January 20, 2021
1. Memory overview
Memory is a memory device in a computer system, used to store programs and data. Currently, semiconductor devices and magnetic materials are mainly used as storage media that constitute memories. The smallest storage unit in a memory is a bistable semiconductor circuit or a CMOS transistor or a storage element of magnetic material, which can store a binary code. A storage unit is composed of several storage elements, and then a memory is composed of many storage units.
2. Memory classification
According to the performance and usage of storage materials, there are various classification methods for storage:
1) According to storage media
Semiconductor memory: A memory composed of semiconductor devices.
Magnetic surface memory: memory made of magnetic materials.
2) According to storage method
Random access memory: The content of any storage unit can be accessed randomly, and the access time has nothing to do with the physical location of the storage unit.
Sequential memory: It can only be accessed in a certain order, and the access time is related to the physical location of the storage unit.
3) According to the read and write functions of the memory
Read only memory (ROM): The stored content is fixed and can only be read but not written into semiconductor memory.
Random access memory (RAM): A semiconductor memory that can be read and written.
4) According to the preservation of information
Non-permanent memory: memory that disappears after power is off.
Permanent memory: A memory that can save information even after power failure.
5) According to the role in the computer system
According to the role played by the memory in the computer system, it can be divided into main memory, auxiliary memory, cache memory, control memory and so on.
3. Hierarchical structure of memory
In order to solve the contradiction between the memory requirements for large capacity, high speed, and low cost, a multi-level memory architecture is usually adopted at present, that is, a cache memory, a main memory and an external memory are used.