Flash Memory: Definition and additional resources from ZDNet

(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 future RAM chips). 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 BIOS chips replaced ROM BIOS chips in PCs so that the BIOS could be updated in place instead of having to be removed and replaced (see ROM BIOS).

NOR and NAND
Flash chips are constructed of either NOR or NAND logic gates. NOR works like a computer's memory, while NAND functions like a hard disk. For example, in a digital camera, a NOR flash chip holds the software, while NAND flash is the chip in the memory card for storing images. See logic gate.

NOR Flash for Software
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 just like computers fetch instructions from main memory.

Before any writing can take place, both NOR and NAND flash cells must be erased in large blocks ranging from 16KB to 128KB. NOR flash has a lifespan of about 100 thousand write cycles.

NAND Flash for Storage
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 functions like a disk rather than memory. Flash Translation Layer (FTL) software makes NAND-based flash memory cards and USB drives look like a regular disk drive to the operating system, which reads and writes them like disk sectors.

NAND cells can be rewritten up to a million times, and "wear leveling" extends write cycles further by evenly distributing writes across the entire chip. In practice, most users will not wear out a memory card or USB drive due to constant use. See USB drive, CompactFlash, SmartMedia, Memory Stick, SD Card and xD Picture Card.

"Digital Film"
Flash memory cards are used to store images on a digital camera. This is a CompactFlash card.

Flash Card Readers
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.

Reproduced with permission from Computer Desktop Encyclopedia.
Copyright (c) 1981-2008 The Computer Language Company Inc.
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Additional Resources

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