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The IBM PC is 30
On 12 August 1981, the world changed
The mythology of the IBM PC's operating system has it that the company approached Digital Research (DR), creator of the CP/M OS, but found it lacking in enthusiasm for the project. Microsoft's Bill Gates got wind of this and pitched MS-DOS instead.
In fact, it's likely IBM selected CP/M-86 but was forced to look, at least temporarily, elsewhere as DR's effort to port CP/M to the Intel 8086/8088 CPU was taking longer than planned. Gates may well have pitched an alternative - IBM was already talking to Microsoft about using the latter's implementation of the Basic programming language - and appears to have won the deal in November 1980.
Not long after, Microsoft licensed 86-DOS from Seattle Computer Products (SCP), specifically to sub-license the OS to IBM in turn. 86-DOS had been created by SCP as a stand-in for CP/M-86 and was thus intentionally compatible with the DR OS.
IBM agreed to use 86-DOS, which in July 1981 was acquired in its entirety by Microsoft and renamed MS-DOS. IBM would bundle it as PC-DOS.
Not just for grown-ups
At launch, IBM said it would offer CP/M-86 for the 5150. "IBM has contracted with Digital Research... to make CP/M-86... available for the IBM Personal Computer," the company said on the 5150's day of launch. "We expect [its] availability will provide the opportunity for many current applications to be transferred to the IBM Personal Computer with minimal modifications."