IBM PC-DOS was an IBM branded version of MS-DOS licensed from Microsoft for use on the IBM PC system. Versions of PC-DOS paralleled that of MS-DOS up until version 6.1, where PC-DOS was independently maintained by IBM up until 7.1, the last version. Although not the only OS available for the IBM PC when it came out, it quickly became popular due to its low cost in comparison to the others. Its direct competitors on the PC at the time were CP/M-86 and UCSD p-System. The major differences between PC-DOS and MS-DOS were only in terms of driver support and built in programs until version 6.1 where development diverged.