If you're wondering why there's all this talk about bits, especially with the upcoming introduction of Mac OS X 10.6, aka Snow Leopard, you're not alone. Everyone seems to be tossing around the term 64-bit, and it can get confusing. The terms "64-bit processor" and "64-bit operating system" get tossed around quite a bit, but the exact definition isn't exactly clear. Let's try to tear apart this mess.
The processor is the brain of your computer, and best deals with information in chunks which are, as you may have guessed, a certain number of bits in length. Let's start with a simple example, the 6502 in the first Apple ][ computers. This processor had 8-bit registers, and had a 16 bit memory address space.
This means registers could represent 256 (2^8) values, and there are 64K (2^16) memory locations, each of which holds an 8-bit byte. Fast forward to the latest Core 2 Duo processors, whose registers can represent 2^64 values, and can address up to 2^64 bytes of (virtual) memory.
An operating system can take advantage of the range of values offered by modern 64-bit processors, but it isn't quite as simple as just creating a new version of the OS after turning on the 64-bit switch. Advances in processor technology take time to be properly integrated into both existing and future systems. Therefore, not all parts of an operating system may use the latest x-bit technologies.
Apple, like others, has gradually introduced portions of 64-bit functionality into their most recent operating systems.
Tiger Developer Overview states that:
"Tiger obliterates that restriction and allows applications to access a 64-bit address space when running on the PowerPC G5. Better yet, this support comes with no compromise in the ability to run current 32-bit applications." Specifically, "Mac OS X Tiger breaks the limitations of 32-bit computing and allows developers to create command-line applications, servers, and computation engines that can work with mind-blowing amounts of memory." Applications in 32-bit operating systems can normally access 4GB of memory (not necessarily RAM) at once.
What this means is that Tiger would allow you to jump into the 64-bit world with applications that didn't directly interact with the user. You could certainly write a program to perform operations in 64-bit mode, including access to a crazy 4TB of physical and 16 exabytes of virtual memory, but they'd still have to deal with the rest of the operating system that was still 32-bit. Fortunately, with enough work, operating systems and processors can support software running in both 32-bit and 64-bit modes.
Leopard introduced additional 64-bit support, such as the Cocoa framework, which is the modern foundation for Mac applications. The Leopard Technology Overview also suggests that you can gain increased performance from transitioning your applications to 64-bits, by using 64-bit registers available in the latest processors such as the Intel Core 2 Duo (not Core Duo, which is a 32-bit processor).
Snow Leopard (10.6)
Snow Leopard promises even more 64-bit support. Nearly all system applications — including the Finder, Mail, Safari, iCal, and iChat — are now built with 64-bit code. Apple also states that all system applications except DVD Player, Front Row, Grapher, and iTunes have been rewritten in 64-bit. Snow Leopard also introduces new 64-bit KEXTs (Kernel Extensions) 64-Bit Drivers and a 64-Bit Kernel. This could introduce some upgrade issues, since software developers need to recompile their kernel extensions and drivers.
Developers may also run into other issues. For example, MenuMeters 1.3 does not run on Snow Leopard because the process that hosts the MenuMeters plugins (SystemUIServer) is now 64-bit. There are a number of subtle 64-bit bugs in MenuMeters, so more than a simple recompile is needed (an update is coming from the developer). iStat Menus isn't 64-bit compatible, either, but a new Snow Leopard version is in development.
What It All Means
Upgrading the deepest levels of OS X to provide 64-bit support is the final step is making Snow Leopard what many can call a true 64-bit operating system.
To summarize, the benefits of a 64-bit operating system are twofold. The first is that you can more effectively deal with large files, such as images or movies, since the 4GB RAM limitation of earlier 32-bit operating system components is no longer a concern. The second is that, if an application takes advantage of 64-bit registers in the latest processors, you can realize a significant gain in performance.