64-bit addressing

64-bit addressing involves computers that can locate es that are 64-bits (4 bytes) wide, allowing for a theoretical maximum of 16 exabytes (16.7 million gigabytes) of memory, though logic board and CPU configurations may add other limitations.

History
The Power Macintosh G5 was first released in June 2003 with the ability to address more than 4GB of RAM with 64-bit addressing, though implementation was limited at the time. Mac OS X 10.2 (Jaguar) allowed up to 8GB of RAM on dual-core G5 models. In 2005, the quad-core G5 running Mac OS X 10.4.x (Tiger) could address up to 16GB of RAM.

The first Macs with Intel Core processors were introduced in January 2006 with support for 32-bit addressing only, though hardware on consumer models could only be upgraded up to 2 GB anyway. Models with Intel Core 2 processors began appearing in October 2006 with full 64-bit support. Mac OS X versions up through 10.5.8 (Leopard) still relied on a 32-bit kernel upon startup. The first time a 64-bit application was launched on a 64-bit processor, a 64-bit framework would be loaded into memory, incurring a performance and memory hit. In Mac OS X 10.6 and later, a 64-bit kernel was introduced, favoring better performance on 64-bit applications and incurring a performance and memory penalty from loading a 32-bit framework when a 32-bit application was launched. Mac OS X 10.8 (Mountain Lion) dropped support for legacy 32-bit kernel extensions. macOS 10.15 (Catalina) eliminated the 32-bit framework and any remaining backward compatibility with 32-bit applications entirely.