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Concurrent DOS 86

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Adapted from the English Wikipedia article Concurrent DOS 86, 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
FateDevelopment ceased by Novell in 1992; further development by VARs
SuccessorDR DOS family
Area served
IBM PC-compatible microcomputers
Key people
Kathryn Strutynski (project manager), Francis "Frank" R. Holsworth (designer)
ProductsMultiuser DOS, Concurrent CP/M-86, Concurrent DOS, FlexOS
OwnerDigital Research; later Novell (from 1991)
ParentDigital Research; Novell (after acquisition in 1991)

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 subsequently enhanced by Novell following its acquisition in 1991. Its lineage can be traced back to the 8-bit Digital Research 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 granted rights to continue independent development, leading to versions such as Datapac Multiuser DOS and System Manager, CCI Multiuser DOS, and IMS Multiuser DOS along with REAL/32. Concurrently, the FlexOS family, which descended 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 faced commercial challenges against IBM’s more affordable PC DOS 1.0. Both systems failed to fully exploit the capabilities of the new 16-bit architecture.

Shortly thereafter, MP/M-86 2.0, launched in September 1981, upgraded the PC into a multi-user platform supporting several users simultaneously through serially connected dumb terminals. This configuration offered considerable cost advantages compared to deploying multiple individual PCs and facilitated shared-data applications like accounting or inventory management before widespread PC networking became common.

The combination of CP/M-86 1.1 (BDOS 2.2) and MP/M-86 2.1 resulted in Concurrent CP/M-86 3.0 (also known as CCP/M-86) with BDOS 3.0, released in late 1982. Development was led by project manager Kathryn Strutynski with design contributions from Francis "Frank" R. Holsworth. Initially a single-user operating system capable of multitasking up to four CP/M-86 compatible programs, it incorporated multi-processor support and "virtual screens" allowing users to switch between running applications. Later iterations introduced multi-user functionality via dumb terminals connected to the CP/M-86 host machine. Concurrent CP/M-86 3.1 (BDOS 3.1) became available 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 incorporated windowing features.

Concurrent CP/M-86/80

This version was developed for computers such as the LSI-M4, LSI Octopus, and CAL PC that combined both 16-bit and 8-bit processors. 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 appropriate processor based on file extensions (.COM or .CMD). It simulated an 8-bit 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 extension for Concurrent CP/M-86 3.1 (BDOS 3.1), which provided basic compatibility with PC DOS 1.1. Demonstrated at COMDEX in December 1983, it was released to hardware manufacturers in March 1984 as Concurrent DOS 3.1 (CDOS). This enabled simple DOS applications—those avoiding direct hardware access—to operate; for instance, console utilities like PKZIP performed effectively and outpaced native CP/M archivers such as ARC. Nonetheless, software requiring direct hardware interaction like WordStar for DOS demanded native versions of Concurrent CP/M or CP/M-86.

While Concurrent DOS versions up through 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 positioned Concurrent DOS 4.1 bundled with the Graphics Environment Manager (GEM) in 1985 as a competitor to IBM’s TopView environment. Compatible with PC DOS 2.x, Concurrent DOS 4.1 supported multiple hardware platforms; its IBM PC-compatible edition 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 applications originally designed for CP/M-86 1.x, Concurrent CP/M-86 3.x, and PC DOS 2.0. It was distributed across multiple hardware platforms, with the IBM PC-compatible BIOS/XIOS variant known as Concurrent PC DOS 3.2. Kathryn Strutynski held the role of product manager for this version.

Concurrent DOS 68K and FlexOS 68K

Starting in 1984, a partnership with Motorola resulted in the creation of Concurrent DOS 68K at the Austin, Texas facility as a successor to CP/M-68K, developed using the C programming language. Francis "Frank" R. Holsworth played a significant role as a lead designer. By early 1985, Concurrent DOS 68K version 1.0 was available for OEM assessment, financed by Motorola with an investment of several million dollars, targeting their 68000 and 68010 processors. Initially deployed on Motorola's VME/10 development platform, it was subsequently ported to Compupro hardware. The release of versions 1.20 and 1.21 in April 1986 was offered to OEMs at approximately . This product line later transitioned into FlexOS 68K by the end of 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 Concurrent DOS 68K, Digital Research introduced Concurrent DOS 286 in January 1985 through collaboration with Intel. Originating from MP/M-286 and Concurrent CP/M-286 developments dating back to 1982, this was a complete rewrite in C featuring a new design that incorporated dynamically loadable device drivers instead of fixed BIOS or XIOS components. Francis "Frank" R. Holsworth again contributed significantly as an architect.

This system was engineered to operate exclusively in native 80286 protected mode, enabling multi-user multitasking while providing support for 8086 emulation mode. Early prototype processors accommodated this functionality; however, production chip issues delayed its launch beyond the initially planned late May date until Intel produced an enhanced chip version. By August, Intel resolved known errata and implemented microcode updates that improved emulation performance via LOADALL instructions.

These processor constraints also impacted FlexOS 286, a reengineered variant developed by Digital Research's Flexible Automation Business Unit based in Monterey beginning in 1986.

Subsequent versions incorporated compatibility with PC DOS versions 2.x and 3.x.

Notable 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 European Development Centre (EDC) located in Hungerford, UK, which then took charge of advancing the Concurrent DOS series starting from version 4.11, including related products such as DOS Plus.

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

In 1987, the system was reengineered into Concurrent DOS 386,[1] retaining the traditional XIOS & BDOS structure but optimized for Intel's 80386 and subsequent processors. It utilized hardware virtualization to run most existing DOS software without modification, even when accessed through terminals. This genuine multiuser operating system allowed simultaneous access to files with record locking mechanisms, permitting multiple users on simple terminals or low-end PCs running terminal emulation software to share a single server without requiring costly workstations or network adapters.

Concurrent DOS 386 demanded approximately 200K of conventional memory—substantially more than MS-DOS required. According to BYTE, with the minimum memory configuration of 512K RAM only the system prompt was accessible; increasing memory to around one megabyte beyond the standard base of 640K enabled running three substantial applications such as Microsoft Word or Multiplan.[2]

The release of Concurrent DOS version 6.0 marked the foundation for the subsequent DR DOS family derived from this platform.

Significant editions included:

  • 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 approximately version ~5.1, January 1987)
  • DR Concurrent DOS XM 5.2 (BDOS approximately ~5.2, September 1987)
  • DR Concurrent DOS XM 6.0 (BDOS version 6.0, November 1987), with incremental updates through version 6.21
  • DR Concurrent DOS versions for Intel386 processors ranging from release version 1.0 (1987) up to version 3.02 (1989)

Concurrent PC DOS XM version 5.0 provided emulation compatible with IBM PC DOS version 2.10,[3] whereas both Concurrent DOS XM version 6.0 and Concurrent DOS386 version 2.0 supported IBM PC DOS version 3.30 compatibility.[4]

Reception

In 1988, BYTE magazine characterized Concurrent DOS 386 as largely compatible with MS-DOS; for instance, Microsoft Word version 3.1 operated correctly, whereas version 4.0 failed due to reliance on undocumented DOS functions. While system performance declined in proportion to the number of concurrently running applications, video input/output performance was reported to be twice as fast compared to standard DOS. The publication concluded that despite the anticipation of more sophisticated 386-based operating systems, Concurrent DOS 386 was available immediately and fully functional.

Variants

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

  • CCI Concurrent DOS 386 releases ranging from version 1.12 (October 1987) up to 3.10 R1 (October 2003)
  • Significant versions such as 3.04 (also known as "CCI Concurrent DOS 4.0") and several updates issued between 1992 and 1998

Additional versions comprise:

  • 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 advanced features from Digital Research's DR DOS 5.0, prompting a rebranding to "Multiuser DOS" (MDOS) starting with release 5.0 (BDOS 6.5) in 1991.

MDOS encountered technical limitations that hindered its ability to compete effectively with PC DOS-based local area networks. The system necessitated specialized device drivers because standard PC DOS drivers lacked multiuser and multitasking capabilities, complicating installation procedures that required relinking the kernel ('nucleus') through SYSGEN.

Support for common PC DOS extensions such as network protocol stacks and emerging hardware technologies—including graphics adapters, sound cards, CD-ROM drives, and pointing devices—was limited. Although graphical terminals allowed operation of CGA, EGA, and VGA software, MDOS was less adaptable than networks composed of individual personal computers as their costs decreased. Despite being more expensive and complex than single-user systems or multitasking utilities like Quarterdeck's DESQview, MDOS provided administrative benefits and reduced total cost of ownership in multiuser settings.

After Novell acquired Digital Research in 1991 and subsequently discontinued MDOS in 1992, three principal Master VARs—DataPac Australasia, Concurrent Controls, and Intelligent Micro Software—secured source code licenses to continue independent development from 1994 onward.

Documented versions include:

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

All Digital Research and Novell MDOS releases were identified as "IBM PC DOS" version 3.31 to maintain compatibility.

DataPac Australasia

DataPac Australasia Pty Limited produced the following known editions:

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

Following DataPac's acquisition by Citrix Systems, Inc. in 1997, System Manager was phased out shortly thereafter. By 2002, the Sydney-based division had been integrated into Citrix's Advanced Products Group.

Concurrent Controls

Concurrent Controls, Inc. (CCI) launched several iterations of their CCI Multiuser DOS series:

  • Versions 7.00 through 7.22, encompassing R1 to R18 GOLD editions, released between 1996 and 2005

Each CCI Multiuser DOS edition identifies itself as "IBM PC DOS" version 3.31, with the displayed version number adjustable via the provided utility.

In 1999, Concurrent Controls, Inc. transitioned to the name Applica, Inc., and subsequently in 2002, Applica Technology evolved into Aplycon Technologies, Inc.

IMS Multiuser DOS

Significant IMS Multiuser DOS releases include:

  • Enhanced Release 5.1 (1992), along with versions 5.11 and 5.14
  • Versions 7.0 and 7.1 (BDOS 6.7, released in 1994)

All these versions identify 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 development and rebranded it as REAL/32 the following year.

IBM licensed REAL/32 version 7.50 in 1995 for use with their 4695 POS terminals.

The REAL/32 release timeline is as follows:

  • Versions 7.50 through 7.54 (BDOS versions 6.8–6.9), issued in the mid-1990s
  • Versions 7.60 to 7.63 (starting February 1997)
  • Versions 7.70 to 7.74 (up to the year 1998)
  • Versions 7.80 to 7.83 (from February 1999 onward)
  • Versions 7.90 to 7.92 (released during 1999)
  • IMS REAL/32 iterations up to version 7.94 under Itera (early 2000s)
  • Integrated Solutions IMS REAL/32 version 7.95 (circa 2010)

Versions up to and including 7.74 identify as "IBM PC DOS" version 3.31; from version 7.80 forward, they report as version 6.20.

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

Following Intelligent Micro Software Ltd.'s bankruptcy in April 2002, Itera Ltd.—a key customer led by Barry Quittenton—acquired ownership of the product but ceased operations in March 2006.

By around 2010, REAL/32 was distributed by Integrated Solutions of Thatcham, UK; this company later redirected its business away from software development.


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." It was designed to support a wide array of hardware platforms ranging from personal computers to x86 multi-processor servers.

Prominent features highlighted in the year 2003 included:

  • Compatibility with Red Hat Linux versions starting from release 7.3
  • Backward compatibility with both DOS and REAL/32 environments
  • Support for up to 65,535 virtual consoles, each functioning as an independent user session
  • Administration accessible via web browser for both local and remote configuration and upgrades
  • An integrated TCP/IP terminal emulator facilitating access for Linux and Windows users
  • Built-in capabilities for printing and file sharing along with drive mapping between Linux systems and REAL/NG servers
  • Enhanced support for hardware and multi-processor configurations
  • Embedded firewall functionality coupled with improved performance through expanded TPA (Transient Program Area)
  • Economical cost per user seat along with reduced total ownership expenses
  • Distribution on CD media bundled together with Red Hat installation CDs

By December of 2003, IMS had issued REALNG versions V1.60 through V1.19 and V1.12, which appear to be the latest available releases; by the year 2005, the realng.com website redirected visitors to IMS's main site where only REAL/32 was mentioned, with no further references to REAL/NG.

Application software

The operating system was enhanced to improve support for DOS applications while allowing software to utilize multiuser capabilities via dedicated function calls. It featured pre-emptive multitasking to prevent inefficient programs from dominating CPU resources. Multiuser DOS maintains compatibility with contemporary SSL/TLS libraries such as wolfSSL.

Its API facilitated both blocking and non-blocking message queues, mutual-exclusion queues, independently executing subprocess threads, and efficient pause mechanisms that avoided the processor waste typical of idle loops in single-user systems. Applications could operate either attached to virtual consoles or detached, enabling background processing with the option to reconnect later if necessary.

Memory management employed a shared memory architecture that separated code and data segments. This design enabled multiple instances of the same program to share code sections, thus optimizing memory consumption.

Software developed for multitasking environments avoided indefinite input wait loops by invoking API calls to "sleep," thereby preserving CPU cycles. Subsequent Concurrent DOS kernels incorporated Idle Detection, which halted inactive processes to enhance multitasking efficiency and power conservation—this concept evolved into the patented DR-DOS Dynamic Idle Detection technology 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

Related profiles

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