GS/OS is an operating system developed by Apple Computer for its Apple IIGS 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 IIGS 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 IIGS.