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Concurrent DOS

From Alfawiki
Digital Research
Company typePrivate
IndustrySoftware
Founded1970
FounderGary Kildall
Headquarters
Pacific Grove, California
,
United States
ProductsMultiuser DOS, Concurrent CP/M-86, Concurrent DOS, FlexOS, DR DOS

Multiuser DOS is a real-time, multi-user, multitasking operating system designed for IBM PC-compatible microcomputers.

Originally created by Digital Research as a progression from Concurrent CP/M-86, Concurrent DOS, and Concurrent DOS 386, the operating system was later acquired and further advanced by Novell in 1991. Its lineage can be traced back to Digital Research's 8-bit platforms CP/M and MP/M, as well as the 16-bit single-tasking CP/M-86.

Following Novell’s discontinuation of development in 1992, three principal value-added resellers (VARs)—DataPac Australasia, Concurrent Controls, and Intelligent Micro Software—received authorization to continue independent development, resulting in Datapac Multiuser DOS/System Manager, CCI Multiuser DOS, and IMS Multiuser DOS/REAL/32 respectively.

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 iteration of CP/M-86 1.0 (incorporating BDOS 2.x) was adapted for the IBM PC in 1982 but did not achieve commercial success compared to IBM’s more affordable PC DOS 1.0. Both systems failed to fully utilize the capabilities of the new 16-bit architecture.

In September 1981, MP/M-86 2.0, the multitasking multi-user variant of CP/M, was launched, allowing PCs to support multiple simultaneous users connected through serial dumb terminals. This cost-effective configuration enabled shared-data applications like accounting and inventory management during an era when PC networking was uncommon and costly.

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) in late 1982 under the leadership of project manager Kathryn Strutynski and designer Francis "Frank" R. Holsworth. Initially designed as a single-user operating system capable of multitasking up to four CP/M-86 programs concurrently, it supported multi-processor environments and provided virtual screens for program switching. Subsequent releases introduced multi-user functionality through dumb terminal access. 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 unveiled 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 LSI-M4, LSI Octopus, and CAL PC systems, which combined both 16-bit and 8-bit processors to take advantage of the wider availability and performance benefits 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 selecting the appropriate processor based on the file extension (.COM or .CMD). It emulated the CP/M environment by translating 8-bit system calls into CP/M-86 calls processed on the 16-bit CPU.

Concurrent DOS

In August 1983, Bruce Skidmore, Raymond D. Pedrizetti, Dave Brown, and Gordon Edmonds developed PC-MODE, an optional module for Concurrent CP/M-86 3.1 (BDOS 3.1), enabling basic compatibility with PC DOS 1.1. Demonstrated publicly at COMDEX in December 1983, it was released in March 1984 as Concurrent DOS 3.1 (CDOS with BDOS 3.1) for hardware manufacturers. This module allowed simple DOS applications that did not directly manipulate hardware—such as console utilities like PKZIP—to operate effectively, offering enhanced functionality compared to the native ARC archiver. However, programs requiring direct screen access, like WordStar for DOS, remained incompatible without dedicated Concurrent CP/M versions.

While Concurrent DOS versions from 3.1 through 4.1 were developed 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 alongside the Graphics Environment Manager (GEM) in 1985 as a competitor to IBM’s TopView. Concurrent DOS 4.1 (BDOS 4.1), compatible with PC DOS versions up to 2.x, was available on various hardware platforms; its IBM PC-compatible edition was marketed as Concurrent PC DOS 4.1.

Concurrent PC DOS

Released in 1984, Concurrent DOS 3.2 (incorporating BDOS 3.2) was designed to be compatible with applications from CP/M-86 1.x, Concurrent CP/M-86 3.x, and PC DOS 2.0. It supported multiple hardware platforms, with the IBM PC-compatible BIOS/XIOS variant known as Concurrent PC DOS 3.2. Kathryn Strutynski held the position of product manager during this period.

Concurrent DOS 68K and FlexOS 68K

Beginning in 1984, Digital Research collaborated with Motorola in Austin, Texas, to create Concurrent DOS 68K as the successor to CP/M-68K. Developed in the C programming language under the leadership of chief architect Francis "Frank" R. Holsworth, this project received substantial financial backing from Motorola and targeted their 68000 and 68010 CPU architectures. The initial implementation ran on Motorola's VME/10 development platform, with Concurrent DOS 68K version 1.0 made available for OEM evaluation by early 1985. Subsequent releases, versions 1.20 and 1.21, appeared in April 1986 and were distributed to OEMs at a set price point. This line later evolved into FlexOS 68K by the end of 1986.

Documented releases:

  • 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

In parallel with Concurrent DOS 68K, Digital Research introduced Concurrent DOS 286 in partnership with Intel in January 1985. This operating system was based on developments from MP/M-286 and Concurrent CP/M-286 dating back to 1982 but was fully rewritten in C. It featured dynamically loadable device drivers instead of relying on a fixed BIOS/XIOS system architecture. Francis R. Holsworth again served as a key architect.

Engineered for native operation on the 80286 processor's protected mode, it supported multitasking for multiple users alongside an emulation mode for the 8086 processor. However, early production Intel chips encountered emulation difficulties that postponed the release beyond May 1985 until Intel resolved these issues through updated stepping revisions and microcode enhancements.

The Flexible Automation Business Unit at Digital Research’s Monterey facility developed FlexOS 286, a redesigned offshoot of Concurrent DOS 286 starting in 1986. Later iterations incorporated support for PC DOS versions 2.x and 3.x.

Known 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 shifted its OEM Support Group to the European Development Centre (EDC) located in Hungerford, United Kingdom, which then assumed responsibility for advancing the Concurrent DOS series from version 4.11 onward, including related products such as DOS Plus.

Developed at the Hungerford site, versions five and six of Concurrent DOS (branded as Concurrent DOS XM, where XM denotes support for Expanded Memory) incorporated bank switching technology allowing access to up to eight megabytes of EEMS. This enabled a real-mode environment capable of running multiple CP/M-86 and DOS applications simultaneously for up to three users—one local user plus two remote users via serial terminals.

By 1987, Concurrent DOS was reengineered as Concurrent DOS 386 while preserving its traditional XIOS & BDOS framework.[1] This iteration exploited the Intel 80386 processor's hardware virtualization features to run most DOS software without modification, including terminal-based clients. The operating system supported concurrent multiuser file access with record locking mechanisms suitable for shared database environments.

Concurrent DOS 386 required approximately 200 KB of conventional memory—substantially more than MS-DOS—and demanded at least half a megabyte of RAM merely to launch the command prompt interface. Running major applications such as Microsoft Word or Multiplan typically necessitated around one megabyte plus the standard base memory.[2]

The release of Concurrent DOS version 6 also marked the foundation of the DR DOS family derived from this codebase.

Significant releases 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 (approximately BDOS version ~5.1, January 1987)
  • DR Concurrent DOS XM 5.2 (approximately BDOS version ~5.2, September 1987)
  • DR Concurrent DOS XM 6.0 (BDOS version 6.0, November 1987), including update version 6.01
  • DR Concurrent DOS XM 6.2 (BDOS version 6.2), including update version 6.21
  • DR Concurrent DOS 386 version 1.0 (approximately BDOS version ~5.0, released in 1987)
  • DR Concurrent DOS 386 version 1.1 (approximately BDOS version ~5.2, September 1987)
  • DR Concurrent DOS 386 version 2.0 (BDOS version 6.0, November 1987), including update version 2.01
  • DR Concurrent DOS 386 version 3.0 (BDOS version 6.2, December 1988–January 1989), including updates versions 3.01 and 3.02

Concurrent PC DOS XM release version 5.0 provided emulation compatible with IBM PC DOS 2.10,[3] whereas versions such as Concurrent DOS XM 6.0 and Concurrent DOS 386 2.0 aligned with IBM PC DOS 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 successfully, whereas version 4.0 failed due to reliance on undocumented DOS calls. While performance declined in proportion to the number of concurrent applications running, video input/output was measured at twice the speed of DOS, albeit requiring more RAM. The publication concluded that despite upcoming more sophisticated 386-based operating systems, Concurrent DOS 386 was immediately available and functional.

Variations

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 featured versions 2.01 (May 1988), 3.04—also known as "CCI Concurrent DOS 4.0" (July 1991)—and several subsequent incremental improvements throughout the late 1990s

Additional variants comprised:

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

Multiuser DOS

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

Multiuser DOS encountered technical constraints that limited its ability to compete with PC DOS-based local area networks. It necessitated custom device drivers because standard PC DOS drivers did not support multiuser or multitasking environments, complicating setup by requiring kernel relinking through the SYSGEN utility.

The platform lacked compatibility with numerous PC DOS enhancements such as network protocol stacks and offered only limited support for newer hardware peripherals like graphics cards, sound devices, CD-ROM drives, and mice. Although some improvements were made—such as graphical terminal support enabling CGA, EGA, and VGA software—Multiuser DOS remained less adaptable than networks composed of individual PCs. Declining prices of personal computers further diminished its market appeal despite advantages in management and lower total cost of ownership.

As a multi-user operating system, it carried a higher price tag than single-user alternatives and demanded specialized drivers, unlike single-user multitasking solutions like Quarterdeck's DESQview. Unlike MP/M, Multiuser DOS never achieved widespread adoption for single-user multitasking scenarios.

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

Documented 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 version 5.13 (BDOS 6.6, released in 1992)

All Digital Research and Novell MDOS editions reported themselves as "IBM PC DOS" version 3.31 within system identification tools.

DataPac Australasia

Versions distributed by DataPac Australasia Pty Limited comprise:

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

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

Concurrent Controls

Concurrent Controls, Inc. (CCI) developed several known releases of CCI Multiuser DOS, spanning versions 7.00 through 7.22. These editions, from R1 launched in September 1996 up to R18 GOLD in 2005, were all built upon BDOS iterations up to version 6.6.

Each CCI Multiuser DOS release identifies itself as "IBM PC DOS" version 3.31 and can be configured using the Multiuser DOS utility.

In 1999, Concurrent Controls rebranded as Applica, Inc., and subsequently transformed into Aplycon Technologies, Inc. in 2002.

IMS Multiuser DOS

The IMS Multiuser DOS lineup includes:

  • Enhanced Release 5.1 (1992)
  • Subsequent versions 5.11 and 5.14
  • Later editions 7.0 and 7.1 (based on BDOS 6.7, released in 1994)

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

REAL/32

In 1994, Intelligent Micro Software Ltd., based in Thatcham, UK, acquired the rights from Novell to continue developing Multiuser DOS, subsequently rebranding it as REAL/32 in 1995.

IBM licensed REAL/32 version 7.50 that same year for its IBM 4695 POS terminals to enhance point-of-sale capabilities.

The REAL/32 product line includes:

  • Versions from 7.50 through 7.54 (utilizing BDOS up to version 6.9; mid-1990s)
  • Releases extending up to version 7.74 (through 1998)
  • Versions between 7.80 and 7.83 (incorporating BDOS 6.10; late 1990s)
  • Editions up to version 7.92 (released in 1999)
  • IMS REAL/32 iterations under ITERA reaching version 7.94 (early 2000s)
  • Integrated Solutions’ IMS REAL/32 release at version 7.95 by around 2010

Versions up to 7.74 identify as "IBM PC DOS" version 3.31; later builds report as version 6.20.

Significantly, support for Logical Block Addressing (LBA) and FAT32 was introduced starting with REAL/32 version 7.90 in 1999.

Following IMS’s bankruptcy filing in April 2002, Itera Ltd.—a key customer led by Barry Quittenton—acquired the product line; Itera ceased operations in March 2006.

By approximately 2010, Integrated Solutions of Thatcham was distributing REAL/32 but later moved away from software development activities.


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," designed for a wide hardware spectrum ranging from standard PCs to x86 multiprocessor servers.

Prominent features highlighted in the product’s 2003 promotion included:

  • Compatibility with Red Hat Linux version 7.3 or newer
  • Backward compatibility with both DOS and REAL/32 applications
  • Support for up to 65,535 virtual consoles, each representing an individual user session
  • Administration and configuration accessible via web browser interfaces, locally or remotely
  • Integrated TCP/IP terminal emulation for users on Linux or Windows platforms
  • Native print and file sharing capabilities along with drive mapping between Linux and REAL/NG servers
  • Advanced multiprocessor and hardware support
  • Embedded firewall functionality
  • Economical pricing per user seat combined with low total cost of ownership
  • Distribution via CD bundled alongside Red Hat installation media

By December of 2003, IMS issued "REALNG V1.60-V1.19-V1.12," which appears to be the final release of this line; by 2005, the realng.com domain redirected visitors to IMS’s main website, where references to REAL/NG were removed in favor of focusing solely on REAL/32 products.


Application software

REAL/32's multiuser functionality was accessed by applications through dedicated function calls and pre-emptive multitasking, which safeguarded system reliability by preventing any malfunctioning software from dominating the CPU. Multiuser DOS maintained compatibility with SSL/TLS libraries such as wolfSSL.

The programming interface included support for both blocking and non-blocking message queues, mutual-exclusion queues, subprocess threads that operated independently of their parent processes, and optimized pause routines that minimized CPU usage. Programs could operate either attached to virtual consoles or detached to run in the background, with the option to reconnect when necessary.

A significant characteristic was the shared memory architecture that separated code from data segments; this design enabled multiple instances of the same application to share code in memory, thereby conserving system resources.

In contrast to single-task systems that waste CPU cycles in idle loops, Concurrent DOS offered API functions for sleep states and integrated Idle Detection to halt inactive processes. This advancement became the foundation for the patented DR-DOS Dynamic Idle Detection power-saving 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
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