flash memory - technical definition

(1) See also Flash.

(2) A very popular non-volatile, rewritable memory chip. Evolving from the EEPROM chip, flash was invented by Toshiba and named after its ability to erase a block of a data "in a flash." Ironically, this block erasing is one of its least desired features and one the industry is trying to eliminate through the development of newer technologies (see charge trap flash and future memory chips). For more about the architecture of the flash cell, see EEPROM.

Extremely durable, flash is widely used for storage modules such as USB drives and digital camera memory cards (see examples below). It is also packaged as stand-alone chips for circuit board mounting. For example, years ago, flash replaced the permanent ROM chip on a PC motherboard that held the BIOS. Instead of having to open the case and physically replace the ROM BIOS chip, updated BIOS code could be downloaded into the flash chip (see BIOS).

NOR and NAND
Flash chips are constructed of either NOR or NAND logic gates. NOR works like a computer's memory, while NAND is like a hard disk. For example, in a digital camera, an internal NOR chip holds the software, while NAND chips are in the removable memory cards. See logic gate.

NOR Flash for Software (Linear Flash)
Developed by Intel in 1988, NOR flash supports one-byte random access, which means machine instructions can be fetched and executed directly from the flash chip (executed in place) just like computers fetch instructions from main memory. NOR flash uses a "linear flash" interface, which is continuous addressable memory.

Before any writing can take place, both NOR and NAND flash cells must be erased in large blocks, typically 128KB in size. NOR flash has a lifespan of about 10 thousand write cycles.

NAND Flash for Storage (ATA Flash)
Developed by Toshiba a year after Intel's NOR flash, NAND is more durable and less expensive. The cells are denser, and erasing and writing are faster. NAND-based flash memory cards and USB drives use the "ATA flash" interface, which resembles a removable ATA hard disk (see IDE). To the operating system, NAND flash is just another disk drive that reads and writes sectors.

NAND cells can be rewritten about 100,000 times; however, "wear leveling" distributes the writing across all the cells and can greatly improve the chip's longevity. In practice, most users will not wear out a memory card or USB drive. In 2008, Japanese scientists prototyped ferroelectric-based NAND flash (FNF) memory, which is said to be denser, use less power and be capable of tens of millions of writes. See USB drive, CompactFlash, SmartMedia, Memory Stick, SD Card and xD Picture Card.