Interacting with the OS

Understanding system calls and privileged operations

This question asks you to explain the mechanism by which user-space programs request actions that require operating-system-level permissions. It tests your grasp of the boundary between user mode and kernel mode, and how the operating system mediates access to protected resources.

The core concept here involves understanding why this separation exists, how the CPU enforces it, and what interface the OS exposes so that unprivileged code can safely request privileged operations. Interviewers in systems roles—including at trading firms building low-latency infrastructure—care whether you understand both the why (security, stability, isolation) and the how (the specific mechanism the kernel provides).

• User mode vs. kernel mode execution
• Hardware-enforced privilege levels
• The trap/exception mechanism
• Protected resource access and permission checks