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In many ways,
our memories make us who we are,
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helping us remember our past,
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learn and retain skills,
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and plan for the future.
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And for the computers that often act
as extensions of ourselves,
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memory plays much the same role,
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whether it's a two-hour movie,
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a two-word text file,
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or the instructions for opening either,
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everything in a computer's memory
takes the form of basic units called bits,
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or binary digits.
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Each of these is stored in a memory cell
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that can switch between two states
for two possible values,
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zero and one.
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Files and programs consist of millions
of these bits,
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all processed in
the central processing unit,
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or CPU,
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that acts as the computer's brain.
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And as the number of bits needing
to be processed grows exponentially,
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computer designers face
a constant struggle
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between size, cost, and speed.
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Like us, computers have short-term memory
for immediate tasks,
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and long-term memory
for more permanent storage.
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When you run a program,
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your operating system allocates area
withing the short-term memory
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for performing those instructions.
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For example, when you press a key
in a word processor,
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the CPU will access one of these locations
to retrieve bits of data.
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It could also modify them,
or create new ones.
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The time this takes is known
as the memory's latency.
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And because program instructions must be
processed quickly and continuously,
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all locations within the short-term memory
can be accessed in any order,
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hence the name random access memory.
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The most common type of RAM
is dynamic RAM, or DRAM.
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There, each memory cell consists
of a tiny transistor and a capacitor
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that story electrical charges,
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a zero when there's no charge,
or a one when charged.
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Such memory is called dynamic
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because it only holds charges briefly
before they leak away,
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requiring periodic recharging
to retain data.
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But even its low latency
of 100 nanoseconds
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is too long for modern CPUs,
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so there's also a small,
high-speed internal memory cache
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made from static RAM.
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That's usually made up
of six interlocked transistors
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which don't need refreshing.
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SRAM is the fastest memory
in a computer system,
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but also the most expensive,
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and takes up three times
more space than DRAM.
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But RAM and cache can only hold data
as long as they're powered.
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For data to remain
once the device is turned off,
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it must be transferred
into a long-term storage device,
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which comes in three major types.
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In magnetic storage,
which is the cheapest,
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data is stored as a magnetic pattern on
a spinning disc coded with magnetic film.
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But because the disc must rotate
to where the data is located
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in order to be read,
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the latency for such drives is 100,000
times slower than that of DRAM.
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On the other hand, optical-based storage
like DVD and BluRay
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also uses spinning discs,
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but with a reflective coating.
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Bits are encoded as light and dark spots
using a dye that can be read by a laser.
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While optical storage media are cheap
and removable,
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they have even slower latencies
than magnetic storage,
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and lower compacity as well.
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Finally, the newest and fastest types of
long-term storage are solid-state drives,
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like flash sticks.
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These have no moving parts,
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instead using floating gate transistors
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that store bits by trapping
or removing electrical charges
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within their specially designed
internal structures.
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So how reliable
are these billions of bits?
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We tend to think of computer memory
as stable and permanent,
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but it actually degrades fairly quickly.
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The heat generated from a device
and its environment
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will eventually demagnetize hard-drives,
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degrade the dye in optical media,
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and cause charge leakage
in floating gates.
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Solid-state drives
also have an additional weakness.
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Repeatedly writing to floating gate
transistors corrodes them,
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eventually rendering them useless.
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With data on most current storage media
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having less than
a ten-year life expectancy,
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scientists are working to exploit
the physical properties of materials
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down to the quantum level
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in the hopes of making
memory devices faster,
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smaller,
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and more durable.
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For now, immortality remains out of reach,
for humans and computers alike.