Concurrent CP/M-86 with Windows
| Industry | Software |
|---|---|
| Founded | 1976 |
| Founder | Gary Kildall |
| Fate | Acquired by Novell in 1991 |
| Successor | Novell (for Multiuser DOS development until 1992) |
Key people | Kathryn Strutynski (project manager), Francis "Frank" R. Holsworth (designer/architect) |
| Products | Multiuser DOS, Concurrent CP/M-86, Concurrent DOS, FlexOS |
Multiuser DOS represents a real-time operating system designed for IBM PC-compatible microcomputers, supporting multiple users and multitasking capabilities. It originated from earlier Digital Research software 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 back to 8-bit platforms like CP/M and MP/M, as well as the 16-bit single-tasking CP/M-86.
Following Novell's termination of Multiuser DOS development in 1992, three principal value-added resellers (VARs)—DataPac Australasia, Concurrent Controls, and Intelligent Micro Software—were granted permission to continue independent enhancements. These companies produced distinct versions including Datapac Multiuser DOS/System Manager, CCI Multiuser DOS, IMS Multiuser DOS, and REAL/32. 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 release of CP/M-86 1.0 (BDOS 2.x) was adapted for the IBM PC in 1982 but faced commercial challenges against IBM's PC DOS 1.0, which provided comparable functionality at a lower price point. Both operating systems underperformed in leveraging the full potential of the new 16-bit processor architecture.
In September 1981, MP/M-86 2.0, a multitasking iteration of CP/M, enabled IBM PCs to support multiple users simultaneously through dumb terminals connected via serial interfaces. This multi-user system significantly lowered expenses compared to deploying multiple standalone PCs and facilitated shared-data applications such as accounting and inventory management before widespread PC networking adoption.
By late 1982, CP/M-86 1.1 (BDOS 2.2) and MP/M-86 2.1 were consolidated into Concurrent CP/M-86 3.0 (BDOS 3.0). Development was overseen by project manager Kathryn Strutynski with design contributions from Francis "Frank" R. Holsworth. Initially intended as a single-user operating system capable of multitasking up to four CP/M-86 programs, it could also be configured for multiprocessor environments and introduced "virtual screens" to switch between active tasks. Subsequent releases incorporated support for dumb terminals to enable true multi-user functionality. Concurrent CP/M-86 3.1 (BDOS 3.1) was officially 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 integrated windowing features into Concurrent CP/M-86.
Concurrent CP/M-86/80
This version targeted systems such as 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) applications by running programs on the appropriate CPU determined by file extensions (.COM for 8-bit executables, .CMD for 16-bit). It created an emulated CP/M environment for the 8-bit programs by translating their system calls into equivalents handled by the 16-bit processor.
Concurrent DOS
In August 1983, Bruce Skidmore, Raymond D. Pedrizetti, Dave Brown, and Gordon Edmonds developed PC-MODE, an optional add-on for Concurrent CP/M-86 3.1 (BDOS 3.1), which provided fundamental compatibility with PC DOS version 1.1. Demonstrated at COMDEX in December of that year, it was released to hardware manufacturers in March 1984 under the name Concurrent DOS 3.1 (CDOS). This enabled basic DOS applications that avoided direct hardware manipulation to operate—for instance, PKZIP ran effectively and outperformed the ARC archiver native to CP/M. However, software requiring screen control like WordStar still needed native versions designed specifically for Concurrent CP/M or CP/M-86.
While initial development of Concurrent DOS took place in the United States, adaptations for OEMs and localization efforts were managed from Digital Research Europe’s OEM Support Group based in Newbury, UK starting in 1983.
In 1985, Digital Research positioned Concurrent DOS 4.1 bundled with GEM as a competitor to IBM’s TopView environment. Concurrent DOS 4.1 (BDOS 4.1) offered compatibility with PC DOS version 2.x across multiple hardware platforms; the 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 run applications compatible with CP/M-86 1.x, Concurrent CP/M-86 3.x, and PC DOS 2.0. This version supported a variety of 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 during this period.
Concurrent DOS 68K and FlexOS 68K
In 1984, Digital Research partnered with Motorola to develop Concurrent DOS 68K in Austin, Texas, as the successor to CP/M-68K. Written in C, this project saw Francis "Frank" R. Holsworth as a leading architect. By early 1985, Concurrent DOS 68K 1.0 was made available for OEM evaluation, backed by Motorola’s multi-million dollar investment targeting their 68000 and 68010 processors. The initial platform was the Motorola VME/10 development system, with adaptations also made for Compupro hardware.
The Concurrent DOS 68K series comprised:
- Concurrent DOS 68K 1.0 (1985)
- Concurrent DOS 68K 1.1
- Concurrent DOS 68K 1.20 (April 1986)
- Concurrent DOS 68K 1.21 (1986)
This product line transitioned into FlexOS 68K by late 1986.
Concurrent DOS 286 and FlexOS 286
Alongside the development of Concurrent DOS 68K, Digital Research introduced Concurrent DOS 286 in collaboration with Intel in January 1985. Building upon technologies from MP/M-286 and Concurrent CP/M-286 since 1982, this operating system was rewritten entirely in C and featured dynamically loadable device drivers instead of a fixed BIOS/XIOS design. Frank Holsworth also played a key architectural role here.
Tailored for native 80286 protected mode operation, it supported multi-user multitasking while maintaining an 8086 emulation mode. Although early prototype processors supported these features, production delays occurred due to hardware errata and microcode optimization needed to enhance emulation performance.
Similar hardware constraints influenced the development of FlexOS 286, a reengineered variant created by Digital Research's Flexible Automation Business Unit based in Monterey starting from 1986.
Subsequent releases improved compatibility with PC DOS versions 2.x and 3.x.
Versions released included:
- 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, UK, which assumed responsibility for developing the Concurrent DOS series from version 4.11 onward, including related products such as DOS Plus.
Concurrent DOS versions 5 and 6—branded as Concurrent DOS XM (with XM denoting support for Expanded Memory)—were developed at Hungerford. These editions employed bank switching techniques to access up to 8 MB of EEMS, enabling a real-mode environment capable of running multiple CP/M-86 and DOS applications simultaneously while supporting up to three users: one local and two via serial terminals.
By 1987, Concurrent DOS was reengineered as Concurrent DOS 386 to leverage the capabilities of Intel's 80386 and subsequent processors. Utilizing hardware virtualization, it could execute most DOS programs without modification across terminals as well as PCs. This genuine multiuser operating system allowed concurrent file access with record locking, enabling multiple users on simple terminals or low-end PCs 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—and required a minimum of 512K RAM to function effectively. With about an additional megabyte beyond the base memory limit of 640K, it could operate three major applications concurrently, such as Microsoft Word or Multiplan.
The release of Concurrent DOS 6.0 also marked the foundation of the DR DOS family derived from this codebase.
Key versions include:
- DR Concurrent PC DOS XM 5.0 (BDOS 5.0)
- DR Concurrent DOS XM 5.0 (BDOS 5.0, Oct 1986)
- DR Concurrent DOS XM 5.1 (BDOS ~5.1?, Jan 1987)
- DR Concurrent DOS XM 5.2 (BDOS ~5.2?, Sept 1987)
- DR Concurrent DOS XM 6.0 (BDOS 6.0, Nov 1987), with subsequent updates through version 6.21
- DR Concurrent DOS 386 1.0 (BDOS ~5.0, released in 1987)
- DR Concurrent DOS 386 1.1 (BDOS ~5.2, Sept 1987)
- DR Concurrent DOS 386 2.0 (BDOS versioned at approximately 6.0, Nov 1987)
- DR Concurrent DOS 386 versions up to 3.02 (BDOS 6.2), available until mid-1989
While Concurrent PC DOS XM 5.0 emulated IBM PC DOS 2.10, both Concurrent DOS XM 6.0 and Concurrent DOS 386 2.0 offered compatibility with IBM PC DOS 3.30.
Reception
In its 1988 review, BYTE 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. The system's performance declined in direct relation to the number of concurrently running applications. Although it demanded more memory, video input/output speeds were approximately twice as fast compared to DOS. The publication summarized that while more sophisticated 386-based operating systems were emerging, Concurrent DOS 386 was "available now and fully operational."
Variants
Concurrent Controls, Inc. developed multiple variants of Concurrent DOS, including:
- CCI Concurrent DOS 386 ranging from version 1.12 (October 1987) up to 3.10 R1 (October 2003)
- Significant releases such as 3.04 (July 1991, also known as "CCI Concurrent DOS 4.0") along with several incremental updates throughout the late 1990s
Additional versions comprised:
- Apricot Concurrent DOS 386 version 2.01 (1987), tailored 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 clone, DR DOS 5.0, which was rebranded as "Multiuser DOS" (MDOS) beginning with release 5.0 (BDOS 6.5) in 1991.
MDOS encountered technical limitations that hindered its ability to compete effectively with PC DOS LAN environments. It necessitated custom device drivers since standard PC DOS drivers lacked multiuser and multitasking capabilities, complicating installation by requiring kernel rebuilding via SYSGEN rather than straightforward file copying and CONFIG.SYS configuration.
Moreover, MDOS did not support many enhancements available in PC DOS, such as network protocol stacks and compatibility with graphics cards, sound devices, CD-ROM drives, and mice. Some improvements were made—for example, graphical terminals supporting CGA, EGA, and VGA software—but MDOS remained less adaptable than networks composed of individual PCs. As the cost of PC hardware declined, MDOS’s competitive edge diminished despite offering administrative advantages and reduced total ownership costs.
Due to its higher price compared to single-user systems and dependence on specialized drivers, MDOS did not gain traction as a single-user multitasking platform, unlike alternatives such as Quarterdeck's DESQview.
Following Novell’s acquisition of Digital Research in 1991 and the subsequent discontinuation of Multiuser DOS in 1992, three Master VARs—DataPac Australasia, Concurrent Controls, and Intelligent Micro Software—secured source code licenses to pursue independent development from 1994 onward.
Documented versions 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 Multiuser DOS editions reported themselves as "IBM PC DOS" version 3.31.
DataPac Australasia
DataPac Australasia Pty Limited released the following versions:
- Datapac Multiuser DOS 5.0
- Datapac Multiuser DOS 5.1 (BDOS 6.6)
- Datapac System Manager 7.0 (August 22, 1996)
After Citrix Systems, Inc. acquired DataPac in 1997, development of System Manager ceased shortly thereafter. In 2002, the Sydney-based division was reorganized into Citrix’s Advanced Products Group.
Concurrent Controls
Concurrent Controls, Inc. (CCI) introduced multiple Multiuser DOS editions, including versions 7.00 through 7.22. These releases featured numerous GOLD updates spanning from Release 4 in 1997 to Release 18 in 2005, all identifying themselves as "IBM PC DOS" version 3.31.
Users could alter the version string through a dedicated Multiuser DOS utility provided by CCI.
In 1999, the company underwent a rebranding to Applica, Inc., and subsequently changed its name to Aplycon Technologies, Inc. in 2002.
IMS Multiuser DOS
The IMS Multiuser DOS variants range from Enhanced Release 5.1 (1992) up to version 7.1 (BDOS 6.7, 1994), consistently reporting as "IBM PC DOS" version 3.31.
REAL/32
In 1994, Intelligent Micro Software Ltd., based in Thatcham, UK, secured licensing rights from Novell to continue Multiuser DOS development, subsequently rebranding the system as REAL/32 in 1995.
IBM adopted REAL/32 version 7.50 in 1995 for integration with its 4695 point-of-sale terminals.
Prominent 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 (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)
All versions up to 7.74 identified as "IBM PC DOS" version 3.31; starting with version 7.80, the reported version changed to 6.20.
REAL/32 introduced support for Logical Block Addressing (LBA) and FAT32 beginning with version 7.90 in 1999.
Following IMS's insolvency filing in April 2002, Itera Ltd., a key client led by Barry Quittenton, acquired the business; Itera itself was dissolved in March 2006.
By around 2010, Integrated Solutions of Thatcham, UK, was distributing REAL/32 but eventually shifted its focus away from software development activities.
REAL/NG
REAL/NG represented IMS's initiative to create the "Next Generation" iteration of REAL/32, marketed as "REAL/32 for the internet age," designed to support an extensive range of hardware platforms from standard PCs up to x86 multiprocessor servers.
As of 2003, notable features included:
- Compatibility with Red Hat Linux versions starting at release 7.3
- Retention of backward compatibility with both DOS and prior REAL/32 versions
- Capability to manage as many as 65,535 virtual consoles, each corresponding to an individual user session
- Administration via a web browser interface that required no Linux system expertise
- An integrated TCP/IP terminal emulator accessible from Linux or Windows clients
- Native support for printing and file sharing along with drive mapping between Linux hosts and REAL/NG servers
- Enhanced hardware compatibility for user devices and multiprocessor environments
- Performance optimizations and expanded transient program area (TPA) capacity
- Built-in firewall functionality
- Economical per-seat licensing and reduced total cost of ownership
- Distribution on CD media bundled alongside Red Hat installation CDs
By December of that year, IMS released "REALNG V1.60-V1.19-V1.12," which appears to be the final update available publicly; by 2005, the realng.com domain redirected visitors to IMS's primary website, where references to REAL/NG were removed in favor of exclusive emphasis on REAL/32.
Application software
REAL/32 enhanced its multitasking functionality to support running DOS programs tailored for multiuser settings through dedicated function calls. It implemented preemptive multitasking to ensure that inefficient applications could not dominate CPU resources. The system continues to maintain compatibility with contemporary security standards, including support for SSL/TLS libraries such as wolfSSL within the Multiuser DOS environment.
The provided API included features like blocking and non-blocking message queues, mutual exclusion queues, subprocess threads that operate independently from their parent processes, and advanced methods to pause execution efficiently—saving processor cycles compared to the busy-wait loops common in single-user operating systems. Programs could execute either attached to virtual consoles or detached, allowing background operation with the possibility of reconnecting to the console later.
Memory management employed a shared memory architecture where executable code segments were separated from writable data sections. This design enabled multiple instances of the same application to share code segments, thereby minimizing overall memory consumption.
To improve multitasking efficiency, applications refrained from using endless loops while waiting for input; instead, Concurrent DOS introduced sleep functions. Subsequent kernel versions added Idle Detection capabilities that tracked DOS API usage and suspended processes when idle, freeing up CPU time. This innovation laid the groundwork for the patented Dynamic Idle Detection power-saving technology, marketed as BatteryMAX by DR-DOS creators Roger Alan Gross and John P. Constant.
See also
- CP/M
- MP/M
- Concurrent DOS V60
- FlexOS
- DR DOS
- PC DOS – IBM's OEM version of MS-DOS
- MS-DOS 4.0 (multitasking)
- PC-MOS/386 – unrelated multitasking DOS clone
- VM/386 – unrelated multitasking DOS environment
- Virtual DOS machine
- Multiuser DOS Federation
- Timeline of operating systems
- List of mergers and acquisitions by Citrix
Further reading
- DRTIPS.ZIP (1992) by Novell Desktop Systems Group, Digital Research/Novell