Operating System Concepts by Silberschatz and Galvin, how much down the edition timeline can I go to safely understand the core-concept I am student from CS background and I have Operating Systems in my upcoming semester. A simple search around the internet revealed that that Operating System Concepts by  Silberschatz and Galvin is one of the best ones to follow.
Now the above text is probably in its 10th edition currently. Now I won't be able to afford a physical copy of the latest edition, so I was looking around for few cheap used copies and found abundance of 5th edition and few 6th edition texts.
Are these two older editions still recommended? If I only want to build my basics and I have no prior knowledge of the above subject. Any help shall be greatly appreciated...
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$\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad$6th edition
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$\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad$5th edition
I don't know this book in particular, but I do know Computer Science text books in general, and offer this advice:
Consider the title: Operating System Concepts. It is about basic and fundamental concepts that underlie operating systems. The basic concepts, the core material of the text, are not going to change much from first edition to the tenth.
You are not being asked to study the text in such detail that you would be able to discover the differences between versions.
Many computer science books do get out of date. If the book was a user manual for Windows or a iMac then having an up to date one might be important, but I suspect not in this case.