Introduction to 64-Bit Transition Guide

This document describes the 64-bit features that are available in OS X v10.4 and v10.5. You should read it to help you determine which of these features to use and how to use them.

What Is 64-Bit Computing?

For the purposes of this document, 64-bit computing is defined as support for a 64-bit address space—that is, support for concurrent use of more than 4 GB of memory by a single executable program—no more, no less.

OS X v10.8 uses a 64-bit kernel and fully supports 64-bit applications. The 64-bit kernel was originally introduced in OS X v10.6 (on some models of Mac hardware), and 64-bit application support was introduced in v10.5. Command-line 64-bit support was introduced in v10.4.

Who Should Read This Document?

Mac app developers should, at a minimum, read the chapter Should You Recompile Your Software as a 64-Bit Executable?. That chapter will help you determine whether it makes sense for your application to take advantage of 64-bit application support in OS X v10.5 and later.

Developers of device drivers and kernel extensions should also read this document. Beginning with v10.6, device drivers and kernel extensions must be compiled with a 64-bit slice to be loadable into a 64-bit kernel. Beginning with v10.8, all kernel device drivers and other extensions must be compiled with a 64-bit slice.

Organization of This Document

This document is organized into the following chapters:

• Should You Recompile Your Software as a 64-Bit Executable?—provides helpful guidance about whether you should recompile your application as a 64-bit executable.

• Major 64-Bit Changes—describes the high-level architectural changes between a 32-bit and 64-bit environment.

• Making Code 64-Bit Clean—explains the general changes needed to make an application 64-bit clean.

• Compiling 64-Bit Code—explains how to compile your application as a 64-bit executable.

• High-Level 64-Bit API Support—summarizes changes to higher level APIs such as Carbon, Cocoa, and QuickTime and includes pointers to more detailed documentation on these changes.

• Cross-Architecture Plug-in Support—describes ways to support legacy plug-ins across architecture boundaries using helper hosts.

• Performance Optimization—gives tips for spotting common performance regressions caused by transitioning your code to 64-bit.

• Kernel Extensions and Drivers—tells how to transition your drivers and other kernel extensions to 64-bit executables.