Motherboard Battery The motherboard battery is used to preserve the computer's time and BIOS settings while the computer is turned off. Some motherboards only need the battery in case of a power outage. They draw the needed electricity from the electrical outlet to power the clock. These motherboards save their BIOS information to EEPROM. EEPROM stands for Electrically Erasable Programmable Read Only Memory. Case Fan Case fans are relatively inexpensive and are extremely important. Computer components generate quite a bit of heat and must be kept as cool as possible. The case fan is the primary source of cooling for most computers. Although the importance of the fan is often overlooked, it is the key to a long life for a computer. Most computer cases are designed to allow a person to add one or more additional case fans. Case Front Panel The front panel of the mid tower case shown above has two external 3.5-inch drive expansion bays, and four 5.25-inch drive expansion bays. Most front panels have an on/off switch, as well as a reset switch that is used in cases where the computer locks up and won't respond. Most front panels also have a power on indicator light and at least one hard drive activity light. Case speakers are often located just behind the front panel. Some front panels also have USB, audio, and other easy to reach port connections. Case Rear Panel In the picture above you can see that there are a lot of openings on the back of a computer case. About halfway down on the left, is where the motherboard's input and output ports are located when installed. In the top left-hand corner you can see where the power cord plugs in from the wall outlet. Below that is the main power supply on/off switch. Barely visible, between the two, is a small switch that allows you to select different voltage types for different countries. Many parts of Europe, as well as other countries, use different electrical voltages than those used in the U.S. The larger circular shaped holes to the right cover the fan for the power supply. This fan not only cools the power supply, but it also helps cool the inside of the case as well. The small holes down the right side provide increased ventilation for the case. There is also a location for a second optional case fan behind the bottom half of these holes. Near the bottom, on the left-hand side of the case rear panel, are seven expansion slot covers. These covers can be removed to connect an AGP video card, PCIe cards, PCI cards, or any other type of expansion cards that might need to be installed in one of these openings. Case Speaker The picture you just clicked on is actually a built in motherboard speaker. The picture above shows what a case speaker looks like. The actual case speaker is hidden behind the front panel. Both are generally only used for computer warning beeps or an occasional rare generic sound from a program. Warning beeps are usually heard during startup if a keyboard is not connected, memory has went bad, or something like that. Otherwise, you seldom ever hear the case speaker. Some sound cards allow you to bypass the case speaker altogether and play the warning beeps or other sounds intended for the case speaker on your regular "good" speakers, but there is nothing wrong with using the case speaker to play these sounds. Chipset Northbridge - Chip 1 of 2 PCI/AGP Controler Why is it called a chipset? Because in the past there were always two or more of these on a motherboard. Some motherboards now have only one chip that performs all of the duties of the chipset, but others still use two or more chips. Chipsets are like the motherboard's traffic cops. They direct the flow of data from one point to another. Each chip in the chipset has its own particular job. This is chip 1 of 2 chips in the 440BX chipset. It is called the "Northbridge." It is the more important of the two, because it links directly to the microprocessor. Above is a picture of the 440BX chipset. Below is Chip 1, the "Intel 82443BX PCI/AGP Controller." It is hidden under a heat sink that protects it from overheating. The 82443BX Northbridge chip controls the data traffic between the microprocessor and the rest of the motherboard. It also controls the traffic to and from the AGP card. Both chips share in the task of controlling the PCI and main memory data flow. In the diagram below you can see the data flow paths mapped out for the chipset. Notice that the paths managed by chip 1 are shown in red. The duties of the Northbridge chip may vary slightly depending on the chipset. For some chipsets, the Northbridge may control the CPU, video, and main memory traffic, while the "Southbridge" chip may control the other traffic. The paths between each component are referred to as "buses." Buses are simply the paths, or wires, that connect one component to another. The chips in the chipset are referred to as bridges, because they bridge the components together and ensure that the data flow is directed to the proper place. The better chipsets are those that can handle data the fastest and most efficiently. Below is a picture of both of the chipsets on the motherboard. Chipset Southbridge - Chip 2 of 2 PCI ISA IDE Xcelerator Why is it called a chipset? Because in the past there were always two or more of these on a motherboard. Some motherboards now have only one chip that performs all of the duties of the chipset, but others still use two or more chips. Chipsets are like the motherboard's traffic cops. They direct the flow of data from one point to another. Each chip in the chipset has its own particular job. This is chip 2 of 2 chips in the 440BX chipset. It is called the "Southbridge." It is slightly less important than the other chip, but out of all the motherboard's components, it is still one of the most important. Without it, the computer could not operate. Above is a picture of the 440BX chipset. The second chip in the chipset is the "Intel 82371EB PCI ISA IDE Xcelerator," also known as the "PIIX4E." The PIIX4E chip controls the data traffic between the ISA slots, USB ports, IDE ports, and the BIOS. Both chips share in the task of controlling the PCI and main memory data flow. In the diagram below you can see the data flow paths mapped out for the chipset. Notice that the paths managed by chip 2 are shown in blue. The duties of the Southbridge chip may vary depending on the chipset. For some chipsets, the Northbridge may control the CPU, video, and main memory traffic, while the Southbridge chip may control the other traffic. The paths between each component are referred to as "buses." Buses are simply the paths, or wires, that connect one component to another. The chips in the chipset are referred to as bridges, because they bridge the components together and ensure that the data flow is directed to the proper place. The better chipsets are those that can handle data the fastest and most efficiently. Below is a picture of both of the chipsets on the motherboard. DIMM Sockets The picture above is of 4 DIMM sockets. DIMM sockets are where the computer's RAM, (or Random Access Memory), is installed. DIMM stands for Dual Inline Memory Module. The reason it is called "Dual" is because both sides of the memory module have completely separate connections from the other side of the module. On the older SIMMs, (or Single Inline Memory Modules), both sides were connected together at each contact point on both sides of the module. Below is a partial picture of a memory module's little gold contact points. These contacts are on both sides of the DIMMs and SIMMs. There are actually twice as many contact points on DIMMs as there are on SIMMs, because the contact points are not connected to the points on the other side of the module. On SIMMs the contact points are connected from one side to the other, which makes them the same connection for both sides. Expansion Slots Expansion slot openings are located on the back of the computer. They look like the ones shown in the picture above. They provide access to the AGP, PCIe, PCI, and ISA expansion slots on the motherboard. The expansion slot opening shown on our Click-N-Learn computer, does not appear to have a PCI, ISA, or AGP expansion slot associated with it. To use this expansion slot opening, a person would need an expansion card like the Sound Blaster Live card shown below, which is actually two cards. The top card plugs into a PCI slot, while the bottom card sends and receives its data through the larger card via the connected cable. The smaller card simply needs an empty expansion slot opening on the back of the case to mount to. The expansion slot opening you clicked on would be perfect for the bottom card. Front Panel Connector Cables The front panel connector cables connect the front panel components to the motherboard. The front panel is where the hard drive activity lights, case speaker, reset button, on/off button, computer power on light, and key lock (if applicable), are normally located. These items must be connected to the motherboard, like everything else, in order to function. Below is a view of the front panel connector cables inside of a different computer. I/O Ports The picture above shows an internal view of some of the I/O ports on the right and a covered view on the left. I/O stands for Input and Output. The most common device for input is the keyboard. When you type, you are putting information into the computer, which is known as input. The most common device for output is the monitor. After the information has made its way through the computer, it is sent out to the monitor for us to see. This is known as output. On the back of computers are several I/O, (or Input/Output), ports. Above, on the very top are two PS/2 ports, normally used for mouse and keyboard connections. Below that are the USB, (or Universal Serial Bus), ports. Below those are two serial ports beside a long parallel port that is often used to connect to a printer. On the bottom right is a game port for joysticks or other game controllers. On the bottom left is a microphone hook up, a speaker hook up, and an additional hook up for another sound input device like a musical keyboard. Below is a picture of the I/O ports on a more recent computer. IDE and Panel Connectors Shown above is the front panel connector on a motherboard, as well as the IDE connectors. Most motherboards have two IDE connectors, which allow two drives to be attached to each connector. One drive is set to master and the other drive is set to slave by using a jumper that is normally located on the back of the drive. This allows a total of four IDE devices, (or drives), to be attached to a typical computer. The motherboard usually also has a floppy connector that supports one or two floppy drives. If a person needs to add additional drives, he or she must purchase an expansion card with an added IDE connector. The front panel connector is simply the location on the motherboard where you attach the front panel wiring. These wires usually include the wiring for the case speaker, hard drive activity lights, computer power on light, reset button, power button, and key lock, (if you have one). IDE Cables IDE stands for Integrated Device, (or Drive), Electronics. EIDE is a later standard of IDE. It stands for Enhanced Integrated Device, (or Drive), Electronics. EIDE is three to four times faster than the older IDE standard. The picture above shows how to connect an IDE cable to two drives. Using jumpers that are normally located on the back of a drive, the top drive should be made the "master" and the bottom drive should be made the "slave." The master drive is the primary drive. It is normally located at the end of a two connection IDE cable. The slave, or secondary drive, is connected to the IDE cable between the master drive and the motherboard IDE connection. Since data can not go to and from each drive at the same time, it is necessary to make one drive the master and the other drive the slave. IDE cables consist of either 40 individual wires or 80 individual wires. Ultra ATA/66 or later devices need the 80 wire cables to operate efficiently. Most of these wires are used to transfer data between the motherboard and the drives. ISA Slot 2 Above is a picture of three ISA slots. These are used for adding ISA expansion cards. ISA stands for Industry Standard Architecture. PCIe and PCI slots are newer and faster than ISA slots. They have replaced the older ISA standard. Below is an ISA card made by Promise called the I/O Max. It has an additional set of serial ports, an additional parallel port, and another IDE connector all on the same card. Microprocessor The microprocessor, (or CPU), is the brain of the computer. The picture above shows a slot 1 processor with heatsinks and a fan, which prevent it from overheating. Below is the processor without the heatsinks and fan, being inserted into a slot 1 motherboard connection. Slot 1 processors have the microprocessor and level 2 cache memory mounted on a circuit board, (or card), which is enclosed inside of a protective shell. The enclosed slot 1 processor card contains the central processing unit, (or CPU), with its level 1 cache memory. The central processing unit also contains the control unit and the arithmetic/logic unit, both working together as a team to process the computer's commands. The control unit controls the flow of events inside the processor. It fetches instructions from memory and decodes them into commands that the computer can understand. The arithmetic/logic unit handles all of the math calculations and logical comparisons. It takes the commands from the control unit and executes them, storing the results back into memory. These 4 steps, (fetch, decode, execute, and store), are what's called the "machine cycle" of a computer. These 4 basic steps are how the computer runs each and every program. The microprocessor's level 1 cache memory, is memory that is contained within the CPU itself. It stores the most frequently used instructions and data. The CPU can access the cache memory much faster than having to access the RAM, (or Random Access Memory). Below is a picture of what's inside of a Pentium 3 processor. The control unit, arithmetic/logic unit, and level 1 cache are contained within the center CPU chip. Level 2 cache memory is visible on the right-hand side of the processor card. Level 1 cache memory is memory that is included inside of the CPU itself. It is usually smaller and faster than level 2 cache memory. Level 2 cache memory is memory between the RAM and CPU. It is used when the level 1 cache memory is full or is too small to hold the intended data. Originally it was not directly on the CPU chip itself. *Read the update at the bottom of this page.* The photo above shows level 2 cache memory on the processor card, beside the CPU. Below are two photos of a CPU. The photo on the bottom is a view of the CPU chip from the outside. The photo on the top is a large map of the inside of the CPU, showing the different areas and what their function is. See if you can find the areas that fetch, decode, and execute the instructions. Can you also find the level 1 cache areas that store information? The pipelined floating point area, logic areas, and superscalar integer execution units area are part of what? Did you guess the arithmetic/logic unit? If so, you're right! At the top you can also see the clock driver. The clock driver is what times, or sets the pace, for the computer. The clock's speed, is how CPUs are rated. Each machine cycle consists of two beats. Each beat the control unit fetches and decodes data, which is called the "instruction cycle." At the same time the arithmetic/logic unit executes and stores data, which is called the "execution cycle." The speed of a clock is rated by how many beats per second it can accomplish. 1 billion beats per second is referred to as 1Ghz. For every beat, (except the very first), a machine cycle is completed. Common CPUs available today perform at 3Ghz and faster. This means that a 3Ghz CPU can execute 3,000,000,000 instructions in a single second! *Update* The slot 1 processor is no longer being produced. Below are two photos of an AMD Athlon 64 FX socket 939 processor and one photo of a Pentium 4 Extreme Edition socket 775 processor. These are later model processors than the slot 1. Currently AMD is using the socket 939, socket 940, and socket 754 processors. Pentium is using the socket 775 and socket 478 processors. All of these processors look similar, but they do have some differences, including the number of contact points, (or pins), that they have. Another difference in some of the newer processors is that the level 2 cache memory is located directly on the CPU chip itself. Any cache memory located outside of a CPU like this is called level 3 cache memory. The usage is still the same though. Level 1 cache memory is still located closest to the core of the CPU and is still usually smaller and faster than the level 2 cache memory. Some of the newer processors even have level 3 cache memory located directly on the CPU itself. Any cache memory located outside of a CPU like this is called level 4 cache memory. As with the other levels of cache memory, the higher the level, the further away from the core of the CPU it is located. The higher levels of cache memory also are usually larger and slower than the smaller levels. The first photo below shows the front and back of a Pentium 4 Extreme Edition socket 775 processor. It has level 3 cache memory located directly on the CPU itself. The second photo below shows the front and back of an AMD Athlon 64 FX socket 939 processor. It has level 2 cache memory located directly on the CPU itself. The third photo below shows the AMD processor installed on a motherboard with a heatsink and fan. Motherboard The motherboard is like a big city with many streets and highways that connect all of the buildings together. Instead of streets and highways, the motherboard uses tiny electrical paths to connect each component of the computer together. These paths are called "buses." The more buses that connect to a component, the faster it can operate. Larger buses are able to operate faster than smaller buses. Buses work just like highways. Wider highways and highways with more lanes are able to carry more traffic than smaller highways and highways with less lanes. Many cities have a freeway. A freeway is designed so that large amounts of traffic can move quickly from one place to another. The motherboard also has a "freeway." It is called the "front side bus," (or FSB). It is the most important bus on the motherboard, because it connects the processor to the main memory and the Northbridge chipset. The faster the FSB is, the faster the computer can operate, since the processor is constantly using the main memory to store and retrieve information. Sometimes the FSB is also called the "system bus." Below is a diagram showing the front side bus in red. Keep in mind that on an actual motherboard, each line representing the buses would actually consist of many electrical paths instead of just one. Older motherboards had FSB speeds of 33MHz or even less. Many newer motherboards have FSB speeds of over 1000MHz. The motherboard in the picture above has a FSB speed that can be set to range from 66MHz to 133MHz. Like a traffic cop, the Chipset, (2 chips on this motherboard), manages and directs the flow of data between each of the components. The BIOS is where the computer's settings are stored and changed. In the first picture of the slot 1 AOpen AX6B motherboard above, you can see most of the connecting slots, ports, and connectors. Some are labeled to show what they are. Motherboards are judged primarily by their chipsets and their front side bus speed. The type of BIOS and the type and amount of expansion slots are also other important things to consider. Below is a picture of the socket 939 Asus A8N-E, a more recent motherboard. It has a FSB speed equivalent to 2000MHz. PCI Audio Accelerator The picture above shows the PCI Audio Accelerator. It is an integrated audio chip. Integrated means that it is "built-in" to the motherboard. Some people might also call it an "onboard" sound chip. Motherboards with integrated audio, (or sound), do not need a sound card. These generic sound chips are capable of producing fairly good quality sound without the need of any additional add-on cards. Some motherboards also have integrated video, which is capable of producing fairly decent quality video without the need for a video card. Integrated audio and video cuts down the cost of a computer. This is why some places are able to sell computers much cheaper than others. It is important to know that integrated audio and integrated video can not replace the higher quality audio and video that most expansion cards provide. If you like video games and have a computer with integrated video, you would most likely see a big difference if you were to add a $50 video card to your computer. If you were to add a $200 GeForce 6600 GT video card, you would see an even bigger difference. It only goes to reason that you usually get what you pay for. The same is true with sound cards. If you're into high quality sound, integrated audio will not perform like a $100 Creative Labs Sound Blaster Audigy 2 card will. Depending on what you do with your computer, integrated audio and video may be just fine. For some people, they are worse than drinking a glass full of sour milk! PCI Slot 1 The photo above shows what a PCI slot looks like. PCI slots can handle 64 bits of data at a time. ISA slots can only handle 32 bits of data at a time. PCI stands for "Peripheral Component Interconnect." A 64-bit PCI slot has 64 connections to the motherboard. Each connection is capable of handling 1 bit of data at a time. A 32-bit ISA slot has 32 connections to the motherboard and can handle only 32 bits of data at a time. Below is a picture of how a PCI card is installed. Note: Older technology ISA slots were 8-bit and 16-bit. The later EISA, (or Extended ISA), slots are capable of 32-bit data transfer. Older PCI technology was 32-bit. The newer PCI technology is 64-bit. As technology changes, improvements to computers and hardware are made. Below is a picture of a Promise PCI expansion card. It allows you to connect up to four additional drives to your computer. This expansion card provides sustained data transfer rates of up to 66,000,000 bits per second, (or 66Mbs/sec). This technology is known as Ultra ATA/66. ATA stands for "Advanced Technology Attachment." Ultra ATA/66 is sometimes referred to as Ultra DMA/66. DMA stands for "Direct Memory Access." Ultra DMA/66 devices can directly access the memory, transferring 66 million bits of data per second. PCI Slot 2 The photo above shows what a PCI slot looks like. PCI slots can handle 64 bits of data at a time. ISA slots can only handle 32 bits of data at a time. PCI stands for "Peripheral Component Interconnect." A 64-bit PCI slot has 64 connections to the motherboard. Each connection is capable of handling 1 bit of data at a time. A 32-bit ISA slot has 32 connections to the motherboard and can handle only 32 bits of data at a time. Below is a picture of how a PCI card is installed. Note: Older technology ISA slots were 8-bit and 16-bit. The later EISA, (or Extended ISA), slots are capable of 32-bit data transfer. Older PCI technology was 32-bit. The newer PCI technology is 64-bit. Below is a picture of Creative's Sound Blaster Live Value PCI sound card. The sound card is what processes a computer's sound data. When you hear music coming from your computer's speakers, the sound card's digital signal processor, (or DSP), is at work along with the digital-to-analog converter, (or DAC), processing and converting digital sound data to analog sound data. When you talk into your computer's microphone, the sound card's DSP works along with the analog-to- digital converter, (or ADC), to process and convert analog sound data to digital sound data. Analog audio is continuous, like the sound waves from a person's voice. Digital audio is broken into pieces that the computer can understand and work with. Better sound cards have better sound. The Sound Blaster Live Value card allows you to connect a sound input device (like a stereo), a microphone, front speakers, rear speakers, and a joystick or MIDI instrument (like a MIDI keyboard). The front and rear speakers can be combined together to produce stereo surround sound. Just like the video card, the sound card uses its own processor to process sound data. PCI Slot 3 The photo above shows what a PCI slot looks like. PCI slots can handle 64 bits of data at a time. ISA slots can only handle 32 bits of data at a time. PCI stands for "Peripheral Component Interconnect." A 64-bit PCI slot has 64 connections to the motherboard. Each connection is capable of handling 1 bit of data at a time. A 32-bit ISA slot has 32 connections to the motherboard and can handle only 32 bits of data at a time. Below is a picture of how a PCI card is installed. Note: Older technology ISA slots were 8-bit and 16-bit. The later EISA, (or Extended ISA), slots are capable of 32-bit data transfer. Older PCI technology was 32-bit. The newer PCI technology is 64-bit. Below is a picture of a SCSI PCI expansion card. SCSI stands for "Small Computer Interface System." With a SCSI expansion card, you can connect up to 15 different devices to one SCSI connection. SCSI is one of the fastest data transfer interfaces available. SCSI cards are available with transfer rates from 4MB per second, up to 320 MB per second. The Ultra320 SCSI-3 interface is the fastest, with a 320MB/sec data transfer rate. Below is a picture of a PCI SCSI expansion card. Power Cable Connectors The power cable connectors connect the power supply to the computer's drives. Once connected to the drives, they provide the electrical power needed in order to operate them. They are also used to provide power to certain video cards and other devices as well. Power Cables The power cables supply electrical power from the power supply to the drives and other devices. The power cables are red, yellow, and black. The yellow wire furnishes 12 volts of power. The red wire furnishes 5 volts of power. The two black wires are the ground wires for the yellow and red wires. Some drives, like the 1.44MB floppy drive, use a smaller cable and connector, but the wires still have the same voltage. Power Supply The power supply supplies the electrical power for a computer. It supplies power to the motherboard, drives, and certain expansion cards. It normally has at least one fan that helps cool the power supply and will assist in the task of cooling the computer. Some power supplies have an additional outlet on the back that can be used to provide power to the monitor. Power supplies come in a variety of wattages. They range anywhere from around 160 watts to about 700 watts. 350 to 400 watt power supplies are probably the most common. Some video cards need quite a bit of power in order to operate effectively. Motherboards that support nVidia's SLI, (or Scalable Link Interface), have the ability of connecting two video cards together. This adds an even greater demand on the power supply. For reasons like this, many people prefer power supplies that have at least 450 to 500 watts. A higher wattage power supply doesn't hurt anything, but a lower wattage power supply can cause problems for people with lots of devices connected to their computer. It is important to make sure that a power supply will connect properly to your motherboard before you purchase it. There are several different types of motherboard power connectors including the older AT type, the ATX 20-pin, the ATX 24-pin, and the EPS 24-pin. Another thing to consider when purchasing a power supply is its PFC, (or Power Factor Correction). PFC protects against things like voltage fluctuations and electrical irregularities that can affect the efficiency of your power supply. There are three types of PFC: active, passive, and non-PFC. Active PFC provides the best protection, while non-PFC provides the worst. A good power supply will cost between $50 and $100. RAM RAM is an abbreviation for Random Access Memory. RAM is the computer's main memory. The computer uses RAM constantly to temporarily store information while it is working with it. The photo above shows what a 128MB SDRAM 100MHz DIMM memory module looks like. SDRAM stands for Synchronous Dynamic Random Access Memory. SDRAM runs synchronously, (or at the same pace), with the processor's front side bus at 66MHz, 100MHz, or 133MHz, depending on what type of SDRAM the computer has. A bus is simply a connection between items on the motherboard. The speed of the the memory, or its data transfer rate, is how fast the data can travel between the RAM and the processor. The speed is measured in MHz, (or megahertz). One megahertz is one million frequency cycles per second. Data travels at a pace of 100 million cycles per second with 100MHz memory. SDRAM that runs at 100MHz is called PC100 memory. SDRAM that runs at 133MHz is called PC133 memory. Memory modules are available in 16MB, 32MB, 64MB, 128MB, 256MB, 512MB, 1GB, and 2GB capacities. Each is able to hold different amounts of temporary data. The 16MB module can hold 16MBs, (or 16,000,000 bytes), of data. The 256MB module can hold 256MBs, (or 256,000,000 bytes), of data. That's 16 times more than the 16MB module! The memory module shown above is a DIMM module. DIMM stands for Dual In-line Memory Module. The term DIMM has nothing to do with the speed or capacity of a memory module. It simply refers to the way the module is designed. DIMM modules consist of several DRAM chips. DIMM modules have separate contact points on both sides of the module. The contact points on the older SIMM, (or Single Inline Memory Module), modules are connected together on both sides of the module. Even though SIMM modules have two sides, with contact points on each, the connections are actually the same on either side. The memory capacity of a single DRAM memory chip on a memory module can be 1MB, 2MB, 4MB, 8MB, 16MB, 32MB, 64MB, or 128MB. Below is a picture of a single DRAM memory chip. Some memory is capable of checking for errors. This memory is called ECC, (or Error Correction Code), memory. If a computer has a lot of memory, it can store a lot of temporary data and operate faster. People with good memories also retain more information and do things faster, because they don't waste a lot of time trying to remember things. Computers commonly have 512MB to 1GB of memory. Here's a question for you. If you have 4 4MB DRAM memory chips on both sides of a SDRAM DIMM module, or 4 8MB DRAM memory chips on one side of a SDRAM DIMM module, how much memory would you have? If you guessed 32MB you're right, because 4 x 4MB x 2 sides = 32MB or 4 x 8MB x 1 side = 32MB. The DDR SDRAM memory module replaced the SDRAM memory module. DDR stands for Double Data Rate. SDRAM runs at the same pace the system clock runs. DDR SDRAM runs at double the pace the system clock runs. After DDR SDRAM came DDR2 SDRAM. DDR2 SDRAM runs at four times the pace the system clock runs. DDR SDRAM is available in speeds from 266MHz up to 600MHz. DDR2 SDRAM is currently available in speeds from 400MHz up to 800MHz. Below is a picture of a 2GB DDR2 SDRAM 400MHz DIMM memory module with ECC. Starting with DDR SDRAM, memory companies often use names like: PC2100, PC2700, PC3200, and etc. to refer to the speed of a memory module. Before DDR SDRAM, companies used names like PC100 to refer to memory with a 100MHz bus speed or PC133 for memory with a 133MHz bus speed. PC2100 refers to memory with a 266MHz bus speed. PC2700 refers to memory with a 333MHz bus speed. At first, this might not make a lot of sense, but after you see how they arrive at these names, maybe it will begin to make a little more sense. What they did was take the bus speed and multiplied it by the bus width, rounding the result to the nearest hundred. The current bus width for SDRAM, DDR SDRAM, and DDR2 SDRAM DIMM modules is 8 bytes wide. If you take 266MHz and multiply it by 8 you get 2128MHz, which rounded to the nearest hundred is 2100MHz for PC2100 memory. 333MHz x 8 = 2664MHz, which rounds to 2700MHz for PC2700 memory. DDR2 SDRAM modules are written with PC2 in front of them. Since 800MHz x 8 = 6400, an 800MHz DDR2 SDRAM module with an 8 byte wide bus would be written PC2 6400 or PC2-6400, using a space or dash to keep from confusing the type of memory with the speed. Since data is able to flow at the same time on the different paths of the RAM bus, this is actually a truer representation of the highest possible data transfer rate for the complete memory module. Shared PCI/ISA Slot If you look at the picture above, you will see two slots: a black ISA expansion slot and a white PCI expansion slot. The problem is that there is only one expansion slot opening available on the back of the case. With this type of expansion slot you must choose whether you want to use an ISA card or a PCI card. You can't use both. That's the reason it's called a shared PCI/ISA slot. This is a good place to use a modem, since modems are available on both ISA and PCI expansion cards. Below is a picture of an ISA modem. Notice that the modem has its own speaker. When you connect to the Internet and hear the modem dialing, it's this speaker that you hear those sounds coming from. Modems convert a computer's digital data into analog waves, so that it can travel through the telephone lines to another computer. The other computer's modem then converts these analog waves back into digital data, so that the other computer can understand it. Modems are rated by how many bits per second that they can receive data. They normally send data at a slightly lower rate, which is around 48Kbps, (or 48,000 bits per second), for a typical 56Kbps modem. Just because a modem is rated at 56Kbps does not necessarily mean that it will be able to receive data at that speed. Phone lines and your Internet service provider also play a big part in how fast your computer is able to receive and send information via the Internet. Some phone lines are only capable of transmitting data at 28.8k. For this reason, even though Internet service providers usually offer a 56K connection, you can only access the Internet at whatever speed your phone lines will permit. Below is a picture of a PCI modem.