BSD Overview

The BSD portion of the OS X kernel is derived primarily from FreeBSD, a version of 4.4BSD that offers advanced networking, performance, security, and compatibility features. BSD variants in general are derived (sometimes indirectly) from 4.4BSD-Lite Release 2 from the Computer Systems Research Group (CSRG) at the University of California at Berkeley. BSD provides many advanced features, including the following:

Preemptive multitasking with dynamic priority adjustment. Smooth and fair sharing of the computer between applications and users is ensured, even under the heaviest of loads.

Multiuser access. Many people can use an OS X system simultaneously for a variety of things. This means, for example, that system peripherals such as printers and disk drives are properly shared between all users on the system or the network and that individual resource limits can be placed on users or groups of users, protecting critical system resources from overuse.

Strong TCP/IP networking with support for industry standards such as SLIP, PPP, and NFS. OS X can interoperate easily with other systems as well as act as an enterprise server, providing vital functions such as NFS (remote file access) and email services, or Internet services such as HTTP, FTP, routing, and firewall (security) services.
Memory protection. Applications cannot interfere with each other. One application crashing does not affect others in any way.

Virtual memory and dynamic memory allocation. Applications with large appetites for memory are satisfied while still maintaining interactive response to users. With the virtual memory system in OS X, each application has access to its own 4 GB memory address space; this should satisfy even the most memory-hungry applications.
Support for kernel threads based on Mach threads. User-level threading packages are implemented on top of kernel threads. Each kernel thread is an independently scheduled entity. When a thread from a user process blocks in a system call, other threads from the same process can continue to execute on that or other processors. By default, a process in the conventional sense has one thread, the main thread. A user process can use the POSIX thread API to create other user threads.

SMP support. Support is included for computers with multiple CPUs.

Source code. Developers gain the greatest degree of control over the BSD programming environment because source is included.

Many of the POSIX APIs.