helping us remember our past,

讓我們想起過去、

learn and retain skills,

學習、維持技能、

and plan for the future.

以及計畫未來。

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

對作為人類延伸角色的電腦來說，

memory plays much the same role,

記憶體的功用大致相同，

whether it's a two-hour movie,

無論 1 部 2 小時的電影、

a two-word text file,

或 2 個單字的文字檔、

or the instructions for opening either,

或是打開它們的指令，

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

無論如何，電腦記憶體 都以「位元」形式存放，

or binary digits.

或稱「二進位」數字。

Each of these is stored in a memory cell

每 1 位元存放於 1 個「記憶元」中，

that can switch between two states for two possible values,

且有 2 個狀態代表 2 個值：

0 and 1.

0 與 1。

Files and programs consist of millions of these bits,

檔案與程式由數百萬個位元組成，

all processed in the central processing unit,

全部都在中央處理器中處理，

or CPU,

稱為 CPU －

that acts as the computer's brain.

作用如同電腦的大腦。

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

隨著處理的位元容量指數成長，

computer designers face a constant struggle

電腦設計師不斷努力處理

between size, cost, and speed.

容量、成本、和處理速度 三方面的難題。

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

如同我們，電腦擁有短暫記憶體， 用於即時任務，

and long-term memory for more permanent storage.

與永久記憶體作為儲存空間。

When you run a program,

當你執行程式時，

your operating system allocates area within the short-term memory

作業作系統會分配短暫記憶體

for performing those instructions.

以便執行程式指令。

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

例如，當你在編輯器中 上按下 1 個鍵時，

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

CPU 立即存取記憶體， 取得資料對應的位元組。

It could also modify them, or create new ones.

可以修改位元組的值， 或是新增位元組。

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

存取所花費的時間稱為「讀取時間」。

And because program instructions must be processed quickly and continuously,

由於程式的指令必須 迅速與連續的進行處理，

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

因此所有短暫記憶體 皆可按照任意順序來存取，

hence the name random access memory.

故又稱為「隨機存取記憶體」 - R A M。

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

最常見的 RAM 為 「動態隨機存取記憶體」- D R A M

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

每個記憶元由微小 電晶體和電容組成，

that store electrical charges,

可以儲存電荷，

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

0 值為不含電荷， 1 值則含電荷。

Such memory is called dynamic

此類記憶體稱為「動態」 的原因是：

because it only holds charges briefly before they leak away,

因為電荷洩漏很快， 所以維持狀態很短暫，

requiring periodic recharging to retain data.

需要高速重複 供應電荷以保持狀態。

But even its low latency of 100 nanoseconds

即便已是相當快速的 0.1 微秒的讀取時間，

is too long for modern CPUs,

對於現代 CPU 來說仍然太慢，

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

所以另一種高速「快取記憶體」

made from static RAM.

由「靜態」RAM 組成- SRAM。

That's usually made up of six interlocked transistors

通常以 6 個相互聯結的電晶體構成，

which don't need refreshing.

不需高速重複充電。

SRAM is the fastest memory in a computer system,

SRAM 是計算機系統 中最快的記憶體，

but also the most expensive,

但也是最貴的，

and takes up three times more space than DRAM.

比 DRAM 多 3 倍的占用空間。

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

RAM 和快取記憶體只能在 供電情況下保持資料。

For data to remain once the device is turned off,

若要在斷電後保留資料，

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

必須將它們搬到「永久」儲存裝置上，

which comes in three major types.

市面上有 3 種儲存裝置。

In magnetic storage, which is the cheapest,

「磁」儲存裝置最便宜，

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

資料以「磁性」形式儲存於 磁膜塗層的轉盤上。

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

因圓盤必須旋轉到資料儲存的位置

in order to be read,

才能讀取得到，

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

所以「磁」儲存裝置的讀取時間 比 DRAM 慢了100,000 倍。

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

第 2 類為「光」儲存裝置， 如： DVD 與 藍光

also uses spinning discs,

同樣也是轉盤設計，

but with a reflective coating.

不過使用的是「光可反射」塗層。

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

位元值使用可被雷射讀取的 顏料編碼成「光點」與「暗點」。

While optical storage media are cheap and removable,

儘管光儲存裝置便宜且可移除，

they have even slower latencies than magnetic storage

但它們的讀取時間比磁儲存裝置更慢

and lower capacity as well.

容量較小。

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

最後一類永久儲存裝置為 最新型的固態硬碟，

like flash sticks.

例如快閃隨身碟。

These have no moving parts,

它們沒有機械組件，

instead using floating gate transistors

而是使用浮動閘極電晶體

that store bits by trapping or removing electrical charges

「捕捉」與「釋放」電荷來 呈現資料的位元值

within their specially designed internal structures.

這是利用內部結構 的特殊設計使然。

So how reliable are these billions of bits?

而這些成千上百億的 位元可靠性如何？

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

我們總認為電腦記憶體 具有穩定性和永久性，

but it actually degrades fairly quickly.

但實際上性能降低相當得快。

The heat generated from a device and its environment

裝置和本身與周邊所產生的熱

will eventually demagnetize hard drives,

最終將導致硬碟消磁，

degrade the dye in optical media,

光碟上的顏料退化，

and cause charge leakage in floating gates.

以及浮動閘極電荷的漏失。

Solid-state drives also have an additional weakness.

固態硬碟還有其他缺點：

Repeatedly writing to floating gate transistors corrodes them,

在不斷重複寫入下， 將造成電晶體的銹蝕，

eventually rendering them useless.

最終導致失效。

With data on most current storage media

當今大絕大多數儲存的資料

having less than a ten-year life expectancy,

預期壽命都不會超過 10 年，

scientists are working to exploit the physical properties of materials

所以科學家正嘗試活用 物質的物理特性

down to the quantum level

深入至量子研究層級，

in the hopes of making memory devices faster,

希望下一代記憶體裝置可以更快、

smaller,

更小、

and more durable.

更耐久。

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

眼下，「永恆」對於人類或是 電腦而言仍是遙不可及。