History
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 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.
Read more about this topic: Universal Binary
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—Thomas Jefferson (1743–1826)