IBM PC DOS, aka IBM personal computer disk operating system, is a discontinued operating system for the IBM Personal Computer, manufactured and sold by IBM from the early 1980s into the 2000s.
The IBM task force assembled to develop the PC decided that critical components of the machine, including the operating system, would come from outside vendors. This radical break from company tradition of in-house development was one of the key decisions that made the IBM PC an industry standard. At that time the private company Microsoft, founded five years earlier by Bill Gates, was eventually selected for the operating system.
IBM wanted Microsoft to retain ownership of whatever software it developed, and wanted nothing to do with helping Microsoft, other than making suggestions from afar. According to task force member Jack Sams:
The reasons were internal. We had a terrible problem being sued by people claiming we had stolen their stuff. It could be horribly expensive for us to have our programmers look at code that belonged to someone else because they would then come back and say we stole it and made all this money. We had lost a series of suits on this, and so we didn't want to have a product which was clearly someone else's product worked on by IBM people. We went to Microsoft on the proposition that we wanted this to be their product.
IBM first contacted Microsoft to look the company over in July 1980. Negotiations continued over the months that followed, and the paperwork was officially signed in early November.
Although IBM expected that most customers would use PC DOS, the IBM PC also supported CP/M-86, which became available six months after PC DOS, and UCSD p-System operating systems. IBM's expectation proved correct: one survey found that 96.3% of PCs were ordered with the $40 PC DOS compared to 3.4% with the $240 CP/M-86.
Over the history of IBM PC DOS, various versions were developed by IBM and/or Microsoft. By the time PC DOS 3.0 was completed, IBM had a team of developers covering the full OS. At that point in time, either IBM or Microsoft completely developed versions of IBM PC DOS going forward. By 1985 the joint development agreement (JDA) between IBM and Microsoft for the development of PC DOS had each company giving the other company a completely developed version. Most of the time branded versions were identical however there were some cases in which each of the companies made minor modifications to their version of DOS. In the fall of 1984, IBM gave all the source code and documentation of the internally developed IBM TopView for DOS to Microsoft so that Microsoft could more fully understand how to develop an Object-oriented operating environment, how to do overlapping windows (for it's development of Windows 2.0) and multitasking (Windows used a cooperative method to share the CPU through the entire life of Windows 1.0.