The iPhone SDK (software development kit) is free to download atwww.apple.com/developer, but only for Mac OS X. It’s based on the same Xcode development suite used to develop many Mac OS X apps, which provides a versatile environment for writing the Objective-C code that underpins iPhone software. It’s complemented by companion tools, such as Instruments, that enable performance analysis and drag-and-drop interface building.

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