Apple has confirmed that an optimized version of the Mac OS X operating system (without unnecessary components) runs on the iPhone, although differences between the operating system (OS X) running on Macs and the iPhone have not been officially explained. As iPhone's CPU is an ARM processor, the version of OS X that runs on iPhone differs from the desktop version in that code written in high-level programming languages is compiled to, and code written in assembly language is written in, instructions from the ARM instruction set architecture (ISA) instead of the x86 and PowerPC ISAs that the Mac version of OS X uses.
The operating system takes up about 300 MB of the device's total 4 or 8 GB storage according to the size of the OS partition. It will be capable of supporting bundled and future applications from Apple.
Apple provides updates to the iPhone's operating system through iTunes, in a similar fashion to the way that iPods are updated, and touts this as an advantage compared to other cell phones.
Widgets, similar to the ones available in Mac OS X v10.4's Dashboard, are included on the iPhone. They include Stocks and Weather widgets.
The iPhone's version of OS X includes the software component "Core Animation" which is responsible for the smooth animations used in its user interface. Core Animation has not yet been released for Macs, but will be part of Mac OS X v10.5.
The build of OS X on at least one iPhone is "OS X 1.0 (1A543a)", as seen in a crash log for the MobileMail application. The application apparently runs as the superuser.