A universal binary is, in Apple parlance, an executable file or application bundle that runs natively on either PowerPC or x86 (Intel)-based Macintosh computers. It was introduced at the 2005 WWDC 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 for the architecture in use. This allows the application to run natively on any supported architecture, with no negative performance impact.
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 can be, and usually is, smaller than both binaries combined, because some non-executable resources are shared by the two architectures. They also do not require extra RAM because only one of those two copies is loaded for execution.
NEXTSTEP, another predecessor of Mac OS X, supported the fat binary so that one application bundle could be run on multiple architectures, including Intel's x86, Sun Microsystems' Sparc and Hewlett-Packard's PA-RISC. The binary format underlying the universal binary, a Mach-O archive, is the same format used 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.
Many software developers have provided universal binary updates for their products since the 2005 WWDC. As of January 2008, Apple's website now lists more than 7,200 Universal applications.
On April 16, 2007, Adobe announced the release of Adobe Creative Suite 3, the first version of the application suite in a Universal Binary format.
As of March 2008, all high-profile applications for Mac OS X have been ported to Universal binary, including QuarkXPress, Apple's own Final Cut Studio, Adobe Creative Suite, Microsoft Office 2008, and Shockwave Player beginning with version 11. Non-Universal products will run on Intel Macs (in most cases), but with non-optimal performance, since they must be translated on-the-fly by Rosetta.