Category Archives: Operating Systems


In the early 70’s, there was a company known as intergalactic Digital Research. The name was just a little too geeky for even them, and was later shortened to Digital Research. The founder, Gary Kildall, created the first Disk Operating System for Microcomputers called CP/M (Control Program for Microcomputers). This became THE operating system for hobbyist microcomputers. A few years later this changed a bit (or was supplemented) when Steve Jobs and Steve Wozniak designed a home computer that came prebuilt. It was a wild concept for the time, not having to build your computer — it was a wildly successful idea as well. The micro(home)-computer bloomed from a small little hobby toy, into something that many small businesses wanted and felt they needed. But there was still the other group of CP/M builders and users.

Digital Research’s original CP/M for the 8-bit Intel 8080 and Z-80 based systems spawned numerous spin-off versions, most notably CP/M-86 for the Intel 8086/8088 family of processors. Although CP/M had dominated the market, and was shipped with the vast majority of non-proprietary-architecture personal computers, the IBM PC in 1981 brought the beginning of what was eventually to be a massive change.

IBM originally approached Digital Research, seeking an x86 version of CP/M. However, there were disagreements over the contract, and IBM withdrew. Instead, a deal was struck with Microsoft, who purchased another operating system, 86-DOS, fromSeattle Computer Products. This became Microsoft MS-DOS and IBM PC DOS. 86-DOS’ command structure and application programming interface imitated that of CP/M. Digital Research threatened legal action, claiming PC DOS/MS-DOS to be too similar to CP/M. IBM settled by agreeing to sell their x86 version of CP/M, CP/M-86, alongside PC DOS. However, PC DOS sold for $40, while CP/M-86 had a $240 price tag. The proportion of PC buyers prepared to spend six times as much to buy CP/M-86 was very small, and the availability of compatible application software, at first decisively in Digital Research’s favor, was only temporary.

Digital Research fought a long losing battle to promote CP/M-86 and its multi-tasking multi-user successor Concurrent CP/M-86, and eventually decided that they could not beat the Microsoft-IBM lead in application software availability, so they modified Concurrent CP/M-86 to allow it to run the same applications as MS-DOS and PC DOS.

First DR DOS version

As requested by several OEMs Digital Research started to plan develop a new DOS operating system addressing the shortcomings left by MS-DOS in 1987. The first DR DOS version was released on 28 May 1988. Version numbers were chosen to reflect features relative to MS-DOS; the first version promoted to the public was DR DOS 3.31, which offered features comparable to Compaq MS-DOS 3.31 with large disk support (FAT16B aka “BIGDOS”). DR DOS 3.31 reported itself as “IBM PC DOS 3.31”, while the internal BDOS (Basic Disk Operating System) kernel version was reported as 6.0, single-user nature, reflecting its origin as derivative of Concurrent DOS 6.0 with the multitasking and multiuser capabilities as well as CP/M API support stripped out and the XIOSreplaced by an IBM-compatible DOS-BIOS. The system files were named DRBIOS.SYS (for the DOS-BIOS) and DRBDOS.SYS (for the BDOS kernel), the disk OEM label used was “DIGITAL␠”.

DR DOS offered some extended command line tools with command line help, verbose error messages, sophisticated command line history and editing (HISTORY directive) as well as support for file and directory passwords built right into the kernel. It was also cheaper to license than MS-DOS, and was ROMable right from the start. The ROMed version of DR DOS was also named ROS (ROM Operating System). DRI was approached by a number of PC manufacturers who were interested in a third-party DOS, which prompted several updates to the system.

At this time, MS-DOS was only available to OEMs bundled with hardware. Consequently, DR DOS achieved some immediate success when it became possible for consumers to buy it through normal retail channels since 3.4x.

Known versions are DR DOS 3.31 (BDOS 6.0, 1988-06, OEM only), 3.32 (BDOS 6.0, 1988-08-17, OEM only), 3.33 (BDOS 6.0, 1988-09-01, OEM only), 3.34 (OEM only), 3.35 (BDOS 6.0, 1988-10-21, OEM only), 3.40, 3.41 (BDOS 6.3, 1989-06, OEM and retail). Like MS-DOS, most of them were produced in several flavors for different hardware. While most OEMs kept the DR DOS name designation, one OEM version is known to be called EZ-DOS 3.41.

Novell DOS was Novell Corporation’s name for DR-DOS during the period when Novell sold DR-DOS, after the acquisition of Digital Research in 1991. Regarding features and performance, it was typically at least one release ahead of MS-DOS.[13] In 1993, PC DOS 6.1, MS-DOS 6.2 and PC DOS 6.3 were trumped by Novell’s DOS 7.0.


Xenix is a discontinued version of the Unix operating system for various microcomputer platforms, licensed by Microsoft from AT&T Corporation in the late 1970s. The Santa Cruz Operation (SCO) later acquired exclusive rights to the software, and eventually replaced it by SCO UNIX (now known as SCO OpenServer).

In the mid-to-late 1980s, Xenix was the most common Unix variant, measured according to the number of machines on which it was installed. Microsoft chairman Bill Gates said in 1996 that for a long time his firm had the highest-volume AT&T Unix license.

Microsoft licensed UNIX from AT&T in 1979. That was an interesting year in Unix history: at the June 1979 USENIX meeting in Toronto, AT&T announced an big price hike ( from $1000 to $7500 for educational institutions; from $5000 to $40,000 for full commercial open source; license was per CPU, which at the time meant per each computer in the orgnization that runs Unix). Essentially AT&T decided to commercialize Unix using simple “make money fast” approach, which quite predictably backfired.

Bill Gates instantly realized that such move opens the door for third party Unix distributors and jumped on this opportunity.  There were rumors that licensing of Unix from AT&T by Microsoft was partially stimulated by behavior of Digital Research which in 1979 broke implicit agreement with Microsoft to concentrate on OS and moved into compiler space by packaging CP/M with CBasic developed by one of Kildall student Gordon Eubanks (future founder of Symantec). This was the area which Microsoft considered its own: at this time Microsoft was the languages compiler/interpreter development company, selling mainly several compilers and first of all Microsoft Basic which was microcomputer standard of the time.  At the same time Microsoft behaved in quite gentleman matter with Digital Research. Not only there was no attempt to encroach into Digital Research turf, Microsoft served as a powerful implicit marketing agent for Digital Research. As for the language company supporting multiple operating systems was a nightmare,  Microsoft was strong promoter of CP/M, helping Digital Research to generate revenue by sending to them the customers of their compilers and interpreters. So Microsoft rightly considered this move of Digital Research as a knife in his back.  It was after that unfortunate episode Bill Gates turned to AT&T and in February 1980 licensed  Unix operating system.

So when in September 1980 Gill Gates picked up the phone and called Kindall to inform him that IBM want to talk to him about creating OS for their new microcomputer, it was more then a gesture of good will — he was sending business to the competitor to tried to steal his share of Basic interpreters market.  This historical meeting happened  just six month after Microsoft got into Xenix business so in a way Microsoft hands were full. The same year SCO was founded and Microsoft XENIX development was outsourced to SCO (which at a time was a two man operation) expecting them to perform the bulk of technical work connected with porting of AT&T codebase to various microprocessors.

XENIX was originally developed on a DEC Virtual Address Extension (VAX) running the Virtual Memory System (VMS) and a PDP-11 running UNIX V7, albeit now using Microsoft’s own in-house minicomputers, and then converted into assembly language specific to the new 16-bit Motorola 68000 and Intel 8086 microprocessors. This put XENIX at the high end of the microcomputer market, which was still dominated by 8-bit machines, but well below the lowest end of the minicomputer market.

In 1979, brothers Doug and Larry Michels founded the Santa Cruz Operation (SCO) as a UNIX porting and consulting company using venture capital from Microsoft, which handed over all further development of Microsoft XENIX to SCO. Doug Michels recalled that the company’s name was a bit of “social engineering” to obscure the fact that it was essentially a two-man operation. “I’d call up and say, ‘This is Doug from the Santa Cruz Operation’ and be pretty sure they wouldn’t catch that the ‘O’ was capitalized and think I was from another branch of their company.”

By 1980, the UNIX family tree had split into three distinct major branches:

  1. AT&T UNIX System III from Bell Labs’ UNIX Support Group (USG).
  2. Berkeley Standard Distribution 4.1 from UCB.
  3. XENIX 3.0 from Microsoft and SCO.

Microsoft XENIX was initially an Intel 8086 port of AT&T UNIX Version 7 with some BSD-like enhancements. This became Microsoft/SCO XENIX 3.0 a year or so later. SCO XENIX 5.0 was updated to conform to AT&T UNIX System V Release 0 (SVR0) in 1983, for which SCO brought its own rights to the UNIX source code. By then, XENIX had the largest installed base of any UNIX system during the early 1980s.


CP/M (Control Program for Microcomputers) was a mass-market operating system created for Intel 8080/85-based microcomputers by Gary Kildall of Digital Research, Inc (originally incorporated as Intergalactic Digital Research). Initially confined to single-tasking on 8-bit processors and no more than 64 kilobytes of memory, later versions of CP/M added multi-user variations and were migrated to 16-bit processors.

The combination of CP/M and S-100 bus computers loosely patterned on the MITS Altair was an early industry standard for microcomputers, and this computer platform was widely used in business through the late 1970s and into the mid-1980s, expanding to include 16-bit CPUs and multiuser capability. By greatly reducing the amount of programming required to install an application on a new manufacturer’s computer, CP/M increased the market size for both hardware and software.

In 1974, Dr. Gary A. Kildall, while working for Intel Corporation, created CP/M as the first operating system for the new microprocessor.  By 1977, CP/M had become the most popular operating system (OS) in the fledgling microcomputer (PC) industry.  The largest Digital Research licensee of CP/M was a small company which had started life as Traf-0-Data, and is now known as Microsoft.  In 1981, Microsoft paid Seattle Software Works for an unauthorized clone of CP/M, and Microsoft licensed this clone to IBM which marketed it as PC-DOS on the first IBM PC in 1981, and Microsoft marketed it to all other PC OEMs as MS-DOS.

In 1991, Gary Kildall and the other shareholders of Digital Research Inc., which Gary & Dorothy Kildall had founded in 1975, sold the closely-held private shares of Digital Research Inc. (DRI) to Novell, Inc., and then on July 23, 1996 all of the Digital Research, Inc. assets were acquired from Novell Inc. by Caldera Inc., a company founded by Bryan Sparks with the assistance of Ray Noorda, former Chairman/CEO of Novell Inc., and on July 24, 1996, Caldera Inc. filed a private Federal Antitrust Lawsuit against Microsoft Corp. for alleged illegal activities and unfair practices in the marketing of MS-DOS and its successors, including Windows 95 and Windows 98, both of which are still Digital Research CP/M at their essential core.  The lawsuit was settled out of court in January 2000 at which time Microsoft Corporation agreed to certain terms and paid certain funds to Caldera Inc.

In 2000, Caldera Inc. owns all trademark and copyright to CP/M, whose successors are Caldera DR-DOS for single-user (client) purposes, and IMS Ltd. REAL/32 for multi-user and networking. Both are the most advanced versions of 32-bit DOS available, and are ideal for Thin Server and Thin Client Server solutions in the office, POS, embedded, communications, and other important emerging markets such as hard real-time for robotic control and full-fledged Video Computing.