Berkeley Software Distribution

Berkeley Software Distribution (BSD) is the name of the UNIX dialects
distributed already in the 1970s from the University of California, Berkeley.

In its infancy AT&T Bell Laboratories permitted Berkeley and other
universities to share the source code to their UNIX operating system.
Berkeley used their software as a research base for a variety of
investigations into operating system design throughout the 1970s and 1980s.
Eventually the sum total of the systems that Berkeley students had developed
from scratch for their research had replaced essentially every component of
the original UNIX kernel, and in the early 1990s the full Berkeley source
code was released publicly with a very generous license called the BSD
License.

BSD pioneered many of the advances of modern computing. Berkeley's Unix was
the first to include library support for the Internet protocol stacks,
Berkeley sockets. By integrating sockets with the UNIX operating system file
descriptors, users of their library found it almost as easy to read and
write data across the network, as it was to put data on a disk. The AT&T
laboratory eventually released their own STREAMS library which incorporated
much of the same functionality in a software stack with better architectural
layers, but the already widely distributed sockets library, together with
the unfortunate omission of a function call for polling a set of open
sockets (an equivalent of the select call in the Berkeley library), made it
difficult to justify porting applications to the new API.

Like AT&T Unix, the BSD kernel is a monolithic kernel, meaning that device
drivers in the kernel run in ring 0, the core of the operating system. Early
versions of BSD were used to form Sun Microsystems' SunOS, founding the
first wave of popular Unix workstations.

Other versions of UNIX that descend from BSD include FreeBSD, NetBSD,
OpenBSD, 386BSD, DragonFlyBSD, and Apple's Darwin (and, hence, Mac OS X).