Flash

Classification of flash memory according to the type of logic gate used:

• NOR flash memory – uses a logical binegation functor (NOR)
• NAND flash memory – uses a logic disjunction functor (NAND)

NOR flash memory allows direct access to each memory cell, but has relatively long write and erase times. For this reason, it is suitable for storing data that does not require frequent updates, such as the firmware of various types of devices. It withstands from 10 to 100 thousand programming cycles. It was used in the first versions of CompactFlash memory cards, but was later replaced by cheaper NAND flash memory.

NAND flash memory has faster write and erase times, higher data density, better cost-to-capacity ratio, and ten times longer endurance than NAND memory. However, the main feature of this type of memory is sequential data access. This limits the range of applications as storage only, e.g. in memory cards. The first memory card based on NAND flash memory was the SmartMedia card. Later, they began to be used in other types of memory cards, e.g. MultiMedia Card, Secure Digital, Memory Stick and xD Picture Card, as well as in USB flash drives.

To save a flash memory cell, you must delete it beforehand. It is not possible to rewrite data to an already saved cell. Although any memory cell can be read and written, the erase operation allows only entire blocks of cells to be erased. A single cell cannot be deleted. For this reason, the data is not fully free. These memories allow you to read and write any cell, but not to write/read the contents freely, as in the case of RAM.

The above limitations cause some difficulties in handling data access in storage. The write must be coordinated with the memory block erasing operation. Typically, if a file is to be updated or overwritten, the memory management system creates a new copy of the file elsewhere, marking only the previous version as unusable. This version of the file still takes up free space, it is freed up if the erase operation is possible, i.e. there is no fragment of another file in a given memory block. In order to erase memory blocks more effectively, it is also possible to move some other files (which do not require modification) to another place, so that the block can be erased. An additional complication is the fact that the erase operation is much longer than the write and read operation.

Standard EEPROMs can only write or erase one memory cell at a time, which means that flash memories are much faster if the system using them writes and reads cells with different addresses at the same time. All types of EEPROMs, including flash memories, have a technologically limited number of erase (write) cycles – exceeding this number causes irreversible damage.

Flash memory is commonly used in all memory cards, USB sticks (flash drives) and SSD memories (SSD).

The following memory cards that use flash memory as a storage medium are currently in use:

• MultiMedia Card (MMC)
• Secure Digital (SD)
• Memory Stick (MS)
• CompactFlash (CF)
• SmartMedia (SM)
• xD Picture Card (xD)

In addition, they are used to store programs and data in embedded devices, where they are used instead of the once popular EPROM and PROM memories.