Understanding Cocoa’s Role in Mac OS X - XNU

The core of Darwin is the XNU kernel. This is a single-server Mach microkernel with a BSD server derived largely from FreeBSD. The distinction between Mach and BSD in the XNU kernel is largely academic, since they both run in the same process. Mach in XNU fills a similar rˆole to the HAL in Windows. It is a small layer of platform-specific code that is easy to port to new architectures. It handles memory and process-related activities, but higher-level interfaces are all handled by the BSD layer. In addition, the BSD layer is responsible for providing a POSIX interface to the Mach functionality. The Mach microkernel is responsible for handling threads, processes, and interprocess communication. Everything else is implemented by the BSD layer. Some things are shared between the two. Although it is possible to interact with Mach threads directly, most of the time developers choose to use the POSIX functions.

The BSD subsystem is responsible for maintaining UNIX process structures that are implemented in terms of Mach tasks and allowing UNIX-like system calls to control the kernel. Device drivers for OPENSTEP were written using DriverKit, an Objective- C framework. It is worth noting that, on a platform with a 25MHz Motorola 68040 CPU, Objective-C was regarded as being fast enough for writing device drivers. On OS X, DriverKit was replaced with IOKit. This framework has a similar design, but is implemented in Embedded C++, a very small subset of C++ omitting a large amount of the language to make it suitable for embedded work.

Source of Information : Addison Wesley - Cocoa Programming Developers Handbook (December 2009)