In computing, GS/OS is the second operating system developed by Apple Computer for its Apple IIGS computer 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 IIGS System Software versions 4.0 through 6.0.1.
GS/OS was a major improvement over ProDOS 16, Apple's first operating system for the IIGS. GS/OS included a facility known as File System Translators (FSTs) which allowed it to elegantly support multiple on-disk file systems in a manner transparent to application programs, an innovation 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 supported a variety of other file systems, including the HFS file system used by the Mac OS. Another advantage of GS/OS over ProDOS 16 was that it was written in 16-bit code for the 65816 processor used in the IIGS, rather than primarily in 8-bit 6502 machine code that did not take advantage of the IIGS's unique features. It also extended the ProDOS file system to provide for resource forks on files similar to those used on the Apple Macintosh, which allowed for programs to be written in a more elegant and flexible way.