8.8 Kernel and User Address Space Layouts
The Mac OS X kernel has a 32-bit virtual address space whether it runs on a 32-bit or 64-bit machine. Beginning with Mac OS X 10.4, it is possible to create 64-bit user programs, although very few user-space APIs are available in 64-bit versions.
On some systems, a portion of every user address space is reserved for use by the kernel. For example, on 32-bit Windows, a user process is given the lower 2GB[14] of its 4GB virtual address space for private use. The remaining 2GB is used by the operating system. Similarly, the Linux kernel divides a 4GB user address space into two parts. The operating system uses its portion by mapping the kernel into the address space of each process, which avoids the overhead ...
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