A brief introduction to RAM, SRAM, SSRAM, DRAM, FLASH, EEPROM
RAM (Random Access Memory) Random access memory. The contents of the storage unit can be freely taken out or stored as needed, and the speed of access is independent of the location of the storage unit. Such a memory will lose its stored content when it is powered off, so it is mainly used to store programs for short-term use.
According to different storage information, random access memory is further divided into static random access memory (SRAM) and dynamic random access memory (DRAM).
SRAM (Static RAM) can save the data stored in it without refreshing the circuit.
SSRAM (Synchronous SRAM) is a synchronous static random access memory. Synchronization means that the Memory work requires a synchronous clock. The transmission of internal commands and the transmission of data are based on it. Random means that data is not stored in a linear order, but data is read and written by a specified address.
All access to the SSRAM is initiated on the rising/falling edge of the clock. Addresses, data inputs, and other control signals are all related to the clock signal.
This is different from asynchronous SRAM, where access to asynchronous SRAM is independent of clock and data input and output are controlled by address changes.
DRAM (Dynamic RAM) is refreshed once every once in a while, otherwise the internal data will disappear.
Therefore, SRAM has higher performance, but SRAM also has its disadvantages, that is, its integration is low, DRAM memory of the same capacity can be designed to be small in size, but SRAM requires a large volume and consumes a large amount of power. . So the SRAM memory on the motherboard takes up a part of the area.
SRAM has high speed and good performance. It mainly has the following applications:
1) Cache between CPU and main memory.
2) L1/L2 inside the CPU or external L2 cache.
SDRAM (Synchronous DRAM) is a synchronous dynamic random access memory. Synchronization means that the Memory work needs to synchronize the clock. The internal command transmission and data transmission are based on it. Dynamic means that the storage array needs constant refresh to ensure data is not lost. Random means that the data is not stored in linear order, but Freely specify the address for data reading and writing.
DRAM evolution and detailed classification:
A. DRAM (Dynamic Random-Access Memory), the most common system memory of dynamic random access memory. DRAM can only keep data for a short time. In order to maintain data, DRAM uses capacitor storage, so it must be refreshed once in a while. If the memory unit is not refreshed, the stored information will be lost.
B. SDRAM: Synchronous Dynamic Random Access Memory, synchronous dynamic random access memory, synchronization means that the memory work requires a step clock, the internal command transmission and data transmission are based on it; dynamic means that the storage array needs constant refresh To ensure that data is not lost; random means that the data is not stored in a linear order, but the data is read and written by the specified address.
C. DDR: SDRAM has gone through four generations since its development: first generation SDR SDRAM, second generation DDR SDRAM, third generation DDR2 SDRAM, fourth generation DDR3 SDRAM.
Both the first and second generation SDRAMs use single-ended (Single-Ended) clock signals. The third and fourth generations use a differential clock signal that reduces interference as a synchronous clock because of the faster operating frequency. The clock frequency of SDR SDRAM is the frequency of data storage. The first generation memory is named with clock frequency, such as pc100, pc133 indicates that the clock signal is 100 or 133MHz, and the data read/write rate is also 100 or 133MHz.
After the second, third, and fourth generation DDR (Double Data Rate) memory uses the data read and write rate as the naming standard, and is preceded by a symbol indicating its DDR algebra, PC - DDR, PC2 = DDR2, PC3 = DDR3 . For example, the PC2700 is DDR333, its operating frequency is 333/2=166MHz, and 2700 means the bandwidth is 2.7G.
DDR read and write frequencies range from DDR200 to DDR400, DDR2 from DDR2-400 to DDR2-800, and DDR3 from DDR3-800 to DDR3-1666.
Many people understand SDRAM errors as the first generation, that is, SDR SDRAM, and as a terminology, they are misleading. SDR is not equal to SDRAM.
A ROM (Read-Only Memory) read-only memory is a solid-state semiconductor memory that can only read data stored in advance. Its characteristic is that once the data is stored, it can no longer be changed or deleted. Usually used in electronic or computer systems that do not require frequent changes to the data, and the data does not disappear because the power is turned off.
FLASH is flash memory. It is a long-lived non-volatile memory that retains stored data information in the event of a power outage. Data deletion is not in a single byte but in fixed blocks (note) :NOR Flash is byte storage.), the block size is generally 256KB to 20MB.
The EEPROM is an electronically erasable read-only memory. Unlike EEPROM, EEPROM can delete and rewrite at the byte level instead of the entire chip, so flash is faster than EEPROM. Since data can still be saved when it is powered off, flash memory is often used to save setup information, such as saving data in a computer's BIOS (basic input and output program), PDA (personal digital assistant), digital camera, and so on.
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