IPod games are in the form of .ipg files, which are actually .zip archives in disguise. When unzipped, they reveal executable files along with common audio and image files, leading to the possibility of third party games. Apple has not publicly released a software development kit (SDK) for iPod-specific development. Apps produced with the iPhone SDK are compatible only with the iOS on the iPod Touch and iPhone, which cannot run clickwheel-based games. All iPods except for the iPod Touch can function in "disk mode" as mass storage devices to store data files but this may not be the default behavior.
IPod Touch date has been introduced with the same release number of iOS as the contemporary iPhone model. Successive updates to iOS since the initial release in 2007 have released additional features. iPhone OS 2.0, released on July 11, 2008, introduced the App Store, which allowed third-party applications for the first time. iPhone OS 3.0, released on June 17, 2009, added features such as cut, copy, and paste, data tethering and push notification support. iOS 4.0, released on June 21, 2010, introduced iBooks, FaceTime, and multitasking. It dropped support for the first generation iPod Touch. In June 2011, iOS 5, the fifth