Operating Systems: Three Easy Pieces
A widely used operating systems textbook from the University of Wisconsin-Madison, free to read chapter by chapter online. It is organised around three ideas, virtualisation, concurrency and persistence, and written in a friendly, question-driven style with homework simulators and projects. It is the best free way to understand processes, scheduling, virtual memory, locks and file systems.
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GitHub (ostep-projects) · Sept 2026
Book
Other
Remzi and Andrea Arpaci-Dusseau
Self paced
Intermediate
Free
What it is
Operating Systems: Three Easy Pieces, usually called OSTEP, is a textbook by Remzi H. Arpaci-Dusseau and Andrea C. Arpaci-Dusseau of the University of Wisconsin-Madison. The current version listed on the official page is 1.10. The authors state that the book is and will always be free in PDF form, and every chapter can be read individually on the site.
The three pieces are virtualisation (processes, CPU scheduling, address spaces, paging and swapping), concurrency (threads, locks, condition variables, semaphores and common bugs) and persistence (I/O devices, disks, file systems, journaling and distributed file systems). Security chapters were contributed by Peter Reiher of UCLA.
Who it is for
It is written for undergraduates taking a first operating systems course, and it works just as well for self-learners. You should be comfortable with basic C and have some idea of how memory and the CPU work; the "How a computer runs a program" stage of a CS roadmap is enough preparation.
Why it is worth your time
Many operating systems textbooks are long and dry. OSTEP is short per chapter and built around a "crux" question that each chapter answers, such as how the OS can virtualise memory efficiently. It explains why designs exist, not just what they are, which is what exam questions and real debugging both need. The homework uses small simulators so you can predict and check scheduling or paging behaviour, and the public projects repository has programming projects, including a Unix shell and work on the xv6 teaching kernel.
Free or paid
Reading online is free. The official page lists paid options for those who want them: a hardcover through Lulu, a softcover on Amazon and a downloadable PDF, with prices shown on the page (as of 2026-09, around $40 for the hardcover, $28 for the softcover and $10 for the PDF). The authors also accept donations.
How to use it
Read the chapters that match your course week by week rather than all at once. After each chapter, do the simulator homework before looking anything up, and try at least one project, such as the shell. Keep the book as your main reference and use lecture slides only as a summary.
Common questions
Is it enough for my university course? For most first courses it covers the syllabus and more. Check your course outline for any topic, such as a specific scheduling algorithm, and read that chapter closely.
Do I need Linux? For the projects, yes, or WSL on Windows. For reading and the simulators, any machine with Python works.
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