Linux OS - 01 - Linux Kernel

An Operating System (OS) is a collection of programs that work together as a whole to support the user’s interaction with the computer by managing hardware resources and running processes.

Operating systems typically comprise many different programs, with the heart of the OS being a single program called the kernel.

• The kernel is loaded into memory when the computer boots and remains resident in memory until the system shuts down.
• Its role is to handle process and resource management, among other tasks.
• The kernel may call upon other OS components to complete certain tasks, which are loaded and run as needed.
• Users may also call upon some of these OS components, which are also loaded and run as needed.

The Operating System Kernel

The kernel is responsible for most of the important tasks of the OS. There are three primary types of kernels:

1. Monolithic Kernel
2. Microkernel
3. Hybrid Kernel

Monolithic Kernel

A monolithic kernel is a single program that operates solely within the computer’s privileged mode and its own memory(address) space.

• Communication between the user space (running applications) and the kernel space is done via system calls.

• A system call invokes a specific portion of the OS kernel. Upon receiving a system call, the kernel switches from user mode to privileged mode.

• The kernel ensures that the user’s request is legitimate and that the appropriate access level is granted for the request to be carried.

• Early Operating Systems used a monolithic kernel, which was small due to limited memory and fewer system resources. As computing capabilities grew, operating systems became more complex, and so did their kernels.

Microkernel

In a microkernel:

• The kernel is smaller and simpler than the monolithic type.
• Communication is similar to the monolithic kernel, but the kernel relies on servers to handle various tasks.
• System calls occur between the kernel and servers, as well as between application software and servers.

Among the server components are: File system servers, Network servers, Display servers, User interface servers, Device driver communication servers

The kernel retains responsibility for: Process scheduling, Memory management (including virtual memory), Interprocess communication between OS components.

Hybrid Kernel

The hybrid kernel is a compromise between monolithic and microkernels. The kernel is kept small, but additional server-like components are added:

• Server components run in kernel mode but typically in the user’s address space.
• This allows processes to call upon the servers more easily than in the microkernel approach, bypassing some time-consuming system calls.

Linux Kernel:

• Modular Monolithic Kernel: Monolithic kernels can be broken into modules, which are loaded as needed. This modular approach allows Linux to remain lightweight based on the modules loaded.
• Linux and System Calls: In Linux, a system call isn’t directly intercepted by the kernel. Instead, it’s handled by a wrapper function, which places the arguments of the function call into appropriate hardware registers before the kernel switches from user mode to privileged mode.
• Portability: Linux is a portable OS. It is not written for any specific hardware but can run on various platforms.

Why Linux?

Advantages:

• Free: Linux is available at no cost.
• Open-source: The code is open, allowing users to enhance and modify it.
• More control over the OS: Provides greater administrative control for system management.
• Educational value: Helps users learn more about computers and operating systems.
• Community-driven: Quick fixes for major issues from the community.
• Portable: Can be used on various hardware platforms.

Disadvantages:

• No commercial support: Organizations might face challenges without guaranteed support.
• Lagging behind on features and security: Sometimes updates and fixes may be slower compared to proprietary systems.

UNIX systems have the same system calls which are described in the POSIX specification.

Features of UNIX

Multiuser System:
Multitasking Sytem:
Building block Approach : Hundreds of command which perform one simple job. Commands can be connected with pipe to manipulate data which are called as filters.
It is through Pipes UNIX implements the “small is beautiful” philosophy. UNIX tools are designed with a requirement that the output of one tool be used as input to another. That’s why the architecture of UNIX had to make commands that throw out excessive verbiage and clutter the output - which is the reason why UNIX Programs are not interactive.
If the output of ls contained headers or prompted users for a specific information, this output couldn’t have been used as useful input to any other commands.

By interconnecting various tools, there can be a large combination of usages.

Roles of OS Kernel