What are the categories of storage devices?

Memory is a component used in computer systems to store data and instructions. According to different classification criteria, memory can be classified into multiple categories. The following is a detailed classification of memory from different dimensions:
1、 Classified by storage medium
Semiconductor Memory:
Features: Using semiconductor devices such as transistors and capacitors to store information, it has the advantages of small size, low power consumption, and fast read and write speed.
Examples: RAM (Random Access Memory), ROM (Read Only Memory), Flash Memory, etc.
Magnetic surface memory:
Characteristic: Using magnetic materials to store information, the information is recorded in the form of magnetization on the storage medium.
Example: Hard disk drive (HDD), floppy disk (gradually phased out), magnetic tape, etc.
Optical storage:
Characteristic: Using optical principles to store information, the information is recorded in small pits carved by laser on the optical disc.
Examples: CD (compact disc), DVD (digital versatile disc), BD (Blu ray disc), etc.
2、 Classified by access method
Random Access Memory (RAM):
Features: It can read or write data from any storage unit at any time, with fast read and write speed, but data will be lost after power failure.
Category:
SRAM (Static RAM): Fast but expensive, typically used for CPU caching.
DRAM (Dynamic RAM): Slow but low-cost, it is the main component of computer main memory.
Read only memory (ROM):
Characteristics: Data is written during manufacturing, can only be read and cannot be written (or can only be written once), and data will not be lost after power failure.
Category:
PROM (Programmable ROM): Users can write data in one go.
EPROM (Erasable Programmable ROM): Data can be erased and rewritten by ultraviolet irradiation.
EEPROM (electrically erasable programmable ROM): Data can be erased and rewritten through electrical signals.
Flash Memory: A special type of EEPROM that has the advantages of fast erase and write speed, low power consumption, and is widely used in storage devices such as USB flash drives and solid-state drives.
Sequential access memory:
Characteristics: Data is accessed sequentially, such as on magnetic tape, which has a slow access speed but a large storage capacity and low cost.
3、 Classified by their functions in computers
Main memory:
Features: Directly exchange data with CPU, fast speed but relatively small capacity.
Example: RAM, ROM, etc.
Auxiliary storage (external storage):
Features: Used for long-term data storage, with large capacity but slow speed.
Examples: hard drives, CDs, USB drives, etc.
Cache:
Features: Located between the CPU and main memory, it is used to cache data and instructions frequently accessed by the CPU to improve access speed.
4、 Classified by volatility
Volatile memory:
Characteristic: Data will be lost after power failure.
Example: RAM, Cache, etc.
Non volatile memory:
Feature: Data will not be lost after power failure.
Example: ROM, Flash Memory, hard drive, CD, etc.
5、 Special type of memory
Register:
Features: Located inside the CPU, used to store intermediate data during CPU operations, with the fastest speed but smallest capacity.
NVMe SSD：
Features: Solid state drives based on NVMe protocol, with extremely fast read and write speeds, suitable for high-performance computing and data center scenarios.
Phase change memory (PCM):
Features: Utilizing the transition of phase change materials between crystalline and amorphous states to store information, it has advantages such as non volatility and fast read/write speed.
Magnetoresistive Random Access Memory (MRAM):
Features: Utilizing the magnetoresistance effect to store information, it has the advantages of non volatility, fast read and write speed, and low power consumption.
Ferroelectric Random Access Memory (FeRAM):
Features: Utilizing the polarization characteristics of ferroelectric materials to store information, it has the advantages of non volatility, fast read and write speed, low power consumption, and strong radiation resistance.