iPhone OS (known as OS X iPhone or iPhone OS X in its early history) is a mobile operating system developed and marketed by Apple Inc. It is the default operating system of the iPhone, the iPod Touch, and the iPad. It is derived from Mac OS X, with which it shares the Darwin foundation, and is therefore a Unix-like operating system by nature. iPhone OS has four abstraction layers: the Core OS layer, the Core Services layer, the Media layer, and the Cocoa Touch layer. The operating system uses less than 500 megabytes of the device's memory.