How computer memory works - Kanawat Senanan

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In many ways, our memories make us who we are, helping us remember our past, learn and retain skills, and plan for the future. And for the computers that often act as extensions of ourselves, memory plays much the same role. Kanawat Senanan explains how computer memory works.

Lesson by Kanawat Senanan, animation by TED-Ed.

In many ways,
our memories make us who we are,

helping us remember our past,

learn and retain skills,

and plan for the future.

And for the computers that often act
as extensions of ourselves,

memory plays much the same role,

whether it's a two-hour movie,

a two-word text file,

or the instructions for opening either,

everything in a computer's memory
takes the form of basic units called bits,

or binary digits.

Each of these is stored in a memory cell

that can switch between two states
for two possible values,

0 and 1.

Files and programs consist of millions
of these bits,

all processed in
the central processing unit,

or CPU,

that acts as the computer's brain.

And as the number of bits needing
to be processed grows exponentially,

computer designers face
a constant struggle

between size, cost, and speed.

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

and long-term memory
for more permanent storage.

When you run a program,

your operating system allocates area
within the short-term memory

for performing those instructions.

For example, when you press a key
in a word processor,

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

It could also modify them,
or create new ones.

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

And because program instructions must be
processed quickly and continuously,

all locations within the short-term memory
can be accessed in any order,

hence the name random access memory.

The most common type of RAM
is dynamic RAM, or DRAM.

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

that store electrical charges,

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

Such memory is called dynamic

because it only holds charges briefly
before they leak away,

requiring periodic recharging
to retain data.

But even its low latency
of 100 nanoseconds

is too long for modern CPUs,

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

made from static RAM.

That's usually made up
of six interlocked transistors

which don't need refreshing.

SRAM is the fastest memory
in a computer system,

but also the most expensive,

and takes up three times
more space than DRAM.

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

For data to remain
once the device is turned off,

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

which comes in three major types.

In magnetic storage,
which is the cheapest,

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

But because the disc must rotate
to where the data is located

in order to be read,

the latency for such drives is 100,000
times slower than that of DRAM.

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

also uses spinning discs,

but with a reflective coating.

Bits are encoded as light and dark spots
using a dye that can be read by a laser.

While optical storage media are cheap
and removable,

they have even slower latencies
than magnetic storage

and lower capacity as well.

Finally, the newest and fastest types of
long-term storage are solid-state drives,

like flash sticks.

These have no moving parts,

instead using floating gate transistors

that store bits by trapping
or removing electrical charges

within their specially designed
internal structures.

So how reliable
are these billions of bits?

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

but it actually degrades fairly quickly.

The heat generated from a device
and its environment

will eventually demagnetize hard drives,

degrade the dye in optical media,

and cause charge leakage
in floating gates.

Solid-state drives
also have an additional weakness.

Repeatedly writing to floating gate
transistors corrodes them,

eventually rendering them useless.

With data on most current storage media

having less than
a ten-year life expectancy,

scientists are working to exploit
the physical properties of materials

down to the quantum level

in the hopes of making
memory devices faster,

smaller,

and more durable.

For now, immortality remains out of reach,
for humans and computers alike.