Detailed explanation of Linux architecture and kernel structure

Linux Architecture

As shown in the figure below, the Linux architecture can be broadly divided into user space (User Space) and kernel space (Kernel Space).

User space includes C libraries and user applications. Some architecture diagrams also include shells. Of course, shell scripts are also an integral part of the Linux system.

The kernel space includes hardware platform, platform dependent code, kernel and system call interface.

In any modern operating system, there are layers. Why is layering necessary? From a programmer's perspective, separating the Linux underlying and applications allows each to focus on its own area, thereby improving efficiency. From a security perspective, layering is about protecting the kernel. Modern CPUs usually implement different working modes. For example, ARM implements 7 working modes. In different modes, the CPU can execute different instructions or access different registers. If all upper-layer applications can call registers at will, the operating system cannot execute stably. Therefore, a new discipline has emerged in operating systems - "modern operating systems".

In the X86 architecture, 4 different levels of permissions are implemented, namely Ring0 to Ring3. Under Ring0 permissions, privileged instructions can be executed and IO devices can be accessed, while under Ring3 permissions there are many restrictions.

Android is even more "crazy". All APK applications run on the Java virtual machine, and the applications are further away from the bottom layer.

In addition, user space and kernel space are two different states of program execution. We can complete the transfer from user space to kernel space through "system call" and "hardware interrupt".

Linux kernel structure

In this section, we analyze the kernel structure.

As shown in the figure below, it is the Linux kernel structure diagram.

SCI layer (System Call Interface), this layer provides a set of standard system call functions for application user space to access Linux. When analyzing the Linux architecture earlier, we introduced that any modern operating system will not allow upper-layer applications to directly access the lower layer. In Linux, the kernel provides a set of standard interfaces through which upper-layer applications can access. Ground floor.

PM (Procees Management), this part includes specifically creating processes (fork, exec), stopping processes (kill, exit), and controlling communication between them (signal, etc.). Also included is process scheduling, which controls how active processes share the CPU. This part is already done by Linux. When writing the driver, you only need to call the corresponding functions to realize these functions, such as creating processes, process communication, etc.

MM (Memory Management), the main function of memory management is to control the safe shared memory area of ​​multiple processes.

VFS (Virtual File Systems), a virtual file system, hides the specific details of various file systems and provides a unified interface for file operations. In Linux, "everything is a file", and these files are implemented through VFS. Linux provides a large general model that encompasses the set of all file system functions. As shown in the figure below, it is the structure diagram of a virtual file system.

Device Drivers device driver, this part is what needs to be learned and mastered. There is a large amount of code in the Linux kernel in the device driver section, which is used to control specific hardware devices.

Linux drivers are generally divided into network devices, block devices, character devices, and miscellaneous devices. We only need to write character devices. Miscellaneous devices are a type of driver that are not easy to classify. There are many overlaps between miscellaneous devices and character devices. .

Network protocol stack, the Linux kernel provides a wealth of network protocol implementations.

Before learning Linux drivers, you need to understand and master the Linux architecture and Linux kernel structure

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