What is Random Access Memory?

Random access memory (English: Random Access Memory, abbreviation: RAM), also called main memory, is an internal memory that directly exchanges data with the CPU. It can be read and written at any time (except when refreshing), and it is very fast. It is usually used as a temporary data storage medium for the operating system or other running programs. The RAM can write (store) or read (remove) information from any designated address at any time during RAM operation. The biggest difference between it and ROM is the volatile nature of the data, that is, the stored data will be lost in the event of a power failure. RAM is used in computers and digital systems to temporarily store programs, data, and intermediate results. ^[1]

Chinese name: Random access memory
Foreign name: random access memory

Storage principle: Store data by trigger
Unit structure: Composition of six NMOS or OS
Short name: RAM

Introduction to Random Access Memory

Memory is a device or component used to store large amounts of information in digital systems, and is an important part of computers and digital devices. Memory can be divided into two categories: random access memory (RAM) and read-only memory (ROM).

Random access memory (RAM) can both store information to and read information from designated units. Any information stored in RAM will be lost after power off, so RAM is volatile memory.

ROM is a read-only memory. In addition to fixed storage of data, tables, and fixed programs, it is also widely used in combinational logic circuits. ^[2]

Random Access Memory Features

Random access memory

The so-called "random access" refers to the time required when the data in the memory is read or written.

Static random access memory Location-independent. In contrast, when reading or writing information in a Sequential Access storage device, the time and location required will be related. It is mainly used to store the operating system, various applications, data, and so on. ^[3]

When the RAM is in normal operation, data can be read from and written to the RAM. Compared with ROM, RAM has the advantages of convenient read / write and flexible use, which is especially suitable for occasions where data is frequently changed quickly. ^[4]

Random access memory volatile

RAM cannot retain data when the power is turned off. If data needs to be saved, it must be written to a long-term storage device (such as a hard disk). ^[3]

The working characteristic of RAM is that data information can be accessed at any location unit at any time after power-on, and internal information disappears after power-off. ^[5]

Random access memory is sensitive to static electricity

Like other delicate integrated circuits, random access memory is very sensitive to the electrostatic charge of the environment. Static electricity can disturb the charge of capacitors in the memory, cause data loss, and even burn out circuits. Therefore, before touching the random access memory, you should first touch the metal ground with your hand. ^[3]

Random access memory access speed

Modern random access memory has the fastest writing and reading speed of almost all access devices, and the access latency is insignificant compared to other storage devices that involve mechanical operations. ^[3]

Random access memory needs to be refreshed (regenerated)

Modern random access memories rely on capacitors to store data. Capacitors represent 1 (binary) when fully charged, and 0 if uncharged. Because capacitors have more or less leakage, data will be lost over time without special treatment. Refresh is to read the state of the capacitor in a specified period, and then recharge the capacitor in the original state to make up for the lost charge. The need to refresh explains the volatile nature of random access memory. ^[3]

Composition of random access memory

The RAM consists of a memory matrix, an address decoder, a read / write controller, input / output, and chip select control. ^[6]

(1) Storage matrix. As shown in the figure, the core part of the RAM is a register matrix, which is used to store information, called a storage matrix. ^[6]

(2) Address decoder. The role of the address decoder is to translate the binary number corresponding to the register address into a valid row selection signal and column selection signal, thereby selecting the storage unit. ^[6]

(3) Read / write controller. When accessing the RAM, whether to perform a read operation or a write operation on the selected register is controlled by reading and writing signals. During the read operation, the data of the selected unit is transmitted to the CPU (Central Processing Unit) via the data line, input / output line; during the write operation, the CPU stores the data into the selected unit via the input / input line and data line. ^[6]

(4) Input / output. The RAM exchanges data with the computer's CPU through the input / input terminal. When it is read, it is the input terminal. When it is written, it is the input terminal. It is dual-purpose. Controlled by read / write control lines. The number of data lines at the input / output end is the same as the number of register bits in an address. Some RAM chips have separate input / output ends. Usually, the output end of the RAM has an open collector or a tri-state output structure. ^[6]

(5) Chip selection control. Due to the limited integration of RAM. A computer's memory system is often a combination of many RAMs. When the CPU accesses the memory, it can only access a certain slice (or slices) of RAM at a time, that is, only one address (or slices) of RAM in the memory accepts the CPU to access and exchange information with it, while other slices of RAM and the CPU do not When contact occurs, the chip selection is used to achieve this control. Usually a piece of RAM has one or more chip select lines. When the chip select line of a certain chip is connected to an effective level, the chip is selected, and the output signal of the address decoder controls the register of a certain address of the chip to be connected to the CPU. When the chip select line is connected to an invalid level, the chip is disconnected from the CPU. ^[6]

Random Access Memory Class

According to the working principle of the storage unit, RAM is divided into static RAM and dynamic RAM. ^[7]

SRAM Random Access Memory Static Random Access Memory (SRAM)

The static storage unit is formed by adding a gate control on the basis of a static trigger. Therefore, it relies on the self-protection function of the trigger to store data. The information stored in the SRAM can be retained for a long time without power failure, and the state is stable. No external refresh circuit is required, thereby simplifying the external circuit design. But because SRAM's basic memory circuit contains more transistors, it has a lower integration and consumes more power. ^[7]

The characteristics of SRAM are as follows:

Storage principle: Data is stored by triggers. ^[8]

Unit structure: Six-tube NMOS or OS. ^[8]

Advantages: fast, simple to use, no refresh required, extremely low static power consumption; often used as cache. ^[8]

Disadvantages: many components, low integration, and high power consumption. ^[8]

Commonly used SRAM integrated chips: 6116 (2K × 8 bits), 6264 (8K × 8 bits), 62256 (32K × 8 bits), 2114 (1K × 4 bits). ^[8]

DRAM Random Access Memory Dynamic Random Access Memory (DRAM)

DRAM uses the principle of capacitor to store charge to save information. The circuit is simple and highly integrated. Since any capacitor has a leakage current, when the capacitor stores a charge, the charge will be lost due to the discharge of the capacitor after a period of time, and the stored information will be lost. The solution is to read and rewrite the DRAM at a certain interval (usually 2ms), so that the charge discharged from the capacitor at the logic level "l" is replenished, and the voltage at the level " The capacitance of 0 remains 0, this process is called DRAM refresh. ^[7]

The refresh operation of DRAM is different from the memory read / write operation, which mainly manifests in the following points:

(1) The refresh address is generated by the refresh address counter, not provided by the address bus. ^[7]

(2) The basic memory circuit of DRAM can be refreshed at the same time in rows, so only row address is needed for refresh, and no column address is needed. ^[7]

(3) The data line of the memory chip is in a high-impedance state during the refresh operation, that is, the on-chip data line is completely isolated from the external data line. ^[7]

Compared with SRAM, DRAM has the advantages of high integration, low power consumption, and low price, so it is widely used in large-capacity memories. The disadvantage of DRAM is that it requires a refresh logic circuit, and normal read and write operations cannot be performed during a refresh operation. ^[7]

DRAM features are as follows:

Storage principle: The principle that the gate capacitance of the MOS tube can store the charge needs to be refreshed (early: three-tube basic unit; after: single-tube basic unit). ^[8]

Refresh (regenerate): In order to replenish the missing charge in time to avoid the loss of stored information, the gate capacitor must be recharged periodically. ^[8]

Refresh time: the time for refresh operation periodically. This time must be less than the time that the gate capacitance naturally holds the information (less than 2ms). ^[8]

Advantages: The integration is much higher than SRAM, the power consumption is low, and the price is low. ^[8]

Disadvantages: The peripheral circuits are complicated due to the need for refresh; refresh also makes the access speed slower than SRAM, so in computers, DRAM is often used as the main memory. ^[8]

Nevertheless, DRAM memory cells have become the mainstream products of large-capacity RAM due to their simple structure, few components, high integration, and low power consumption. ^[8]

Random access memory difference

The difference between random access memory and read-only memory

In a computer, both RAM and ROM are data memories. RAM is a random access memory, which is characterized by volatile, that is, power loss amnesia.

Dynamic random access memory ROM usually refers to solid-state memory (write once, read repeatedly), which has the opposite characteristics to RAM. For example, if there is a sudden power failure or the file is closed without saving, the ROM can randomly save the previously unsaved files but the RAM will make the previously unsaved files disappear. ^[8]

The difference between random access memory and memory

A very important part of the composition of a computer is the memory. Memory is a component used to store programs and data.

Laptop memory As far as computers are concerned, only with the memory can the memory function be guaranteed to work normally. There are many types of memory. According to its purpose, it can be divided into main memory and auxiliary memory. Main memory is also called internal memory (referred to as memory), and auxiliary memory is also called external memory (referred to as external memory). External storage is usually magnetic media or optical disks, such as hard disks, floppy disks, magnetic tapes, CDs, etc., which can store information for a long time and do not rely on electricity to save information, but driven by mechanical components, the speed is much slower compared to the CPU. Memory refers to the storage components on the motherboard. The components that the CPU directly communicates with and uses to store data store the data and programs that are currently in use (that is, executing). Its physical essence is that one or more groups have data. The integrated circuit with input and output and data storage functions, the memory is only used to temporarily store programs and data. Once the power is turned off or a power failure occurs, the programs and data in it will be lost. ^[8]

From the moment you have a computer, you have memory. The development of memory has also experienced many technical improvements to today. From the earliest DRAM to FPMDRAM, EDODRAM, SDRAM, etc., the speed of memory has been increasing and the capacity has been increasing. What kind of memory does the server use today? Registered ECC SDRAM is commonly used in IA-based servers

Fast cycle random access memory . ^[8]

Since the memory is used to store the data and programs that are currently being used (that is, executing), how does it work? The memory of the computer we usually refer to is dynamic memory (ie DRAM). The so-called "dynamic" in dynamic memory refers to the time when we write data to DRAM. After a period of time, the data will be lost. An additional circuit is set for memory refresh operation. ^[8]