Reprinted from http://blog.csdn.net/yuanlulu/article/details/6163106
Before a variety of memory has been not very clear, to summarize today.
Memory is divided into two main categories: RAM and ROM.
Ram is not spoken, and today the main discussion ROM.
Rom initially can not be programmed, the factory what content is always what content, not flexible. Later the prom, can write their own once, if the wrong, can only change a piece, consider himself unlucky. Human civilization continues to progress, and finally appeared to be able to erase the written eprom, every time erase to take the chip to the ultraviolet radiation, think of you to a single-chip computer up and down a program found a place to add a word, for this you have to put the single-chip microcomputer under the ultraviolet light for half an hour, and then the next time, A day like this will not change a few times. The Wheel of history is moving forward, the great eeprom appeared, saved a large number of programmers, finally can modify the contents of the ROM at will.
The full name of the EEPROM is "electrically erasable programmable read-only memory", i.e. electrically erasable Programmable read-only memory. is relative to the UV erase ROM. But today there are many variants of EEPROM that become a generic term for a class of memory.
Narrow-sense EEPROM:
The feature of this ROM is that it can randomly access and modify any byte, and can write 0 or 1 to each bit. This is the most traditional kind of EEPROM, the data is not lost after power loss, can be saved for 100 years, can be erased 100w times. Has high reliability, but the circuit complex/cost is also high. So the current eeprom is tens kilometers bytes to several hundred bytes, rarely more than 512K.
Flash
Flash belongs to the generalized EEPROM because it is also an electric erase ROM. But in order to distinguish it from the general write-by-byte EEPROM, we call it flash.
The improvement of Flash is that the erase is no longer in bytes, but in blocks, once simplifies the circuit, the data density is higher, reduces the cost. The ROM on M is generally flash.
Flash is divided into nor flash and NAND flash. Nor flash data cable and address line separate, you can implement RAM-like random addressing function, can read any one byte. But the erase still has to be wiped by the block.
NAND Flash is also erased by block, but the data and address lines are multiplexed and cannot be randomly addressed using address lines. Reads can only be read by page. (Nandflash to erase by block, read by page, Norflash no page)
Because the Nandflash pin is multiplexed, the read speed is slower than nor flash, but the erase and write speeds are much faster than nor flash. NAND Flash internal circuitry is simpler, so the data density is large, small size, and low cost. As a result, large volumes of flash are NAND-shaped. Small capacity of the 2~12m flash is nor type.
The number of erase times for NAND Flash is or is several times the lifetime of the service. and NAND flash can flag bad blocks, allowing the software to skip bad blocks. Nor flash can be used again once it is damaged.
Because nor flash can be byte addressable, the program can run in NOR flash. Embedded systems use a small or flash storage boot code with a large volume of NAND flash to store file systems and cores.
Even copied the tape, wrote the last blog year ago. Take the train home at night! Ha ha!
The difference between Flash and EEPROM