The architecture of Linux can be understood at several levels, including the hardware architecture it supports, the kernel architecture, and the overall system architecture. 

Architecture

#1 Hardware Architecture

Linux supports a wide range of hardware architectures, including:

• x86 and x86-64 (Intel and AMD processors)
• ARM (commonly used in embedded systems and mobile devices)
• PowerPC
• SPARC
• MIPS
• RISC-V
• Others (like Itanium, Alpha, etc.)

#2 Kernel Architecture

At its core, Linux consists of the Linux kernel, which serves as the central component managing system resources and providing essential services. The kernel architecture includes:

1. Process Management: Linux supports multitasking and multiprocessing, managing processes through a scheduler that allocates CPU time efficiently.
   1. Process Creation and Termination.
   2. Process Scheduling.
   3. Inter-Process Communication (IPC).
2. Memory Management: The kernel handles memory allocation, virtual memory management, and memory protection mechanisms.
   1. Virtual Memory.
   2. Memory Allocation.
   3. Memory Protection.
3. File System: Linux supports various file systems, including ext4, Btrfs, XFS, and others, providing support for storage and file organization.
4. Device Drivers: Linux includes a wide range of device drivers to support different hardware components, such as network interfaces, storage devices, input/output devices, and more.
   1. Network Interfaces
   2. Storage Devices
   3. Input/Output Devices
   4. Graphics Drivers
   5. Sound Drivers
5. Networking: Linux offers extensive networking capabilities, supporting protocols such as TCP/IP, UDP, IPv4, IPv6, and providing features like routing, firewalling, and network device configuration.
   1. TCP/IP Stack
   2. Network Device Drivers
   3. Routing
   4. Firewalling
   5. Network Configuration

#3 System Architecture

Beyond the kernel, Linux systems typically include additional components to form a complete operating environment. This system architecture may include:

• Shell: Linux typically uses a shell (command-line interface) for interacting with the system. Common shells include Bash, Zsh, and Fish.
• User Space: User space consists of programs and libraries that run outside the kernel, providing higher-level functionality such as user interfaces, applications, and system utilities.
  • Libraries
    • GNU C Library (glibc)
    • Standard C Library (libc)
    • Others
  • System Utilities
    • Coreutils
    • Text Utilities (awk, sed, grep)
    • Compression Utilities (gzip, tar)
    • Others
• Init System: Historically, Linux systems used init as the first process launched by the kernel, responsible for initializing the system. More modern distributions may use alternatives such as systemd.
• Desktop Environment or Window Manager: For desktop systems, Linux often includes a desktop environment (e.g., GNOME, KDE, Xfce) or a window manager (e.g., Openbox, i3) to provide graphical user interfaces.
• Package Management: Linux distributions use package managers (e.g., apt, yum, pacman) to install, update, and manage software packages, ensuring dependencies are satisfied and software is securely distributed.
• System Libraries: Linux relies on various libraries (e.g., GNU C Library, glibc) to provide common functions and interfaces for applications.
• Utilities and Applications: Linux offers a vast ecosystem of utilities and applications for diverse purposes, including web browsing, office productivity, multimedia, development tools, and more.

Different types of Kernel

1 Monolithic Kernel:

The monolithic kernel, pioneered by Unix, is characterized by its design where all kernel services exist and execute in kernel space. These kernels manage all system resources and provide services such as process management, memory management, file system handling, and device drivers within a single monolithic binary. Examples include the Linux kernel and traditional Unix kernels. Monolithic kernels tend to be efficient and fast due to their tight integration but can suffer from stability issues since a bug in one component can affect the entire system.

2 Microkernel:

Microkernels take a minimalist approach by implementing only the most essential functionalities in kernel space, such as inter-process communication (IPC) and basic scheduling. Additional services, including device drivers, file systems, and networking protocols, are implemented as user-space processes called servers. This design promotes modularity, scalability, and fault isolation, enhancing system stability and security. Examples include the Mach kernel and GNU Hurd. Microkernels excel in environments where reliability and extensibility are paramount but may incur performance overhead due to increased communication between user-space servers.

3 Hybrid Kernel:

Hybrid kernels combine elements of both monolithic and microkernel designs, offering a compromise between performance and modularity. In hybrid kernels, critical system services such as process management and memory management remain in kernel space for efficiency, while non-essential services like device drivers and file systems operate in user space. This approach aims to strike a balance between the benefits of monolithic and microkernel architectures, providing flexibility, performance, and maintainability. Examples include the Windows NT kernel and macOS's XNU kernel.

4 Exokernel:

Exokernels push the concept of minimalism even further by exposing hardware resources directly to applications, giving them fine-grained control over system resources. Unlike traditional kernels, exokernels do not enforce higher-level abstractions such as file systems or process management, leaving these tasks to user-level libraries called library operating systems (libOSes). Exokernels prioritize performance and customization, making them suitable for specialized and resource-constrained environments. However, they require applications to handle resource management and protection, which can complicate software development. Examples include ExOS and Nemesis.

Main Subsystems of Kernel

+-------------------------------------------------------------+
|                      Linux Kernel                            |
|                                                             |
|    +------------------+       +-------------------------+   |
|    |  Process         |       |   Memory Management     |   |
|    |  Management      |       |                         |   |
|    |                  |       |                         |   |
|    |  Scheduling      |       |   Virtual Memory        |   |
|    |  Context Switch  |       |   Memory Allocation     |   |
|    |  Inter-Process   |       |   Memory Protection     |   |
|    |  Communication   |       |                         |   |
|    +------------------+       +-------------------------+   |
|                                                             |
|    +------------------+       +-------------------------+   |
|    |  File System     |       |   Device Drivers        |   |
|    |                  |       |                         |   |
|    |  Ext4, Btrfs,    |       |   Network Interfaces   |   |
|    |  XFS, etc.       |       |   Storage Devices      |   |
|    |  File I/O        |       |   Input/Output Devices |   |
|    |  Directory       |       |   Graphics Drivers     |   |
|    |  Management      |       |   Sound Drivers        |   |
|    +------------------+       +-------------------------+   |
|                                                             |
|    +------------------+       +-------------------------+   |
|    |  Networking      |       |   System Calls          |   |
|    |                  |       |                         |   |
|    |  TCP/IP Stack    |       |   Interface to User     |   |
|    |  Routing         |       |   Applications          |   |
|    |  Firewalling     |       |   Interact with Kernel  |   |
|    |  Network Device  |       |                         |   |
|    |  Configuration   |       |                         |   |
|    +------------------+       +-------------------------+   |
|                                                             |
|    +------------------+       +-------------------------+   |
|    |  Security        |       |   Virtualization        |   |
|    |                  |       |                         |   |
|    |  Access Control  |       |   Hypervisor Support    |   |
|    |  Authentication  |       |   Virtual Device        |   |
|    |  Encryption      |       |   Drivers               |   |
|    |  Integrity       |       +-------------------------+   |
|    +------------------+                                     |
+-------------------------------------------------------------+

1. Process Management:
   1. Handles the creation, scheduling, and termination of processes.
   2. Manages process states, context switching, and inter-process communication (IPC).
   3. Includes components like the scheduler, process control block (PCB), and process scheduler.
2. Memory Management:
   1. Manages system memory, including allocation, deallocation, and virtual memory.
   2. Implements mechanisms such as paging, swapping, and memory protection.
   3. Includes components like the memory manager, page allocator, and virtual memory manager.
3. File System:
   1. Provides support for file systems, enabling storage and retrieval of data on storage devices.
   2. Implements file system types such as ext4, Btrfs, XFS, and others.
   3. Includes components for file I/O, directory management, and file system drivers.
4. Device Drivers:
   1. Interfaces with hardware devices, enabling communication between software and hardware components.
   2. Includes drivers for devices such as network interfaces, storage controllers, input/output devices, and graphics cards.
   3. Consists of device drivers, bus drivers, and hardware abstraction layers (HALs).
5. Networking:
   1. Manages network interfaces, protocols, and communication between systems.
   2. Implements networking protocols such as TCP/IP, UDP, IPv4, IPv6, and others.
   3. Includes components for network stack, socket handling, and network device drivers.
6. Inter-Process Communication (IPC):
   1. Facilitates communication and data exchange between processes.
   2. Supports mechanisms like pipes, sockets, signals, and shared memory.
   3. Includes components for IPC primitives, synchronization, and message passing.
7. Kernel Security:
   1. Enforces security policies and controls access to system resources.
   2. Implements mechanisms such as access control lists (ACLs), capabilities, and mandatory access control (MAC).
   3. Includes components for user authentication, privilege management, and system integrity.
8. System Calls:
   1. Provides an interface for user-space processes to interact with the kernel.
   2. Includes a set of functions (system calls) for performing operations like file I/O, process management, and memory management.
   3. Bridge between user-space applications and kernel space.
9. Power Management:
   1. Manages system power states and optimizes power consumption.
   2. Implements features such as CPU frequency scaling, sleep states (e.g., suspend, hibernate), and device power management.
   3. Includes components for power management policies, device power states, and power management drivers.
10. Virtualization:
    1. Supports virtualization technologies, enabling multiple virtual machines (VMs) to run concurrently on a single physical machine.
    2. Implements features such as hardware virtualization support (e.g., Intel VT-x, AMD-V), hypervisor support, and virtual device drivers.
    3. Includes components for virtual machine monitor (VMM), guest kernel support, and virtualization APIs.