Why does MacOS X 10.6 "Snow Leopard" boot in 32-bit mode by default? How can I boot in 64-bit mode? What are the advantages of 64-bit mode? Are there any disadvantages?

When originally shipped on August 28, 2009, Apple had MacOS X 10.6 "Snow Leopard" boot in 32-bit mode to ensure compatibility with non-Apple kernal extensions and drivers. As a result, most applications and peripherals that had yet to be updated to run in 64-bit mode would continue to operate without problems. Meanwhile, new 64-bit applications still would run with "Snow Leopard" booting in 32-bit mode and may be faster than they were running with "Leopard".

Since that time, Apple continued -- and still continues (as of the date last updated, see top) -- to ship most Macs booting in 32-bit mode by default. However, Apple ships the "Mid-2010" Mac Pro models to boot in 64-bit by default.

The advantage of booting in 64-bit mode is performance. Some 64-bit applications -- particularly those able to utilize more than 4 GB of RAM -- are significantly faster when the operating system is booting in 64-bit mode. To startup in 64-bit mode for models that are not enabled by default, just hold down 6 and 4 when booting. You will have to do this each time you startup, so if you discover that your system is fast and stable running in 64-bit mode -- and you have no out-of-date third-party kernal extensions or drivers -- you may wish to either make the change persistent or try Startup Mode Selector. This application makes it possible to set your Mac to boot in 64-bit mode each time.

Please note that the author is providing Startup Mode Selector as "donationware" and if you find the program useful, please contribute to his chosen charity. Please note that Apple does not support Startup Mode Selector.

Does MacOS X 10.6 "Snow Leopard" improve the performance of 32-bit Intel Macs or just 64-bit ones?

MacOS X 10.6 "Snow Leopard" provides the greatest possible performance boost for 64-bit applications running "Snow Leopard" booting in 64-bit mode. However, 64-bit applications often are faster running "Snow Leopard" booting in 32-bit mode than the same applications running under "Leopard".

32-bit Macs typically will see modestly improved performance running "Snow Leopard" compared to "Leopard", but the gains will not be as substantial as they are for Macs capable of booting in 64-bit mode.

Also see:

• How much faster is MacOS X 10.6 "Snow Leopard" in "real-world" tests compared to MacOS X 10.5 "Leopard"?
• Which Intel Macs have 64-bit processors? Which have a 64-bit EFI? Which are capable of booting MacOS X 10.6 "Snow Leopard" in 64-bit mode?
• What is "OpenCL"? Which Macs support the "OpenCL" technology provided by MacOS X 10.6 "Snow Leopard"?
• What is "Grand Central Dispatch"? Which Macs support the "Grand Central Dispatch" technology provided by MacOS X 10.6 "Snow Leopard"?

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