(1) See also Flash.|
(2) A very popular non-volatile, rewritable memory chip used for storage. Extremely durable, flash memory is used in USB drives, digital camera memory cards (see examples below) and increasingly as a hard disk replacement in laptops. Years ago, flash replaced the permanent read-only-memory (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). See firmware.
Evolving from the EEPROM, 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.
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 never wear out the flash memory in their devices. 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.
Flash memory cards are used to store images on a digital camera. This is a CompactFlash card.
Memory card readers function like disk drives to the computer and come in a variety of models. The earlier reader (top) supported only CompactFlash and SmartMedia, while newer units (bottom) handle all popular flash memory formats.