Multi-Level-Cell-Flash
Multi-level-cell flash stores more than one bit by distinguishing several cell states. In common product terminology, MLC usually means two bits per cell. Two bits require four distinguishable states, whereas one bit requires two. The broader idea of multiple levels also underlies later three-bit and four-bit technologies, commonly called TLC and QLC.
The controller reads a state through electrical thresholds rather than visible binary switches. Programming changes the cell’s threshold-voltage distribution, and reading compares the cell against appropriate references. A four-state diagram represents information categories, not four perfectly sharp physical voltages. Real distributions have width and change with use and retention conditions.
More information per cell trades physical margin for density. Fitting more states into a usable electrical range leaves less separation between neighboring distributions. Programming precision, sensing, error correction, and management become important to reliable storage. A density benefit should not be described as free capacity with unchanged endurance and performance under every condition.
NOR and NAND histories should be identified separately. KIOXIA’s technical history identifies its 2001 1 Gbit MLC NAND product as a NAND milestone. That statement should not be used as the invention date of every multilevel flash concept or NOR implementation. The architecture and the meaning of the reported milestone must remain explicit.
The practical specification extends beyond bits per cell. Compare capacity, program and read behavior, retention requirements, endurance, and controller responsibilities for the actual product. A cell encoding explains how density is increased; it does not by itself establish the reliability or sustained performance of a card, drive, or embedded storage system.
Sources and further reading. [KIOXIA multilevel technology](https://www.kioxia.com/en-jp/rd/technology/multi-level-cell.html); [Micron NAND selection guide](https://www.micron.com/products/storage/nand-flash/choosing-the-right-nand).
Read Multi-level-cell flash as a density technique that encodes multiple bits in carefully separated electrical states.