Privilege separation keeps applications from directly performing protected operations. System calls provide controlled entry to kernel services.
Before you start
You should understand processes, threads, memory and basic file operations. Track ownership and state changes over time. For concurrent scenarios, write down at least two possible execution orders; the same instructions can behave differently when scheduling or resource availability changes.
The practical goal is to reason through this situation: Reading a file crosses a defined system-call boundary. Read the walkthrough first, then try the interview exercise before opening its answer. The important part is explaining the decision and its consequences, rather than remembering a definition alone.
Step-by-step walkthrough
Step 1: Identify protected work
File and device access crosses an operating-system-controlled boundary.
Step 2: Trace the system call
The kernel validates context and performs the permitted operation.
Step 3: Handle returned outcomes
The application receives data or an error rather than direct privileged access.
Worked scenario
Reading a file crosses a defined system-call boundary.
A file read asks the kernel to locate an open resource and obtain bytes under applicable permissions. The operation may wait for storage and later resume the application. Crossing into kernel mode does not grant the caller arbitrary kernel execution; the interface limits the requested operation and its inputs.
Common mistake
Kernel mode is not simply another application thread with unrestricted user control.
Verify the behavior
Explain permission failure and delayed I/O separately from normal data return.
Interview exercise
Explain a file read.
Answer and reasoning
Trace application request, kernel permission and filesystem handling, then the return of data or an error.
Continue learning
Compare the scenario with the Operating Systems interview questions and test your understanding with the Operating Systems MCQs. For terminology and implementation details, consult the reference material.