64-bit refers to the number of bits that make up the structure of various parts of a processor’s architecture. These structures usually include the data bus, address bus, and internal registers.
As used with Apple Macintosh computers, 64-bit can refer to the processor architecture as well as the operating system’s architecture. The PowerMac G5s introduced in June of 2003 were the first desktop 64-bit computers available to consumers. But while the G5 had a 64-bit architecture, the operating system (OS X 10.2.7) had limited support for 64-bit. Apple promised that each new OS X upgrade would bring further support for the 64-bit architecture.
The first full 64-bit version of OS X was Snow Leopard OS X 10.6, released in September of 2009. While Snow Leopard was a 64-bit operating system, Apple by this time had moved away from the G5 and its PowerPC architecture, and had instead embraced Intel as its processor of choice. As a result, Snow Leopard was designed to run only on Intel Macs. The desktop G5 will never enjoy the benefit of a full 64-bit OS.
The 64-bit architecture used in Snow Leopard and most Intel Macs will provide for a memory space of 16 exabytes (that’s 16 billion gigabytes). Of course, the Macs of today can’t physically accommodate that much memory, but future Macs may very well use very large amounts of RAM or other alternate memory technology.
The 64-bit architecture used in Macs also provides for a potential performance boost. While the data bus and the internal registers of the processor are 64-bits wide, not all of the instructions need 64 bits. In some cases, it will be possible to pack more than one instruction into those registers and execute multiple instructions at the same time.
