Concurrent DOS 68K

Digital Research
Company type	Private
Industry	Software
Founded	1974; 52 years ago
Founder	Gary Kildall
Fate	Acquired by Novell in 1991
Successor	Novell
Headquarters	Pacific Grove, California, United States
Products	Multiuser DOS, Concurrent CP/M-86, Concurrent DOS, FlexOS

Multiuser DOS is a real-time, multiuser, 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, the system was acquired by Novell in 1991, who continued its development. Its lineage can be traced back to the 8-bit CP/M and MP/M operating 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 (VARs)—DataPac Australasia, Concurrent Controls, and Intelligent Micro Software—were licensed to independently advance the platform. This led to the emergence of Datapac Multiuser DOS and System Manager, CCI Multiuser DOS, and IMS Multiuser DOS along with REAL/32.

The related FlexOS series, derived 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 (using 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 capabilities of the new 16-bit hardware.

Shortly thereafter, MP/M-86 2.0 was introduced in September 1981, converting PCs into multiuser platforms that supported several simultaneous users through dumb terminals connected via serial ports. This innovation reduced costs and facilitated multiuser 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) and MP/M-86 2.1 were combined into Concurrent CP/M-86 3.0 (CCP/M-86) featuring BDOS 3.0. Development was led by project manager Kathryn Strutynski with designer Francis "Frank" R. Holsworth. Originally a single-user system, it enabled true multitasking of up to four CP/M-86-compatible programs and could be configured for multi-processor operation. The OS introduced "virtual screens," allowing users to switch between multiple running applications seamlessly. Subsequent releases added support for dumb terminals in multiuser configurations. Concurrent CP/M-86 3.1 (BDOS 3.1) was launched 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 features.

Concurrent CP/M-86/80

This version targeted computers such as LSI-M4, LSI Octopus, and CAL PC that combined both 16-bit and 8-bit processors to utilize the larger software base and efficiency of 8-bit applications. Concurrent CP/M-86/80 allowed execution of both CP/M (8-bit) and CP/M-86 (16-bit) programs by directing software to the appropriate processor based on file extensions (.COM for 8-bit and .CMD for 16-bit). It achieved CP/M environment emulation by translating system calls for the 16-bit processor.

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), designed to offer fundamental compatibility with PC DOS 1.1. This module was showcased at COMDEX in December 1983 and released in March 1984 under the name Concurrent DOS 3.1 (CDOS). It allowed basic DOS programs that avoided direct hardware access to function correctly. For instance, console utilities like PKZIP ran efficiently, surpassing native CP/M archivers in performance; however, software that manipulated the display directly, such as WordStar for DOS, required native Concurrent CP/M versions.

While development of Concurrent DOS versions up to 4.1 took place within the United States, adaptations for OEMs and localized versions were managed by Digital Research Europe’s OEM Support Group based in Newbury, UK starting from 1983.

In 1985, Digital Research marketed Concurrent DOS 4.1 combined with GEM as an alternative to IBM’s TopView. Compatible with PC DOS 2.x and supporting multiple hardware platforms, the IBM PC-compatible variant was identified as Concurrent PC DOS 4.1.

Concurrent PC DOS

Concurrent DOS 3.2 (BDOS 3.2), released in 1984, provided support for applications running on CP/M-86 1.x, Concurrent CP/M-86 3.x, and PC DOS 2.0 environments. Distributed across a range of hardware systems, the IBM-PC-compatible BIOS/XIOS version carried the name Concurrent PC DOS 3.2. Kathryn Strutynski held the role of product manager for this line.

Concurrent DOS 68K and FlexOS 68K

A partnership with Motorola initiated in 1984 resulted in Concurrent DOS 68K, developed in Austin, Texas as a successor to CP/M-68K, with its codebase written in C. Francis "Frank" R. Holsworth played a leading role as architect of this system. The first version, Concurrent DOS 68K 1.0, was available for OEM evaluation by early 1985 and was financially backed by Motorola with a multimillion-dollar investment. It targeted Motorola’s 68000/68010 processors and initially operated on Motorola's VME/10 development system before being ported to other hardware platforms.

Released versions 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 operating system evolved into FlexOS 68K.

Concurrent DOS 286 and FlexOS 286

In parallel with Concurrent DOS 68K development, Digital Research introduced Concurrent DOS 286 in January 1985 through collaboration with Intel. Built upon MP/M-286 and Concurrent CP/M-286 foundations and rewritten in C, it featured a redesigned architecture employing dynamically loadable device drivers instead of fixed BIOS/XIOS components. Francis "Frank" R. Holsworth again served as a principal architect.

This OS was engineered to operate natively on the 80286 processor’s protected mode, enabling multi-user multitasking while maintaining support for emulating the earlier 8086 processor mode. Early prototypes functioned on initial chip stepping levels; however, production chips exhibited emulation challenges that postponed release until Intel resolved these issues through microcode updates.

Subsequent releases expanded compatibility to include PC DOS versions 2.x and 3.x.

Identified releases are:

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)

The Flexible Automation Business Unit of Digital Research in Monterey, California began developing FlexOS 286 in 1986 as a reengineered evolution of Concurrent DOS 286.

Concurrent DOS XM and Concurrent DOS 386

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

The Concurrent DOS versions 5 and 6, known as Concurrent DOS XM (with XM denoting Expanded Memory support), were developed at Hungerford. These iterations introduced bank switching capabilities for up to 8 MB of EEMS, enabling a real-mode platform to execute multiple CP/M-86 and DOS applications simultaneously. The system accommodated up to three users concurrently—one local user plus two connected via serial terminals.

In 1987, Concurrent DOS was redesigned as Concurrent DOS 386 to exploit the Intel 80386 processor’s hardware virtualization features. This redesign allowed unmodified DOS programs to operate concurrently across multiple terminals, facilitating multiuser file access and permitting several users to share a single database through record locking mechanisms. This innovation reduced hardware expenses by allowing less powerful terminals or low-end PCs running terminal emulation software to replace costly workstations and network interface cards.

Operating Concurrent DOS 386 demanded approximately 200K of conventional memory—substantially more than MS-DOS—and required a minimum of 512K RAM just for basic functionality. For practical use with major applications like Microsoft Word or Multiplan, around 1.6MB of total RAM was necessary.

Concurrent DOS 6.0 also served as the foundation for the DR DOS family, which evolved from this version.

Significant releases included:

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 subsequent updates such as 6.01
DR Concurrent DOS XM 6.2 (BDOS 6.2), including version 6.21
DR Concurrent DOS 386 releases from version 1.0 (1987) through to 3.02 (1989), with BDOS evolving from approximately 5.0 to 6.2

Concurrent PC DOS XM 5.0 emulated IBM PC DOS version 2.10, while both Concurrent DOS XM 6.0 and Concurrent DOS 386 version 2.0 maintained compatibility with IBM PC DOS version 3.30.

Reception

In a 1988 review, 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 its reliance on undocumented DOS calls. Performance declined proportionally with the number of concurrent applications running; however, despite increased memory demands, video input/output performance was twice as fast compared to standard DOS systems. The publication concluded that while more sophisticated operating systems for the 386 processor were anticipated soon, Concurrent DOS 386 was already available and functional.

Adaptations

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

CCI Concurrent DOS 386 versions ranging from 1.12 (October 1987) to 3.10 R1 (October 2003)
Progression through BDOS versions from 5.0 up to and beyond BDOS 6.2
Incremental updates spanning from release 3.05 R1 (1992) through to release 3.07 R7 (1998)

Additional adaptations comprised:

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

Multiuser DOS

Subsequent iterations of Concurrent DOS 386 incorporated elements from Digital Research's single-user PC DOS alternative, DR DOS 5.0, which was renamed "Multiuser DOS" (MDOS) starting with version 5.0 (BDOS 6.5) released in 1991.

Technical constraints limited Multiuser DOS's ability to compete with PC DOS-based LAN solutions. It demanded unique device drivers because standard PC DOS drivers lacked support for multiuser and multitasking environments, making installation more complex by requiring kernel relinking through SYSGEN.

Initially, the platform did not accommodate many typical PC DOS enhancements such as network protocol stacks, graphics cards, audio devices, CD-ROM drives, and mouse support. Although graphical terminals eventually allowed use of CGA, EGA, and VGA applications, Multiuser DOS remained less adaptable compared to networks of separate PCs. As the cost of PC hardware dropped, Multiuser DOS’s financial advantage declined despite providing administrative advantages and reduced total ownership expenses in multi-user setups.

Its cost exceeded that of single-user systems, and it necessitated specialized drivers unlike single-user multitasking products like Quarterdeck's DESQview. Unlike MP/M, Multiuser DOS failed to gain traction as a single-user multitasking solution.

After Novell acquired Digital Research in 1991 and discontinued Multiuser DOS development in 1992, three primary Master VARs—DataPac Australasia, Concurrent Controls, and Intelligent Micro Software—obtained source code licenses in 1994 to pursue independent enhancements.

Documented 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 MDOS editions internally identified themselves as "IBM PC DOS" version 3.31.

DataPac Australasia

DataPac Australasia Pty Limited produced several versions:

Datapac Multiuser DOS 5.0
Datapac Multiuser DOS 5.1 (BDOS 6.6)
Datapac System Manager 7.0 (released on 1996-08-22)

Following DataPac’s acquisition by Citrix Systems, Inc. in 1997, System Manager was phased out soon afterward. In 2002, the Sydney-based division was spun off into Citrix’s Advanced Products Group.

Concurrent Controls

Concurrent Controls, Inc. (CCI) developed multiple versions under the CCI Multiuser DOS brand:

CCI Multiuser DOS versions 7.00, 7.10, 7.21, and 7.22
Releases ranging from R1 (September 1996) up to R18 GOLD (2005), featuring BDOS 6.6 and later

All editions report their version as "IBM PC DOS" version 3.31; this can be modified using the Multiuser DOS utility.

In 1999, CCI changed its name to Applica, Inc., and later rebranded as Aplycon Technologies, Inc., in 2002.

IMS Multiuser DOS

Recognized IMS variants of Multiuser DOS include:

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

All versions identify themselves as "IBM PC DOS" version 3.31 internally.

REAL/32

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

IBM incorporated REAL/32 version 7.50 into their 4695 point-of-sale terminals under a licensing agreement established in 1995.

The IMS REAL/32 product line includes the following releases:

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 up to 7.94 (2002–2003)
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.

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

Following IMS’s insolvency declaration in April 2002, Itera Ltd.—a primary client led by Barry Quittenton—acquired the REAL/32 product line; Itera ceased operations in March 2006.

By the year 2010, Integrated Solutions of Thatcham, UK, had taken over distribution of REAL/32, although the company eventually transitioned its focus away from software development toward building services.

REAL/NG

REAL/NG represented IMS’s initiative to create a "Next Generation" iteration of REAL/32, marketed as "REAL/32 for the internet age." Its design targeted compatibility across a wide range of hardware platforms, spanning from standard PCs to x86 multiprocessor servers.

Prominent 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
Administration accessible via web browsers for both local and remote management
An integrated TCP/IP terminal emulator for Linux and Windows clients
Native print and file sharing capabilities with drive mapping between Linux and REAL/NG servers
Enhanced hardware support for users and multiprocessor configurations
Performance improvements alongside an expanded transient program area (TPA)
Embedded firewall functionality coupled with a low total cost of ownership
Distribution on CD bundled with Red Hat installation media

By December 2003, IMS had issued REALNG updates up to version V1.60, which appears to be the last release for this product line. By 2005, the realng.com website redirected visitors to IMS’s main site, where references to REAL/NG were removed in favor of focusing exclusively on REAL/32.

Application software

Applications developed for this platform utilized multiuser capabilities via dedicated function calls within the operating system's API framework. The OS implemented pre-emptive multitasking to maintain stable process scheduling and prevent any single poorly designed program from monopolizing CPU resources.

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

The API provided support for blocking and non-blocking message queues, mutual exclusion queues, and independently running subprocess threads, along with efficient pause mechanisms that conserved CPU cycles compared to the busy-wait loops typical in single-user environments.

Programs could operate either "attached" to a virtual console or detach to run as background services with optional reattachment capabilities.

Memory management employed a shared memory architecture that separated code segments from data areas, allowing multiple instances of the same application to share code segments and thereby reduce overall memory consumption.

To enhance multitasking efficiency, processes avoided continuous polling loops by using API calls that put them into a sleep state, conserving CPU usage during idle periods.

Later kernel iterations introduced Idle Detection technology that monitored DOS API activity to suspend processes when idle, further improving multitasking performance.

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

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