Storage:
1. Main memory (main memory or memory): It is an important part of computer hardware, its function is to store instructions and data, and can be directly and randomly accessed by the central processing unit (CPU). It is characterized by fast access speed, but small capacity and high price.
1.1 ROM (Read Only Memory): It works in a non-destructive readout mode, and can only read and cannot write information. Once the information is written, it is fixed. Even if the power is turned off, the information will not be lost, so it is also called fixed memory.
There are five types of ROM. Mask-programmed ROM (Mask-programmedROM) cannot be written by users. It is "written" when manufactured by the manufacturer. Programmable ROM (Programmable ROM) can be written once and erased by users. Programmable read-only memory EPROM (Erasable Programmable ROM) can be programmed multiple times, electrically erasable, programmable read-only memory EEPROM (Elecrically Erasable Programmable ROM) and flash memory (Flash Memory).
1.2 RAM (Random Access Memory): It is the main part of the memory and the internal memory that directly exchanges data with the CPU. It can read and write at any time (except when refreshing), and it is very fast, and is usually used as a temporary data storage medium for the operating system or other running programs. The biggest difference between it and ROM is the volatility of data, that is, the stored data will be lost once the power is off. RAM is used in computers and digital systems to temporarily store programs, data, and intermediate results.
RAM is divided into SRAM (static random access memory) and DRAM (dynamic random access memory). The information stored in the SRAM can be retained for a long time without power failure, the state is stable, and no external refresh circuit is required, which simplifies the external circuit design. However, due to the large number of transistors contained in the basic storage circuit of SRAM, the integration level is low and the power consumption is high. Compared with SRAM, DRAM has the advantages of high integration, low power consumption, and low price, so it is widely used in large-capacity memory. The disadvantage of DRAM is that the logic circuit needs to be refreshed, and normal read and write operations cannot be performed during refresh operations.
2. Auxiliary storage (auxiliary storage or external storage): refers to storage other than computer memory and CPU cache, this type of storage generally can still save data after power failure. Common external storage devices include hard disks, floppy disks, CDs, U disks, etc. The characteristic is large capacity, low price, but slow access speed.
3. Cache: It is a kind of high-speed cache memory, which is an important technology used to solve the speed mismatch between CPU and main memory. Cache is a small-capacity memory between the CPU and the main memory. It is composed of a static memory chip (SRAM), but its access speed is faster than that of the main memory and is close to the speed of the CPU.