C. 32- and 64-Bit
At the time of the first edition of this book, Apple was in the process of moving OS X to 64-bit. That transition is now almost complete. Xcode and Clang build OS X programs as 64-bit by default. With Lion (OS X 10.7) new Macs boot a 64-bit kernel by default. While it is still possible to build 32-bit Objective-C programs for OS X, 32-bit OS X programs use the legacy runtime and cannot use the newer features of the Objective-C language such as ARC, synthesized instance variables, and instance variables that are declared in class extensions.
iOS and iOS devices are 32-bit only. The material in the rest of this appendix refers to OS X.
Note
The earliest Intel Macs (those with Core Duo or Core Solo processors) are 32-bit only. ...
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