Книга: Embedded Linux Primer: A Practical, Real-World Approach

2.3.2. NAND Flash

2.3.2. NAND Flash

NAND Flash is a relatively new Flash technology. When NAND Flash hit the market, traditional Flash memory such as that described in the previous section was referred to as NOR Flash. These distinctions relate to the internal Flash memory cell architecture. NAND Flash devices improve upon some of the limitations of traditional (NOR) Flash by offering smaller block sizes, resulting in faster and more efficient writes and generally more efficient use of the Flash array.

NOR Flash devices interface to the microprocessor in a fashion similar to many microprocessor peripherals. That is, they have a parallel data and address bus that are connected directly[7] to the microprocessor data/address bus. Each byte or word in the Flash array can be individually addressed in a random fashion. In contrast, NAND devices are accessed serially through a complex interface that varies among vendors. NAND devices present an operational model more similar to that of a traditional hard drive and associated controller. Data is accessed in serial bursts, which are far smaller than NOR Flash block size. Write cycle lifetime for NAND Flash is an order of magnitude greater than for NOR Flash, although erase times are significantly smaller.

In summary, NOR Flash can be directly accessed by the microprocessor, and code can even be executed directly out of NOR Flash (though, for performance reasons, this is rarely done, and then only on systems in which resources are extremely scarce). In fact, many processors cannot cache instruction accesses to Flash like they can with DRAM. This further impacts execution speed. In contrast, NAND Flash is more suitable for bulk storage in file system format than raw binary executable code and data storage.

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