Memory is the ability of the brain to store, retain, and subsequently recall information. Although traditional studies of memory began in the realms of philosophy, the late nineteenth and early twentieth century put memory within the paradigms of cognitive psychology. In the recent decades, it has become one of the principal pillars of a new branch of science that represents a marriage between cognitive psychology and neuroscience, called cognitive neuroscience.
There are several ways of classifying memories, based on duration, nature and retrieval of information. From an information processing perspective there are three main stages in the formation and retrieval of memory:
•
Encoding (processing and combining of received information)
• Storage (creation of a permanent record of the encoded information)
•
Retrieval/Recall (calling back the stored information in response to some cue for use in some process or activity)

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The two main properties with which to assess the computer and human memory should be capacity and speed of retrieval. It is difficult to assess the human mind in terms of capacity, as its limits have not been proven. A computers capacity is steadily increasing with the more powerful ones being able to store hundreds of millions of bytes of information; a byte can roughly be equated to a single letter or number. This capacity pales in comparison to the
50,000,000,000 or so neurons in the brain. The speed at which a computer is able to retrieve information is measured in milliseconds,
The human mind is capable of times similar to this but in general the time varies considerably. The human minds' retrieval times seem to be dependant on the type of information being recalled.
The decay of information in the human mind can distort the information or loose it altogether (eventually), again, dependant on how it was learnt and the importance attached to it. Computer stores everything verbatim and is able to recall anything within milliseconds without loosing any information.

The model of human memory is like a computer-like information processing system. To remember any event requires that we
1.Get information into the brain (encoding)
2.Retain the information (storage)
3.Get the information back out later (retrieval)
A computer first translates input (keystrokes) into an electronic language, much as the brain encodes sensory information into neural language. The computer permanently stores vast amounts of information on a disk. From this information store it can retrieve a file or document into working memory, which can also receive information from the keyboard. Part of this working memory is visible on the screen.

We store vast amounts of information in long-term memory. And from our memory store we can retrieve information into an active working memory, part of which is displayed on our mental screen as short-term memory. Just as a computers screen-saver program blanks the screen after a period of inactivity, activated human memories rapidly decay unless kept active.

Random access memory (RAM) is the best-known form of computer memory. RAM is considered "random access" because you can access any memory cell directly if you know the row and column that intersect at that cell.
The opposite of RAM is serial access memory (SAM). SAM stores data as a series of memory cells that can only be accessed sequentially (like a cassette tape). If the data is not in the current location, each memory cell is checked until the needed data is found.
SAM works very well for memory buffers , where the data is normally stored in the order in which it will be used (a good example is the texture buffer memory on a video card).
RAM data, on the other hand, can be accessed in any order.
Read-only memory (ROM), also known as firmware , is an integrated circuit programmed with specific data when it is manufactured. ROM chips are used not only in computers, but in most other electronic items as well. In this edition of HowStuffWorks , you will learn about the different types of ROM and how each works. This article is one in a series of articles dealing with computer memory, including:

If you have been shopping for a computer, then you have heard the word "cache."
Modern computers have both L1 and L2 caches. You may also have gotten advice on the topic from well-meaning friends, perhaps something like "Don't buy that Celeron chip, it doesn't have any cache in it!"
It turns out that caching is an important computer-science process that appears on every computer in a variety of forms. There are memory caches, hardware and software disk caches, page caches and more. Virtual memory is even a form of caching. In this article, we will explore caching so you can understand why it is so important.

SRAM
Static random access memory uses multiple transistors, typically four to six, for each memory cell but doesn't have a capacitor in each cell. It is used primarily for cache.
DRAM
Dynamic random access memory has memory cells with a paired transistor and capacitor requiring constant refreshing.
FPM DRAM
Fast page mode dynamic random access memory was the original form of DRAM. It waits through the entire process of locating a bit of data by column and row and then reading the bit before it starts on the next bit. Maximum transfer rate to L2 cache is approximately 176 Mops.
EDO DRAM
Extended data-out dynamic random access memory does not wait for all of the

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 processing of the first bit before continuing to the next one. As soon as the address of the first bit is located, EDO DRAM begins looking for the next bit. It is about five percent faster than FPM.
Maximum transfer rate to L2 cache is approximately 264 MBps.
SDRAM
Synchronous dynamic random access memory takes advantage of the burst mode concept to greatly improve performance. It does this by staying on the row containing the requested bit and moving rapidly through the columns, reading each bit as it goes. The idea is that most of the time the data needed by the CPU will be in sequence. SDRAM is about five percent faster than EDO RAM and is the most common form in desktops today. Maximum transfer rate to L2 cache is approximately 528 MBps.
DDR SDRAM
Double data rate synchronous dynamic RAM is just like
SDRAM except that is has higher bandwidth, meaning greater speed. Maximum transfer rate to L2 cache is approximately
1,064 MBps (for DDR SDRAM 133 MHZ).

RDRAM
Rambus dynamic random access memory is a radical departure from the previous DRAM architecture. Designed by Rambus, RDRAM uses a Rambus in-line memory module (RIMM) , which is similar in size and pin configuration to a standard DIMM. What makes RDRAM so different is its use of a special high-speed data bus called the Rambus channel. RDRAM memory chips work in parallel to achieve a data rate of 800 MHz, or 1,600 MBps. Since they operate at such high speeds, they generate much more heat than other types of chips. To help dissipate the excess heat Rambus chips are fitted with a heat spreader, which looks like a long thin wafer. Just like there are smaller versions of DIMMs, there are also SO-RIMMs, designed for notebook computers.
Credit Card Memory
Credit card memory is a proprietary self-contained DRAM memory module that plugs into a special slot for use in notebook computers

PCMCIA Memory Card
Another self-contained DRAM module for notebooks, cards of this type are not proprietary and should work with any notebook computer whose system bus matches the memory card's configuration.
CMOS RAM
CMOS RAM is a term for the small amount of memory used by your computer and some other devices to remember things like hard disk settings -- see Why does my computer need a battery? for details. This memory uses a small battery to provide it with the power it needs to maintain the memory contents.
VRAM
Video RAM , also known as multi port dynamic random access memory
(MPDRAM), is a type of RAM used specifically for video adapters or 3-D accelerators.
The "multi port"

part comes from the fact that VRAM normally has two independent access ports instead of one, allowing the CPU and graphics processor to access the RAM simultaneously.
VRAM is located on the graphics card and comes in a variety of formats, many of which are proprietary. The amount of VRAM is a determining factor in the resolution and color depth of the display. VRAM is also used to hold graphics-specific information such as 3-D geometry data and texture maps. True multi port VRAM tends to be expensive, so today; many graphics cards use SGRAM (synchronous graphics RAM) instead. Performance is nearly the same, but SGRAM is cheaper.
Electronic memory comes in a variety of forms to serve a variety of purposes. Flash memory is used for easy and fast information storage in such devices as digital cameras and home video game consoles. It is used more as a hard drive than as RAM. In fact,
Flash memory is considered a solid-state storage device. Solid-state means that there are no moving parts -- everything is electronic instead of mechanical.

Here are a few examples of Flash memory:
•Your computer's BIOS chip
•Compact Flash (most often found in digital cameras)
•Smart Media (most often found in digital cameras)
•Memory Stick (most often found in digital cameras)
•PCMCIA Type I and Type II memory cards (used as solid-state disks in laptops)
•Memory cards for video game consoles
Removable Flash Memory Cards
While your computer's bios chip is the most common form of flash memory, removable solidstate storage devices are becoming increasingly popular
.
Smart Media and Compact Flash cards are both well known, especially as "electronic film" for digital cameras. Other removable
Flash memory products include Sony's Memory Stick , PCMCIA memory cards, and memory cards for video game systems such as Nintendo's N64, Sega's Dream cast and Sony's PlayStation.
We will focus on Smart Media and Compact Flash, but the essential idea is the same for all of these products. Every one of them is simply a form of Flash memory.

There are several reasons to use Flash memory instead of a hard disk:
•Flash memory is noiseless.
•It allows faster access.
•It is smaller in size.
•It is lighter.
•It has no moving parts
Virtual memory is a common part of most operating systems on desktop computers. It has become so common because it provides a big benefit for users at a very low cost.
In this article, you will learn exactly what virtual memory is, what your computer uses it for and how to configure it on your own machine to achieve optimal performance

One of the most common uses of Flash memory is for the basic input/output system of your computer, commonly known as the BIOS (pronounced "bye-ose"). On virtually every computer available, the BIOS make sure all the other chips, hard drives, ports and CPU function together.
Every desktop and laptop computer in common use today contains a microprocessor as its central processing unit. The microprocessor is the hardware component. To get its work done, the microprocessor executes a set of instructions known as software (see How Microprocessors
Work for details). You are probably very familiar with two different types of software:
•The operating system - The operating system provides a set of services for the applications running on your computer, and it also provides the fundamental user interface for your computer. Windows 98 and Linux are examples of operating systems. (See How Operating
Systems Work for lots of details.)
•The applications - Applications are pieces of software that are programmed to perform specific tasks. On your computer right now you probably have a browser application, a word processing application, an e-mail application and so on. You can also buy new applications and install them.
It turns out that the BIOS is the third type of software your computer needs to operate successfully. In this article, you'll learn all about BIOS -- what it does, how to configure it and what to do if your BIOS needs updating.

The term computer memory refers to the parts of a digital computer, which retain physical state (data) for some interval of time.
In its most common usage, "memory" refers to very fast storage, which does not retain its stored data when the power is turned off. Compare this to "storage", such as hard drive space, which is slow but keeps its data even without power. An analogy is to think of the storage as human memory, with the hard disk as long-term memory, and the memory as short-term memory.
In a home computer, memory will often take the form of:
• Random access memory , or RAM , which is used to temporarily store things such as programs and data while the computer, is using them. Since RAM can be accessed at very high speeds, it is well suited for this task.
•Cache memory is a small amount of very high-speed dedicated memory. Cache memory is used to allow quicker access to data, which ordinarily is slow to retrieve. Because of cache memory's high-speed nature, storing data into cache memory before it is actually accessed can allow quicker response times. Cache memory is found in microprocessors, hard drives and many other places.

In some ways human and computer memory are similar. Humans have short term memory (although this assumption is doubted now), and so do computers in the form of RAM (Random Access Memory). In fact, it is believed that in place of STM, humans now have a working memory which is what RAM is anyway.
Also similar is long term memory in humans and Hard Drive storage in computers. Information is held in the brain or on the disk and can be added to over time and as the brain or drive gets older, deterioration makes it harder to access this data.
There are other similarities. Like humans, computers memory is strengthened with practice. Depending on the software being used, a computer remembers the most common tasks performed and can be run quicker and quicker over time.
Viruses affect memory on humans and computers too in similar ways. In humans, diseases such as
Alzheimer's can obliterate the memory in much the same way a computer virus clears the drive of a computer.
But of course, human memory and computer memory is different. The human brain has the ability to fill in the gaps if it recognises prompts (this is what deja vu is, where the brain recalls some features of a situation and tries to fill in the blanks). Computers can't do this, working as it does in binary, it is either in memory or it isn't.
Where computer memory can be said to be better than humans is in the field of Expert Systems. Expert
Systems are steady, unemotional, and provide a complete response at all times. This may be very important in real-time and emergency situations when a human expert may not operate at peak efficiency because of stress or fatigue.