A common property of mass storage systems based on magnetic or optic technology is that physical motion, such as spinning disks, moving read/write heads, and aiming laser beams, is required to store and retrieve data. This means that data storage and retrieval is slow compared to the speed of electronic circuitry. Flash memory technology has the potential of alleviating this drawback. In a flash memory system, bits are stored by sending electronic signals directly to the storage medium where they cause electrons to be trapped in tiny chambers of silicon dioxide, thus altering the characteristics of small electronic circuits.
Since these chambers are able to hold their captive electrons for many years, this technology is suitable for off-line storage of data. Although data stored in flash memory systems can be accessed in small byte-size units as in RAM applications, current technology dictates that stored data be erased in large blocks. Moreover, repeated erasing slowly damages the silicon dioxide chambers, meaning that current flash memory technology is not suitable for general main memory applications where its contents might be altered many times a second.
However, in those applications in which alterations can be controlled to a reasonable level, such as in digital cameras, cellular telephones, and hand-held PDAs, flash memory has become the mass storage technology of choice. Indeed, since flash memory is not sensitive to physical shock (in contrast to magnetic and optic systems) its potential in portable applications is enticing. Flash memory devices called flash drives, with capacities of up to a few hundred GBs, are available for general mass storage applications.
These units are packaged in small plastic cases approximately three inches long with a removable cap on one end to protect the unit’s electrical connector when the drive is off-line. The high capacity of these portable units as well as the fact that they are easily connected to and disconnected from a computer make them ideal for offline data storage. However, the vulnerability of their tiny storage chambers dictates that they are not as reliable as optical disks for truly long term applications. Another application of flash technology is found in SD (Secure Digital) memory cards or just SD Card). These provide up to two GBs of storage and are packaged in a plastic rigged wafer about the size a postage stamp (SD cards are also available in smaller mini and micro sizes), SDHC (High Capacity) memory cards can provide up to 32 GBs and the next generation SDXC (Extended Capacity) memory cards may exceed a TB. Given their compact physical size, these cards conveniently slip into slots of small electronic devices. Thus, they are ideal for digital cameras, smartphones, music players, car navigation systems, and a host of other electronic appliances.