Flash Memory

Flash memory isnon-volatile, which means that it does not need power to maintain the information stored in the chip. In addition, flash memory offers fast readaccess times(although not as fast as volatileDRAMmemory used for main memory in PCs) and better kinetic shock resistance thanhard disks.These characteristics explain the popularity of flash memory for applications such as storage on battery-powered devices. Another feature of flash memory is that when packaged in a "memory card", it is enormously durable, being able to withstand intense pressure, extremes of temperature and immersion in water¹

闪存的分类

NOR型与NAND型闪存的区别很大，打个比方说，NOR型闪存更像内存，有独立的地址线和数据线，但价格比较贵，容量比较小；而NAND型更像硬盘，地址线和数据线是共用的I/O线，类似硬盘的所有信息都通过一条硬盘线传送一般，而且NAND型与NOR型闪存相比，成本要低一些，而容量大得多。因此，NOR型闪存比较适合频繁随机读写的场合，通常用于存储程序代码并直接在闪存内运行，手机就是使用NOR型闪存的大户，所以手机的“内存”容量通常不大；NAND型闪存主要用来存储资料，我们常用的闪存产品，如闪存盘、数码存储卡都是用NAND型闪存。

这里我们还需要端正一个概念，那就是闪存的速度其实很有限，它本身操作速度、频率就比内存低得多，而且NAND型闪存类似硬盘的操作方式效率也比内存的直接访问方式慢得多。因此，不要以为闪存盘的性能瓶颈是在接口，甚至想当然地认为闪存盘采用USB2.0接口之后会获得巨大的性能提升。

前面提到NAND型闪存的操作方式效率低，这和它的架构设计和接口设计有关，它操作起来确实挺像硬盘(其实NAND型闪存在设计之初确实考虑了与硬盘的兼容性)，它的性能特点也很像硬盘：小数据块操作速度很慢，而大数据块速度就很快，这种差异远比其他存储介质大的多。这种性能特点非常值得我们留意。

Computerized systems, from simple appliances to complex networks, contain many different parts—processors, displays, software, drives, keyboards and mice, printed circuit boards, switches, modems, and, of course, memory—to name a few.

But what exactly is memory, and more specifically, what is flash memory? Memory is capable of retaining digital information under certain conditions. This retained material might be operational code or data files or a combination of the two. The ideal memory subsystem optimizes density, preserves critical material in a nonvolatile condition, is easy to program and re-program, can be read fast, and is cost-effective for the application. Some memory technologies meet one or more of these requirements very well, but offsetting limitations can prevent the product from becoming a genuine solution, especially in newer applications.

Flash memory is a nonvolatile memory using NOR technology, which allows the user to electrically program and erase information. Intel® Flash memory uses memory cells similar to an EPROM(可擦可编程只读存储器,电可编程只读存储器)