Hardware Reference
In-Depth Information
carrier injection into the floating gate. Electrons become embedded into the float-
ing gate, which places a negative charge internal to the flash transistor. The embed-
ded negative charge increases the voltage necessary to turn on the flash transistor,
and by testing whether or not the channel turns on with a high or low voltage, it is
possible to determine whether the floating gate is charged or not, resulting ina0or
1 value for the flash cell. The embedded charge remains in the transistor, even if
power is removed from the system, making the flash memory cell nonvolatile.
Because SSDs are essentially memory, they have superior performance to spin-
ning disks and have zero seek time. While a typical magnetic disk can access data
up to 100 MB/sec, a SSD can operate two to three times faster. And because the
device has no moving parts, it is particularly suited for use in notebook computers,
where jarring and movement will not affect its ability to access data. The downside
of SSDs, compared to magnetic disks, is their cost. While magnetic disks cost pen-
nies/gigabyte, a typical SSD will cost one to three dollars/gigabyte, making their
use appropriate only for smaller drive applications or situations that are not cost
sensitive. The cost of SSDs is dropping, but they still have a long way to go to
catch up to cheap magnetic disks. So while SSDs are replacing magnetic disks in
many computers, it will likely be a long time before the magnetic disk goes the
way of the dinosaur (unless another big meteorite strikes the earth, in which cases
the SSDs are probably not going to survive either).
Another disadvantage of SSDs compared with magnetic disks is their failure
rate. A typical flash cell can be written only about 100,000 times before it will no
longer function. The process of injecting electrons into the floating gate slowly
damages it and the surrounding insulators, until it can no longer function. To in-
crease the lifetime of SSDs, a technique called
wear leveling
is used to spread
writes out to all flash cells in the disk. Every time a new disk block is written, the
destination block is reassigned to a new SSD block that has not been recently writ-
ten. This requires the use of a logical block map inside the flash drive, which is one
of the reasons that flash drives have high internal storage overheads. Using wear
leveling, a flash drive can support a number of writes equal to the number of writes
a cell can sustain times the number of blocks on the disk.
Some SSDs are able to encode multiple bits per byte using multilevel flash
cells. The technology carefully controls the amount of charge placed into the float-
ing gate. An increasing sequence of voltages is then applied to the control gate to
determine how much charge is stored in the floating gate. Typical multilevel cells
will support four charge levels, yielding two bits per flash cell.
Optical disks were originally developed for recording television programs, but
they can be put to more esthetic use as computer storage devices. Due to their
large capacity and low price optical disks are widely used for distributing software,
books, movies, and data of all kinds, as well as making backups of hard disks.
