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Universal Binary

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A universal binary is, in Apple parlance, an application bundle that runs natively on either PowerPC- or x86 (Intel)-based Macintosh computers. It was introduced at the 2005 Worldwide Developers Conference as a means to ease the transition from the existing PowerPC architecture to systems based on Intel processors, which began shipping in 2006. Universal binaries typically include both PowerPC and x86 versions of a compiled application. The operating system detects a universal binary by its header, and executes the appropriate section depending on the architecture in use. This allows the application to run natively on any supported architecture, with no performance impact.

There are two general alternative solutions. The first is to simply provide two separate binaries, one compiled for the x86 architecture and one for the PowerPC architecture. However, this can be confusing to software users unfamiliar with the difference between the two. The other alternative is to rely on emulation of one architecture by a system running the other architecture. This approach results in lower performance, and is generally regarded an interim solution to be used only until universal binaries or specifically compiled binaries are available (See Rosetta).

Universal binaries are usually larger than single-platform binaries, because multiple copies of the compiled code must be stored. The size of the resulting universal binary is usually not double as a significant portion will be non-executable resources which are shared between the two architectures. They do not require extra RAM, however, because only one of those two copies is loaded for execution.

Apple previously used a similar technique during the transition from 68k processors to PowerPC in the mid-1990s. These dual-platform executables were called "fat binaries", referring to their larger file size.

NEXTSTEP, another predecessor of Mac OS X, supported the fat binary so that one application bundle could be run on multiple platforms, including Intel, Sparc and PA-RISC. In fact, the binary format underlying the universal binary is exactly the same as the one for the fat binary in NEXTSTEP.

Apple's Xcode 2.1 supports the creation of these files, a new feature in that release. A simple application developed with processor-independence in mind might require very few changes to compile as a universal binary, but a complex application designed to take advantage of architecture-specific features might require substantial modification. Applications originally built using other development tools might require additional modification. These reasons have been given for the delay between the introduction of Intel-based Macintosh computers and the availability of third-party applications in universal binary format. Apple's delivery of Intel-based computers several months ahead of their previously announced schedule is another factor in this gap.

Apple's Xcode 2.4 takes the concept of universal binaries even further, by allowing four-architecture binaries to be created (32 and 64 bit for both Intel and PowerPC), therefore allowing a single executable to take full advantage of the CPU capabilities of any Mac OS X machine.

[edit] Universal applications

Many software developers have provided universal binary updates for their products since the 2005 WWDC. As of February 2007, Apple's website now lists more than 6,000 Universal applications. However, such updates are yet to be released for some high-profile applications, such as Microsoft Office 2004. This product been reported to run correctly on Intel macs using the Rosetta emulation layer, although with non-optimal performance.

On April 16, 2007, Adobe announced the release of Photoshop CS3, the first version of the application in a universal binary format. [1]

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[edit] External links

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