This can get pretty complex pretty quickly, but the short version is that if you are running Snow Leopard or later, on a Mac that is running on a Core 2 Duo Intel chip (or newer) then you can run 64 bit apps.
You should note that there is no distinction between the 32bit and 64bit versions of OSX, the difference is in the EFI which is essentially the firmware controlling the interface between your OS and your hardware. Some older Macs come with a 32 bit EFI, some with 64 bit. In this exmaple you will see mine is running a 64 bit EFI.
StuffeMac:~ stuffe$ ioreg -l -p IODeviceTree | grep firmware-abi
| | "firmware-abi" = <"EFI64">
So, up to now you know that if you have a 64 bit EFI on the right CPU hardware, you are good to go with 64 bit, and it will boot into a 64 bit kernel at the operating system level, and allow you to run 64 bit apps.
However, there are exceptions, in that some fully 64 bit Macs still boot into 32bit mode by default. Whatever your system default, you can attempt the opposite by holding down
2 on bootup, or
4, however this was very much a short term issue for certain macs when running Snow Leopard, and is largely a non issue now on newer macs/OS.
You can see which kernel you are using in System Profiler:
- Choose About This Mac from the Apple () menu.
- Click More Info.
- Click System Report (Optional step dependant on OS level)
- Select Software in the Contents pane.
- Look for "64-bit Kernel and Extensions: Yes (or No)" under the System Software Overview heading.
And now for the final kicker, any Mac running on the above CPU spec, irregardless of if they have a 32 bit or 64 bit EFI, and irregardless of if they are then booted into 32 bit or 62 bit kernels can still run 64 bit apps on top of a 32 bit OS kernel(!), which is pretty damn cool, not to mention highly unusual for most desktop class OS, as this note from a good review of 10.6 shows when discussing the 64 bit question.:
Finally, this is worth repeating: please keep in mind that you do not need to run the 64-bit kernel in order to run 64-bit applications or install more than 4GB of RAM in your Mac. Applications run just fine in 64-bit mode on top of the 32-bit kernel, and even in earlier versions of Mac OS X it's been possible to install and take advantage of much more than 4GB of RAM.