Free versions are often old editions or poorly scanned copies with missing pages or illegible diagrams. Better Ways to Access the Material
If you are studying from this book, focuses on these core topics as per the 3rd Edition content outline: Computer System Overview
I recently accessed the PDF version of "Operating System" by P. Balakrishna Prasad, and here's my review:
OS requires solving scheduling algorithms and memory management calculations. Prasad’s book is renowned for its step-by-step solved numericals, which is a goldmine for students preparing for competitive exams like GATE or university practicals.