Transcript Flash memory

Primary storage
Primary storage is directly connected to the central processing unit
of the computer. It must be present for the CPU to function correctly,
just as in a biological analogy the lungs must be present (for oxygen
storage) for the heart to function (to pump and oxygenate the blood).
As shown in the diagram, primary storage typically consists of three
kinds of storage:
•In contemporary usage, memory usually refers to a form of solid
state storage known as random access memory (RAM) and
sometimes other forms of fast but temporary storage.
•Random-access memory (usually known by its acronym,
RAM) is a type of data store used in computers that allows the
stored data to be accessed in any order — that is, at random,
not just in sequence. In contrast, other types of memory devices
(such as magnetic tapes, disks, and drums) can access data on
the storage medium only in a predetermined order due to
constraints in their mechanical design.
Memory / USB Flash Memory
• Flash memory is a form of non-volatile computer memory that can
be electrically erased and reprogrammed.
– In contemporary usage, memory usually refers to a form of solid
state storage known as random access memory (RAM) and
sometimes other forms of fast but temporary storage.
– Random-access memory (usually known by its acronym, RAM)
is a type of data store used in computers that allows the stored
data to be accessed in any order — that is, at random, not just in
sequence. In contrast, other types of memory devices (such as
magnetic tapes, disks, and drums) can access data on the
storage medium only in a predetermined order due to constraints
in their mechanical design.
• Unlike EEPROM, it is erased and programmed in blocks consisting
of multiple locations (in early flash the entire chip had to be erased
at once).
• Flash memory costs far less than EEPROM and therefore has
become the dominant technology wherever a significant amount of
non-volatile, solid-state storage is needed.
•Flash memory is a form of non-volatile computer memory that can be
electrically erased and reprogrammed.
•Unlike EEPROM, Flash Memory is erased and programmed in
blocks consisting of multiple locations (in early flash the entire
chip had to be erased at once).
• Flash memory costs far less than EEPROM and therefore has become
the dominant technology wherever a significant amount of non-volatile,
solid-state storage is needed.
•Flash memory is also used in USB flash drives (thumb drives , handy
drive), which are used for general storage and transfer of data between
computers. It has also gained some popularity in the gaming market,
where it is often used instead of EEPROMs or battery-powered SRAM for
game save data.
Comparison with EPROM (Erasable Programmable Read Only Memory)
EPROM has two significant limitations:
it has to be taken out of the circuit and put under an ultraviolet light source to
erase it
higher (non-TTL) voltages are generally required to program it. These
voltages are normally not available in regular computer circuit boards.
EEPROM can be programmed and erased electrically.
The difference between EPROM and EEPROM lies in the thickness of the
insulating layer — in an EPROM it is about 3 nanometers thick whereas in
an EEPROM it is much thinner — typically around 1 nanometer. This thinner
insulating layer allows for lower voltages to be used for programming.
An EEPROM (also called an E2PROM) or Electrically Erasable
Programmable Read-Only Memory, is a non-volatile storage chip used in
computers and other devices to store small amounts of volatile
(configuration) data. When larger amounts of more static data are to be
stored (such as in USB flash drives) other memory types like flash memory
are more economic. SEEPROM, meaning Serial EEPROM, is an EEPROM
chip that uses a serial interface to the circuit board.
While RAM has no limitations on rewrites to memory, EEPROMs are
limited in that repeated write and erase cycles eventually damage the
thin insulating layer, a process called 'wear out'. New models specify
100,000 erase-write cycles or more.
Flash memory is cheaper than EEPROM but will wear out faster,
typically after 10,000 erase-write cycles. Another reason why
EEPROM is more effective for storing configuration data is that
Flash memory has to erase multiple memory locations at a time.
Changing a single byte is only possible by rewriting a whole block, which
causes the flash memory to wear out more quickly than one might
otherwise expect.
Principles of operation
Flash memory stores information in an array of floating gate
transistors, called "cells", each of which traditionally stores one bit of
information. Newer flash memory devices, sometimes referred to as
multi-level cell devices, can store more than 1 bit per cell, by using
more than two levels of electrical charge, placed on the floating gate
of a cell.
NAND Flash uses tunnel injection for writing and tunnel release for
erasing. NAND flash memory forms the core of the removable USB
interface storage devices known as USB flash drives.
Tunnel injection is the quantum tunneling effect, also called FowlerNordheim tunnel injection, when charge carriers are injected to an
electric conductor through a thin layer of an electric insulator.
It is used to program NAND flash memory. (Cf. tunnel release, used
for its erasing.)
An alternative to tunnel injection is the spin injection.
Essential components
There are typically three parts to a flash drive:
Male type-A USB connector - provides an interface to the host
computer.
USB mass storage controller - implements the USB host controller and
provides a linear interface to block-oriented serial flash devices while
hiding the complexities of block-orientation, block erasure, and wear
balancing, or wear levelling, although drives that actually perform this in
hardware are rare. The controller contains a small RISC microprocessor
and a small amount of on-chip ROM and RAM.
NAND flash memory chip - stores data. NAND flash is typically also
used in digital cameras.
Crystal oscillator - produces the device's main 12 MHz clock signal and
controls the device's data output through a phase-locked loop.
USB Flash Memory
USB flash drives are NAND-type flash memory data storage devices
integrated with a USB interface. They are typically small, lightweight,
removable and rewritable. Memory capacity typically ranges from
128 megabytes up to 64 gigabytes
NAND Type:
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. The blocks are
typically 512 or 2048 bytes in size. Associated with each block are a few
bytes (typically 12–16 bytes) that should be used for storage of an error
detection and correction block checksum.
NAND devices typically have software-based bad block management.
This means that when a logical block is accessed it is mapped to a
physical block, and the device has a number of blocks set aside for
compensating bad blocks and for storing primary and secondary mapping
tables.
Strengths and Weaknesses
•Like all flash memory devices, flash drives can sustain only a limited
number of write and erase cycles before failure.
Mid-range flash drives under normal conditions will support several
hundred thousand cycles, although write operations will gradually slow as
the device ages. This should be a consideration when using a flash drive
to run application software or an operating system. To address this, as
well as space limitations, some developers have produced special
versions of operating systems (such as Linux) or commonplace
applications (such as Mozilla Firefox) designed to run from flash drives.
These are typically optimized for size and configured to place temporary
or intermediate files in memory rather than store them temporarily on the
flash drive.