The OSL (Operating System Layer) allows cross platform access to common operating system features but that are accessed differently depending on the platform. The OSL consists of a series of C++ classes, backed by a C-based API for maximum portability. This was necessary when Star Division started because many of the things needed, such as mutexes, condition variables, threading, time functions and a variety of other operating system specific mechanisms hadn't been produced by libraries such as the STL, and certainly what there was couldn't be used as a cross platform solution.