Ram, SRAM, SDRAM, Rom, EPROM, EEPROM, flash memory

Source: Internet
Author: User


I have never been clear about the difference between flash and Rom and Ram, today I have nothing to do, reproduced an article; http://blog.chinaunix.net/space.php? Uid = 22342666 & Do = Blog & id = 1774747

Common memory concepts: Ram, SRAM, SDRAM, Rom, EPROM, EEPROM, and Flash memory can be divided into many types, including RAM (Random Access Memory) based on whether power loss data is lost) and Rom (read-only memory), where Ram access speed is relatively fast, but data will be lost after power loss, and data will not be lost after Rom power loss.
Rom and RAM are both semiconductor memory, Rom is short for read only memory, and Ram is short for random access memory. ROM can still maintain data when the system stops power supply, while Ram usually loses data after power loss. A typical Ram is the computer memory.

Ram can also be divided into SRAM (static RAM/static memory) and DRAM (Dynamic RAM/dynamic memory ). SRAM uses a bistability trigger to store information. As long as no power loss occurs, the information will not be lost. DRAM uses the MOs (metal oxide semiconductor) capacitor to store information, so it must maintain information by constantly charging the capacitor, therefore, DRAM costs, integration, and power consumption are much better than those of SRAM. The speed of SRAM is very fast. It is currently the fastest reading and writing storage device, but it is also very expensive, so it is only used in demanding places, such as the first-level buffer of CPU and second-level buffer. DRAM retains data for a short period of time and is slower than SRAM, but it is faster than any Rom, but DRAM is much cheaper than SRAM in terms of price, the computer memory is dram.

What we usually call SDRAM is a dram, which synchronizes dynamic memory and uses a single system clock to synchronize all address data and control signals. Using SDRAM not only improves system performance, but also simplifies design and provides high-speed data transmission. It is often used in embedded systems.

There are also many types of Rom. The difference between the prom and the EPROM is that the prom is one-time, that is, after the software is filled in, it cannot be modified. This is an early product and cannot be used now. The EPROM is used to erase the original program through ultraviolet radiation and is a general memory. Another type of EEPROM is an electronic interface. It has a high price, a long write time, and a low write speed.

Flash is also a kind of non-volatile memory (which won't be lost after power loss), which is easy to erase and fast to access, and has greatly replaced the status of the traditional EPROM. Because it has the same power loss as Rom, it will not be lost, so many people call it Flash ROM. Flash Memory, also known as flash memory, combines the strengths of ROM and Ram, not only has the electronic Erasable Programmable (EEPROM) performance, data will not be lost after power failure, and data can be quickly read (NVRAM advantage), which is used in USB flash drives and MP3. In the past 20 years, embedded systems have been using ROM (EPROM) as their storage devices. However, in recent years, Flash has completely replaced the position of RoM (EPROM) in embedded systems, it is used to store bootloader, operating system or program code, or directly when the hard disk is used (u
Disk ).

Currently, there are two types of FLASH: nor flash and nadn flash. The reading of nor flash is the same as that of our common SDRAM. Users can directly run the code loaded in nor flash, which can reduce the capacity of SRAM and save costs. NAND Flash does not adopt the random read Technology of memory. It reads 512 bytes at a time, usually in a low cost. Users cannot directly run the code on NAND Flash. Therefore, many development boards that use NAND Flash use a small nor flash to run the startup code in addition to NAND flah.

Nor flash is usually used for small capacity. Because of its fast reading speed, it is used to store important information such as the operating system, while NAND Flash is used for large capacity, the most common nand flash application is the DOC (Disk On Chip) used in the embedded system and the "flash disk" we usually use, which can be erased online. Flash on the market is mainly from Intel, AMD, Fujitsu and toshba, while the main manufacturers of NAND Flash are Samsung and toshba.

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