If you call it writing when Gates merely changed enough things to make it "legal." What follows is a bit of the story that can be found here: http://inventors.about.com/science/inventors/library/weekly/aa033099.htm?iam=dpile&terms=%2Bmicrosoft+%2Bhistory *IBM tried to contact Kildall for a meeting, executives met with Mrs. Kildall who refused to sign a non-disclosure agreement. IBM soon returned to Bill Gates and gave Microsoft the contract to write the new operating system, one that would eventually wipe Kildall's CP/M out of common use. *The "Microsoft Disk Operating System" or MS-DOS was based on QDOS, the "Quick and Dirty Operating System" written by Tim Paterson of Seattle Computer Products, for their prototype Intel 8086 based computer. *QDOS was based on Gary Kildall's CP/M, Paterson had bought a CP/M manual and used it as the basis to write his operating system in six weeks, QDOS was different enough from CP/M to be considered legal. *Microsoft bought the rights to QDOS for $50,000, keeping the IBM deal a secret from Seattle Computer Products. *Gates then talked IBM into letting Microsoft retain the rights, to market MS DOS separate from the IBM PC project, Gates proceeded to make a fortune from the licensing of MS-DOS.