0:00:06.879,0:00:10.030
In many ways, [br]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[br]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[br]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[br]for two possible values,

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0 and 1.

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Files and programs consist of millions[br]of these bits,

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all processed in [br]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[br]to be processed grows exponentially,

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computer designers face [br]a constant struggle

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between size, cost, and speed.

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Like us, computers have short-term memory[br]for immediate tasks,

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and long-term memory [br]for more permanent storage.

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When you run a program,

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your operating system allocates area[br]within the short-term memory

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for performing those instructions.

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For example, when you press a key[br]in a word processor,

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the CPU will access one of these locations[br]to retrieve bits of data.

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It could also modify them,[br]or create new ones.

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The time this takes is known [br]as the memory's latency.

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And because program instructions must be[br]processed quickly and continuously,

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all locations within the short-term memory[br]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 [br]is dynamic RAM, or DRAM.

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There, each memory cell consists[br]of a tiny transistor and a capacitor

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that store electrical charges,

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a 0 when there's no charge,[br]or a 1 when charged.

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Such memory is called dynamic

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because it only holds charges briefly[br]before they leak away,

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requiring periodic recharging[br]to retain data.

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But even its low latency [br]of 100 nanoseconds

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is too long for modern CPUs,

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so there's also a small, [br]high-speed internal memory cache

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made from static RAM.

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That's usually made up [br]of six interlocked transistors

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which don't need refreshing.

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SRAM is the fastest memory [br]in a computer system,

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but also the most expensive,

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and takes up three times [br]more space than DRAM.

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But RAM and cache can only hold data[br]as long as they're powered.

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For data to remain[br]once the device is turned off,

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it must be transferred [br]into a long-term storage device,

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which comes in three major types.

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In magnetic storage, [br]which is the cheapest,

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data is stored as a magnetic pattern on[br]a spinning disc coated with magnetic film.

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But because the disc must rotate [br]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 [br]times slower than that of DRAM.

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On the other hand, optical-based storage[br]like DVD and Blu-ray

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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[br]using a dye that can be read by a laser.

0:03:28.029,0:03:31.151
While optical storage media are cheap[br]and removable,

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they have even slower latencies [br]than magnetic storage

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and lower capacity as well.

0:03:37.236,0:03:42.871
Finally, the newest and fastest types of[br]long-term storage are solid-state drives,

0:03:42.871,0:03:44.025
like flash sticks.

0:03:44.025,0:03:45.957
These have no moving parts,

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instead using floating gate transistors

0:03:48.627,0:03:53.134
that store bits by trapping [br]or removing electrical charges

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within their specially designed [br]internal structures.

0:03:56.453,0:03:59.739
So how reliable [br]are these billions of bits?

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We tend to think of computer memory[br]as stable and permanent,

0:04:03.463,0:04:06.363
but it actually degrades fairly quickly.

0:04:06.363,0:04:09.000
The heat generated from a device[br]and its environment

0:04:09.000,0:04:11.739
will eventually demagnetize hard drives,

0:04:11.739,0:04:13.991
degrade the dye in optical media,

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and cause charge leakage [br]in floating gates.

0:04:17.115,0:04:20.081
Solid-state drives [br]also have an additional weakness.

0:04:20.081,0:04:24.095
Repeatedly writing to floating gate [br]transistors corrodes them,

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eventually rendering them useless.

0:04:26.705,0:04:29.215
With data on most current storage media

0:04:29.215,0:04:31.958
having less than [br]a ten-year life expectancy,

0:04:31.958,0:04:36.333
scientists are working to exploit[br]the physical properties of materials

0:04:36.333,0:04:38.649
down to the quantum level

0:04:38.649,0:04:40.998
in the hopes of making [br]memory devices faster,

0:04:40.998,0:04:42.063
smaller,

0:04:42.063,0:04:43.609
and more durable.

0:04:43.609,0:04:49.045
For now, immortality remains out of reach,[br]for humans and computers alike.