Understanding Kernel in OS, With Examples

Operating System Access Connection Interface Concept

Understanding Kernel in OS, With Examples

Key Points

  • A kernel is an interface between the hardware and software in a computer, helping the CPU run processes efficiently and handling tasks such as input/output operations, memory management, resource management, device management, and process scheduling.
  • Monolithic kernels handle processes entirely within the kernel space, providing high performance but making the system vulnerable to security issues and harder to maintain.
  • Microkernels delegate services to the user space, making them more reliable and modular, but can be more complex and resource-intensive.
  • Hybrid kernels combine aspects of both microkernels and monolithic kernels, balancing modularity and stability with performance, but can be more complex and vulnerable to security attacks.
  • Exokernels provide applications with direct control over resources, offering flexibility, customization, and fine control of system resources, but can be more complicated to develop and maintain.

There are a vast number of parts in a computer, and some are more familiar than others. While most of us have a basic understanding of CPUs, GPUs, and RAM, especially when it comes to upgrading our systems, the kernel is something that often flies under the radar. Although kernels aren’t as talked about as other aspects of a PC, they play a pivotal role in helping computers to function smoothly. In this article, we’re going to explore what is the kernel in OS and the common types of kernels, along with advantages and disadvantages.

What Is a Kernel?

In simple terms, you can think of a kernel as an interface between the hardware and software in a computer. The CPU is in charge of computations and executing instructions, but the kernel helps the CPU run processes efficiently. Tasks such as input/output operations, memory management (usually through paging), resource management, device management, and scheduling processes are all handled by the kernel. As such, the relationship between the CPU and the kernel is crucial for the proper functioning of the computer as a whole. The kernel is also preserved in a protected memory space so that it doesn’t interfere with user data.

Although the kernel isn’t the operating system itself, it is the major part of the OS. The kernel takes its name from a seed shell, since it works within the OS. The OS provides the environment for the kernel and other components to work in.

Example of a Kernel

To illustrate how kernels work, we’re providing a simple example in C. Take a look at the following code:

#include <stdio.h>

void kernel_main() {
    printf("Kernel initialized.n");

    printf("Performing kernel tasks...n");

    printf("Shutting down kernel.n");

int main() {

    return 0;

In a nutshell, this example initializes a kernel, performs tasks, and then terminates the kernel.

First, we include the standard input/output library and define the “kernel_main()” function with void return type.

We use the “printf()” function to print the initialization message, with “n| designating a newline. We also print the “Performing kernel tasks…” message, as well as the termination message.

To finish, we define the “main()” function, which is the program’s start point. The program calls the kernel function, which represents the core functionality of the kernel. By returning the 0 value, we verify the success of the program. You can see the output in the image. Note that we haven’t provided any specific tasks here, but the kernel would perform these tasks as instructed by the kernel method.

Example of using a kernel in C
Kernels handle input/output operations, and resource, memory, and device management.


What Types of Kernels Are There?

We’ve covered the basic definition of the kernel, so it’s time to jump into the different types of kernels you may come across.


Monolithic refers to a singular entity, which is apt for this type of kernel. A monolithic kernel is one that handles processes entirely within the kernel space. This is a protected space where the kernel has unrestricted and privileged access to hardware resources. While monolithic kernels can be less modular than the alternatives, they generally provide high performance and are simpler as they don’t have to switch between kernel and user space as much. The downside to this is that any security issues or bugs in the kernel space can disrupt the whole system. This can also make it more difficult to maintain the kernel. Examples of monolithic kernels include OpenBSD, FreeBSD, and Linux.


The objective of a microkernel is to keep the kernel space as small as possible. To this end, the kernel delegates a lot of the services to the user space, which has restricted access to system resources. By their nature, microkernels are more reliable than monolithic kernels, because they’re more modular. A single change is less likely to affect the entire system. This also makes it easier to maintain a microkernel, as well as add or delete particular services or port the operating system. However, due to the increased communication between the kernel and user spaces, microkernels can be more complex and introduce additional overheads, as well as require more resources. Examples of microkernels include MINIX, K42, L4, and QNX.


As you may expect, a hybrid kernel combines aspects of both microkernels and monolithic kernels. Some components are kept in the kernel space, while others are moved to the user space. The objective of this is to balance the modularity and stability of a microkernel with the performance of a monolithic kernel. While hybrid kernels can be more compatible with a wider range of drivers, they can have their own drawbacks. Since they incorporate elements of microkernels, hybrid kernels can be more complex than monolithic kernels. Likewise, they can be more vulnerable to security attacks and harder to maintain because they include monolithic elements. Examples include BeOS, Netware, and Windows NT.


Where microkernels delegate many services to the user space, exokernels take this a step further. Exokernels provide applications with direct control over resources. This removes unnecessary levels of abstraction and allows the applications to access hardware more efficiently. Because they have fewer layers of abstraction, exokernels have to enforce strict security policies to make sure applications don’t interfere with each other. The plus side is that exokernels have the most flexibility and opportunities for customization, and the finer control of system resources can offer more security than a typical kernel. However, exokernels can be more complicated to develop and debug than a traditional kernel, as resource allocation needs to be carefully considered. Maintenance can also be tricky since exokernels are still a relatively new technology with relatively limited support available. Examples of exokernels are ExOS and Nemesis.


If exokernels are more minimalist than microkernels, then nanokernels take this to a whole new level. The core goal of nanokernels is to provide efficiency and simplicity while using as few resources as possible. This is done by minimizing both the code required and the kernel size. Many processes are delegated to the user space, much like with microkernels. But nanokernels delegate to a greater degree, including non-essential processes. While nanokernels can theoretically provide greater modularity and efficiency than typical kernels, their functionality is limited and maintenance can be harder. They may also not be compatible with the software and hardware configurations you need. It’s also notable that nanokernel isn’t as widely accepted a term as monolithic and microkernels. It’s sometimes considered a type of microkernel. Nanokernel examples include EROS and ERIKA.

How Are Kernels Optimized?

Since kernels work together with a lot of components, there are multiple ways in which they can be optimized. The particular algorithms within the kernel can be optimized since these majorly affect overall performance. This can include reducing unnecessary operations and improving the algorithm’s efficiency. We can also optimize kernels by improving the performance of hardware and the compiler, or by optimizing I/O operations, memory management, and the overall kernel configuration. Generally, optimization requires cautious testing and aims to strike a balance between performance and stability.

Wrapping Up

In summary, the kernel is central to managing system resources and effective communication between hardware and software. While kernel development is a highly specialized and complex field, a basic understanding can give you an appreciation for how operating systems and system optimization work.

Summary Table

Type of KernelDescriptionExamples
MonolithicHandles processes entirely within the kernel space, providing high performance and simplicity.OpenBSD, FreeBSD, Linux
MicroKeeps the kernel space small, delegating many services to the user space for modularity and reliability.MINIX, K42, L4, QNX
HybridCombines aspects of microkernels and monolithic kernels, balancing modularity, stability, and performance.BeOS, Netware, Windows NT
ExoProvides applications with direct control over resources, removing unnecessary abstraction layers for flexibility and customization.ExOS, Nemesis
NanoMinimizes code and kernel size for efficiency and simplicity, delegating many processes to the user space.EROS, ERIKA

Frequently Asked Questions

What are kernels?

Kernels are the major component of the operating system that provide an interface between hardware and software. They manage resources and memory allocation, and help the CPU to perform its tasks.

What kinds of kernels are there?

There are a lot of types of kernel, but the most common are monolithic, micro, nano, and exo.

How do microkernels differ from monolithic kernels?

Whereas monolithic kernels primarily run in kernel space, microkernels offload many processes to the user space to increase modularity and reliability.

What's the difference between user space and kernel space?

Kernel space is a protected area of memory where kernels perform most of their operations. They run in privileged mode, as they have unrestricted access to resources. User space, however, has restrictions on its access, and is typically the space where applications are run.

How are kernels optimized?

Kernels can be optimized through improvements to their algorithms and compilers, as well as hardware and software improvements and overall configuration.

What languages are used to write kernels?

C is the most popular language used for kernel development due to its low-level functionality, but we can also write kernels in assembly languages or other languages, i.e. Rust.

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