Hardware Reference
In-Depth Information
Flash memory can be considered sort of a cross between random access memory (RAM)
and read-only memory (ROM). Just like RAM, flash memory can be written directly in
the system, yet just like ROM it is completely nonvolatile, meaning that it retains data
after the power has been turned off (and without a battery like the Complementary Met-
al Oxide Semiconductor [CMOS] RAM). Besides being nonvolatile, there is one other
big difference between flash memory and conventional RAM: The system cannot rewrite
Flash memory; it must always erase it first.
When erased, flash memory cells are in a low-voltage state that carries a logical 1 value.
The act of writing to (or programming) flash places a charge in the transistor's floating
gate,whichchangesthe1toa0.Onceaflashcellisprogrammed(thatis,changedtoa0),
the only way it can be changed back to a 1 is by erasing it. The problem with this is that,
althoughyoucanprogramindividualcellsorpages,youcanonlyerasecellsorpagesona
blockbasis,andablockusuallyconsistsofthousandsofcells(512KBinmostcases).The
actual programming and erasing process coaxes electrons into and out of the transistor's
floating gate by a process known as Fowler-Nordheim tunneling.
The two major types of flash memory technology are called NOR (Not OR) and NAND
(Not AND). Both use the same basic transistor (cell) design, but they differ in how the
cells are interconnected. NOR flash works more like dynamic RAM (DRAM), providing
high-speed random-access capabilities with the ability to read or write data in single byte
quantities. NOR flash is the type of memory used for flash ROMs, such as those found in
motherboards, cell phones, and most devices that have updatable firmware.
On the other hand, NAND flash works more like a storage device, reading and writing
data in pages or blocks instead of individual bytes. NAND flash is used in devices that
storefile-orienteddata,suchasSSDs,USBkeyorthumbdrives,digitalcamerasanddigit-
al film media, music players, and more. NAND flash is more dense than NOR flash, stor-
ing more data in a given amount of die space and costing less overall for a given amount
of storage.
The speed, low power requirements, and compact size of recent flash memory and SSD
devices have made flash memory a perfect counterpart for portable devices such as laptop
computers and digital cameras, which often refer to flash memory devices as so-called
“digital film.” Unlike real film, digital film can be erased and reshot. Ultra-compact, USB
flashmemorydriveshaveallbutreplacedtraditionalfloppydrives,Zip/SuperDiskdrives,
and even optical discs for transporting data between systems.
Several types of flash memory devices have been popular, including the following:
• CompactFlash (CF)
• SmartMedia (SM)
