Computer memory


Информатика, кибернетика и программирование

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.



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Computer memory

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.

RAM Basics
Similar to a microprocessor, a memory chip is an integrated circuit (IC) made of millions of transistors and capacitors. In the most common form of computer memory, dynamic random access memory (DRAM), a transistor and a capacitor are paired to create a memory cell, which represents a single bit of data. The capacitor holds the bit of information -- a 0 or a 1 (see How Bits and Bytes Work for information on bits). The transistor acts as a switch that lets the control circuitry on the memory chip read the capacitor or change its state.

A capacitor is like a small bucket that is able to store electrons. To store a 1 in the memory cell, the bucket is filled with electrons. To store a 0, it is emptied. The problem with the capacitor's bucket is that it has a leak. In a matter of a few milliseconds a full bucket becomes empty. Therefore, for dynamic memory to work, either the CPU or the memory controller has to come along and recharge all of the capacitors holding a 1 before they discharge. To do this, the memory controller reads the memory and then writes it right back. This refresh operation happens automatically thousands of times per second.

This refresh operation is where dynamic RAM gets its name. Dynamic RAM has to be dynamically refreshed all of the time or it forgets what it is holding. The downside of all of this refreshing is that it takes time and slows down the memory.

Memory cells are etched onto a silicon wafer in an array of columns (bitlines) and rows (wordlines). The intersection of a bitline and wordline constitutes the address of the memory cell.

Memory is made up of bits arranged in a two-dimensional grid.
In this figure, red cells represent 1s and white cells represent 0s.
In the animation, a column is selected and then rows are charged to write data into the specific column.

DRAM works by sending a charge through the appropriate column (CAS) to activate the transistor at each bit in the column. When writing, the row lines contain the state the capacitor should take on. When reading, the sense-amplifier determines the level of charge in the capacitor. If it is more than 50 percent, it reads it as a 1; otherwise it reads it as a 0. The counter tracks the refresh sequence based on which rows have been accessed in what order. The length of time necessary to do all this is so short that it is expressed in nanoseconds (billionths of a second). A memory chip rating of 70ns means that it takes 70 nanoseconds to completely read and recharge each cell.

Memory cells alone would be worthless without some way to get information in and out of them. So the memory cells have a whole support infrastructure of other specialized circuits. These circuits perform functions such as:

  •  Identifying each row and column (row address select and column address select)
  •  Keeping track of the refresh sequence (counter)
  •  Reading and restoring the signal from a cell (sense amplifier)
  •  Telling a cell whether it should take a charge or not (write enable)

Other functions of the memory controller include a series of tasks that include identifying the type, speed and amount of memory and checking for errors.

Static RAM uses a completely different technology. In static RAM, a form of flip-flop holds each bit of memory (see How Boolean Logic Works for details on flip-flops). A flip-flop for a memory cell takes four or six transistors along with some wiring, but never has to be refreshed. This makes static RAM significantly faster than dynamic RAM. However, because it has more parts, a static memory cell takes up a lot more space on a chip than a dynamic memory cell. Therefore, you get less memory per chip, and that makes static RAM a lot more expensive.

So static RAM is fast and expensive, and dynamic RAM is less expensive and slower. So static RAM is used to create the CPU's speed-sensitive cache, while dynamic RAM forms the larger system RAM space.

Memory Modules
Memory chips in desktop computers originally used a pin configuration called dual inline package (DIP). This pin configuration could be soldered into holes on the computer's motherboard or plugged into a socket that was soldered on the motherboard. This method worked fine when computers typically operated on a couple of megabytes or less of RAM, but as the need for memory grew, the number of chips needing space on the motherboard increased.

The solution was to place the memory chips, along with all of the support components, on a separate printed circuit board (PCB) that could then be plugged into a special connector (memory bank) on the motherboard. Most of these chips use a small outline J-lead (SOJ) pin configuration, but quite a few manufacturers use the thin small outline package (TSOP) configuration as well. The key difference between these newer pin types and the original DIP configuration is that SOJ and TSOP chips are surface-mounted to the PCB. In other words, the pins are soldered directly to the surface of the board, not inserted in holes or sockets.

Memory chips are normally only available as part of a card called a module. You've probably seen memory listed as 8x32 or 4x16. These numbers represent the number of the chips multiplied by the capacity of each individual chip, which is measured in megabits (Mb), or one million bits. Take the result and divide it by eight to get the number of megabytes on that module. For example, 4x32 means that the module has four 32-megabit chips. Multiply 4 by 32 and you get 128 megabits. Since we know that a byte has 8 bits, we need to divide our result of 128 by 8. Our result is 16 megabytes!

The type of board and connector used for RAM in desktop computers has evolved over the past few years. The first types were proprietary, meaning that different computer manufacturers developed memory boards that would only work with their specific systems. Then came SIMM, which stands for single in-line memory module. This memory board used a 30-pin connector and was about 3.5 x .75 inches in size (about 9 x 2 cm). In most computers, you had to install SIMMs in pairs of equal capacity and speed. This is because the width of the bus is more than a single SIMM. For example, you would install two 8-megabyte (MB) SIMMs to get 16 megabytes total RAM. Each SIMM could send 8 bits of data at one time, while the system bus could handle 16 bits at a time. Later SIMM boards, slightly larger at 4.25 x 1 inch (about 11 x 2.5 cm), used a 72-pin connector for increased bandwidth and allowed for up to 256 MB of RAM.

From the top: SIMM, DIMM and SODIMM memory modules

As processors grew in speed and bandwidth capability, the industry adopted a new standard in dual in-line memory module (DIMM). With a whopping 168-pin connector and a size of 5.4 x 1 inch (about 14 x 2.5 cm), DIMMs range in capacity from 8 MB to 128 MB per module and can be installed singly instead of in pairs. Most PC memory modules operate at 3.3 volts, while Mac systems typically use 5 volts. Another standard, Rambus in-line memory module (RIMM), is comparable in size and pin configuration to DIMM but uses a special memory bus to greatly increase speed.

Many brands of notebook computers use proprietary memory modules, but several manufacturers use RAM based on the small outline dual in-line memory module (SODIMM) configuration. SODIMM cards are small, about 2 x 1 inch (5 x 2.5 cm), and have 144 pins. Capacity ranges from 16 MB to 512 MB per module. An interesting fact about the Apple iMac desktop computer is that it uses SODIMMs instead of the traditional DIMMs.

Error Checking
Most memory available today is highly reliable. Most systems simply have the memory controller check for errors at start-up and rely on that. Memory chips with built-in error-checking typically use a method known as parity to check for errors. Parity chips have an extra bit for every 8 bits of data. The way parity works is simple. Let's look at even parity first.

When the 8 bits in a byte receive data, the chip adds up the total number of 1s. If the total number of 1s is odd, the parity bit is set to 1. If the total is even, the parity bit is set to 0. When the data is read back out of the bits, the total is added up again and compared to the parity bit. If the total is odd and the parity bit is 1, then the data is assumed to be valid and is sent to the CPU. But if the total is odd and the parity bit is 0, the chip knows that there is an error somewhere in the 8 bits and dumps the data. Odd parity works the same way, but the parity bit is set to 1 when the total number of 1s in the byte are even.

The problem with parity is that it discovers errors but does nothing to correct them. If a byte of data does not match its parity bit, then the data are discarded and the system tries again. Computers in critical positions need a higher level of fault tolerance. High-end servers often have a form of error-checking known as error-correction code (ECC). Like parity, ECC uses additional bits to monitor the data in each byte. The difference is that ECC uses several bits for error checking -- how many depends on the width of the bus -- instead of one. ECC memory uses a special algorithm not only to detect single bit errors, but actually correct them as well. ECC memory will also detect instances when more than one bit of data in a byte fails. Such failures are very rare, and they are not correctable, even with ECC.

The majority of computers sold today use nonparity memory chips. These chips do not provide any type of built-in error checking, but instead rely on the memory controller for error detection.

Common RAM Types

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.

Dynamic random access memory has memory cells with a paired transistor and capacitor requiring constant refreshing.

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 MBps.

Extended data-out dynamic random access memory does not wait for all of the 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.

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.

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).

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.

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 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.

VideoRAM, also known as multiport dynamic random access memory (MPDRAM), is a type of RAM used specifically for video adapters or 3-D accelerators. The "multiport" 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 multiport 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.

For a comprehensive examination of RAM types, including diagrams and speed tables, check out the PDF document A Basic Overview of Commonly Encountered Types of Random Access Memory.

How Much Do You Need?
It's said that you can never have enough money, and the same seems to hold true for RAM, especially if you do a lot of graphics-intensive work or gaming. Next to the CPU itself, RAM is the most important factor in computer performance. If you don't have enough, adding RAM can make more of a difference than getting a new CPU!

If your system responds slowly or accesses the hard drive constantly, then you need to add more RAM. If you are running Windows 95/98, you need a bare minimum of 32 MB, and your computer will work much better with 64 MB. Windows NT/2000 needs at least 64 MB, and it will take everything you can throw at it, so you'll probably want 128 MB or more.

Linux works happily on a system with only 4 MB of RAM. If you plan to add X-Windows or do much serious work, however, you'll probably want 64 MB. Apple Mac OS systems will work with 16 MB, but you should probably have a minimum of 32 MB.

The amount of RAM listed for each system above is estimated for normal usage -- accessing the Internet, word processing, standard home/office applications and light entertainment. If you do computer-aided design (CAD), 3-D modeling/animation or heavy data processing, or if you are a serious gamer, then you will most likely need more RAM. You may also need more RAM if your computer acts as a server of some sort (Web pages, database, application, FTP or network).

Another question is how much VRAM you want on your video card. Almost all cards that you can buy today have at least 8 MB of RAM. This is normally enough to operate in a typical office environment. You should probably invest in a 32-MB graphics card if you want to do any of the following:

  •  Play realistic games
  •  Capture and edit video 
  •  Create 3-D graphics 
  •  Work in a high-resolution, full-color environment
  •  Design full-color illustrations

When shopping for video cards, remember that your monitor and computer must be capable of supporting the card you choose.

How to Install RAM
Most of the time, installing RAM is a very simple and straightforward procedure. The key is to do your research. Here's what you need to know:

  •  How much RAM you have
  •  How much RAM you wish to add
  •  Form factor
  •  RAM type
  •  Tools needed
  •  Warranty
  •  Where it goes

In the previous section, we discussed how much RAM is needed in most situations. RAM is usually sold in multiples of 16 megabytes: 16, 32, 64, 128, 256, 512. This means that if you currently have a system with 64 MB RAM and you want at least 100 MB RAM total, then you will probably need to add another 64 MB module.

Once you know how much RAM you want, check to see what form factor (card type) you need to buy. You can find this in the manual that came with your computer, or you can contact the manufacturer. An important thing to realize is that your options will depend on the design of your computer. Most computers sold today for normal home/office use have DIMM slots. High-end systems are moving to RIMM technology, which will eventually take over in standard desktop computers as well. Since DIMM and RIMM slots look a lot alike, be very careful to make sure you know which type your computer uses. Putting the wrong type of card in a slot can cause damage to your system and ruin the card.

You will also need to know what type of RAM is required. Some computers require very specific types of RAM to operate. For example, your computer may only work with 60ns-70ns parity EDO RAM. Most computers are not quite that restrictive, but they do have limitations. For optimal performance, the RAM you add to your computer must also match the existing RAM in speed, parity and type. The most common type available today is SDRAM.

Before you open your computer, check to make sure you won't be voiding the warranty. Some manufacturers seal the case and request that the customer have an authorized technician install RAM. If you're set to open the case, turn off and unplug the computer. Ground yourself by using an anti-static pad or wrist strap to discharge any static electricity. Depending on your computer, you may need a screwdriver or nut-driver to open the case. Many systems sold today come in toolless cases that use thumbscrews or a simple latch.

To install more RAM, look for memory modules on your computer's motherboard. At the left is a Macintosh G4 and on the right is a PC.

The actual installation of the memory module does not normally require any tools. RAM is installed in a series of slots on the motherboard known as the memory bank. The memory module is notched at one end so you won't be able to insert it in the wrong direction. For SIMMs and some DIMMs, you install the module by placing it in the slot at approximately a 45-degree angle. Then push it forward until it is perpendicular to the motherboard and the small metal clips at each end snap into place. If the clips do not catch properly, check to make sure the notch is at the right end and the card is firmly seated. Many DIMMs do not have metal clips; they rely on friction to hold them in place. Again, just make sure the module is firmly seated in the slot.

Once the module is installed, close the case, plug the computer back in and power it up. When the computer starts the POST, it should automatically recognize the memory. That's all there is to it!

For more information on RAM, other types of computer memory and related topics, check out the links on the next page.

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:

  •  How Computer Memory Works 
  •  How RAM Works 
  •  How Virtual Memory Works 
  •  How Flash Memory Works 
  •  How BIOS Works 

Let's start by identifying the different types of ROM.

ROM Types
There are five basic ROM types:

  •  ROM 
  •  PROM 
  •  EPROM 
  •  EEPROM 
  •  Flash memory 

Each type has unique characteristics, which you'll learn about in this article, but they are all types of memory with two things in common:

  •  Data stored in these chips is nonvolatile -- it is not lost when power is removed.
  •  Data stored in these chips is either unchangeable or requires a special operation to change (unlike RAM, which can be changed as easily as it is read).

This means that removing the power source from the chip will not cause it to lose any data.

ROM at Work
Similar to RAM, ROM chips (Figure 1) contain a grid of columns and rows. But where the columns and rows intersect, ROM chips are fundamentally different from RAM chips. While RAM uses transistors to turn on or off access to a capacitor at each intersection, ROM uses a diode to connect the lines if the value is 1. If the value is 0, then the lines are not connected at all.

Figure 1. BIOS uses Flash memory, a type of ROM.

A diode normally allows current to flow in only one direction and has a certain threshold, known as the forward breakover, that determines how much current is required before the diode will pass it on. In silicon-based items such as processors and memory chips, the forward breakover voltage is approximately 0.6 volts. By taking advantage of the unique properties of a diode, a ROM chip can send a charge that is above the forward breakover down the appropriate column with the selected row grounded to connect at a specific cell. If a diode is present at that cell, the charge will be conducted through to the ground, and, under the binary system, the cell will be read as being "on" (a value of 1). The neat part of ROM is that if the cell's value is 0, there is no diode at that intersection to connect the column and row. So the charge on the column does not get transferred to the row.

As you can see, the way a ROM chip works necessitates the programming of perfect and complete data when the chip is created. You cannot reprogram or rewrite a standard ROM chip. If it is incorrect, or the data needs to be updated, you have to throw it away and start over. Creating the original template for a ROM chip is often a laborious process full of trial and error. But the benefits of ROM chips outweigh the drawbacks. Once the template is completed, the actual chips can cost as little as a few cents each. They use very little power, are extremely reliable and, in the case of most small electronic devices, contain all the necessary programming to control the device. A great example is the small chip in the singing fish toy. This chip, about the size of your fingernail, contains the 30-second song clips in ROM and the control codes to synchronize the motors to the music.

Creating ROM chips totally from scratch is time-consuming and very expensive in small quantities. For this reason, mainly, developers created a type of ROM known as programmable read-only memory (PROM). Blank PROM chips can be bought inexpensively and coded by anyone with a special tool called a programmer.

PROM chips (Figure 2) have a grid of columns and rows just as ordinary ROMs do. The difference is that every intersection of a column and row in a PROM chip has a fuse connecting them. A charge sent through a column will pass through the fuse in a cell to a grounded row indicating a value of 1. Since all the cells have a fuse, the initial (blank) state of a PROM chip is all 1s. To change the value of a cell to 0, you use a programmer to send a specific amount of current to the cell. The higher voltage breaks the connection between the column and row by burning out the fuse. This process is known as burning the PROM.

Figure 2

PROMs can only be programmed once. They are more fragile than ROMs. A jolt of static electricity can easily cause fuses in the PROM to burn out, changing essential bits from 1 to 0. But blank PROMs are inexpensive and are great for prototyping the data for a ROM before committing to the costly ROM fabrication process.

Working with ROMs and PROMs can be a wasteful business. Even though they are inexpensive per chip, the cost can add up over time. Erasable programmable read-only memory (EPROM) addresses this issue. EPROM chips can be rewritten many times. Erasing an EPROM requires a special tool that emits a certain frequency of ultraviolet (UV) light. EPROMs are configured using an EPROM programmer that provides voltage at specified levels depending on the type of EPROM used.

Once again we have a grid of columns and rows. In an EPROM, the cell at each intersection has two transistors. The two transistors are separated from each other by a thin oxide layer. One of the transistors is known as the floating gate and the other as the control gate. The floating gate's only link to the row (wordline) is through the control gate. As long as this link is in place, the cell has a value of 1. To change the value to 0 requires a curious process called Fowler-Nordheim tunneling. Tunneling is used to alter the placement of electrons in the floating gate. An electrical charge, usually 10 to 13 volts, is applied to the floating gate. The charge comes from the column (bitline), enters the floating gate and drains to a ground.

This charge causes the floating-gate transistor to act like an electron gun. The excited electrons are pushed through and trapped on the other side of the thin oxide layer, giving it a negative charge. These negatively charged electrons act as a barrier between the control gate and the floating gate. A device called a cell sensor monitors the level of the charge passing through the floating gate. If the flow through the gate is greater than 50 percent of the charge, it has a value of 1. When the charge passing through drops below the 50-percent threshold, the value changes to 0. A blank EPROM has all of the gates fully open, giving each cell a value of 1.

To rewrite an EPROM, you must erase it first. To erase it, you must supply a level of energy strong enough to break through the negative electrons blocking the floating gate. In a standard EPROM, this is best accomplished with UV light at a frequency of 253.7. Because this particular frequency will not penetrate most plastics or glasses, each EPROM chip has a quartz window on top of it. The EPROM must be very close to the eraser's light source, within an inch or two, to work properly.

An EPROM eraser is not selective, it will erase the entire EPROM. The EPROM must be removed from the device it is in and placed under the UV light of the EPROM eraser for several minutes. An EPROM that is left under too long can become over-erased. In such a case, the EPROM's floating gates are charged to the point that they are unable to hold the electrons at all.

EEPROMs and Flash Memory
Though EPROMs are a big step up from PROMs in terms of reusability, they still require dedicated equipment and a labor-intensive process to remove and reinstall them each time a change is necessary. Also, changes cannot be made incrementally to an EPROM; the whole chip must be erased. Electrically erasable programmable read-only memory (EEPROM) chips remove the biggest drawbacks of EPROMs.


  •  The chip does not have to removed to be rewritten.
  •  The entire chip does not have to be completely erased to change a specific portion of it.
  •  Changing the contents does not require additional dedicated equipment.

Instead of using UV light, you can return the electrons in the cells of an EEPROM to normal with the localized application of an electric field to each cell. This erases the targeted cells of the EEPROM, which can then be rewritten. EEPROMs are changed 1 byte at a time, which makes them versatile but slow. In fact, EEPROM chips are too slow to use in many products that make quick changes to the data stored on the chip.

Manufacturers responded to this limitation with Flash memory, a type of EEPROM that uses in-circuit wiring to erase by applying an electrical field to the entire chip or to predetermined sections of the chip called blocks. Flash memory works much faster than traditional EEPROMs because it writes data in chunks, usually 512 bytes in size, instead of 1 byte at a time.

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.

A Simple Example
Caching is a technology based on the memory subsystem of your computer. The main purpose of a cache is to accelerate your computer while keeping the price of the computer low. Caching allows you to do your computer tasks more rapidly.

To understand the basic idea behind a cache system, let's start with a super-simple example that uses a librarian to demonstrate caching concepts. Let's imagine a librarian behind his desk. He is there to give you the books you ask for. For the sake of simplicity, let's say you can't get the books yourself -- you have to ask the librarian for any book you want to read, and he fetches it for you from a set of stacks in a storeroom (the library of congress in Washington, D.C., is set up this way). First, let's start with a librarian without cache.

The first customer arrives. He asks for the book Moby Dick. The librarian goes into the storeroom, gets the book, returns to the counter and gives the book to the customer. Later, the client comes back to return the book. The librarian takes the book and returns it to the storeroom. He then returns to his counter waiting for another customer. Let's say the next customer asks for Moby Dick (you saw it coming...). The librarian then has to return to the storeroom to get the book he recently handled and give it to the client. Under this model, the librarian has to make a complete round trip to fetch every book -- even very popular ones that are requested frequently. Is there a way to improve the performance of the librarian?

Yes, there's a way -- we can put a cache on the librarian. Let's give the librarian a backpack into which he will be able to store 10 books (in computer terms, the librarian now has a 10-book cache). In this backpack, he will put the books the clients return to him, up to a maximum of 10. Let's use the prior example, but now with our new-and-improved caching librarian.

The day starts. The backpack of the librarian is empty. Our first client arrives and asks for Moby Dick. No magic here -- the librarian has to go to the storeroom to get the book. He gives it to the client. Later, the client returns and gives the book back to the librarian. Instead of returning to the storeroom to return the book, the librarian puts the book in his backpack and stands there (he checks first to see if the bag is full -- more on that later). Another client arrives and asks for Moby Dick. Before going to the storeroom, the librarian checks to see if this title is in his backpack. He finds it! All he has to do is take the book from the backpack and give it to the client. There's no journey into the storeroom, so the client is served more efficiently.

What if the client asked for a title not in the cache (the backpack)? In this case, the librarian is less efficient with a cache than without one, because the librarian takes the time to look for the book in his backpack first. One of the challenges of cache design is to minimize the impact of cache searches, and modern hardware has reduced this time delay to practically zero. Even in our simple librarian example, the latency time (the waiting time) of searching the cache is so small compared to the time to walk back to the storeroom that it is irrelevant. The cache is small (10 books), and the time it takes to notice a miss is only a tiny fraction of the time that a journey to the storeroom takes.

From this example you can see several important facts about caching:

  •  Cache technology is the use of a faster but smaller memory type to accelerate a slower but larger memory type.
  •  When using a cache, you must check the cache to see if an item is in there. If it is there, it's called a cache hit. If not, it is called a cache miss and the computer must wait for a round trip from the larger, slower memory area.
  •  A cache has some maximum size that is much smaller than the larger storage area.
  •  It is possible to have multiple layers of cache. With our librarian example, the smaller but faster memory type is the backpack, and the storeroom represents the larger and slower memory type. This is a one-level cache. There might be another layer of cache consisting of a shelf that can hold 100 books behind the counter. The librarian can check the backpack, then the shelf and then the storeroom. This would be a two-level cache.

Computer Caches
A computer is a machine in which we measure time in very small increments. When the microprocessor accesses the main memory (RAM), it does it in about 60 nanoseconds (60 billionths of a second). That's pretty fast, but it is much slower than the typical microprocessor. Microprocessors can have cycle times as short as 2 nanoseconds, so to a microprocessor 60 nanoseconds seems like an eternity.

What if we build a special memory bank, small but very fast (around 30 nanoseconds)? That's already two times faster than the main memory access. That's called a level 2 cache or an L2 cache. What if we build an even smaller but faster memory system directly into the microprocessor's chip? That way, this memory will be accessed at the speed of the microprocessor and not the speed of the memory bus. That's an L1 cache, which on a 233-megahertz (MHz) Pentium is 3.5 times faster than the L2 cache, which is two times faster than the access to main memory.

There are a lot of subsystems in a computer; you can put cache between many of them to improve performance. Here's an example. We have the microprocessor (the fastest thing in the computer). Then there's the L1 cache that caches the L2 cache that caches the main memory which can be used (and is often used) as a cache for even slower peripherals like hard disks and CD-ROMs. The hard disks are also used to cache an even slower medium -- your Internet connection.

Your Internet connection is the slowest link in your computer. So your browser (Internet Explorer, Netscape, Opera, etc.) uses the hard disk to store HTML pages, putting them into a special folder on your disk. The first time you ask for an HTML page, your browser renders it and a copy of it is also stored on your disk. The next time you request access to this page, your browser checks if the date of the file on the Internet is newer than the one cached. If the date is the same, your browser uses the one on your hard disk instead of downloading it from Internet. In this case, the smaller but faster memory system is your hard disk and the larger and slower one is the Internet.

Cache can also be built directly on peripherals. Modern hard disks come with fast memory, around 512 kilobytes, hardwired to the hard disk. The computer doesn't directly use this memory -- the hard-disk controller does. For the computer, these memory chips are the disk itself. When the computer asks for data from the hard disk, the hard-disk controller checks into this memory before moving the mechanical parts of the hard disk (which is very slow compared to memory). If it finds the data that the computer asked for in the cache, it will return the data stored in the cache without actually accessing data on the disk itself, saving a lot of time.

Here's an experiment you can try. Your computer caches your floppy drive with main memory, and you can actually see it happening. Access a large file from your floppy -- for example, open a 300-kilobyte text file in a text editor. The first time, you will see the light on your floppy turning on, and you will wait. The floppy disk is extremely slow, so it will take 20 seconds to load the file. Now, close the editor and open the same file again. The second time (don't wait 30 minutes or do a lot of disk access between the two tries) you won't see the light turning on, and you won't wait. The operating system checked into its memory cache for the floppy disk and found what it was looking for. So instead of waiting 20 seconds, the data was found in a memory subsystem much faster than when you first tried it (one access to the floppy disk takes 120 milliseconds, while one access to the main memory takes around 60 nanoseconds -- that's a lot faster). You could have run the same test on your hard disk, but it's more evident on the floppy drive because it's so slow.

To give you the big picture of it all, here's a list of a normal caching system:

  •  L1 cache - Memory accesses at full microprocessor speed (10 nanoseconds, 4 kilobytes to 16 kilobytes in size)
  •  L2 cache - Memory access of type SRAM (around 20 to 30 nanoseconds, 128 kilobytes to 512 kilobytes in size)
  •  Main memory - Memory access of type RAM (around 60 nanoseconds, 32 megabytes to 128 megabytes in size)
  •  Hard disk - Mechanical, slow (around 12 milliseconds, 1 gigabyte to 10 gigabytes in size)
  •  Internet - Incredibly slow (between 1 second and 3 days, unlimited size)

As you can see, the L1 cache caches the L2 cache, which caches the main memory, which can be used to cache the disk subsystems, and so on.

Cache Technology
One common question asked at this point is, "Why not make all of the computer's memory run at the same speed as the L1 cache, so no caching would be required?" That would work, but it would be incredibly expensive. The idea behind caching is to use a small amount of expensive memory to speed up a large amount of slower, less-expensive memory.

In designing a computer, the goal is to allow the microprocessor to run at its full speed as inexpensively as possible. A 500-MHz chip goes through 500 million cycles in one second (one cycle every two nanoseconds). Without L1 and L2 caches, an access to the main memory takes 60 nanoseconds, or about 30 wasted cycles accessing memory.

When you think about it, it is kind of incredible that such relatively tiny amounts of memory can maximize the use of much larger amounts of memory. Think about a 256-kilobyte L2 cache that caches 64 megabytes of RAM. In this case, 256,000 bytes efficiently caches 64,000,000 bytes. Why does that work?

In computer science, we have a theoretical concept called locality of reference. It means that in a fairly large program, only small portions are ever used at any one time. As strange as it may seem, locality of reference works for the huge majority of programs. Even if the executable is 10 megabytes in size, only a handful of bytes from that program are in use at any one time, and their rate of repetition is very high. Let's take a look at the following pseudo-code to see why locality of reference works (see How C Programming Works to really get into it):

Output to screen « Enter a number  between 1 and 100 »

Read input from user

Put value from user in variable X

Put value 100 in variable Y

Put value 1 in variable Z

Loop Y number of time

  Divide Z by X

  If the remainder of the division = 0

     then output « Z is a multiple of X »

  Add 1 to Z

Return to loop


This small program asks the user to enter a number between 1 and 100. It reads the value entered by the user. Then, the program divides every number between 1 and 100 by the number entered by the user. It checks if the remainder is 0 (modulo division). If so, the program outputs "Z is a multiple of X" (for example, 12 is a multiple of 6), for every number between 1 and 100. Then the program ends.

Even if you don't know much about computer programming, it is easy to understand that in the 11 lines of this program, the loop part (lines 7 to 9) are executed 100 times. All of the other lines are executed only once. Lines 7 to 9 will run significantly faster because of caching.

This program is very small and can easily fit entirely in the smallest of L1 caches, but let's say this program is huge. The result remains the same. When you program, a lot of action takes place inside loops. A word processor spends 95 percent of the time waiting for your input and displaying it on the screen. This part of the word-processor program is in the cache.

This 95%-to-5% ratio (approximately) is what we call the locality of reference, and it's why a cache works so efficiently. This is also why such a small cache can efficiently cache such a large memory system. You can see why it's not worth it to construct a computer with the fastest memory everywhere. We can deliver 95 percent of this effectiveness for a fraction of the cost.

Removable Flash Memory Cards
While your computer's BIOS chip is the most common form of Flash memory, removable solid-state storage devices are becoming increasingly popular. SmartMedia and CompactFlash 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 Dreamcast and Sony's PlayStation. We will focus on SmartMedia and CompactFlash, 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.

So why don't we just use Flash memory for everything? Because the cost per megabyte for a hard disk is drastically cheaper, and the capacity is substantially more.

The solid-state floppy-disk card (SSFDC), better known as SmartMedia, was originally developed by Toshiba.

SmartMedia card

SmartMedia cards are available in capacities ranging from 2 MB to 128 MB. The card itself is quite small, approximately 45 mm long, 37 mm wide and less than 1 mm thick. This is amazing when you consider what is packed into such a tiny package!

As shown below, SmartMedia cards are elegant in their simplicity. A plane electrode is connected to the Flash-memory chip by bonding wires. The Flash-memory chip, plane electrode and bonding wires are embedded in a resin using a technique called over-molded thin package (OMTP). This allows everything to be integrated into a single package without the need for soldering.

The OMTP module is glued to a base card to create the actual card. Power and data is carried by the electrode to the Flash-memory chip when the card is inserted into a device. A notched corner indicates the power requirements of the SmartMedia card. Looking at the card with the electrode facing up, if the notch is on the left side, the card needs 5 volts. If the notch is on the right side, it requires 3.3 volts.

SmartMedia cards erase, write and read memory in small blocks (256- or 512-byte increments). This approach means that they are capable of fast, reliable performance while allowing you to specify which data you wish to keep. They are small, lightweight and easy to use. They are less rugged than other forms of removable solid-state storage, so you should be very careful when handling and storing them.

CompactFlash cards were developed by Sandisk in 1994, and they are different from SmartMedia cards in two important ways:

  •  They are thicker.
  •  They utilize a controller chip.

CompactFlash consists of a small circuit board with Flash-memory chips and a dedicated controller chip, all encased in a rugged shell that is several times thicker than a SmartMedia card.

As shown below, CompactFlash cards are 43 mm wide and 36 mm long, and come in two thicknesses: Type I cards are 3.3 mm thick, and Type II cards are 5.5 mm thick.

CompactFlash card

CompactFlash cards support dual voltage and will operate at either 3.3 volts or 5 volts.

The increased thickness of the card allows for greater storage capacity than SmartMedia cards. CompactFlash sizes range from 8 MB to 192 MB. The onboard controller can increase performance, particularly on devices that have slow processors. The case and controller chip add size, weight and complexity to the CompactFlash card when compared to the SmartMedia card.

Both of these types of removable storage, as well as PCMCIA Type I and Type II memory cards, adhere to standards developed by the Personal Computer Memory Card International Association (PCMCIA). Because of these standards, it is easy to use CompactFlash and SmartMedia products in a variety of devices. You can also buy adapters that allow you to access these cards through a standard floppy drive, USB port or PCMCIA card slot (like the one you find on a laptop computer). Sony's Memory Stick is available in a large array of products offered by Sony, and is now showing up in products from other manufacturers as well.

Although standards are flourishing, there are many Flash-memory products that are completely proprietary in nature, such as the memory cards in video game systems. But it is good to know that as electronic components become increasingly interchangeable and learn to communicate with each other (by way of technologies such as Bluetooth), standardized removable memory will allow you to keep your world close at hand.

Virtual memory

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.

What is Virtual Memory?
Most computers today have something like 32 or 64 megabytes of RAM available for the CPU to use (see How RAM Works for details on RAM). Unfortunately, that amount of RAM is not enough to run all of the programs that most users expect to run at once.

For example, if you load the operating system, an e-mail program, a Web browser and word processor into RAM simultaneously, 32 megabytes is not enough to hold it all. If there were no such thing as virtual memory, then once you filled up the available RAM your computer would have to say, "Sorry, you can not load any more applications. Please close another application to load a new one." With virtual memory, what the computer can do is look at RAM for areas that have not been used recently and copy them onto the hard disk. This frees up space in RAM to load the new application.

Because this copying happens automatically, you don't even know it is happening, and it makes your computer feel like is has unlimited RAM space even though it only has 32 megabytes installed. Because hard disk space is so much cheaper than RAM chips, it also has a nice economic benefit.

Speed Concerns
The read/write speed of a hard drive is much slower than RAM, and the technology of a hard drive is not geared toward accessing small pieces of data at a time. If your system has to rely too heavily on virtual memory, you will notice a significant performance drop. The key is to have enough RAM to handle everything you tend to work on simultaneously -- then, the only time you "feel" the slowness of virtual memory is is when there's a slight pause when you're changing tasks. When that's the case, virtual memory is perfect.

When it is not the case, the operating system has to constantly swap information back and forth between RAM and the hard disk. This is called thrashing, and it can make your computer feel incredibly slow.

The area of the hard disk that stores the RAM image is called a page file. It holds pages of RAM on the hard disk, and the operating system moves data back and forth between the page file and RAM. On a Windows machine, page files have a .SWP extension.

Configuring Virtual Memory
Take Windows 98 as an example of a typical operating system that has virtual memory. Windows 98 has an intelligent virtual memory manager that uses a default setting to help Windows allocate hard drive space for virtual memory as needed. For most circumstances, this should meet your needs, but you may want to manually configure virtual memory, especially if you have more than one physical hard drive or speed-critical applications.

To do this, open the "Control Panel" window and double-click on the "System" icon. The system dialog window will open. Click on the "Performance" tab and then click on the "Virtual Memory" button.

Click on the option that says, "Let me specify my own virtual memory settings." This will make the options below that statement become active. Click on the drop-down list beside "Hard disk:" to select the hard drive that you wish to configure virtual memory for. Remember that a good rule of thumb is to equally split virtual memory between the physical hard disks you have.

In the "Minimum:" box, enter the smallest amount of hard drive space you wish to use for virtual memory on the hard disk specified. The amounts are in megabytes. For the "C:" drive, the minimum should be 2 megabytes. The "Maximum:" figure can be anything you like, but one possible upper limit is twice physical RAM space. Windows default is normally 12 megabytes above the amount of physical RAM in your computer. To put the new settings into effect, close the dialog box and restart your computer.

The amount of hard drive space you allocate for virtual memory is important. If you allocate too little, you will get "Out of Memory" errors. If you find that you need to keep increasing the size of the virtual memory, you probably are also finding that your system is sluggish and accesses the hard drive constantly. In that case, you should consider buying more RAM to keep the ratio between RAM and virtual memory about 2:1. Some applications enjoy having lots of virtual memory space but do not access it very much. In that case, large paging files work well.

One trick that can improve the performance of virtual memory (especially when large amounts of virtual memory are needed) is to make the minimum and maximum sizes of the virtual memory file identical. This forces the operating system to allocate the entire paging file when you start the machine. That keeps the paging file from having to grow while programs are running, which improves performance. Many video applications recommend this technique to avoid pauses while reading or writing video information between hard disk and tape.

Another factor in the performance of virtual memory is the location of the pagefile. If your system has multiple physical hard drives (not multiple drive letters, but actual drives), you can spread the work among them by making smaller pagefiles on each drive. This simple modification will significantly speed up any system that makes heavy use of virtual memory.


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