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Flash Memory: Theory and Applications
Low-level access to a physical
flash memory by device driver software is different from accessing common
memories. Whereas a common RAM will simply respond to read and write operations
by returning the contents or altering them immediately, flash memories need
special considerations, especially when used as program memory to a read-only
memory (ROM).
While reading data can be
performed on individual addresses on NOR memories (not on NAND memories)
unlocking (making available for erase or write), erasing and writing operations
are performed block-wise on all flash memories. A typical block size will be 64,
128, or 256 KiB.
The read-only mode of NOR
memories is similar to reading from a common memory, provided address and data
bus is mapped correctly, so NOR flash memory is much like any address-mapped
memory. NOR flash memories can be used as execute-in-place memory, meaning it
behaves as a ROM memory mapped to a certain address.
When unlocking, erasing or
writing NOR memories, special commands are written to the first page of the
mapped memory. These commands are defined as the common flash interface
(defined by Intel) and the flash circuit will provide a list of all available
commands to the physical driver.
Apart from being used as a ROM,
the NOR memories can of course also be partitioned with a file system and used
as any storage device.
NAND flash memories cannot
provide execute-in-place due to their different construction principles. These
memories are accessed much like block devices such as hard disks or memory
cards. When executing software from NAND memories, virtual memory strategies are
used: memory contents must first be paged into memory-mapped RAM and executed
there, making the presence of a memory management unit (MMU) on the system
absolutely necessary.
NOR
offers faster read speed and random access capabilities, making it suitable for
code storage in devices such as PDAs and cell phones. However, with NOR
technology, write and erase functions are slow compared to NAND. NOR also
has a larger memory cell size than NAND, limiting scaling capabilities and
therefore achievable bit density compared to NAND.
Conversely,
NAND offers fast write/erase capability and is slower than NOR in the area of
read speed. NAND is, however, more than sufficient for a majority of
consumer applications such as digital video, music or data storage.
NAND’s fast write/erase speed combined with its higher available densities and
a lower cost-per-bit than NOR make it the favored technology for file storage in
a host of consumer applications. Offering users the ability to rewrite
data quickly and repeatedly, NAND is typically used for storing large quantities
of information in devices such as Flash drives, MP3 players, multi-function cell
phones, digital cameras and USB drives.
NAND
vs. NOR
NAND's
advantages are fast write (program) and erase operations, while NOR's advantages
are random access and byte write capability. NOR's random access ability allows
for execute in place (XiP) capability, which is often a requirement in embedded
applications. The disadvantages for NAND are slow random access, while NOR is
hampered by are slow write and erase performance. NAND is better suited for file
applications. However, more processors include a direct NAND interface and can
boot directly from NAND (without NOR).
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