There are two main types of memory: RAM and ROM.
1. RAM
Random Access Memory (RAM) is the main component of the main memory in a computer. In a random access device, a memory cell address can be used to randomly access a data item without having to access all data items preceding it. The term is sometimes confused with ROM because ROM can also be randomly accessed. The difference between RAM and ROM is that the user can read and write RAM, i.e. the user can write information in RAM. Afterwards, the original information can be easily erased by overwriting. Another characteristic of RAM is volatility. Information (programs or data) will be lost when the system is powered off. In other words, when the computer loses power, the information stored in RAM will be deleted. RAM technology can be divided into two categories: SRAM and DRAM.
（1） SRAM
Static RAM (SRAM) technology uses traditional flip-flop gate circuits to store data. These gates maintain state (0 or 1), which means that data is always present when power is applied and does not need to be refreshed. SRAM is fast, but expensive.
（2）DRAM
Dynamic Random Access Memory (DRAM) technology uses capacitors. If the capacitor is charged, the state at this time is 1; if it is discharged, the state is 0. Because capacitors lose some of their power over time, memory cells need to be refreshed periodically. DRAM is slower, but cheaper.
Common ones such as computer memory, mobile phone memory, CPU cache, etc. belong to RAM.
2. ROM
The contents of the read only memory ( ROM ) ( Read only Memory ) are written by the manufacturer. The user can only read but not write, and its advantage is that it is non-volatile: when the power is cut off, the data will not be lost. It is usually used to store programs or data that cannot be lost after shutdown. For example, ROM is used to store programs that run at boot time.
The common ones that belong to ROM are: the boot program started by the computer, the corresponding program codes of some electronic products such as mobile phones, MP3 , MP4 , digital cameras, etc.
3.PROM
A type of ROM called Programmable Read Only Memory ( PROM ) . This memory is blank when the computer is shipped. Computer users can store programs on it with the help of some special equipment. When the program is stored, it cannot be rewritten like a ROM. That is, computer users can use it to store some specific programs.
4.EPROM
A type of PROM called Erasable Programmable Read Only Memory (EPROM). It can be programmed by the user, but it must be erased with a special device that emits ultraviolet light. The EPROM memory needs to be removed and reinstalled.
5.EEPROM
A type of EPROM called Electrically Erasable Programmable Read-Only Memory (EEPROM). It can be programmed and erased with electronic pulses without removing it from the computer.
U disk belongs to flash memory and also belongs to EEPROM. It can be written and erased at any time, not RAM or ROM, but FLASH-ROM.
DRAM and SRAM
In addition to the main memory, a lot of memory in CNOOC exists in the computer. Such as the second level of cache memory in the CPU. The data of the CPU is provided by the main memory, but the memory controller is required between the CPU and the main memory. If some frequently used programs or data can be placed inside the CPU, then the CPU does not need to repeatedly read from the main memory, which can improve performance. This is the design concept of L2 cache. The relationship between the second-level cache and the main memory and CPU is shown in the following figure:
因为第二层快取(L2 cache)整合到CPU內部，因此這個L2记忆体的速度必須要CPU时脉相同。 使用DRAM是无法达到这个时脉速度的，此时就需要静态随机存取记忆体(Static Random Access Memory, SRAM)的帮忙了。 SRAM在设计上使用的电晶体数量较多，价格较高，且不易做成大容量，不过由于其速度快， 因此整合到CPU內成為快取记忆体以加快资料的存取是个不错的方式。新一代的CPU都有內建容量不等的L2快取在CPU內部， 以加快CPU的运作效能。