What is the kernel?

Learn the kernel concept and user/kernel mode.

What is the kernel?

The kernel is the heart of the operating system; it manages hardware and serves processes. C programs run in user mode and cannot touch hardware directly.

When needed they switch to kernel mode. The switch happens via a system call (syscall). The kernel validates the request and, if safe, performs it. This separation keeps a buggy program from crashing the whole system. The POSIX kernel interface is declared in <unistd.h>.

Mode switch and trap

The switch from user mode to kernel mode happens through a trap instruction. When a process invokes syscall, the CPU raises its privilege level and jumps into kernel code.

After the kernel finishes, control returns to the same instruction. The switch is costly; user-space libraries such as libc cache or batch calls. On modern x86 CPUs the SYSCALL and SYSRET instructions speed up this transition. C programmers usually call wrappers like read and write; direct syscall is rarely needed.

Exercise Print kernel modes

Define an enum cpu_mode with USER_MODE = 0, KERNEL_MODE = 1. Use printf to print each mode on its own line as "USER_MODE=0" and "KERNEL_MODE=1".

Show example solution

#include <stdio.h>

enum cpu_mode { USER_MODE = 0, KERNEL_MODE = 1 };

int main(void) {
    printf("USER_MODE=%d\n", USER_MODE);
    printf("KERNEL_MODE=%d\n", KERNEL_MODE);
    return 0;
}

Test yourself

In which mode can hardware be accessed directly?
- Kernel mode
- User mode
- Fault mode
- Idle mode
Show answer

Kernel mode — Kernel mode can run hardware instructions directly.

Complete the cpu_mode enum

enum cpu_mode { ____ = 1 };

Show answer

enum cpu_mode { KERNEL_MODE = 1 };

Match kernel concepts

user_mode

Application mode
kernel_mode

Kernel mode
trap

Mode switch instruction
syscall

Kernel service request

Next lesson: Syscall

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