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Kernel Architecture Overview

Mac OS X provides many benefits to the Macintosh user and developer communities. These benefits include improved reliability and performance, enhanced networking features, an object-based system programming interface, and increased support for industry standards.

In creating Mac OS X, Apple has completely re-engineered the Mac OS core operating system. Forming the foundation of Mac OS X is the kernel. Figure 3-1 illustrates the Mac OS X architecture.

Figure 3-1  Mac OS X architecture

The kernel provides many enhancements for Mac OS X. These include preemption, memory protection, enhanced performance, improved networking facilities, support for both Macintosh (Extended and Standard) and non-Macintosh (UFS, ISO 9660, and so on) file systems, object-oriented APIs, and more. Two of these features, preemption and memory protection, lead to a more robust environment.

In Mac OS 9, applications cooperate to share processor time. Similarly, all applications share the memory of the computer among them. Mac OS 9 is a cooperative multitasking environment. The responsiveness of all processes is compromised if even a single application doesn’t cooperate. On the other hand, real-time applications such as multimedia need to be assured of predictable, time-critical, behavior.

In contrast, Mac OS X is a preemptive multitasking environment. In Mac OS X, the kernel provides enforcement of cooperation, scheduling processes to share time (preemption). This supports real-time behavior in applications that require it.

In Mac OS X, processes do not normally share memory. Instead, the kernel assigns each process its own address space, controlling access to these address spaces. This control ensures that no application can inadvertently access or modify another application’s memory (protection). Size is not an issue; with the virtual memory system included in Mac OS X, each application has access to its own 4 GB address space.

Viewed together, all applications are said to run in user space, but this does not imply that they share memory. User space is simply a term for the combined address spaces of all user-level applications. The kernel itself has its own address space, called kernel space. In Mac OS X, no application can directly modify the memory of the system software (the kernel).

Although user processes do not share memory by default as in Mac OS 9, communication (and even memory sharing) between applications is still possible. For example, the kernel offers a rich set of primitives to permit some sharing of information among processes. These primitives include shared libraries, frameworks, and POSIX shared memory. Mach messaging provides another approach, handing memory from one process to another. Unlike Mac OS 9, however, memory sharing cannot occur without explicit action by the programmer.

Contents:

Darwin
Architecture
Kernel Extensions

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