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The Apple Watch is using "most" of iOS 8.2, with a new subsystem called "Carousel" in place of the Springboard homescreen found on iPhones and iPads, developer Steve Troughton-Smith said on Thursday, also suggesting that the Watch's S1 processor appears to be equivalent to the Apple A5.
Posting on Twitter, Troughton-Smith noted that the Watch firmware contains a PowerVR SGX543 driver. The SGX543 is the same graphics component found in the A5, which Apple first shipped with the iPad 2 and iPhone 4S back in 2011.
The S1 though is a much more compressed chip design, building everything the device needs into a system-in-package surrounded by resin for extra protection. Apple has said relatively little about the technology beyond that, making it difficult to gauge performance.
The Watch firmware is not officially considered a version of iOS. Since it has to interact with a paired iPhone, however, it makes sense for it to be founded in the same general codebase.
Troughton-Smith further observed that all of the Watch Faces are stored in a single framework, called NanoTimeKit. Those Faces use SpriteKit graphics, but animated faces like Mickey Mouse apply OpenGL for rendering.