Computer Operating System Study Notes (1) --- OPERATING SYSTEM INTRODUCTION

I. The role of the operating system is as follows:
1. OS serves as an interface between the user and the Computer Hardware System

2. OS as the manager of computer system resources

Computer system resources can be divided into four categories:

(1) Processors

(2) Storage

(3) I/O devices

(4) information (data and procedures)

The main function of the OS is to effectively manage these four types of resources, corresponding:

(1) Process Management

(2) Memory Management

(3) device management

(4) File Management

3. OS used as an extended Machine
Generally, software-covered machines are called extended machines or virtual machines. When bare metal machines are covered with several levels of OS, a multi-layer extended machine or multi-layer Virtual Machine with significantly enhanced functionality can be obtained.

2. There are four types of operating systems:
1. Offline (Off-Line) I/O manual operations
Both program and data input and output are completed under the control of the peripheral machine. They are performed without the host, so they are called offline input/output methods. Otherwise, the input/output method under the direct control of the host is called online (On-Line) input/output. In fact, the disk plays a buffer role here.

2. Single-channel Batch Processing System
Input a batch of jobs offline to the tape (Disk). The computer system automatically processes the jobs in sequence until all the jobs on the tape (Disk) are completed. Its features are automatic, ordered, and single-track.
3. Multi-Channel Batch Processing System
A job generally uses (CPU) and (memory and I/O devices), but can only use (CPU) and (memory and I/O devices) at one time, therefore, it reduces the (CPU) and (memory and I/O devices) utilization. Multiple batch processing systems enable cross-use (CPU) of multiple jobs) and (memory and I/O devices ). For example, when A is used (CPU), B is used (memory and I/O devices), and A is used (memory and I/O devices), B is used (CPU ). This way (CPU) and (memory and I/O devices) are used by jobs at every moment, thus improving (CPU) and (memory and I/O devices) to increase the system throughput.
Its features include multi-channel, no-order, and scheduling.
Advantages: high resource utilization and high system throughput.
Disadvantages: long average turnaround time and no interaction capability. (The turnover of a batch processing system is long because jobs need to be queued)
4. Time-Sharing System

A time-sharing system is used to connect multiple terminals with monitors and keyboards on a single host, and allow multiple users to use computers interactively and share resources in the host through their own terminals.

Time-sharing system: first, you must be able to accept user requirements in a timely manner, that is, the job should be placed on the memory, not on the external storage, and the job cannot run in the External Storage

Second, you need to be able to process user commands in a timely manner, that is, each job system will allocate a fixed time slice for it. After it is used up, it will be the turn of other jobs to run, and you have to wait for the next time

Its features include: multiplexing, independence, timeliness, and interactivity.

5. Real-Time System
It is mainly used for real-time control, such as automatic artillery control systems, aircraft autonomous systems, missile guidance systems, and real-time information processing, such: ticket Booking System and Information Retrieval System for airplanes or trains
Comparison between real-time systems and time-sharing systems:
(1) multiplexing: the time-sharing system waits for (end-user) requests and then provides services for the end-user. The real-time system automatically performs (Multiple-way) and then provide services for multiple channels;
(2) independence: the time-sharing system initiates service requests to (hosts) for multiple (end users), and operates independently without mutual interference. The real-time system is (host)-To (multi-channel) collect information and control objects without interfering with each other;
(3) Timeliness: real-time systems are more timely than time-based systems;
(4) Interactivity: A time-sharing system provides data processing services and resource sharing services for (end users), while a real-time system can only provide (multiple channels) provides specific specialized service programs;
(5) Reliability: the real-time system is more reliable than the time-based system;

III. Basic Features of the operating system are as follows:
1. Concurrency:
Difference between parallelism and concurrency: parallelism refers to the occurrence of two or more times at the same time, and parallel release refers to the occurrence of two or more times at the same time interval;
2. Share:

Mutual Exclusion and simultaneous access;

Mutually Exclusive sharing: You can only wait for one time before the next time

Simultaneous access: Cross-use is allowed, that is, concurrent access is allowed.

3. Virtual:
The so-called virtual means that a physical entity is converted into several logical counterparts through a certain technology (for example, there is only one physical CPU, the virtual processor technology virtualizes it into multiple logical CPUs. from the user's point of view, it seems that multiple CPUs are in use, but actually that physical CPU is still in use, but the user cannot detect it ). Virtual technologies are used to implement virtual technologies.
Virtual processor technology, virtual memory technology, and virtual device technology.
4. Asynchronization:
When multiple processes run concurrently, they generally use a single resource in the computer. Therefore, most processes do not run in one breath, but stop and take a walk, it is possible that the final job is completed first, so the process is pushed forward in an unpredictable direction. This process is asynchronous.

Iv. Architecture Design of the Operating System
1. The passthrough operating system structure includes: non-structured operating system, modular OS structure, and hierarchical OS structure.
2. The advanced operating system structure includes the microkernel structure.
The micro-kernel OS structure can effectively support multi-processor operation and is very suitable for distributed system environments. The so-called microkernel technology refers to a small kernel that is well-designed and capable of implementing the core functions of modern OS. Unlike general operating systems, it is smaller and more refined, and not only runs in the core state, in addition, the resident memory is not swapped out due to memory shortage after the instance is started. The microkernel is not a complete OS, but an important foundation for building a general-purpose OS.
Basic Features of the microkernel:
(1) Process Management
(2) Memory Management
(3) Process Communication Management
(4) I/O device management

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