Apple DOS (disk operating system) was Apple's first major operating system and deployed on the Apple II, an 8-bit computer the company sold between 1978 and 1983. The first version was Apple DOS 3.1 and ran, as the name indicates, from a "disk"; its predecessors loaded from tape. The operating system was written by Apple co-founder Steve Wozniak and Randy Wigginton, the latter being Apple's sixth employee. DOS was updated to versions 3.2 and 3.3, discontinued in 1983, and replaced by ProDOS, a product based on Apple III's Sophisticated Operating System (SOS) which debuted with the Apple IIe in 1983. ProDOS was later renamed to GS/OS.