Introduction to the classification of memory and the classification of various memory functions

Memory: used to store all information in the computer: including programs, raw data, intermediate results and final results of operations, etc.
Read-only memory (ROM): When in use, read-only memory can only be read but not written. The information in ROM will not be lost after power failure. Therefore, it is generally used to store some fixed programs, such as monitoring programs, subroutines, fonts and data tables. ROM can be divided into the following types according to the method of storing information:

1. Mask ROM:
Mask ROM is also called fixed ROM. It is programmed by the manufacturer and written into ROM (called curing) for users to use. Users cannot change the internal program. Its characteristic is that it is cheap.
2. Programmable read-only memory (PROM): 
Its content can be written once by the user according to the program he/she has programmed. Once written, it can only be read out and cannot be changed. This type of memory is now also called OTP (Only Time Programmable).
3. Rewritable read-only memory EPROM:
The first two types of ROM can only be written once, so users rarely use them. The more popular ROM chip is EPROM. Because its content can be completely erased by ultraviolet irradiation, and new programs can be rewritten after erasure. 
4. Electrically rewritable read-only memory (EEPROM):
EEPROM can write and clear its content by electrical methods. Its programming voltage and clearing voltage are the same as the 5V working voltage of the microcomputer CPU, and no additional voltage is required. It has the advantages of simple read and write operations like RAM, and data will not be lost due to power failure, so it is extremely convenient to use. This type of memory is now the most widely used.
Random Access Memory (RAM): 
This type of memory is also called read-write memory. It can not only read the data stored in the storage unit, but also write new data at any time. After writing, the original data will be lost. All the information in RAM will be lost after power failure. Therefore, RAM is often used to store information such as programs or intermediate calculation results that need to be changed frequently.
RAM can be divided into static and dynamic types according to the way of storing information.
1. Static SRAM: Its characteristic is that as long as there is power applied to the memory, the data can be stored for a long time. 
2. Dynamic DRAM: The written information can only be saved for a few ms, so it must be rewritten every certain period of time to keep the original information unchanged.
Field-rewritable non-volatile memory:
The characteristics of this type of memory are: in principle, they belong to ROM-type memory, and in terms of function, they can rewrite information at any time, and their function is equivalent to RAM. Therefore, the definition and division of ROM and RAM have gradually lost their meaning. 
1. Flash Erasable Memory (FLASH)
This type of memory is a non-volatile memory produced on the basis of the manufacture of EPROM and EEPROM. It has high integration and lower manufacturing cost than DRAM. It has both the flexibility of SRAM reading and writing and faster access speed, and the characteristic of ROM that it does not lose information after power failure, so it has developed rapidly.
2. Ferroelectric memory FRAM  
uses the polarization direction of ferroelectric materials to store data. It is characterized by high integration, fast reading and writing speed, low cost, and short reading and writing cycle.