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IBM PC DOS (full name: The IBM Personal Computer Disk Operating System) is a DOS system for the IBM Personal Computer and compatibles, manufactured and sold by IBM from the 1980s to 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 the key decision that made the IBM PC an industry standard. But it was done out of necessity, to save time. Microsoft was 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 next months, and the paperwork was officially signed in early November. A source at Microsoft said Microsoft licensed the first version of DOS to IBM for $15,000. Microsoft also received royalties as part of the license, although the royalty arrangement has always been a closely guarded secret.[1]

Microsoft first licensed, then purchased 86-DOS from Seattle Computer Products(SCP), which was modified for the IBM PC by Microsoft employee Bob O'Rear with assistance from SCP (later Microsoft) employee Tim Paterson. O'Rear got 86-DOS to run on the prototype PC in February 1981. 86-DOS had to be converted from 8-inch to 5.25-inch floppy disks and integrated with the BIOS, which Microsoft was helping IBM to write.[2] IBM had more people writing requirements for the computer than Microsoft had writing code. O'Rear often felt overwhelmed by the number of people he had to deal with at the Entry Level Systems facility in Boca Raton. 86-DOS was rebranded IBM PC DOS 1.0 for its August 1981 release with the IBM PC. Toward the end of 1981, Paterson went to work on an upgrade, which was called PC DOS 1.1. It allowed data to be written on both sides of a diskette, thus doubling the capacity of the IBM machine from 160K to 320K, and was finished in March 1982. Later, a group of Microsoft programmers (primarily Paul Allen, Mark Zbikowski and Aaron Reynolds)[2] began work on PC DOS 2.0, the next version for the IBM PC/XT, the first PC to store data on a hard disk. A much more sophisticated program than 1.0, it had 20,000 lines of assembly language code, compared to about 4,000 lines in the first version. It was officially announced in March 1983. In March 1984, the IBM PCjr shipped. It ran PC DOS 2.1, which supported PCjr's ability to run programs from ROM cartridges and slightly different disk-controller architecture.[2] In August 1984, IBM introduced the IBM PC/AT, a computer built around the Intel 80286 processor. It ran on Microsoft's PC DOS 3.0, which supported the computer's larger hard drives and high density (1.2 MB) diskettes.[1]

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