Apple Developer

With Snow Leopard, Apple completes the Macs transition to 64-bit computing. Mac OS X ships as a single version for both 32-bit and 64-bit computers, with bundled applications running as 64-bit processes on capable hardware. This enhances performance and security, while adding the ability to easily manage massive amounts of memory.

Now a 64-Bit Kernel

Snow Leopard also makes available a 64-bit kernel. The larger address space enables much faster system calls, which dramatically boosts the performance of network services and other I/O-intensive applications important to professional customers.

Making Applications 64-Bit

Although Mac OS X is already 64-bit capable in many ways, Snow Leopard takes the next big step by rewriting nearly all system applications in 64-bit code and by enabling the Mac to address massive amounts of memory. 64-bit computing shatters the 4 GB barrier of 32-bit computing by enabling applications to address a theoretical 16 billion gigabytes of memory, or 16 exabytes. It can also enable Macs to crunch twice the data per clock cycle, which can dramatically speed up numeric calculations and other tasks.

Additional Security Benefits

Another benefit of the 64-bit applications in Snow Leopard is that they’re even more secure from hackers and malware than the 32-bit versions. That's because 64-bit applications can use more advanced security techniques to fend off malicious code. First, 64-bit applications can keep their data out of harm's way thanks to a more secure function argument-passing mechanism and the use of hardware-based execute disable for heap memory. In addition, memory on the system heap is marked using strengthened checksums, helping to prevent attacks that rely on corrupting memory.

To ensure simplicity and flexibility, Mac OS X still comes in one version that runs both 64-bit and 32-bit applications. So you don’t need to update everything on your system just to run a single 64-bit program.