19-V1.12 released by December 2003; by 2005, the realng.com domain redirected to IMS’s primary website, which no longer referenced REAL/NG and instead concentrated solely on REAL/32.


Application software

While supporting standard DOS applications, the operating system also enabled programs to utilize its multiuser environment through specialized function calls. It incorporated pre-emptive multitasking to prevent any single poorly designed application from dominating CPU resources.

Multiuser DOS maintains compatibility with modern SSL/TLS libraries such as wolfSSL.

The provided API included features like blocking and non-blocking message queues, mutual exclusion queues, subprocess threads capable of running independently from parent processes, and efficient sleep functions that conserved CPU cycles compared to conventional single-user idle loops.

Applications could operate either attached to virtual consoles or detached for background execution with optional reattachment later.

Memory management employed a shared memory architecture that separated code and data segments; since code segments were read-only, they could be shared among multiple instances of the same program, thereby reducing overall memory consumption.

To enhance multitasking efficiency, applications avoided continuous polling loops by using API calls that put them to sleep while waiting for input.

Later iterations of Concurrent DOS kernels incorporated Idle Detection technology that suspended inactive processes, freeing CPU time for active applications.

This advancement culminated in the patented Dynamic Idle Detection power management feature marketed under the name BatteryMAX.

See also

Further reading

  • DRTIPS.ZIP (1992) 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
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