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Developer Digital Research
Written in C
Working state Discontinued
Initial release 1986; 32 years ago (1986)
Latest release 2.33 / 1998-05
Latest preview 2.34 / 1999
Marketing target Industrial, PoS
Platforms 80286, 80386, 68000, V60, V70
Kernel type modular
Default user interface various frontends, X/GEM
Preceded by Concurrent DOS 286, Concurrent DOS 68K
Succeeded by 4680 OS, 4690 OS

FlexOS is a discontinued modular real-time multi-user multi-tasking operating system (RTOS) designed for computer-integrated manufacturing, laboratory, retail and financial markets. It was developed by Digital Research's Flexible Automation Business Unit in Monterey, California[1] since 1986 and was marketed since January 1987 as a reengineered derivation of Digital Research's Concurrent DOS 286 multiuser multitasking operating system.[2]


FlexOS was re-written in C (see also: Portable CP/M) for higher portability across hardware platforms, and it featured very low interrupt latency and fast context switching.[3]

The original protected mode FlexOS 286 version 1.0 was designed for host machines equipped with 286 CPUs, and with adaptations for NEC V60, NEC V70 and Motorola 68000 processors planned.[2][4] FlexOS 286 executables using the system's native INT DCh (INT 220) application program interface had the filename extension .286. A CP/M API front end (FE) was available as well,[5] using the extension .CMD for executables. (A filename extension of .68K was reserved for FlexOS 68K, presumably derived from Concurrent DOS 68K as of 1986.[5])

In May 1987, FlexOS version 1.31 was released for 80286 machines. The developer version required an IBM PC/AT-compatible machine with 640 KB of conventional and 512 KB of extended memory, and either a (monochrome) CGA or an EGA graphics adapter.

FlexOS supported a concept of dynamically loadable and unloadable subdrivers, and it came with driver prototypes for floppies, hard disks, printers, serial interfaces, RAM disks, mice and console drivers.

During boot, the FLEX286.SYS kernel would load the resource managers and device drivers specified in the CONFIG.SYS binary file (not to be mixed up with the similarly named CONFIG.SYS configuration file under DOS), and its shell (COMMAND.286) would execute a CONFIG.BAT startup batch job instead of the common AUTOEXEC.BAT.

FlexOS's optional DOS emulator provided limited PC DOS 2.1 compatibility for DOS .COM and .EXE programs. Certain restrictions applied since these programs were executed in the processor's protected mode. Due to bugs in earlier steppings of the Intel 80286, the FlexOS 286 DOS front-end required at least the 80286 E2 stepping to function properly (see LOADALL). These problems had already caused delays in the delivery of Concurrent DOS 286 earlier.[6][7]

The system optionally supported a multitasking GEM VDI for graphical applications.[8]

FlexOS 1.31 could be linked with none, either or both of these two modules. FlexOS 1.31 also supported FlexNet.

By June 1987 there were also versions 1.0 of FlexOS 386 (for hosts) and FlexOS 186 (for remote cell controllers).[3]

FlexOS 286 and FlexOS 386 versions 2.0 were registered on 3 July 1989.

Among the major FlexOS customers in 1990/1991 were FANUC, IBM, ICL, Nixdorf, Siemens, TEC, Thorn EMI Software and Micrologic.[1][9]

Novell bought Digital Research in July 1991.[10][1][11]

X/GEM for FlexOS release 1.0 (a.k.a. X/GEM FlexOS 286 and 386) and FlexNet were registered on 21 December 1992.

FlexOS was used as the primary test platform for the new Novell Embedded Systems Technology (NEST).[12]

When Novell decided to abandon further development of the various Digital Research operating systems such as Multiuser DOS (a successor to Concurrent DOS) and Novell DOS (a successor to DR DOS), they sold FlexOS off to the Santa Clara, California-based Integrated Systems, Inc. (ISI) for US$3,000,000 in July 1994. The deal comprised a direct payment of half this sum as well as shares representing 2% of the company. The company already had pSOS+, another modular real-time multitasking operating system for embedded systems, but they continued to maintain FlexOS as well. FlexOS version 2.33 was current as of May 1998 and with FlexOS 2.34 to be released soon after with added support for faster CPUs, 64 MB of memory, EIDE and ATAPI CDROM drives.

Integrated Systems was bought by their competitor Wind River Systems in 1999, who are owned by Intel since 2009.


IBM 4680 OS[edit]

Named IBM 4680 OS Version 1, IBM originally chose DR Concurrent DOS 286 as the basis of their IBM 4680 computer for IBM Plant System products and Point-of-Sale terminals in 1986.[2][5][13][14] The last release of the IBM 4680 OS has been Version 4, before it was replaced by IBM 4690 Version 1.[15]


  • IBM 4680 Operating System Version 1 Release 1
  • IBM 4680 Operating System Version 1 Release 2
  • IBM 4680 Operating System Version 1 Release 3
  • IBM 4680 Operating System Version 2 Release 1
  • IBM 4680 Operating System Version 3 Release 1
  • IBM 4680 Operating System Version 4 Release 1

IBM and Toshiba 4690 OS[edit]

In July 1993, IBM announced the adoption of FlexOS version 2.32 as the basis of their IBM 4690 OS Version 1, to be pre-released on 24 September 1993[14] and generally made available from 25 March 1994.[15][16] FlexOS 2.32 supported 286 and 386 modes, had more efficient memory management, better console and pipe systems, and brought overall quality and performance improvements compared to the version that came with IBM 4680 OS Version 4.[15] Further, it removed limits on the number of applications running concurrently due to its more efficient use of KOSPOOL.[15]

In order to support Java IBM 4690 OS Version 2 brought support for long filenames by means of a virtual filesystem (VFS) architecture and it introduced FAT32 volumes.

According to "The Year of the Store?", IHL Consulting Group/RIS News, IBM 4690 OS still had a market share of 12% in the POS register/client market in June 2005, when IBM was starting to phase it out in favour to IBM Retail Environment for SUSE (IRES).[14]

IBM will continue to maintain 4690 OS up to April 2015, with the most recent version released by IBM in May 2012 being IBM 4690 OS Version 6 Release 3.

Meanwhile, Toshiba has released Toshiba 4690 OS Version 6 Release 4 in January 2014.

Siemens S5-DOS/MT[edit]

Siemens used and still maintains FlexOS in their factory automation equipment as well. For example, their Simatic (de) S5 STEP-5 operating system S5-DOS/MT is based on FlexOS 386 with X/GEM, FlexNet and Btrieve, whereas the smaller S5-DOS system, also present on these systems, is a variant of Digital Research's Personal CP/M-86.

See also[edit]


  1. ^ a b c "Novell and Digital Research sign definitive merger agreement". Business Wire. 1991-07-17. Archived from the original on 2018-08-18. Retrieved 2017-01-24. 
  2. ^ a b c CBR Staff Writer (1987-01-15). "Digital Research launches FlexOS 286 Real-Time Manufacturing Operating System". 
  3. ^ a b CBR, ed. (1987-06-03). "Digital Research shows off Real-Time FlexOS 386". Retrieved 2011-09-06. 
  4. ^ Glass, Brett (1991-05-06). Answer Line. InfoWorld. p. 72. ISSN 0199-6649. 
  5. ^ a b c Digital Research. IBM selects Concurrent DOS-286 for PC AT retail system, Digital Research European Review, March 1986, Issue 18, p. 1 ([1])
  6. ^ Edward Foster. Super DOS awaits new 80286 - Concurrent DOS 286 - delayed until Intel upgrades chip - offers Xenix's power and IBM PC compatibility. InfoWorld Media Group, InfoWorld, Volume 7, No. 19, 1985-05-13, ISSN 0199-6649, pages 17–18, ([2]).
  7. ^ Edward Foster. Intel shows new 80286 chip - Future of DRI's Concurrent DOS 286 still unclear after processor fixed. InfoWorld Media Group, InfoWorld, Volume 7, No. 34, 1985-08-26, ISSN 0199-6649, page 21 ([3]).
  8. ^ CBR (1988-04-05). "Digital Research, profitable for fiscal 1987, comes out of the closet". 
  9. ^ Digital Research (1990-06-04). "Sold on FlexOS". Computerworld (advertisement). Retrieved 2017-01-24. 
  10. ^ Scott, Karyl (1991-07-29). "Novell/DRI merger to reap better client management". InfoWorld: 33. Retrieved 2017-01-21. 
  11. ^ Allchin, Jim (1992-05-27) [1991-07-17]. "Novell/Digital Research reach definitive agreement..." (PDF) (Court document). Plaintiff's exhibit 828, Comes v. Microsoft. Archived (PDF) from the original on 2016-11-19. Retrieved 2017-01-21. 
  12. ^ Novell. Novell Announces RTOS Vendor Program Which Brings Integrated Networking Solutions to the Embedded Systems Marketplace. Press Release, 1995-05-23 ([4]): '"Because Novell used Integrated Systems' FlexOS during the development and testing of NEST, we are in the unique position of supporting it through both our real-time product lines pSOSystem for deeply embedded markets, and FlexOS for point of sale," said Moses Joseph, vice president of marketing for Integrated Systems. "Developers using the FlexOS development kit and the expanded pSOSystem/NEST package for everything from home security and entertainment to office automation and global communications applications, now have quick and easy access to the widest variety of standard networking protocols.'
  13. ^ Calvo, Melissa; Forbes, Jim (1986-02-10). InfoWorld, ed. IBM to use a DRI operating system. Retrieved 2011-09-06. 
  14. ^ a b c Novell (2005). How to choose the best OS for your point-of-sale. 360Commerce ([5]). Comment: This PDF has a short history of POS operating systems, also mentioning IBM 4680 and IBM 4690.
  15. ^ a b c d IBM (ed.). "IBM 4690 OPERATING SYSTEM VERSION 1 - Announcement Letter Number 293-281 dated 1 June 1993 - Brief description of announcement, charges, and availability". Retrieved 2011-09-06. 
  16. ^ IBM (ed.). "IBM 4690 Operating System". Archived from the original on 2011-09-30. Retrieved 2011-09-06.