Apple ProDOS followed by Apple GS/OS
ProDOS is the name of two similar operating systems for the Apple II series of personal computers.  The original ProDOS, renamed ProDOS 8 in version 1.2, is the last official operating system usable by all 8-bit Apple II series computers, and was distributed from 1983 to 1993.  The other, ProDOS 16, was a stop-gap solution for the 16-bit Apple IIGS that was replaced by GS/OS within two years.ProDOS was marketed by Apple as meaning Professional Disk Operating System, and became the most popular operating system for the Apple II series of computers 10 months after its release in January 1983.