Windows Predecessor is a term used to refer to the operating systems that preceded Microsoft Windows. The two most common predecessors are DOS (Disk Operating System) and MS-DOS (Microsoft Disk Operating System). DOS was developed by Microsoft in 1981 and was the first operating system for IBM-compatible PCs. MS-DOS was released in 1983 and was the first version of DOS to be sold commercially. Both DOS and MS-DOS were popular operating systems until the release of Windows 95 in 1995.