32 bit vs. 64 bit

If you have a Core 2 Duo or later model then the hardware supports running 64-bit software with Snow Leopard. However, if your model does not have an EFI64-bit boot ROM, then it cannot run the 64-bit kernel of Snow Leopard. That means it will not support more than 4 GBs of RAM and some 64-bit software will run slower.

Leopard is not a 64-bit OS, but Snow Leopard is.

Your 2007 MacBook has a Core 2 Duo intel CPU. It will run 64-bit applications.

4GB of physical RAM is NOT an issue for you, as your MacBook's Southbridge CPU to memory interface chip is lucky if it can handle 3GB of RAM (I think 2GB is the official max).

Your signature says you have Leopard (10.5) installed. If you have a 64-bit application, and it says it runs on Leopard, then it can run as a 64-bit app. You can use Applications -> Utilities -> Activity Monitor -> Kind to see if it is running as a 64-bit application (it will say 64bit if it is running as a 64-bit app).

Your 64-bit app can use more (lots more) than 4GB of "Virtual" memory (via paging and swapping to disk), but since your MacBook cannot hold 4GB of physical memory your app may not see much benefit from running a 64-bit app on your MacBook.

If you upgrade your MacBook to Snow Leopard (10.6), and look at Activity Monitor, you will see that many of the Mac OS X provided applications and daemons run as 64-bit applications (most of them do not need lots of memory, they just run as 64-bit apps because they can).

If you get a newer Mac which supports stuffing more than 4GB of physical memory inside, then any application which can run as a 64-bit app will be able to consume more than 4GB of physical RAM during its operation (assuming the app cares to use that much memory). The current MacBook Pros, the iMacs, Mac minis, and Mac Pros all support 8GB or more of memory. The MacBook line is limited to 4GB physical RAM in the current generation.

As for booting the Mac OS X kernel in 64-bit mode, most consumers do not need this feature. It is useful to developers writing new 64-bit drivers, customers that MUST install and run a specific 64-bit driver (very rare), or customers that need to squeeze every last ounce of performance out of their Mac and tests show the application they are using will do that.

The downside to booting the Mac OS X 64-bit kernel, is that not all developers have updated their drivers for 64-bits. If you depend on a specific 3rd party driver that has not been updated, then booting the Mac OS X 64-bit kernel will keep that driver from loading.

Kappy said "If you have a Core 2 Duo or later model"
So presumably my earlier Core Duo model is not capable of 64 bit mode?

Kappy said "If you have a Core 2 Duo or later model"
So presumably my earlier Core Duo model is not capable of 64 bit mode?

Unless you can put 64GB of RAM into your Core Duo Mac, it is unlikely you will miss being able to boot a 64-bit kernel.

The latest Xserve and Mac Pros are the first Macs that need to boot a 64-bit kernel in order to manage 64GB of RAM. All other Macs can manage their maximum memory with a 32-bit kernel.

Assuming RAM densities double every 18 months to 2 years, then in the default 4GB Mac might start to have a default of 64GB in 6 to 8 years. That is when those Macs will need that 64-bit kernel.

The goal of Snow Leopard was to give developers time to update and test their 64-bit drivers, and to have something for those high end Xserve and Mac Pros.

Your 32-bit Core Duo will most likely continue to work just fine until you decide you want to get a new Mac.

Thanks. However, the reason I asked is because I saw an application from realmacsoftware called Courier; it states that it requires Snow Leopard and a 64-bit Mac so it would appear that I cannot run the program. Based on your reply, it seems strange that they'd require 64-bits so soon. Hopefully not too many programs will go down that route, because you're right, my iMac is working fine and I expect it to for years to come.

Thanks. However, the reason I asked is because I saw an application from realmacsoftware called Courier; it states that it requires Snow Leopard and a 64-bit Mac so it would appear that I cannot run the program.

I took a peak, and I'm guessing that since this application looks like they put everything in there, "Including" the kitchen sink, they needed the extra address space (or think they need it).

Based on your reply, it seems strange that they'd require 64-bits so soon. Hopefully not too many programs will go down that route, because you're right, my iMac is working fine and I expect it to for years to come.

I agree. And Apple does make it easy to build and package apps with PowerPC, 32-bit intel, and 64-bit intel.