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 chargesāa 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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