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OS (Lecture 1, Lecture 2, Lecture 3), OS auxiliary modules are usually…

- - - - Operating systems provide, firstly, the ease of use of the computer system, and secondly, the efficiency and reliability of its work. The first function is characteristic of the OS as an extended machine, the second — the OS as a manager of hardware resources.
      - One of the main tasks of an operating system is to hide hardware and existing programs (and their developers) under beautiful, elegant, persistent abstractions that are created instead for normal operation.
      - The task of the operating system is to provide orderly and controlled
        distribution of processors, memory and input-output devices between different programs that claim to use them.
- - - - A process is a program during its execution. Each process is associated with its address space - a list of addresses of memory cells from zero to some maximum, from where the process can read and where it writes data.
        
        A process (including a terminated one) consists of its own address space, which is usually called a memory image, and an entry in the process table, with the contents of its registers, as well as other information necessary for further recovery of the process.
      - One of the most important functions of the OS is the execution of application programs.
        
        The operating system allocates resources between processes. Such resources include processor time, memory, input-output devices, disk space in the form of files. When memory is allocated, each process is associated with its address space - a set of memory addresses to which it is allowed to access.
      - During the execution of the program code, the processor takes instructions and data from the operational (main) memory of the computer. At the same time, such memory is displayed as an array of bytes, each of which has an address.
        
        The main memory is one of the types of resources distributed between
        processes. The OS is responsible for allocating memory for the protected address space of the process and for releasing the memory after the execution of the process is completed. The amount of memory available to the process can change during execution, in this case we speak of dynamic memory allocation.
    - - A file is a set of data in a file system that is accessed by name. The term "file system" can be used for two concepts: the principle of organizing data in the form of files, and a specific set of data (usually a corresponding part of a disk) organized according to such a principle. Simultaneous support of several file systems can be implemented within the framework of the OS.
        
        File systems are considered at the logical and physical levels.
        
        The logical level defines the external presentation of the system as a collection of files (which are usually located in directories), as well as the execution of operations on files and directories (creation, removal, etc.).
        
        The physical layer defines the principles of placing file system data structures on a disk or other device.
- - - - The mechanism shows what is implemented by the component, and the policy shows how it can be used. When different components are responsible for the implementation of the mechanism and the policy (the mechanism is separated from the policy), the development of the system is simplified and its flexibility increases.
    - - 1 - the kernel is the modules that perform the main functions of the OS
        
        The kernel includes functions that solve the internal system tasks of the organization of the computing process, such as the following
        like process switching, loading/unloading, interrupt handling.
        
        The kernel functions that can be called by applications will form an application programming interface (API).
        
        The kernel is the driving force of all computing processes in a computer system, and the collapse of the kernel is equivalent to the collapse of the entire system. Therefore, OS developers pay special attention to the reliability of the kernel codes, as a result, the process of debugging them can drag on for many months.
      - 2 - modules providing auxiliary functions of the OS.
    - - The multi-level kernel is another development direction of the monolithic kernel architecture. The main idea is to organize the OS as a hierarchy of layers.
    - - Nanokernel is the architecture of the OS kernel, in the framework of which the extremely simplified kernel performs only one task - processing of hardware interrupts generated by computer devices. After processing interrupts from the hardware, the nanokernel, in turn, sends information about the processing results to the upstream software using the same interrupt mechanism.