Compaq's efforts were possible because IBM had used mostly off-the-shelf parts for the PC and published full technical documentation for it, and because Microsoft had kept the right to license MS-DOS to other computer manufacturers. The only difficulty was the BIOS, because it contained IBM's copyrighted code. Compaq solved this problem by producing a clean room workalike that performed all documented functions of the IBM PC BIOS, but was completely written from scratch.