Advanced Search

Log In | Not a Member? Contact ADC

Architecture

The foundation layer of Darwin and Mac OS X is composed of several architectural components, as shown in Figure 3-3. Taken together, these components form the kernel environment.

Figure 3-3  Mac OS X kernel architecture

“A kernel, in traditional operating-system terminology, is a small nucleus of software that provides only the minimal facilities necessary for implementing additional operating-system services.” — from The Design and Implementation of the 4.4 BSD Operating System, McKusick, Bostic, Karels, and Quarterman, 1996.

Similarly, in traditional Mach-based operating systems, the kernel refers to the Mach microkernel and ignores additional low-level code without which Mach does very little.

In Mac OS X, however, the kernel environment contains much more than the Mach kernel itself. The Mac OS X kernel environment includes the Mach kernel, BSD, the I/O Kit, file systems, and networking components. These are often referred to collectively as the kernel. Each of these components is described briefly in the following sections. For further details, refer to the specific component chapters or to the reference material listed in the bibliography.

Because Mac OS X contains three basic components (Mach, BSD, and the I/O Kit), there are also frequently as many as three APIs for certain key operations. In general, the API chosen should match the part of the kernel where it is being used, which in turn is dictated by what your code is attempting to do. The remainder of this chapter describes Mach, BSD, and the I/O Kit and outlines the functionality that is provided by those components.

In this section:

Mach
BSD
I/O Kit

Mach

Mach manages processor resources such as CPU usage and memory, handles scheduling, provides memory protection, and provides a messaging-centered infrastructure to the rest of the operating-system layers. The Mach component provides

• untyped interprocess communication (IPC)

• remote procedure calls (RPC)

• scheduler support for symmetric multiprocessing (SMP)

• support for real-time services

• virtual memory support

• support for pagers

• modular architecture

General information about Mach may be found in the chapter “Mach Overview”. Information about scheduling can be found in the chapter “Mach Scheduling and Thread Interfaces”. Information about the VM system can be found in “Memory and Virtual Memory”.

BSD

Above the Mach layer, the BSD layer provides “OS personality” APIs and services. The BSD layer is based on the BSD kernel, primarily FreeBSD. The BSD component provides

• file systems

• networking (except for the hardware device level)

• UNIX security model

• syscall support

• the BSD process model, including process IDs and signals

• FreeBSD kernel APIs

• many of the POSIX APIs

• kernel support for pthreads (POSIX threads)

The BSD component is described in more detail in the chapter “BSD Overview”.

Networking

Mac OS X networking takes advantage of BSD’s advanced networking capabilities to provide support for modern features, such as Network Address Translation (NAT) and firewalls. The networking component provides

• 4.4BSD TCP/IP stack and socket APIs

• support for both IP and DDP (AppleTalk transport)

• multihoming

• routing

• multicast support

• server tuning

• packet filtering

• Mac OS Classic support (through filters)

More information about networking may be found in the chapter “Network Architecture”.

File Systems

Mac OS X provides support for numerous types of file systems, including HFS, HFS+, UFS, NFS, ISO 9660, and others. The default file-system type is HFS+; Mac OS X boots (and “roots”) from HFS+, UFS, ISO, NFS, and UDF. Advanced features of Mac OS X file systems include an enhanced Virtual File System (VFS) design. VFS provides for a layered architecture (file systems are stackable). The file system component provides

• UTF-8 (Unicode) support

• increased performance over previous versions of Mac OS.

More information may be found in the chapter “File Systems Overview”.

I/O Kit

The I/O Kit provides a framework for simplified driver development, supporting many categories of devices.The I/O Kit features an object-oriented I/O architecture implemented in a restricted subset of C++. The I/O Kit framework is both modular and extensible. The I/O Kit component provides

• true plug and play

• dynamic device management

• dynamic (“on-demand”) loading of drivers

• power management for desktop systems as well as portables

• multiprocessor capabilities

The I/O Kit is described in greater detail in the chapter “I/O Kit Overview”.

Did this document help you? Yes: Tell us what works for you. It’s good, but: Report typos, inaccuracies, and so forth. It wasn’t helpful: Tell us what would have helped.

	Get information on Apple products. Visit the Apple Store online or at retail locations. 1-800-MY-APPLE Copyright © 2007 Apple Inc. All rights reserved. | Terms of use | Privacy Notice