Apple GS/OS

GS/OS is an operating system developed by Apple Computer for its Apple II personal computer that uses the ProDOS file system. It provides facilities for accessing the file system, controlling input/output devices, loading and running program files, and a system allowing programs to handle interrupts and signals. GS/OS was included as a component of Apple II System Software versions 4.0 through 6.0.1. GS/OS, unlike its predecessor, is written entirely in 16-bit code and was the first true 16-bit operating system for the II.
GS/OS includes a facility known as file system translators (FSTs) which allows it to support multiple on-disk file systems in a manner transparent to application programs, a feature not found in ProDOS or most other microcomputer operating systems at the time. It was usually used with the ProDOS file system (which was the only one from which it could be booted), but GS/OS also supports a variety of other file systems, including the Hierarchical File System used by the Mac OS. Other file system translators, in addition to ProDOS and HFS, include those for MS-DOS, High Sierra/ISO-9660, Apple DOS 3.3, and Pascal, albeit read-only (full read/write support had been planned but never completed). Another advantage of GS/OS over ProDOS 16 is that it was written in 16-bit code for the 65816 processor used in the II, rather than primarily in 8-bit 6502 machine code that does not take advantage of the II's unique features. It also extends the ProDOS file system to provide for resource forks on files similar to those used on the Apple Macintosh, which allows for programs to be written in a more flexible way.
The big change from the previous versions of ProDOS is that GS/OS incorporates many features of the Macintosh System 5. In particular, GS/OS has a Finder, loadable fonts, plug-in device drivers (modem, printer, etc.). A command-line development environment called APW (Apple Programmer's Workshop) is available; much like the Macintosh Programmer's Workshop.