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There are two types of flash memory, NAND and NOR. The names refer to the type of logic gate used in each memory cell. (Logic gates are a fundamental building block of digital circuits). NOR flash was first introduced by Intel in 1988. NAND flash was introduced by Toshiba in 1989. The two chips work differently. NAND has significantly higher storage capacity than NOR. NOR flash is faster, but it's also more expensive. Some mobile devices use both NAND and NOR. A pocket PC, for instance, may use embedded NOR to boot up the operating system and a removable NAND card for all its other memory/storage requirements. Generally speaking, however, when someone talks about a flash solid state drive, they are referring to NAND flash memory.
NAND flash has a finite number of read-write cycles, though the total number continues to increase with each generation of chips. An important goal of NAND flash development has been to reduce the cost per bit and increase maximum drive capacity, performance and endurance so that flash memory can compete with magnetic hard disk drives in the enterprise. Although flash SSDs are much faster than spinning disks, they are not as fast as RAM-based solid-state disk when handling write input/outputs (I/Os).