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  • 7 months ago
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.
Transcript
00:00In many ways, our memories make us who we are,
00:10helping us remember our past, learn and retain skills,
00:14and plan for the future.
00:16And for the computers that often act as extensions of ourselves,
00:20memory plays much the same role.
00:22Whether it's a two-hour movie, a two-word text file,
00:25or the instructions for opening either,
00:28everything in a computer's memory takes the form of basic units called bits,
00:33or binary digits.
00:36Each of these is stored in a memory cell that can switch between two states
00:40for two possible values, zero and one.
00:44Files and programs consist of millions of these bits,
00:47all processed in the central processing unit, or CPU,
00:51that acts as the computer's brain.
00:54And as the number of bits needing to be processed
00:57grows exponentially, computer designers face a constant struggle
01:02between size, cost, and speed.
01:06Like us, computers have short-term memory for immediate tasks,
01:10and long-term memory for more permanent storage.
01:14When you run a program, your operating system allocates area
01:17within the short-term memory for performing those instructions.
01:21For example, when you press a key in a word processor,
01:24the CPU will access one of these locations to retrieve bits of data.
01:30It could also modify them or create new ones.
01:34The time this takes is known as the memory's latency.
01:39And because program instructions must be processed quickly and continuously,
01:44all locations within the short-term memory can be accessed in any order,
01:49hence the name random access memory.
01:51The most common type of RAM is dynamic RAM, or DRAM.
01:56There, each memory cell consists of a tiny transistor and a capacitor
02:01that store electrical charges—a zero when there's no charge,
02:05or a one when charged.
02:07Such memory is called dynamic because it only holds charges briefly before they leak away,
02:13requiring periodic recharging to retain data.
02:16But even its low latency of 100 ns is too long for modern CPUs.
02:22So there's also a small, high-speed internal memory cache made from static RAM.
02:28That's usually made up of six interlocked transistors, which don't need refreshing.
02:33SRAM is the fastest memory in a computer system, but also the most expensive,
02:38and takes up three times more space than DRAM.
02:42But RAM and cache can only hold data as long as they're powered.
02:46For data to remain once the device is turned off,
02:49it must be transferred into a long-term storage device,
02:53which comes in three major types.
02:55In magnetic storage, which is the cheapest,
02:58data is stored as a magnetic pattern on a spinning disk coated with magnetic film.
03:03But because the disk must rotate to where the data is located in order to be read,
03:08the latency for such drives is 100,000 times slower than that of DRAM.
03:14On the other hand, optical-based storage like DVD and Blu-ray also uses spinning disks,
03:20but with a reflective coating.
03:23Bits are encoded as light and dark spots using a die that can be read by a laser.
03:28While optical storage media are cheap and removable,
03:31they have even slower latencies than magnetic storage, and lower capacity as well.
03:37Finally, the newest and fastest types of long-term storage are solid-state drives, like flash sticks.
03:43These have no moving parts, instead using floating gate transistors
03:48that store bits by trapping or removing electrical charges
03:53within their specially designed internal structures.
03:57So how reliable are these billions of bits?
04:00We tend to think of computer memory as stable and permanent,
04:03but it actually degrades fairly quickly.
04:06The heat generated from a device and its environment will eventually demagnetize hard drives,
04:12degrade the die in optical media, and cause charge leakage in floating gates.
04:17Solid-state drives also have an additional weakness.
04:20Repeatedly writing to floating gate transistors corrodes them,
04:24eventually rendering them useless.
04:27With data on most current storage media having less than a 10-year life expectancy,
04:32scientists are working to exploit the physical properties of materials down to the quantum level,
04:38in the hopes of making memory devices faster, smaller, and more durable.
04:43For now, immortality remains out of reach for humans and computers alike.
05:02Many other people can have excellent maps to the shore.
05:06I guess they're getting the soil built by new people in other places.
05:07How many objects in robots experience so that they can normally do?
05:11They may not be able to find the job in their first few years.
05:14They may not be able to push the molecules, but not mind-based.
05:15They may not be able to push the molecules in their own lives.
05:18They may not be able to push the molecules into the room.
05:20They may not be able to push the molecules, but not mind-by-book.
05:24The image is often all the way behind them being able to push the molecules in the water.
05:26They may not be able to push them to keep the molecules and how they can re-e-sour they will.
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