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The Kernel

The core of any operating system is its kernel. Though Mac OS X shares much of its underlying architecture with BSD, the Mac OS X kernel, known as XNU, differs significantly. XNU is based on the Mach microkernel design, but it also incorporates BSD features. It is not technically a microkernel implementation, but still has many of the benefits of a microkernel, such as Mach interprocess communication mechanisms and a relatively clean API separation between various parts of the kernel.

Why is it designed like this? With pure Mach, the core parts of the operating system are user space processes. This gives you flexibility, but also slows things down because of message passing performance between Mach and the layers built on top of it. To regain that performance, BSD functionality has been incorporated in the kernel alongside Mach. The result is that the kernel combines the strengths of Mach with the strengths of BSD.

How does this relate to the actual tasks the kernel must accomplish? Figure 9-1 illustrates how the kernel’s different personalities are manifested.

The Mach aspects of the kernel handle

• Memory management

• Mach messaging and Mach inter process communication (IPC)

• Device drivers

The BSD components

• Manage users and permissions

• Contain the networking stack

• Provide a virtual file system

• Maintain the POSIX compatibility layer

See Inside Kernel Programming Guide for more information on why you would (or wouldn’t) want to program in the kernel space, including a discussion on the kernel extension (KEXT) mechanism.

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