Operating System Structure

1) Single structure: 

Different functions are classified into different function blocks. Each function block is relatively independent. 

They are connected in a certain way. Different function blocks can call each other's services. In this way, the entire operating system is like a huge single body, running in the kernel state of the system to provide services for users.

            Due to the complex structure between the functional blocks of a single-structure system, modifying a certain functional block will result in modification of other functional blocks, resulting in difficulty in designing and developing the operating system.

                Example: MS-DOS operating system.

  2) Hierarchical structure: 

The operating system is divided into several layers, the low-level functions provide services for the functions of the previous level, and the high-level layers provide services for the higher-level functions.

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        All functions of the operating system run in kernel mode, and the conversion from user mode to kernel mode requires a certain time cost, which causes the operating system efficiency. 

In addition, kernel-run programs can access all system resources, and as operating systems grow larger, designing reliable and secure operating systems becomes extremely difficult.

   3) Microkernel: 

The goal is to build a minimal kernel based on the messaging mechanism, in order to build a simulation layer for other operating systems to simulate the characteristics of other operating systems.

            The advantage of the microkernel is that it greatly improves the compatibility of the operating system, enabling the microkernel-based operating system to be able to minimize the features of other operating systems, thereby supporting many applications running on other operating systems.

            Another advantage of the microkernel is that it increases the scalability of the system. One goal of the microkernel design is that the kernel only needs to provide the functions absolutely necessary for the operating system, while leaving other functions belonging to the traditional operating system kernel to the user state process.

                Example: Unix operating system.

4) Modular approach: 

 Create a modular kernel using object-oriented technology.

            An operating system with a modular structure consists of a collection of structural modules of different functions, each implementing a specific function, such as scheduling, file system, device driver, etc., and these modules can be automatically loaded and unloaded as needed. Essentially, a module is an object file that the kernel can link or break at runtime.

                Example: Linux operating system.