From: manish (manish_mittal_at_hotmail.com)
Date: Tue Jan 06 2004 - 18:28:04 PST
This paper authored by Ritchie and Thompson is a high level introduction to the Unix Operating System, that ran on the DEC PDP-11/40 and /45 systems. The paper describes the Unix's hierarchical file system, process management and synchronization, the shell and other operating system internals in greater detail.
Principal features presented in the paper are as below-
Hierarchical file system- The explanation of mountable file system, linking of files, directories, special files and protection strategy is very concise.
Compatible file, device, and inter-process I/O- This is a big advantage since it allows programmers to access devices and other processes with the same interface as regular files. Due to this the tedious work of device access is pushed onto the operating system, freeing the programmer from having to worry about the numerous devices his/her program would use. Special file & I/O calls are very good examples of this.
Ability to initiate asynchronous processes- This feature enabled a multiprogramming environment and also improved performance.
System command language - This allows the user to communicate with the operating system.
Many of the design decisions about the file system can still be seen in modern UNIX's, such as protection bits, i-nodes and i-numbers, the directory layout and implementation, hard linking, I/O calls etc. The shell described here has similar semantics to the current sh. Additionally, it is inspired by MULTICS. The shell supports Filters, I/O redirection, background processes (using the \& operator), init and basic control structures. The paper describes in greater detail the implementation of File Systems and the shell. The most important characteristics of this system are its simplicity, elegance and ease of use. The Reliability results provided in the article are interesting. 98% up time for the system for the entire year is very impressive.
The system described in this paper has a few shortcomings too:
Although the system supports inter process communications using the Pipe, it doesn't support inter process communication between unrelated processes. Pipe is presumably only useful between the parent and child process.
The author mentions that the system has sufficient internal interlocks to maintain the logical consistency of the file system when two users are writing on the same file, but there is no explanation supporting this. This problem seems to exist even in the newer versions of Unix.
Overall, the paper is very useful in understanding various features of UNIX. Ideas are presented very clearly with supporting analysis. It helps to understand the Unix operating system. The article clearly suggests that the system was designed for the programmers to write/run programs easily.
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