What is the difference between ROM, RAM, DRAM, SRAM and FLASH

Time：2016-10-26

There are two major types of RAM, one is called Static RAM (SRAM). SRAM is very fast. It is the fastest storage device for reading and writing, but it is also very expensive, so it is only used in demanding places. For example, the primary buffer of the CPU, the secondary buffer. The other is called dynamic RAM (DRAM). DRAM keeps data for a short time and slower than SRAM, but it is faster than any ROM, but DRAM is cheaper than SRAM in terms of price. Many, computer memory is DRAM.

There are many types of DRAM. Common ones are FPRAM/FastPage, EDORAM, SDRAM, DDR RAM, RDRAM, SGRAM, and WRAM. One of them is described here.

DDR RAM (Date-Rate RAM) is also called DDR SDRAM. This improved RAM and SDRAM are basically the same, except that it can read and write data twice in one clock, which doubles the data transfer speed. . This is the most used memory in the computer, and it has a cost advantage, in fact defeating Intel's other memory standard - Rambus DRAM. On many high-end graphics cards, high-speed DDR RAM is also used to increase bandwidth, which can greatly improve the pixel rendering capability of 3D accelerator cards.

How memory works:

Memory is used to store the data and programs that are currently in use (ie, in execution). The memory of the computer we usually refer to is dynamic memory (ie, DRAM). The so-called "dynamic" in dynamic memory refers to When we write data to DRAM, after a while, the data is lost, so an extra circuit is needed for the memory refresh operation.

The specific working process is as follows: whether a DRAM memory cell stores 0 or 1 depends on whether the capacitor has a charge, a charge representing 1 and no charge representing 0. But for a long time, the capacitor representing 1 will discharge, and the capacitor representing 0 will absorb the charge, which is the reason for data loss; the refresh operation periodically checks the capacitor. If the power is greater than 1/2 of the full charge, it is considered to be 1 And fully charge the capacitor; if the charge is less than 1/2, it is considered to represent 0, and the capacitor is discharged, thereby maintaining data continuity.

There are also many kinds of ROMs. The difference between PROM is programmable ROM, PROM and EPROM (erasable programmable ROM) is that PROM is one-time, that is, after the software is poured, it cannot be modified. This is early The product is now impossible to use, and EPROM is a general-purpose memory that wipes out the original program by ultraviolet light. Another type of EEPROM is electronically erased, which is expensive, has a long write time, and is very slow to write.

For example, the mobile phone software is usually placed in the EEPROM, we call, some of the last number dialed, temporarily stored in the SRAM, not immediately written through the record (the call record is saved in the EEPROM), because there is a very important job at the time (Call) To do, if written, the long wait is for the user to endure.

FLASH memory, also known as flash memory, combines the advantages of ROM and RAM, not only with electronically erasable programmable (EEPROM) performance, but also does not power off data and can quickly read data (NVRAM advantage), U disk This memory is used in MP3. In the past 20 years, embedded systems have used ROM (EPROM) as their storage device. However, in recent years, Flash has completely replaced the position of ROM (EPROM) in embedded systems, as a storage bootloader and operating system or program. The code is used directly by the hard disk (U disk).

Currently Flash has two main types of NOR Flash and NADN Flash.

The reading of NOR Flash is the same as the reading 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 take random reading technology of memory. Its reading is performed in the form of reading one block at a time, usually reading 512 bytes at a time. Flash using this technology is relatively cheap. Users can't run the code directly on NAND Flash, so many NAND Flash development boards use a small NOR Flash to run the boot code in addition to NAND Flah.

Generally, NOR Flash is used for small capacity. Because it is fast, it is used to store important information such as operating system. For large-capacity NAND FLASH, the most common NAND FLASH application is DOC (Disk On Chip) used in embedded systems. And the "flash disk" we usually use, can be erased online. Currently, FLASH is mainly from Intel, AMD, Fujitsu and Toshiba, while the main manufacturers of NAND Flash are Samsung and Toshiba.

Comparison of NAND Flash and NOR Flash

NOR and NAND are the two main non-volatile flash technologies on the market today. Intel first developed NOR flash technology in 1988, completely changing the situation that was originally dominated by EPROM and EEPROM. Then, in 1989, Toshiba announced a NAND flash architecture that emphasizes lower cost per bit, higher performance, and can be easil