Computer Operating System learning notes _ 1 _ Operating System Overview

Operating System Overview
1. Concepts, features, functions, and services of the Operating System 1. concepts of the Operating System
The operating system is the most important and basic system software in computer systems. The operating system is located between hardware and user programs.
For users: it can provide users with interfaces for using computers;
From the perspective of resource management: it can manage computer software and hardware resources and improve its utilization;
Furthermore, virtual machine technologies (such as wmware, virtualbox, and javavirtual machine) are used to expand the functions and scope of use of computers.
Therefore, the operating system is defined as: the operating system controls and manages computer software and hardware resources, A system program that provides services for different users and their applications in the most reasonable and efficient way possible.
 
2. Operating System features
The operating system has concurrency (the difference from Parallelism: parallelism is the real thing at the same time, while concurrency is the macro one at the same time, and the microscopic one is the alternative), sharing, virtualization, and uncertainty (caused by concurrency, the uncertainty of the system should be minimized.
Among them, the most important is the concurrency feature, and the other three features are on the premise of concurrency.

3. Operating System Functions
The operating system provides four main functions: process management, storage management, file management, and device management.

4. services available to the Operating System
The operating system provides a series of services for user programs and system programs. These services allow computer users to complete their work more quickly, efficiently, and simply.

1. Public Service (Command Control Interface: Command Line shell, Gui, Nui [natural user interface], script)
Provides a human-machine interaction platform.
2. System Call Service
Provides programming system services, including creation, opening, closing, reading, writing, and deletion.
[Note: The API is a combination of multiple system calls and some code, which is more powerful than the system call function]

Ii. Development and classification of Operating Systems 1. Development of Operating Systems
The development of operating systems is currently diversified:
Large computers and giant computers need to meet their cluster computing and high-performance computing needs;
Military Computers and industrial control computers hope the operating system can respond in real time;
Embedded Computers require streamlining and specific functions;
Portable devices require power saving and battery endurance.
Therefore, the operating system will develop faster and more powerful with the support of hardware as users constantly demand the system.

2. Categories of Operating Systems
1). single-user operating system
2). Batch Processing Operating System
Batch Processing Systems are divided into the following two types (online and offline Operating Systems ):
(1) single-channel Batch Processing System
(2) Multi-Channel Batch Processing System
3). time-sharing operating system (now common operating system)
4). Real-Time System (hard real-time, soft real-time)
5). Network Operating System
Features: A. More powerful resource sharing capability compared with common operating systems
B. Enhanced communication functions
6). Distributed Operating System
7). Parallel Operating System

Iii. Operating System Environment 1. kernel state and user State [the operating system did not strictly distinguish kernel state from user State at the beginning, but was proposed with concurrency]
Most systems divide the processor into kernel and user States.
Kernel State [run privileged commands] generally refers to the operating system management program running state, with a high level of privilege, also known as privileged state, system state or tube State [although a high level, however, it cannot access user content, reflecting the western philosophy of respecting private property.]
User-State [Access Control Command] generally refers to the state when the user is running, with a low level of privilege, also known as the common state and object state.
[Kernel mode and user mode: "water is not a waste of water." Hardware provides related technologies].

2. interruption and exception
Interrupt refers to the processor's response to asynchronous events in or out of the system.
An exception (also known as a trap) is a definite service process initiated by the system. It is equivalent to a Soft Interrupt. It is a piece of code called in the form of an interrupt.
The Difference and connection between interruptions and exceptions: From a general point of view, interruptions are forced and exceptions are voluntary. Interruptions are generally external and exceptions are initiated by programs, the service is interrupted in all programs. Exceptions are generally the service of programs that issue exceptions.

3. System Call
The processing process of system calls is like this. When a system call occurs, the processor uses a special mechanism, usually interrupt or exception handling, transfer the control process to some specific locations in the monitoring program. At the same time, the processor mode is changed to the privileged mode. Second, the monitoring program executes the requested functional code. This function code represents the execution of a standard program segment to complete the requested function. Third, after the processing is complete, the monitoring program restores the site before the system call; restores the running mode from the privileged mode to the user mode; and finally transfers the control to the original user program.
System calls are different from general program calls (APIS:
(1) Running in different system states. The called program runs in the user State, and the called program runs in the system state.
(2) The access method is different. The process call statement jumps directly to the called process, and the system call must run the system call command.
(3) return methods are different. Process calls are directly returned; system calls are not directly returned, and there is a rescheduling process.
(4) different code levels. Process calls are user-level programs, while system calls are system-level programs.
(5) system calls generally cannot be nested or recursive.
 
Iv. Operating System Architecture common operating system architectures include the integral structure, hierarchical structure, and microkernel (customer/Server) structure.
 
1). integral structure [no structure, such as MS-DOS]
First, determine the overall functions of the operating system, and then divide the total functions into several sub-functions. Programs that implement each sub-function are called modules. Its main advantages are: tight structure, simple and direct interfaces, and high system efficiency.
2) Hierarchical Structure
The hierarchical structure is to sort all functional modules of the operating system into several layers according to the calling order of the function flow diagram, modules between layers can only be one-way dependency or one-way call relationship. In this way, the operating system structure is clear and does not constitute a loop.
3). microkernel (Client/Server) structure [typical representative: Unix operating system]
In this mode, the kernel provides the core and key parts of all operating systems, such as thread scheduling, Processor Scheduling, and processor allocation, to form a kernel, these components usually adopt hierarchies and constitute the basic operating system. All other parts are loaded in other ways.
[Corresponding to the microkernel, with a strong kernel structure, such as Windows]
4). modular (Object-Oriented) Structure