Foundations
What an OS is, what it's for, and the mechanisms (user/kernel mode, limited direct execution) that make it possible. Read these first.
An operating system has three jobs: virtualize hardware, manage concurrency, and persist state. Every topic in this workbook is a deep dive on one of those jobs, and the Foundations topic is the orientation — what an OS actually does, why the design choices are what they are, and the small set of hardware mechanisms (privilege levels, traps, timer interrupts) that make a modern OS possible at all.
For interview prep, internalise the three-jobs framing. It's the answer to 'why do operating systems exist?' and the structure most reasonable interviewers will let you build a discussion on.
Key concepts
- An OS virtualizes the CPU, memory, and I/O so each process gets a clean abstraction
- It manages concurrency — threads, locks, atomicity — because shared state breaks naive code
- It persists state through file systems and storage stacks that survive crashes and corruption
- Hardware support (mode bits, traps, MMU, timer) is what makes all of the above tractable
- Most design questions resolve as a trade-off between safety, performance, and complexity
Reference template
// Mental model for any OS topic
1. What's the abstraction? (what does the user see?)
2. What's the mechanism? (what does the hardware/OS do?)
3. What's the policy? (when there's a choice, what drives it?)
4. What's the overhead? (latency, memory, CPU cost)
5. What's the failure mode? (what goes wrong and how is it recovered?)Adapt to your problem; the structure is the load-bearing part.
Common pitfalls
- Confusing mechanism with policy —
pagingis a mechanism,LRUis a policy - Treating 'user space' and 'kernel space' as buzzwords rather than a privilege boundary
- Forgetting that almost every OS feature exists to virtualize something the hardware does only one of
- Skipping the history — every modern design has a predecessor whose failure motivated it
Related topics
Items (4)
- What Is an Operating System?
The three jobs of an OS — virtualize hardware, manage concurrency, persist state — and why each one matters.
Concept Foundational - OS Design Goals
Abstractions, low overhead, protection and isolation, reliability. The criteria every kernel decision is weighed against.
Concept Foundational - User Mode vs Kernel Mode
Hardware privilege levels, system calls, trap-and-emulate, and the cost of crossing the boundary.
Concept Foundational - Limited Direct Execution
How the CPU runs user code at full speed while the OS still keeps control — traps, trampolines, timer interrupts.
Concept Intermediate