From: ahemavathy (ahemavathy_at_hotmail.com)
Date: Tue Jan 06 2004 - 22:31:35 PST
The paper gives a brief description of the UNIX operating system and its major features. For a person familiar with UNIX this must have been an easy read. Personally I've not had much exposure to UNIX and so I found the paper enlightening although I couldn't relate to all the features described.
The paper written in 1974 says that the important characteristics of the system are its simplicity, elegance and ease of use. The first version was written by Thompson as he was dissatisfied with the available computer facilities and wanted a more hospitable environment. That left me wondering how those "computer facilities" were ...as a Windows user even Unix is hostile to me. Perhaps those were the very first attempts to build an interactive system.
The paper starts with the description of the hardware and software environment. The OS was running on a PDP - 11/45 and was written in C. It occupies just 42K bytes and a minimal system capable of running it requires as little as 50K bytes of core. Then the author walks through the file system. He discusses the various types of files and how to access the files. Every file has an i-number which is used as an index in a system table (i-list) to find the i-node which contains important descriptions of the file.The system has a bufferring mechanism which reduces greatly the no. of I/O operations. This sounded similar to the cache-memory transfer such as current write -back mechanisms.
The next half of the paper is about the command line interpretor Shell and commonly used commands. Soon after initialization the Shell process is started. All commands that follow are analyzed by Shell and it starts (forks) appropriate child processes. Shell itself is a child process started by 'init'.
I felt there is casualness in the author's tone when describing system "crashes". One crash a day seems to be an accepted issue, attributed to inexplicable processor interrupts to random locations !
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