What's the real deal with 64-bit computing in Snow Leopard?

Is Snow Leopard a 64-bit operating system? Well, yes and no.

One of the biggest points of confusion around Apple's newest version of Mac OS X is about whether it's really a 64-bit or a 32-bit operating system. Apple bills Snow Leopard as supporting 64-bit from top to bottom, while some industry watchers say it's not a true 64-bit OS. What gives?

What's all the fuss about 64-bit?

Both Apple and Microsoft have been chasing 64-bit computing as if it were the Holy Grail -- and in some ways it is. 64-bit computing opens a lot of doors for hardware and software developers. Because 64-bit processors have twice the number of registers to work with, they can process twice as much information per clock cycle as 32-bit processors. That obviously means much better performance.

Another big advantage to 64-bit computing is addressing memory. Applications have traditionally been written to address the memory space available to 32-bit processing, which tops out at 4GB. If a system has more RAM than that, any applications that are designed to work with 32-bit processing will not be able to interact with any memory beyond 4GB.

By contrast, applications written for 64-bit operation can address and store data in up to 16 exabytes -- that's 16 billion GB -- of RAM. For memory-hungry applications that perform complex and data-intensive tasks (such as high-end graphics, video, or scientific computing tools), this alone can make a huge difference in performance, regardless of the number of registers that the processor has to offer.

In fact, 64-bit computing even offers some additional security because the routines used to interact with the processor are more secure than in 32-bit computing. What's more, the system heap (which is shared memory available to applications) is designed with both hard protections and stronger checksum algorithms that help protect against attempts to corrupt the addressable memory used by the operating system and applications.

Is Snow Leopard 64-bit?

Given the superiority of 64-bit computing in performance, memory use and security, it's no surprise that Apple is touting Snow Leopard's 64-bit capabilities as a selling point of the OS. However, over the past few weeks, bloggers from the likes of OSNews, Gizmodo and Ars Technica have pointed out that in pre-release versions of the operating system, the kernel, which provides the core functions of the OS, runs as a 32-bit process on almost all Mac systems that it is installed on.

This is true: The kernel in Snow Leopard is designed to load in 32-bit operation by default on most Mac systems. However, focusing on the kernel, which facilitates very low-level functions that generally don't benefit from 64-bit operation, is missing the real point of 64-bit computing in Snow Leopard.

As anyone using Snow Leopard on a Mac with a 64-bit processor can see by opening the System Profiler utility (located in the Utilities or accessible from the About This Mac dialog under the Apple menu), the vast majority of applications, kernel extensions and application frameworks in Snow Leopard run in 64-bit operation. This means that nearly all the core applications that ship with Snow Leopard (the Finder, Spotlight, directory services, the Dock, etc.) are running in 64-bit mode.

System Profiler showing 64-bit frameworks
All of these application frameworks support 64-bit processing -- only a few frameworks, applications and kernel extensions load in 32-bit mode on Macs capable of 64-bit processing.
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