Summary

The early advantages of UNIX were that this system was

written is a high-level language, was distributed in source

form, and had provided powerful operating-system

primitives on an inexpensive platform. This advantages led to

UNIX's popularity at educational, research and government

institution, and eventually in the commercial world. This

popularity first produce many strains of UNIX with variant

and improved facilities. Market pressures are currently

leading to the consolidation of these versions. One of the

most influential versions is 4.3BSD, developed at Berkeley for

the VAX, and later ported to many other platforms.
 
 

UNIX provides a file system with tree\endash structured

directories. Files are supported by the kernel as unstructured

sequences of bytes. Direct access and sequential access are

supported through system calls and library routines.
 
 

Files are stored as an array of fixed-size data blocks with

perhaps a trailing fragment. The data blocks are found by

pointers in the inode. Directory entries point to inodes. Disk

space is allocated from cylinder groups to minimize

movement and to improve performance.
 
 

UNIX is a multiprogrammed system. Processes can easily

create new processes with the fork system call. Processes can

communicate with pipes or, more generally, sockets. They

may be grouped into jobs that maybe controlled with signals.
 
 

Processes are represented by two structures : the process

structure and the user structure. CPU scheduling is a priority

algorithm with dynamicly computed priorities that reduces to

round-robin scheduling in the extreme case.
 
 

4.3BSD memory management is swapping supported by

paging. A pagedaemon process users a modified

second-chance page-replacement algorithm to keep enough

free frames to support executing processes.
 
 

Page and file I/O users a block buffer cache to minimize the

amount of actual I/O. Terminal device use a separate

character buffering system.
 
 

Networking support is one of the most important features in

4.3BSD. The socket concept provides the programming

mechanism to access other processes, even across a network.

Sockets provide an interface to several sets of protocols.