FLASH memory

EEPROM (short for Electrically-Erasable Programmable Read-Only Memory) is a type of non-volatile computer memory. It is also referred to as E²PROM. Unlike EPROM memory, EEPROM can only be erased using electric current. The number of write and erase cycles is limited, depending on the type and manufacturer of the memory, up to 100,000 cycles. Exceeding this value can damage the memory. The number of read cycles is unlimited. It is used to store a small amount of data that must be accessible after power loss. An extension of EEPROM is Flash memory, which increases the writing speed to memory by using buffers.

Classification of flash memory depending on the type of logic gate used:

• NOR flash memory – uses the NOR logic function
• NAND flash memory – uses the NAND logic function

NOR flash memory NOR 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 firmware for various 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 NAND, compared to NOR memory, has shorter write and erase times, greater data density, a more favorable cost-to-capacity ratio, and ten times greater durability. However, the main feature of this type of memory is sequential access to data. This limits its applications only as storage memory, e.g., in memory cards. The first memory card based on NAND flash memory was the SmartMedia card. Later, they were also used in other types of memory cards, such as MultiMedia Card, Secure Digital, Memory Stick, and xD Picture Card, as well as in USB memory (pendrive).

To write to a flash memory cell, it must first be erased. It is not possible to rewrite data to an already written 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 erased. For this reason, data writing is not fully flexible. These memories allow reading and writing of any cell, but not free writing/reading of content, as is the case with RAM.

The above limitations cause certain difficulties in managing data access in storage memories. Writing must be coordinated with the operation of erasing memory blocks. Typically, if a file is to be updated or overwritten, the memory management system creates a new copy of the file in another location, marking only the previous version as useless. Such a version of the file still occupies free space, which is released if the erase operation is possible, meaning there is no fragment of another file in that memory block. To facilitate more efficient erasure of memory blocks, it is also possible to move parts of other files (not requiring modification) to another location so that the block can be erased. An additional complication is that the erase operation is significantly longer than the write and read operations.

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

Flash memories are commonly used in all types of memory cards, USB memory (pendrive), and SSDs (SSD drives).

Currently, the following memory cards using flash memory as a data carrier are in use:

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

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