Introduction to PC

Introduction to PC’s

How the PC works

The computers primary jobs:

All computers, from the first room-sized mainframes, to today's powerful desktop, laptop and even hand-held PCs, perform the same general operations on information. What changes over time is the information handled, how it is handled, how much is moved around, and how quickly and efficiently it can be done.

How the Computer Computes

At its simplest, a computer is a device that manipulates information, sometimes also called "data". Information can and does take many forms. You can see these different forms every time you use your computer. The words you are reading right now, the signals from the keys you press on your keyboard, the files you load on your hard disk--all are different types of information that your computer manipulates.

Example: What Happens When You Press A Key

To illustrate how the computer works, let's take a very simple example. Let's suppose you are working in your word processor and you type the letter "M". Here's what happens, in general terms, when you press the "M" button:

The keyboard sends an electrical signal, called a scan code, to the computer saying that a button was pressed.

The keyboard controller interprets the scan code and determines that the letter pressed was an "M". It stores this "M" in a special memory location until the processor is ready to deal with it.

The controller sends a signal to the processor, called an interrupt. An interrupt tells the processor that some part of the computer has information for it to process and wants its attention. In this case, the keyboard controller wants the processor to look at the key you just pressed.

The processor is almost always doing many things, sharing its time among many tasks. As a result, most every event must wait its turn. The processor services interrupts based on their priority. When it is time to deal with the keypress, the processor routes it to the program for the operating system that you are using.

Assuming you are using a multi-tasking operating system like Windows, the operating system software decides which window you pressed the key in and sends a message to that window telling it a key was pressed.

The window decides what to do with the keypress. Since in this case it's your word processor window, and the key you pressed was an ordinary letter, the word processor will add that letter to its working area for the file you have open. The letter will take one byte of your computer's memory (RAM). Other keys could be handled differently (for example, if you pressed the key to tell the word processor to exit).

The window will then call the operating system to display the letter on the screen.

The operating system will display the letter on the screen by adding it to your video card's video memory.

The next time the video card refreshes your monitor (re-displays what is in its video memory) the letter will appear on the screen. Most video cards refresh the monitor between 60 and 100 times per second.

Wow, a lot happens even in a simple example like this! This all appears to occur instantaneously because the computer is simply operating at a much faster speed than humans can readily perceive. But despite the illusion created by the speed of the PC, a lot of activity is going on inside the box for even the most basic activity.

In fact, even in the description above, I omitted many steps and details. To list every single step could take dozens of pages, even for just this simple example! The processor itself is handling many thousands of chores every second, and every part of the computer has a job to do on an ongoing basis. This hopefully gives you some idea of how the computer processes, moves, and stores information. Notice that in this example all three activities occurred.

You might feel a bit overwhelmed by this, and think that maybe computers are just too complicated to understand. Fortunately, we don't really need to understand every little detail that goes on inside them to buy and use them, or even to build them. It is usually enough to know what the parts of the computer are and how they interact.

Overview of Systems and Components

One of the great strengths of the PC platform, that has led to its overwhelming success in the marketplace, is its modularity. Most PCs are made up of many different individual components, which can be mixed and matched in thousands of different configurations. This lets you customize the PC you either buy or build to meet your exact needs.

System Case

The system case, sometimes called the chassis or enclosure,  is the metal and plastic box that houses the main components of the computer. Most people don't consider it a very important part of the computer (perhaps in the same way they wouldn't consider their own skin a very important body organ). While the case isn't as critical to the system as some other computer components (like the processor or hard disk), it has several important roles to play in the functioning of a properly-designed and well-built computer.

The case doesn't appear to perform any function at all, at first glance. (I mean, it's a box!) However, this definitely isn't true; the case is in fact much more than just a box. The case has a role to play in several important areas:

• Structure: The motherboard mounts into the case, and all the other internal components mount into either the motherboard or the case itself. The case must provide a solid structural framework for these components to ensure that everything fits together and works well.


• Protection: The case protects the inside of your system from the outside world, and vice-versa. Vice versa? Yes, although most people don't think about that. With a good case, the inside of your computer is protected from physical damage, foreign objects and electrical interference. Everything outside of your computer is protected from noise created by the components inside the box, and electrical interference as well. In particular, your system's power supply, due to how it works, generates a good deal of radio-frequency (RF) interference, which without a case could wreak havoc on other electronic devices nearby.

• Cooling: Components that run cool last longer and give much less trouble to their owner. Cooling problems don't announce themselves; you won't get a "System Cooling Error" on your screen, you'll get random-seeming lockups and glitches with various parts of your system. You'll also have peripherals and drives failing months or years before they do on your friend's computer, and you'll never even dream that poor cooling is the cause. Making sure that your system is cooled properly is one good way to save yourself time, trouble and money.

Note: A spacious, well laid-out case is a critical part of proper system cooling. Small cases require components to be packed close together, which worsens cooling in two ways. First, air flow through the case is reduced because it is blocked by the components. Second, the parts are closer together so there is less space for heat to radiate away from the devices that are generating it. This procedure has tips about how to properly lay out a new PC in the case.

• Organization and Expandability: The case is key to a physical system organization that makes sense. If you want to add a hard disk, CD-ROM, tape backup or other internal device to your PC, the case is where it goes. If your case is poorly designed or too small, your upgrade or expansion options will be limited.


• Aesthetics: The system case is what people see when they look at your computer. For some people this isn't important at all; for others it's essential that their machine look good, or at least fit somewhat into their decor. In an office environment, PCs that all look different can give a work center a "hodge-podge" appearance that some consider unprofessional, for example.


• Status Display: The case contains lights that give the user information about what is going on inside the box (not a lot, but some). Some of these are built into the case and others are part of the devices that are mounted into the case.

Power

Your computer is obviously an electronic device, and its many components of course require power. Like the case, most people don't give much thought to the power supplied to the system. The power supply in your PC can be compared to the officials at a baseball game: if they are doing their jobs properly nobody really notices them, but if they aren't, everybody knows it and isn't very happy about it.

There are two aspects to power in the PC:

• External Power: External power refers to the power that is delivered to the back of the system case. There are several considerations regarding this power and how it is supplied that will determine if your internal power supply is going to work the way it should.


• Power Supply: The power supply is the small box that sits inside your case and takes the external power you supply to the computer. Its main job is to transform this power into a form the rest of the computer can use.

Power Supply Form Factors

The form factor of the power supply refers to its general shape and dimensions. The form factor of the power supply must match that of the case that it is supposed to go into, and the motherboard it is to power. You may not find too many people discussing form factors as they relate to power supplies--this is because power supplies normally come included in system cases, so people talk about the form factor of the case instead. This is changing as the power supply starts to get more of the attention it really deserves. Also, newer power supply form factors can often work with more than one type of case, and vice-versa.

AT Form Factor

In 1984 IBM introduced the IBM PC/AT, "AT" standing for "advanced technology", an abbreviation whose use still survives to this day in some contexts. Very similar in overall physical design to the PC and XT models that preceded it, the power supply in these units was increased in size and changed slightly in shape, establishing it as a distinct form factor.

Baby AT Form Factor

The Baby AT form factor is so named because it is a smaller version of the original AT form factor. It has the same height and depth, but is about 2" narrower. Since it is "similar but smaller", the Baby AT power supply will fit both in Baby AT form factor cases and in full-size AT cases as well, in both tower and desktop styles. It has the same output motherboard and drive connectors as the AT. Due to this flexibility, and the fact that it was introduced at around the time that PCs began to really grow in popularity, the Baby AT form factor reigned as the most popular design for over a decade--far longer than any other.

Form Factor	Typical Dimensions (W x D x H, mm)	Usual Style(s)	Motherboard Connectors	Match to Case Form Factor	Match to Motherboard Form Factor
PC/XT	222 x 142 x 120	Desktop	AT Style	PC/XT	PC/XT
AT	213 x 150 x 150	Desktop or Tower	AT Style	AT	AT, Baby AT
Baby AT	165 x 150 x 150	Desktop or Tower	AT Style	Baby AT, AT, AT/ATX Combo	AT, Baby AT, AT/ATX Combo
LPX	150 x 140 x 86	Desktop	AT Style	LPX, some Baby AT, AT/ATX Combo	LPX, AT, Baby AT, AT/ATX Combo
ATX/NLX	150 x 140 x 86	Desktop or Tower	ATX Style	ATX, Mini-ATX, Extended ATX,  NLX, microATX, AT/ATX Combo	ATX, Mini-ATX, Extended ATX, NLX, microATX, FlexATX
SFX	100 x 125 x 63.5 *	Desktop or Tower	ATX Style	microATX, FlexATX, ATX, Mini-ATX, NLX	microATX, FlexATX, ATX, Mini-ATX, NLX
WTX	150 x 230 x 86 (single fan) 224 x 230 x 86 (double fan)	Tower	WTX Style	WTX	WTX

ATX Form Factor



The ATX power supply design differs from the previous market standards(like AT and Baby AT) in several important ways:

• True Standard: The ATX form factor is a standard, as opposed to the "de facto standards" of prior form factors.


• +3.3 V Power: ATX systems were the first to include +3.3 V power directly, avoiding the need for voltage regulators to provide it on the motherboard.


• Soft Power: ATX systems were the ones where the +5 Standby and Power On signals were introduced. These signals are used along with a change to the way the power switch works, as part of the "Soft Power" feature that enables features such as allowing the operating system to turn off the PC.


• Additional Signals: ATX defines several additional signals used for fan control, IEEE 1394 compatibility, and more.


• Changed Motherboard Connectors: Breaking with 15 years of tradition created by the PC/XT, AT, Baby AT and LPX form factors, Intel specified new motherboard connectors for the ATX form factor. This was in part due to the additional signals used by the ATX power supply and motherboards. For compatibility, some motherboards include both the new and old style of connector.


• Modified Fan Direction and Placement: One of the goals of the original ATX specification was to change the way the power supply fan worked. At around the time ATX was introduced, cooling fans were becoming the standard for the newer, faster CPUs on the market. Instead of exhausting air out the back of the case as had always been the norm, Intel wanted to use this exhaust air to cool the processor directly, saving the cost of a cooling fan. Therefore, the ATX specification calls for the fan to run in the opposite direction and be placed near the CPU's location on the motherboard, to blow on it for cooling. The other advantage of this method is that it keeps the system cleaner, since air entering the case all comes from one place, and can be filtered if necessary.
  
  Unfortunately, while a good idea, this hasn't worked out quite the way Intel hoped. The primary problem is that newer CPUs continue to generate more and more heat as they get faster, and a regular power supply fan doesn't have enough flow to cool them properly. This problem is compounded by the fact that the air blowing on the CPU is warmed by the components in the power supply itself, so it is several degrees above ambient temperature before it ever gets near the CPU. Thus, newer versions of the ATX specification make the fan direction optional. The newest ATX power supplies have gone back to the old style of placing the fan on the back of the power supply and exhausting air to the outside.

Motherboard and System Devices

The motherboard is the base of the modern computer system. It is amazing how little attention this critical component gets in mainstream circles, considering how much it does--though this situation is now improving, fortunately. If the processor is the "brain" of the computer, then the motherboard is the central nervous system and circulatory system, plus much more, all rolled into one. Here are the main parts of the motherboard and its related devices:

The motherboard plays an important role in the following important aspects of your computer system (notice how many there are here):

• Organization: In one way or another, everything is eventually connected to the motherboard. The way that the motherboard is designed and laid out dictates how the entire computer is going to be organized.


• Control: The motherboard contains the chipset and BIOS program, which between them control most of the data flow within the computer.


• Communication: Almost all communication between the PC and its peripherals, other PCs, and you, the user, goes through the motherboard.


• Processor Support: The motherboard dictates directly your choice of processor for use in the system.


• Peripheral Support: The motherboard determines, in large part, what types of peripherals you can use in your PC. For example, the type of video card your system will use (ISA, VLB, PCI) is dependent on what system buses your motherboard uses.


• Performance: The motherboard is a major determining factor in your system's performance, for two main reasons. First and foremost, the motherboard determines what types of processors, memory, system buses, and hard disk interface speed your system can have, and these components dictate directly your system's performance. Second, the quality of the motherboard circuitry and chipset themselves have an impact on performance.


• Upgradability: The capabilities of your motherboard dictate to what extent you will be able to upgrade your machine. For example, there are some motherboards that will accept regular Pentiums of up to 133 MHz speed only, while others will go to 200 MHz. Obviously, the second one will give you more room to upgrade if you are starting with a P133.

Motherboard Form Factors

The form factor of the motherboard describes its general shape, what sorts of cases and power supplies it can use, and its physical organization. For example, a company can make two motherboards that have basically the same functionality but that use a different form factor, and the only real differences will be the physical layout of the board, the position of the components, etc. In fact, many companies do exactly this, they have for example a baby AT version and an ATX version.

System Chipset and Controllers:

The system chipset and controllers are the logic circuits that are the intelligence of the motherboard. They are the "traffic cops" of the computer, controlling data transfers between the processor, cache, system buses, peripherals--basically everything inside the computer. Since data flow is such a critical issue in the operation and performance of so many parts of the computer, the chipset is one of the few components that have a truly major impact on your PC's quality, feature set, and speed.

What exactly is a "chipset"? It sounds like something very complex but really is not, although many of the functions it performs are. A chipset is just a set of chips. (He ducks to avoid the flying vegetables. :^) ) At one time, most of the functions of the chipset were performed by multiple, smaller controller chips. There was a separate chip (often more than one) for each function: controlling the cache, performing direct memory access (DMA), handling interrupts, transferring data over the I/O bus, etc. Over time these chips were integrated to form a single set of chips, or chipset, that implements the various control features on the motherboard. This mirrors the evolution of the microprocessor itself: at one time many of the features on a Pentium for example were on separate chips.

There are several advantages to integration, but the two primary ones are cost reduction and better compatibility (the more things that are done by a single chip or group of chips from one manufacturer, the simpler the design is, and the less chance of a problem). Sometimes the chipset chips are referred to as "ASICs" (application-specific integration circuits).

System Buses:

The components inside your computer talk to each other in various different ways. Most of the internal system components, including the processor, cache, memory, expansion cards and storage devices, talk to each other over one or more "buses".

A bus, in computer terms, is simply a channel over which information flows between two or more devices (technically, a bus with only two devices on it is considered by some a "port" instead of a bus). A bus normally has access points, or places into which a device can tap to become part of the bus, and devices on the bus can send to, and receive information from, other devices. The bus concept is rather common, both inside the PC and outside in the real world as well. In fact, your home telephone wiring is a bus: information flows through the wiring that goes through your house, and you can tap into the "bus" by installing a phone jack, plugging in the phone and picking it up. All the phones can share the "information" (voice) on the bus.

Bus Hierarchy

The PC has a hierarchy, in a way, of different buses. Most modern PCs have at least four buses. I consider them a hierarchy because each bus is to some extent further removed from the processor; each one connects to the level above it, integrating the various parts of the PC together. Each one is also generally slower than the one above it (for the pretty obvious reason that the processor is the fastest device in a modern PC):

• The Processor Bus: This is the highest-level bus that the chipset uses to send information to and from the processor.


• The Cache Bus: Higher-level architectures, such as those used by the Pentium Pro and Pentium II, employ a dedicated bus for accessing the system cache. This is sometimes called a backside bus. Conventional processors using fifth-generation motherboards and chipsets have the cache connected to the standard memory bus.


• The Memory Bus: This is a second-level system bus that connects the memory subsystem to the chipset and the processor. In some systems the processor and memory buses are basically the same thing.


• The Local I/O Bus: This is a high-speed input/output bus used for connecting performance-critical peripherals to the memory, chipset, and processor. For example, video cards, disk storage devices, high-speed networks interfaces generally use a bus of this sort. The two most common local I/O buses are the VESA Local Bus (VLB) and the Peripheral Component Interconnect Bus (PCI).


• The Standard I/O Bus: Connecting to the above three buses is the "good old" standard I/O bus, used for slower peripherals (mice, modems, regular sound cards, low-speed networking) and also for compatibility with older devices. On almost all modern PCs this is the Industry Standard Architecture (ISA) bus.

The system chipset is the conductor that controls this orchestra of communication, and makes sure that every device in the system is talking properly to every other one.

Some newer PCs actually use an additional "bus" that is specifically designed for graphics communications only. The word "bus" is in quotes because it isn't actually a bus, it's a port: the Accelerated Graphics Port (AGP). The distinction between a bus and port is that a bus is generally designed for multiple devices to share the medium, while a port is only for two devices.

Data and Address Buses

Every bus is composed of two distinct parts: the data bus and the address bus. The data bus is what most people refer to when talking about a bus; these are the lines that actually carry the data being transferred. The address bus is the set of lines that carry information about where in memory the data is to be transferred to or from.

In addition, there are a number of control lines that, well, control how the bus functions, and allow users of the bus to signal when data is available. These are sometimes refered to as the control bus, though often they are simply not mentioned.

Bus Width

A bus is a channel over which information flows. The wider the bus, the more information can flow over the channel, much as a wider highway can carry more cars than a narrow one. The original ISA bus on the IBM PC was 8 bits wide; the universal ISA bus used now is 16 bits. The other I/O buses (including VLB and PCI) are 32 bits wide. The memory and processor buses on Pentium and higher PCs are 64 bits wide.

The address bus width can be specified independently of the data bus width. The width of the address bus dictates how many different memory locations that bus can transfer information to or from.

Bus Speed

The speed of the bus reflects how many bits of information can be sent across each wire each second. This would be analogous to how fast the cars are driving on our analogical highway. :^) Most buses transmit one bit of data per line, per clock cycle, although newer high-performance buses like AGP may actually move two bits of data per clock cycle, doubling performance. Similarly, older buses like the ISA bus may take two clock cycles to move one bit, halving performance.

Bus Bandwidth

Bandwidth, also called throughput, refers to the total amount of data that can theoretically be transferred on the bus in a given unit of time. Using the highway analogy, if the bus width is the number of lanes, and the bus speed is how fast the cars are driving, then the bandwidth is the product of these two and reflects the amount of traffic that the channel can convey per second.

The table below shows the theoretical bandwidth of most of the common I/O buses on PCs today. Note the italics on the word "theoretical"; most buses can't actually transmit anywhere near these maximum numbers because of command overhead and other factors. This is especially true of older buses. For example, the theoretical bandwidth of the 8-bit ISA bus might be about  MBytes/sec, but in reality there are wait states inserted during I/O that drop this figure down dramatically.

Most of these buses can run at many different speeds; the speed listed is the one most commonly used for the bus type. See here for a similar table showing processor and memory bus bandwidth for various processors.

Bus	Width (bits)	Bus Speed (MHz)	Bus Bandwidth (MBytes/sec)
8-bit ISA	8	8.3	7.9
16-bit ISA	16	8.3	15.9
EISA	32	8.3	31.8
VLB	32	33	127.2
PCI	32	33	127.2
64-bit PCI 2.1	64	66	508.6
AGP	32	66	254.3
AGP (x2 mode)	32	66x2	508.6
AGP (x4 mode)	32	66x4	1,017.3

Note: You may be somewhat confused by the bandwidth numbers I have listed in the table above. For example, shouldn't the bandwidth of standard PCI be 32/8*33.3=133.3 MB/sec? This is how most people and even companies write it, but this is not technically correct, because of the old problem of different definitions of what "M" stands for. The "M" in "MHz" is 1,000,000 (10^6), but the "M" in "MBytes/second" is 1,048,576 (2^20). So the bandwidth of the PCI bus is more properly stated as 32/8*33.3*1,000,000/1,048,576=127.2 MBytes/second.



A few words on the last four entries. In theory, the PCI bus can be extended to 64 bits in width, and 66 MHz in speed. However (here it comes again) for compatibility reasons almost all PCI buses and the devices they run in, are rated for only 33 MHz at 32 bits. AGP is based upon this theoretical standard and does run at 66 MHz, but remains only 32 bits wide. AGP has additional modes, dubbed x2 and x4, that allow the port to perform data transfers two or four times per clock cycle, respectively, leading to an effective bus speed of 133 or 266 MHz.

Bus Interfacing

On a system that has multiple buses, circuitry must be provided by the chipset to connect the buses and allow devices on one to talk to devices on the other. This device is called a "bridge", the same name used to refer to a piece of networking hardware that connects two dissimilar networks. By far the most commonly found bridge is the PCI-ISA bridge, which is part of the system chipset on a Pentium or Pentium Pro PC. The PCI bus also has a bridge to the processor bus; you can see these devices under "System devices" in the Device Manager in Windows 95.

Bus Mastering

On the higher-bandwidth buses, a great deal of information is flowing through the channel every second. Normally, the processor is required to control the transfer of this information. In essence, the processor is a "middleman", and as with many similar cases in the real world, it is far more efficient to "cut out" the middleman and perform the transfer directly. This is done by having capable devices take control of the bus and do the work themselves; devices that can do this are called bus masters. In theory, the processor can do other work simultaneously; in practice there are several complicating factors. In order to do bus mastering properly, a facility to arbitrate between requests to "take over the bus" must exist; this is provided by the chipset. Bus mastering is also called "first party" DMA since the work is controlled by the device doing the transfer.

Currently most bus mastering in the PC world is done on the

• BIOS: The system BIOS (which stands for Basic Input/Output System and is pronounced "bye-oss" or "bye-ose") is a computer program that is built into the PC's hardware. It is the lowest-level program that runs on your computer. Its job is to act as an intermediary between your system hardware (the chipset, motherboard, processor and peripherals) and your system software (the operating system). By doing this, the operating system doesn't have to be made different for every machine, which is why DOS will load on any PC. The BIOS is what runs when you turn on your computer, and what loads your operating system (for example, DOS). The BIOS also allows you to set or change many different parameters that control how your computer will function. For example, you tell the BIOS what sort of hard drives you have so it knows how to access them.


• Cache: The system cache is a small, high-speed memory area that is placed between the processor and the system memory. The value of the cache is that it is much faster than normal system memory. Each time the processor requests a piece of data from the memory, the system first checks the cache to see if the information is there. If it is, then the value is read from cache instead of memory, and the processor can get back to work that much sooner. If it isn't, then the data is read from memory and given to the processor, but it is also placed into the cache in case the processor needs it again in the near future.


• System Resources: System resources are not actual physical devices; they are nothing you can reach into the machine and touch. But they are very important for two reasons. First, they dictate how your PC organizes its access to various memory areas and devices. Second, they are one of the most common areas where people have problems with the setup of their PCs: so-called resource conflicts. These are the four types of resources that various parts of your computer can sometimes decide to fight over:




• Interrupts (IRQs): As described in the example in the chapter on how the PC works, a device requests time from the processor using these interrupt requests. Under traditional designs, each device has a different IRQ number. If two try to use the same one, a conflict can result. Newer technologies can allow multiple devices to share an IRQ channel.


• Direct Memory Access (DMA) Channels: Some devices have the ability to read and write directly from the system memory, instead of asking the processor to do it for them. Cutting the "middle man" out in this manner improves the efficiency of the system. Each device that does this needs its own DMA channel.


• Input/Output (I/O) Addresses: Devices exchange information with the system by putting data into certain specific memory addresses. For example, when we pressed the letter "M" in the example mentioned above, the keypress was stored in a certain memory address until it was time for the processor to deal with it. Any time information goes into or out of the machine, to your modem or hard drive or printer for example, it uses these I/O addresses. Again, each device needs its own memory area.


• Memory Addresses: Similar to I/O addresses, many devices use blocks of memory as part of their normal functioning. For example, they may map hardware programs (BIOS code) into memory, or use a memory area to hold temporary data they are using.

The Processor

The processor (really a short form for microprocessor and also often called the CPU or central processing unit) is the central component of the PC. It is the brain that runs the show inside the PC. All work that you do on your computer is performed directly or indirectly by the processor. Obviously, it is one of the most important components of the PC, if not the most important. It is also, scientifically, not only one of the most amazing parts of the PC, but one of the most amazing devices in the world of technology.

The processor plays a significant role in the following important aspects of your computer system:

• Performance: The processor is probably the most important single determinant of system performance in the PC. While other components also play a key role in determining performance, the processor's capabilities dictate the maximum performance of a system. The other devices only allow the processor to reach its full potential.


• Software Support: Newer, faster processors enable the use of the latest software. In addition, new processors such as the Pentium with MMX Technology, enable the use of specialized software not usable on earlier machines.


• Reliability and Stability: The quality of the processor is one factor that determines how reliably your system will run. While most processors are very dependable, some are not. This also depends to some extent on the age of the processor and how much energy it consumes.


• Energy Consumption and Cooling: Originally processors consumed relatively little power compared to other system devices. Newer processors can consume a great deal of power. Power consumption has an impact on everything from cooling method selection to overall system reliability.


• Motherboard Support: The processor you decide to use in your system will be a major determining factor in what sort of chipset you must use, and hence what motherboard you buy. The motherboard in turn dictates many facets of your system's capabilities and performance.

System Memory

The system memory holds all of the "active" information that the computer is using. When you turn the computer on the memory is empty. Each program or data file you load uses part of the system memory. When you close a program the memory is freed up for other uses. Generally, the more memory your system has, the more things you can do with it simultaneously. Increasing the amount of memory in the system also improves performance in most cases.

Following are the different types:

DRAM (Dynamic Random Access Memory)

The DRAM can only hold data for a short period of time and must be refreshed periodically. DRAMs are measured by storage capability and access time.

Storage is rated in megabytes (8 MB, 16 MB, etc).

Access time is rated in nanoseconds (60ns, 70ns, 80ns, etc) and represents the amount of time to save or return information. With a 60ns DRAM, it would require 60 billionths of a second to save or return information. The lower the nanospeed, the faster the memory operates.

DRAM chips require two CPU wait states for each execution and can only execute either a read or write operation at one time.

FPM (Fast Page Mode)

At one time, this was the most common and was often just referred to as DRAM. It offered faster access to data located within the same row.

EDO (Extended Data Out)

Newer than DRAM (1995) and requires only one CPU wait state. You can gain a 10 to 15% improvement in performance with EDO memory.

BEDO (Burst Extended Data Out)

A step up from the EDO chips. Also called as Non EDO memory. It requires zero wait states and provides at least another 13 percent increase in performance.

SDRAM (Static RAM)

Introduced in late 1996, retains memory and does not require refreshing. It synchronizes itself with the timing of the CPU. It also takes advantage of interleaving and burst mode functions. SDRAM is faster and more expensive than DRAM. It comes in speeds of 66, 100, 133, 200, and 266MHz.

DDR SDRAM (Double Data Rate Synchronous DRAM)

Allows transactions on both the rising and falling edges of the clock cycle. It has a bus clock speed of 100MHz and will yield an effective data transfer rate of 200MHz.

Direct Rambus

Extraordinarily fast. By using doubled clocked provides a transfer rate up to 1.6GBs yielding an 800MHz speed over a narrow 16-bit bus.

The type of memory module that can be added is depended on the type of the memory slot. Following are the different types of memory slots:



SIMM (Single In-line Memory Modules)

SIMMs is used to store a single row of DRAM, EDO or BEDO chips where the module is soldered onto a PCB. One SIMM can contain several chips. When you add more memory to a computer, most likely you are adding a SIMM.

The first SIMMs transferred 8 bits of data at a time and contained 30 pins. When CPU's began to read 32-bit chunks, a wider SIMM was developed and contained 72 pins.

72 pin SIMMS are 3/4" longer than 30 pin SIMMs and have a notch in the lower middle of the PCB. 72 pin SIMMs install at a slight angle.

DIMM (Dual In-line Memory Modules)

DIMMs allow the ability to have two rows of DRAM, EDO or BEDO chips. They are able to contain twice as much memory on the same size circuit board. DIMMs contain 168 pins and transfer data in 64 bit chunks.

DIMMs install straight up and down and have two notches on the bottom of the PCB.

SODIMM (Small Outline DIMM)

SO DIMMs are commonly used in notebooks and are smaller than normal DIMMs. There are two types of SO DIMMs. Either 72 pins and a transfer rate of 32 bits or 144 pins with a transfer rate of 64 bits.

RDRAM – RIMM

Rambus, Inc, in conjunction with Intel has created new technology, Direct RDRAM, to increase the access speed for memory. RIMMs appeared on motherboards sometime during 1999. The in-line memory modules are called RIMMs. They have 184 pins and provide 1.6 GB per second of peak bandwidth in 16 bit chunks. As chip speed gets faster, so does the access to memory and the amount of heat produced. An aluminum sheath, called a heat spreader, covers the module to protect the chips from overheating.

SO RIMM

Similar in appearance to a SODIMM and uses Rambus technology.

ECC:

ECC is an acronym for Error Checking and Correcting. ECC is used in several areas of computer operations, but the focus of this paper is on ECC in main memory.

ECC is similar to parity. However, where parity can only detect errors, ECC can actually correct most errors. By correcting memory errors, mission critical computer operations can continue.

What kinds of errors occur in RAM?

The most common memory errors are: Single-Bit, Multi-Bit, Column, and Row.

Single-bit errors are the most common and are characterized by a single bit of data being incorrect when reading a complete byte or word. A multi-bit error is the result of more than one bit being erroneous within the same byte or word. A single column or row error would appear as single-bit errors in multiple words.

ECC memory uses extra bits to store an encrypted code with the data. When the data is written to memory, the ECC code is simultaneously stored. Upon being read back, the stored ECC code is compared to the ECC code generated when the data was read. If the codes don't match, they are decrypted to determine which bit in the data is incorrect. The erroneous bit is "flipped" and the memory controller releases the corrected data.

Errors are corrected "on-the-fly," and corrected data is rarely placed back in memory. If the same corrupt data is read again, the correction process is repeated. Replacing the data in memory would require processing overhead that could accumulate and significantly diminish system performance. If the error occurred because of random events and isn't a defect in the memory, the memory address will be cleaned of the error when the data is overwritten with other data.

You may receive customers asking about the type of memory that they can install in the machines. The following web site gives you good explanation about the difference between ECC and Parity:

http://www.crucial.com/gsa/pvtcontent/memorytype.asp?memtype=ECC_VS_PARITY

You can visit the following web site to see the illustrated error message:

http://simone.seeto.com/NMI_error/index.html

More information about the memory errors is available at:

http://www.ece.umd.edu/courses/enee759h.S2003/references/memory4.pdf

Video Cards

Your video card performs the function of displaying the screen you see on the monitor. Inside the video card is a special kind of memory called video memory, where information is stored that represents what you see on the screen. If you look closely at the screen you can see that it is made up of many dots, or pixels. Each pixel's color and brightness is stored in the video memory.

When the computer wants to display something, it calculates how it needs to change the color and brightness of the different pixels, and changes the values in the video memory. The video card then presents the new pixels to you on the monitor. In modern computers, this calculating job is shared between the processor and the video card itself. Having the video card do the calculation can often be much faster, because the video card is specialized to do these types of calculations. Also, while the video card is doing this work, the processor can go on to other things.

Monitors

In simple terms, the monitor, sometimes also called a CRT after the main technology used in making them, is a specialized, high-resolution screen, similar to a high-quality television. Many times per second, your video card sends the contents of its video memory out to your monitor. The screen is made up of a matrix of red, green and blue dots. The information your video card sends controls which dots are lit up and how bright they are, which determines the picture you see.

Hard Disk Drives

The hard disk drive in your system is the "data center" of the PC. It is here that all of your programs and data are stored between the occasions that you use the computer. Your hard disk (or disks) are the most important of the various types of permanent storage used in PCs (the others being floppy disks and other storage media such as CD-ROMs, tapes, removable drives, etc.) The hard disk differs from the others primarily in three ways: size (usually larger), speed (usually faster) and permanence (usually fixed in the PC and not removable).

Hard disk drives are almost as amazing as microprocessors in terms of the technology they use and how much progress they have made in terms of capacity, speed, and price in the last 20 years. The first PC hard disks had a capacity of 10 megabytes and a cost of over $100 per MB. Modern hard disks have capacities approaching 100 gigabytes and a cost of less than 1 cent per MB! This represents an improvement of 1,000,000% in just under 20 years, or around 67% cumulative improvement per year. At the same time, the speed of the hard disk and its interfaces have increased dramatically as well.

Top view of a 36 GB, 10,000 RPM, IBM SCSI server hard disk, with its top cover removed. Note the height of the drive and the 10 stacked platters. (The IBM Ultrastar 36ZX.)

Your hard disk plays a significant role in the following important aspects of your computer system:

• Performance: The hard disk plays a very important role in overall system performance, probably more than most people recognize (though that is changing now as hard drives get more of the attention they deserve). The speed at which the PC boots up and programs load is directly related to hard disk speed. The hard disk's performance is also critical when multitasking is being used or when processing large amounts of data such as graphics work, editing sound and video, or working with databases.


• Storage Capacity: This is kind of obvious, but a bigger hard disk lets you store more programs and data.


• Software Support: Newer software needs more space and faster hard disks to load it efficiently. It's easy to remember when 1 GB was a lot of disk space; heck, it's even easy to remember when 100 MB was a lot of disk space! Now a PC with even 1 GB is considered by many to be "crippled", since it can barely hold modern (inflated) operating system files and a complement of standard business software.


• Reliability: One way to assess the importance of an item of hardware is to consider how much grief is caused if it fails. By this standard, the hard disk is the most important component by a long shot. As I often say, hardware can be replaced, but data cannot. A good quality hard disk, combined with smart maintenance and backup habits, can help ensure that the nightmare of data loss doesn't become part of your life.

Floppy Disk Drives

Floppy disks are your computer's smallest and slowest form of long-term storage. Floppy disks provide a simple, convenient way to transfer information, install new software, and back up small amounts of files. Floppy disks are not as important a part of the computer as they were many years ago. This is largely because the floppy disk still holds the same amount it did five years ago, while most users' needs for storage, software installation and backup, have increased ten-fold or more in that period of time. One great advantage floppy drives have is universality: virtually 100% of PCs made in the last 10 years use a standard 1.44 MB floppy drive.

CD-ROM Drives

CD-ROM stands for Compact Disk - Read Only Memory. As the name implies, CD-ROM drives use compact disks, similar to the ones that hold music, to hold computer information. And also as the name implies, they are a read-only medium. You can read information from them but not write to them (except for some special exceptions). CD-ROMs are currently the most popular way that computer companies distribute applications and games, and are ideal for multimedia information like videos, music and large graphics files.

Peripheral I/O

Peripherals are external devices that you connect to your PC. (Well, technically your hard drive and CD-ROM etc. are peripherals too, but often people use the term to refer to devices outside the main box). There are two main ways that you can connect peripherals to your machine: through a serial connection, or through a parallel connection:

• Serial Communications: A serial connection sends information over the line one bit at a time. It is a simple way to send information in or out of the computer, but is not as fast as other ways the computer can communicate. Serial connections are typically used for devices such as mice and modems.


• Parallel Communications: A parallel connection is faster than a serial one because it sends many bits in parallel. The advantage of this is that it is faster, but the disadvantage is that it is more complicated to do. Parallel connections are used most often for printers and removable storage drives, which need more speed than serial peripherals.

Keyboards

The keyboard is the main input device for most computers. It is used to input textual information to the PC. Keyboards are pretty much standard affairs these days, although they can vary greatly in quality and appearance, and some have significant additional features.

Mice

Until the invention of graphical operating systems, the keyboard was the only way that most people input information into their PCs. Mice are used in graphical environments to let users provide simple "point and click" instructions to the computer. The main advantage of a mouse over the keyboard is simplicity. There are also some operations that are much easier to perform with a mouse than a keyboard (such as picking an item on a screen or choosing from a list of options).