What is a system call and why do we need user/kernel mode?

Deep dive

System calls enforce safety and resource control:

• User mode cannot access hardware directly.
• Kernel mode validates parameters and permissions.
• Each syscall has overhead (mode switch, checks).
• Batching or async I/O reduces syscall overhead.

Examples

File read flow:

read() -> kernel validates FD -> copy data -> return to user

Common pitfalls

• Assuming syscalls are free; they are expensive in hot paths.
• Passing unchecked user pointers in kernel code.
• Doing per-byte syscalls instead of buffering.

Interview follow-ups

• How do syscalls affect performance?
• What is the difference between trap and interrupt?
• How does mmap reduce syscall count?