UNIX ( [#!Troy90!#], [#!Sobell91!#]) is an operating system originating at AT&T Bell Labs in the 1970s. UNIX was one of the first operating systems written in the high level language, C, and intended to be machine hardware independent. Many versions of UNIX exist and the influence of UNIX on other operating systems is dramatic.
Versions of UNIX included virtual memory, multi-processing, symmetric multi-processor, a file system, networking, the X windowing systems, and a script-based user interface shells. Many versions of UNIX are free or near free and the source code is readily available.
UNIX is very popular in university and research environments because of the low cost, advanced features, and readily available source code. A large customer community has contributed massive number of applications, free for the asking. The UNIX distribution includes thousands of user applications.
The key architectural feature that has allowed UNIX to last for such a long time is the communication subsystem design. Everything in the communication subsystem has an index entry called the inode. Given an inode entry, an application can read and write bytes of data. The inode entry may be associated with a network, file system, a process, or a keyboard. Inodes give UNIX applications hardware I/O device independence and allows for dynamic redirection of I/O. A new I/O device is easy to install. Just create an inode, write a device driver, and almost like magic, a new I/O device is now on the system.
See tables 7.15
and 7.16 starting on page
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