With modern PCs having a very high level of standardization and compatibility, today it is easy to see how Microsoft can market complete packaged operating systems that will install and work unmodified on practically any PC you can purchase or build. Without the standardization and compatibility we have come to depend on, different specific "flavors" of a given operating system would be required for specific different hardware.

That is exactly how things were back in the early '80s when the IBM PC was introduced. Many of the Intel x86 processor based PCs in the early '80s were not fully compatible with the IBM PC, and IBM's PC DOS would not run on those systems right out of the box. If a given system would not run PC DOS, the manufacturer could license MS-DOS from Microsoft and produce a custom version for their computer that would run.

For DOS versions up through 3.1 there were only private-labeled OEM (Original Equipment Manufacturer) versions such as PC DOS, Compaq DOS, Zenith-DOS, and so on. Private labeled DOS names and version numbers could vary, even for releases based on the same set of Microsoft code. For example, the code base that Microsoft internally called MS-DOS 1.25 was called IBM PC DOS 1.1 by IBM, and Columbia DOS 2.0 by Columbia Data Products.

In the early to mid-'80s there were many systems that were partially compatible with the IBM PC, but which also differed from the PC in many ways. For example, the Texas Instruments Professional Computer used an 8088 processor, had the same 5.25-inch 360KB floppy drives as an IBM PC (and could read and write the same 360KB disks); however, it also had a different ROM BIOS, an internally different hardware and software interrupt structure, and a higher resolution graphics processor. Because of the differences in system design, IBM's PC DOS would not boot and run on the TI PC.

As you learned in the previous section, the development of PC DOS was a cooperative project between Microsoft and IBM. Microsoft was responsible for producing the core system code, while IBM helped specify the functionality, did testing, and added several additional utility programs to the system to both enhance functionality as well as to work specifically with IBM's hardware. The development agreement between Microsoft and IBM allowed Microsoft to license the Microsoft-developed portions of the PC DOS product (essentially the core system code) to other OEM's, which Microsoft called MS-DOS. As such, MS-DOS was not a complete (finished) product; rather, it was only a core set of code that could be licensed by a computer manufacturer to run on its systems.

To actually have a finished version of MS-DOS for end users, a given computer manufacturer such as Texas Instruments would have to license the core MS-DOS code from Microsoft, test and if necessary modify that code to work properly on its hardware, write their own versions of any or all of the utility programs that IBM had written for PC DOS (as well as possibly write any additional utilities they wanted), and finally write and print the manuals, copy the disks, and package it all together into a finished retail product. If the manufacturer was Texas Instruments, the finished MS-DOS product might be called Texas Instruments DOS, and would only be guaranteed to run on the Texas Instruments computers for which it was designed.

As an end user, once you had the TI version of MS-DOS running on a TI PC, any program strictly written to interface with MS-DOS would work on the system. Unfortunately many programs at the time were designed to go around the operating system and talk directly to the hardware for certain functions in order to improve performance. For example, the popular Lotus 1-2-3 spreadsheet program accessed the IBM graphics hardware directly and would not work on the TI PC. For Lotus 1-2-3 to run on the TI PC, TI had to work with Lotus to produce a special version of the program rewritten to work with the modified graphics on their system.

Besides TI, many other manufacturers at the time were also producing systems that were not 100% compatible with the IBM PC, and therefore also had to license and then produce custom versions of MS-DOS specifically designed for those systems. Owners of those few systems that were 100% compatible with the IBM PC could simply purchase PC DOS from IBM and run that. For example, I used a Compaq Portable PC for a short time, and although Compaq did produce its own custom version of MS-DOS, I ran IBM's PC DOS instead, and it worked perfectly.

As time progressed, most computer manufacturers realized that producing systems that were 100% compatible with the IBM PC was necessary in order to run all of the software that was becoming available for the IBM PC, which in turn became critical for success in the marketplace. Also, PC components such as motherboards became available, enabling smaller computer dealers or even individuals to build their own systems. Although an individual who built a 100% IBM compatible system could simply go to the IBM dealer and purchase a copy of PC DOS, most of the smaller computer manufacturers (who were really just system builders or assemblers) did not want to bundle IBM DOS with their systems, nor did they have the capability to license MS-DOS from Microsoft, write the additional utilities, or produce the manuals and packaging to create a finished product.

What was needed was a generic but complete shrink-wrapped packaged product that a smaller computer manufacturer or assembler could buy from Microsoft and sell with its computers. To oblige, Microsoft wrote its own versions of the utilities provided by IBM in PC DOS, and in August 1986 released Microsoft MS-DOS 3.2, the first Microsoft labeled "shrink-wrapped" packaged version of DOS for smaller OEMs or system builders. This became known as the Microsoft OEM version. On the box, the labeling stated that it was "For Personal Computers Compatible with IBM Personal Computers." This version was technically not sold retail, but was sold through what became known as the Microsoft OEM System Builder program. In fact, later versions of the packaged MS-DOS product contained the statement "Not for retail sale except with a computer system" right on the box. Microsoft was afraid to sell MS-DOS retail because then it would have to support it on the myriad of different systems out there. Instead Microsoft sold it only to system builders, who were responsible for testing the DOS to work properly on their systems, and then provide any and all necessary support to the end user.

IBM and Microsoft had signed a JDA (Joint Development Agreement) in June of 1985 to collaborate on what was originally called Advanced DOS, but which would later be known as OS/2. Although the JDA was centered around OS/2, it also brought on a major change in DOS development. Starting with DOS 3.3, IBM became the main development center for DOS (both the core and the utilities) while Microsoft focused mainly on OS/2. As a result of the JDA, Microsoft gained the right to redistribute the PC DOS utilities written by IBM. This meant that PC DOS 3.3 and the Microsoft MS-DOS 3.3 OEM packaged product version that followed were now almost identical code, with only a few minor exceptions.

Note that many of the larger computer manufacturers continued to license MS-DOS and produce their own custom versions. For example, after the IBM and Microsoft versions of DOS 3.3 were released, Compaq released Compaq DOS 3.31, which included the implementation of support for larger than 32MiB hard disk partitions that would officially appear in DOS 4.0.

As with 3.3, DOS 4.0 was also initially developed at IBM and subsequently released by Microsoft. There were several bugs in the first release, and by the time the Microsoft OEM packaged version came out it had been updated to version 4.01.

During 1991 the joint development agreement between IBM and Microsoft fell apart, which resulted in IBM taking over full responsibility and development for OS/2, and primary development of DOS 5.0 became Microsoft's responsibility. The MS-DOS 5.0 OEM packaged version was released on June 6, 1991, five days before IBM released PC DOS 5.0. This was somewhat significant as up until DOS 5, PC DOS had always been on the market first, and in some cases with a fairly long lead over the same relative version of MS-DOS. For the first time Microsoft also began selling MS-DOS as a retail product in the form of a lower cost upgrade version.

The growing rift forming between IBM and Microsoft after the dissolution of their joint development agreement caused several different and somewhat confusing releases of DOS 6.x. For example, Microsoft developed MS-DOS 6.0 and released its OEM and upgrade versions first. Rather than merely introducing the same thing later, IBM made some changes and subsequently released PC DOS 6.1, skipping a version number in the process (there was no PC DOS 6.0). Microsoft then developed its next version and also skipped a number, calling it MS-DOS 6.2 to eliminate confusion with the IBM product (there was no MS-DOS 6.1). IBM followed suit and called their subsequent release PC DOS 6.3 (there was no PC DOS 6.2).

The last official standalone MS-DOS release from Microsoft was 6.22, while IBM subsequently released PC DOS 7.0 and finally PC DOS 2000 (7.1). IBM's PC DOS 2000 was the last official release of any standalone version of MS-DOS. Later versions of MS-DOS 7.0, 7.1, and 8.0 came with Windows 95, Windows 98, and Windows Me respectively; however, those DOS versions were never released separately as standalone products.