IPhone 4 models. In fall 2010, pentalobe screws started to replace the Philips screws used in post-repair units in the US and in production units in Japan. The iPhone 4 shipped with iOS 4, which was the newest version of Apple's iOS mobile operating system at the time. iOS 4 introduced a multitasking system, allowing apps to remain "suspended" in memory, perform certain operations in the background, and allowing users to switch between apps using an on-screen menu. It also introduced FaceTime, which allows users to perform video calls between other iPhone, iPad, iPod Touch, and OS X users. The iPhone 4
IPhone 5S iOS 7 and OS X Yosemite (version 10.10) noticeably departs from skeuomorphic elements such as green felt in Game Center, wood in Newsstand, and leather in Calendar, in favor of flat, colorful design. iOS 7 adds AirDrop, an ad-hoc Wi-Fi sharing platform. Users can share files with the iPhone 5 onwards, the iPod Touch (5th generation), iPad (4th generation) onwards, or iPad Mini (1st generation) onwards. The operating system also adds Control Center, a control panel accessed by swiping up from the bottom of the screen. Control Center contains a number of commonly used functions, such as volume and brightness