Information Technology Reference
In-Depth Information
Source
Control
Drain
Control
Gate
Floating
Gate
Source
Drain
Figure12.5: A floating gate transistor.
in place. NAND flash storage is wired to allow reads and writes of a page at a
time, where a page is typically 2 KB to 4 KB. NAND flash is more dense than
NOR flash, so NAND is used in the storage systems we will consider.
Flash storage access and performance
Flash storage is accessed using
three operations.
Erase erasure block. Before flash memory can be written, it must be
erased by setting each cell to a logical \1". Flash memory can only be
erased in large units called erasure blocks. Today, erasure blocks are often
Denition: erasure blocks
128 KB to 512 KB. Erasure is a slow operation, usually taking several
milliseconds.
Erasing an erasure block is what gives flash memory its name for its re-
semblance to the flash of a camera.
Write page. Once erased, NAND flash memory can be written on a
page by page basis, where each page is typically 2048-4096 bytes. Writing
a page typically takes tens of microseconds.
Read page. NAND flash memory can be read on a page by page basis.
Reading a page typically takes tens of microseconds.
Notice that to write a page, its entire erasure block must first be erased.
This is a challenge both because erasure is slow and because erasure affects a
large number of pages. Flash drives implement a flash translation layer (FTL)
Denition: flash
translation layer
that maps logical flash pages to different physical pages on the flash device.
Then, when a single logical page is overwritten, the FTL writes the new version
