System memory introduction and memory management

1、 Why need to manage memory ？

Computer memory resources are limited , The processes running in the system are infinite , So , The computer cannot run too many processes , If there are too many running processes , There will be electricity Brain performance decreases , Even due to limited resources , The new process does not work properly .

2、 What is memory management ？

Memory management is system management The memory space between each running process is independent , It can run normally .

The spatial address allocation mechanism of each process is existing , Each process is in Kernel mode and User mode Each has a Stack , When the process is running in user mode , The user state space used by the process ; When running in kernel mode , When using kernel space .

a: What is kernel state ？ What is user mode ？

In a nutshell , Kernel mode operation permission is high , The operation permission of user mode is low . Kernel mode can directly operate all resources of the system , Such as operating various hardware devices . User status can be achieved through API Function to operate hardware devices and memory . The mechanism of user mode and kernel mode provides the stability and availability of the operating system .

b: How does the system manage memory ？

The system goes through Virtual memory _ Baidu Encyclopedia (baidu.com) Technology to manage memory , How virtual memory works 、 Please refer to Baidu Encyclopedia for scheduling methods . Virtual memory mechanism is equivalent to memory manager , The main things to do are ：（1） Allocate independent virtual space to each process , The memory addresses between each process do not affect each other .（2） Realize the mapping mechanism from virtual space address to actual hardware physical address .（3） The page missing exception mechanism is introduced to realize the inert allocation of memory .（4） Get into swap Mechanism , Switch inactive data to disk .

The mapping relationship between virtual address and physical address is reflected by page table , A page table is equivalent to a map , Memory management unit received CPU After the virtual address of , The corresponding physical address can be determined by querying the page table . For specific introduction, please refer to 40 Zhang diagram ｜ Linux The mysterious past behind the memory

There is a good article on memory management, see ： That's great ！ 20 Here's a picture to uncover the fog of memory management , Suddenly the light came out

3、 Introduction to basic knowledge

The memory distribution is as follows ：

 

I'll add later .