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Concurrent DOS XM

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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; further development by DataPac Australasia, Concurrent Controls, and Intelligent Micro Software
Area served
IBM PC-compatible microcomputers
Key people
Kathryn Strutynski (project manager), Francis "Frank" R. Holsworth (design lead)
ProductsMultiuser DOS, Concurrent CP/M-86, Concurrent DOS variants, FlexOS
OwnerDigital Research (original), Novell (from 1991)
ParentDigital Research

Multiuser DOS is a real-time, multitasking operating system designed for IBM PC-compatible microcomputers, supporting multiple users simultaneously. 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 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 major value-added resellers (VARs)—DataPac Australasia, Concurrent Controls, and Intelligent Micro Software—secured licenses to independently continue its development. These efforts resulted in Datapac Multiuser DOS/System Manager, CCI Multiuser DOS, and IMS Multiuser DOS/REAL/32 respectively.

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

Concurrent CP/M-86

The first iteration of CP/M-86 1.0 (BDOS 2.x) was adapted for the IBM PC in 1982; however, it faced commercial challenges competing with IBM’s more affordable PC DOS 1.0. Both operating systems underexploited the potential of the new 16-bit architecture.

In September 1981, MP/M-86 2.0 introduced multitasking capabilities, converting PCs into multiuser systems that supported several simultaneous users connected through serial dumb terminals. This approach provided substantial cost advantages over deploying multiple PCs and facilitated shared-data applications such as accounting or inventory management before PC networking became commonplace.

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 overseen by manager Kathryn Strutynski with design led by Francis "Frank" R. Holsworth. Initially intended as a single-user OS enabling true multitasking for up to four CP/M-86 programs, it also supported multiprocessor configurations and featured "virtual screens" to switch between running programs. Subsequent releases added multiuser functionality via dumb terminals connected to the CP/M-86 host system. Concurrent CP/M-86 3.1 (BDOS 3.1) launched on 21 February 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, which incorporated windowing features into Concurrent CP/M-86.

Concurrent CP/M-86/80

This variant targeted systems such as LSI-M4, LSI Octopus, and CAL PC machines that combined both 16-bit and 8-bit processors to capitalize on the broader availability and often enhanced performance of 8-bit software. Concurrent CP/M-86/80 allowed execution of both CP/M (8-bit) and CP/M-86 (16-bit) applications by dispatching commands to the appropriate processor based on file extensions (.COM for 8-bit programs, .CMD for 16-bit). It simulated the CP/M environment for 8-bit applications by converting their system calls into equivalent CP/M-86 instructions processed 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 add-on for Concurrent CP/M-86 3.1 (BDOS 3.1) that offered fundamental compatibility with PC DOS 1.1. This feature was showcased at COMDEX in December 1983 and subsequently released to hardware manufacturers as Concurrent DOS 3.1 (CDOS) in March 1984. It enabled basic DOS programs that avoided direct hardware access to operate—for instance, console utilities like PKZIP ran efficiently, surpassing the performance of the native CP/M ARC archiver. Nevertheless, DOS software requiring direct screen manipulation, such as WordStar, necessitated running native versions of Concurrent CP/M or CP/M-86.

While the US team handled development of Concurrent DOS versions up to 4.1, from 1983 onward, Digital Research Europe’s OEM Support Group based in Newbury, UK took charge of OEM customizations and regional adaptations.

In 1985, Digital Research promoted Concurrent DOS 4.1 combined with the Graphics Environment Manager (GEM) as a competitor to IBM’s TopView. Concurrent DOS 4.1 (BDOS 4.1) provided compatibility with PC DOS 2.x across diverse hardware systems; the variant featuring an IBM PC compatible BIOS/XIOS was marketed under the name Concurrent PC DOS 4.1.

Concurrent PC DOS

Released in 1984, Concurrent DOS 3.2 (incorporating BDOS 3.2) supported software designed for CP/M-86 1.x, Concurrent CP/M-86 3.x, and PC DOS 2.0 environments. It was distributed for multiple hardware platforms, with the IBM–PC-compatible BIOS/XIOS edition labeled Concurrent PC DOS 3.2. The product management role was held by Kathryn Strutynski.

Concurrent DOS 68K and FlexOS 68K

Beginning in 1984, Digital Research collaborated with Motorola to develop Concurrent DOS 68K in Austin, Texas as a successor to CP/M-68K. Written in the C programming language, Francis "Frank" R. Holsworth played a leading role in its architecture. By early 1985, Concurrent DOS 68K version 1.0 was available for OEM evaluation, backed by multimillion-dollar funding from Motorola and tailored for their 68000/68010 CPU series. Initially deployed on Motorola VME/10 development platforms, it was subsequently ported to Compupro systems. Versions 1.20 and 1.21 were offered to OEMs around April 1986 at an estimated price point . This line later 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

Alongside the development of the 68K variant, Digital Research introduced Concurrent DOS 286 in partnership with Intel in January 1985. This system built upon MP/M-286 and Concurrent CP/M-286 work dating back to 1982 and was completely rewritten in C to incorporate dynamically loadable device drivers replacing static BIOS/XIOS components.

Francis "Frank" R. Holsworth again contributed significantly as a principal architect. Designed specifically for native Intel 80286 operation mode, it offered protected mode multitasking alongside multi-user capabilities with integrated 8086 emulation. Early prototypes functioned on initial chip steppings; however, release was postponed due to processor-level issues until Intel resolved compatibility problems through revised chip versions and microcode updates enhancing emulation efficiency.

These hardware limitations similarly impacted FlexOS 286, a redesigned iteration developed by Digital Research’s Flexible Automation Business Unit based in Monterey from 1986 onward.

Subsequent editions improved support for PC DOS versions ranging from 2.x to 3.x.

Known releases:

  • 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. From version 4.11 onward, this center assumed responsibility for advancing the Concurrent DOS series, including associated products such as DOS Plus.

At the Hungerford facility, versions 5 and 6—known as Concurrent DOS XM (with XM indicating Expanded Memory)—were developed to support bank switching of up to 8 MB of EEMS. This allowed a real-mode operating environment capable of running multiple CP/M-86 and DOS applications simultaneously, accommodating up to three users: one local and two connected via serial terminals.

In 1987, Concurrent DOS underwent a complete rewrite resulting in Concurrent DOS 386,[1] which preserved the traditional XIOS & BDOS framework. Designed specifically for Intel 80386 processors and beyond, it utilized hardware-assisted virtualization to execute most DOS software without modification, even on terminal devices. The system facilitated multiuser file access with record locking, enabling multiple users to work concurrently on shared databases as if each had a dedicated PC linked to a network server. This approach provided an economical multiuser environment by employing simple terminals or low-end PCs running terminal emulation, thereby eliminating the need for costly workstations and network interface cards.

Concurrent DOS 386 demanded roughly 200K of conventional memory—substantially more than MS-DOS—and a minimum of 512K RAM was necessary just to reach the command prompt. Having approximately an additional 1MB above the standard 640K memory limit allowed operation of three significant applications such as Microsoft Word or Multiplan.[2]

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

The known 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
  • DR Concurrent DOS 386 series spanning versions from 1.0 (1987) through 3.02 (1989), with BDOS versions approximately between 5.0 and 6.2

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

Reception

In a 1988 review by BYTE, Concurrent DOS 386 was characterized as "largely compatible" with MS-DOS; for example, Microsoft Word version 3.1 operated correctly, while version 4.0 failed due to reliance on undocumented DOS functions. System performance diminished proportionally as more applications ran concurrently. Although it required more RAM than typical MS-DOS setups, video input/output operations were twice as fast compared to standard DOS systems. The publication concluded that despite newer advanced operating systems for the 386 being imminent, Concurrent DOS 386 "is available now and fully operational."

Adaptations

Concurrent Controls, Inc., created multiple variants based on Concurrent DOS technology, including:

  • CCI Concurrent DOS 386 versions ranging from 1.12 (October 1987) through to release 3.10 R1 (October 2003)
  • Key releases encompassed BDOS versions from approximately 5.0 up to BDOS 6.6 across the period from 1987 to 1998
  • Additional incremental updates and revisions persisted into the mid-1990s

Other notable adaptations comprise:

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

Multiuser DOS

Subsequent editions of Concurrent DOS 386 incorporated elements from Digital Research's DR DOS 5.0, adopting the name "Multiuser DOS" (MDOS) starting with the 1991 release of version 5.0.

MDOS encountered technical limitations that constrained its ability to compete with PC DOS-based LAN environments. It necessitated dedicated device drivers because the standard PC DOS drivers lacked multiuser and multitasking capabilities. Installing these drivers was a complicated process requiring kernel relinking through SYSGEN instead of straightforward file copying.

Furthermore, MDOS did not support numerous common PC DOS extensions, including network protocol stacks, and initially offered restricted compatibility with advancing hardware such as graphics cards, sound devices, CD-ROM drives, and pointing devices. Although some progress was made—for example, graphical terminals were developed to support CGA, EGA, and VGA software—MDOS remained less adaptable than networks composed of individual PCs. As the cost of PC hardware declined, the benefits MDOS provided in terms of administration and overall ownership expenses became less significant.

With a price point exceeding that of single-user systems and the requirement for specialized drivers (unlike single-user multitasking solutions like Quarterdeck's DESQview), MDOS failed to achieve widespread adoption for single-user multitasking purposes.

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 in 1994 to pursue independent development efforts.

Notable 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 iterations from Digital Research and Novell identified themselves as "IBM PC DOS" version 3.31.

DataPac Australasia

DataPac Australasia Pty Limited produced several versions such as:

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

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

Concurrent Controls

Concurrent Controls, Inc. (CCI) launched multiple editions under the CCI Multiuser DOS brand:

  • Versions ranging from CCI Multiuser DOS 7.00 up to 7.22, including releases from R1 (September 1996) through R18 GOLD (2005)

Each version self-reports as "IBM PC DOS" version 3.31, with the displayed version string modifiable via the DOSVER utility.

In 1999, CCI changed its name to Applica, Inc., which later became Aplycon Technologies, Inc. in 2002.

IMS Multiuser DOS

Releases include:

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

All report themselves as "IBM PC DOS" version 3.31.

REAL/32

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

In 1995, IBM obtained a license for REAL/32 version 7.50 to bundle it with their 4695 POS terminal systems.

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 versions 7.93 and 7.94 (2002–2003)
  • Integrated Solutions IMS REAL/32 version 7.95 (circa 2010)

Versions up to 7.74 identified themselves as "IBM PC DOS" version 3.31; beginning with version 7.80, they reported as version 6.20.

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

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

By around 2010, Integrated Solutions Ltd., also located in Thatcham, UK, was distributing REAL/32, although the company eventually transitioned away from software development toward building 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 was designed to support a wide array of hardware, ranging from personal computers to x86 multiprocessor servers.

Highlighted features promoted in 2003 included:

  • Compatibility with Red Hat Linux version 7.3 or newer
  • Full backward compatibility with DOS and REAL/32 applications
  • Support for up to 65,535 virtual consoles, each corresponding to an individual user session
  • Administration via web browser interfaces accessible locally or remotely
  • Included TCP/IP terminal emulator compatible with Linux and Windows platforms, aligned with user license counts
  • Integrated printing and file sharing capabilities along with drive mapping between Linux and REAL/NG servers
  • Enhanced hardware compatibility and multiprocessor support
  • Embedded firewall functionality
  • Performance improvements and significantly increased transient program area (TPA)
  • Cost-effective licensing per user seat and overall ownership expenses
  • Distribution on CD media bundled alongside Red Hat installation CDs

By December 2003, IMS issued REALNG versions V1.60-V1.19-V1.12, which appear to be the final updates released. By 2005, the realng.com website redirected visitors to IMS’s main homepage, removing references to REAL/NG and emphasizing REAL/32 instead.

Application software

Software developed for this environment capitalized on its multiuser architecture through specialized function calls. The system utilized preemptive multitasking to prevent individual applications from monopolizing CPU resources due to poor design. Multiuser DOS maintains compatibility with contemporary SSL/TLS libraries such as wolfSSL.

The programming interface provided support for blocking and non-blocking message queues, mutual-exclusion queues, subprocess threads independent of their parent processes, and efficient pause mechanisms that conserved CPU cycles more effectively than the idle loops typical of single-user operating systems. Applications generally operated attached to virtual consoles but could also detach to run in the background and reconnect when necessary.

Memory management employed a shared memory architecture that separated code segments from data areas. Because executable code contained no mutable data, multiple instances of the same program could share code sections concurrently, thereby optimizing memory utilization.

Programs adapted for multitasking avoided infinite loops waiting for input by invoking API calls that allowed them to "sleep" efficiently. Subsequent Concurrent DOS kernels introduced an Idle Detection feature that suspended inactive processes to release CPU capacity—this technology laid the groundwork for DR-DOS’s patented Dynamic Idle Detection power management system 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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