Norflash replace rom

refer:

? ??http://bbs.csdn.net/topics/370053495

? ??http://www.myslidepresenter.com/blog/tag/nor-flash

3、nor?flash和nandflash都属于电可擦除存储器，它存储的数据可以擦除、修改，但是要有特定的访问程序（步骤）和/或外部电路的跳线设置。由于flash存储器可多次修改，又能长久保持数据，因而现在广泛用在原先使用原始ROM的场合，使系统具有在线更新数据、程序的功能。
4、你可以在“维基百科”中查阅说明

Flash memory or a flash RAM is a type of nonvolatile semiconductor memory device where stored data exists even when memory device is not electrically powered. It’s an improved version of electrically erasable programmable read-only memory (EEPROM). The difference between Flash Memory and EEPROM are,EEPROM erases and rewrite its content one byte at a time or in other words,at byte level. Where as Flash memory erases or writes its data in entire blocks,which makes it a very fast memory compared to EEPROM.?Flash memory?can’t replace DRAM and SRAM because the speed at which the DRAM/SRAM can access data and also their ability to address at byte level can’t be matched by Flash.

The flash memory is also termed as Solid-state Storage Device (SSD) due to the absence of moving parts in comparison to traditional computer hard disk drive.

Flash memory types:

The two main types of flash memory are the NOR Flash & NAND Flash. Intel is the first company to introduce commercial (NOR type) flash chip in 1988 and Toshiba released world’s first NAND-flash in 1989.

NOR-flash?is slower in erase-operation and write-operation compared to NAND-flash. That means the NAND-flash has faster erase and write times. More over NAND has smaller erase units. So fewer erases are needed. NOR-flash can read data slightly faster than NAND.

NOR offers complete address and data buses to randomly access any of its memory location (addressable to every byte). This makes it a suitable replacement for older ROM BIOS/firmware chips,which rarely needs to be updated. Its endurance is 10,000 to 1,000,000 erase cycles. NOR is highly suitable for storing code in embedded systems. Most of the today’s microcontrollers comes with built in flash memory.

NAND-flash?occupies smaller chip area per cell. This maker NAND available in greater storage densities and at lower costs per bit than NOR-flash. It also has up to ten times the endurance of NOR-flash. NAND is more fit as storage media for large files including video and audio. The USB thumb drives,SD cards and MMC cards are of NAND type.

NAND-flash does not provide a random-access external address bus so the data must be read on a block-wise basis (also known as page access),where each block holds hundreds to thousands of bits,resembling to a kind of sequential data access. This is one of the main reasons why the NAND-flash is unsuitable to replace the ROM,because most of the microprocessors and microcontrollers require byte-level random access.