Printer

Printers are still accessories, for now at least. They perform a process, transferring information onto paper, that is truly external to the other functions of the computer. On a home computer, the printer is usually wired directly to the computer, but it is common to find printers in businesses where the printer is off away from the computer and performing printing operations for several computers, rather than just one.

External Modem

Unless you have a really old computer, the modem is almost certain to be built in, inside the CPU Box, although technical people persist in thinking of it as an accessory. However, in the old days, modems really were exotic accessories that had to be attached through lots of wires and plugs.

External Disk Drives

Another kind of accessory is an external disk drive. This is becoming more rare, as the computer manufacturers are building specialized disk drives, such as CD drives, into the computer's CPU Box and you don't have to attach them with lots of wires and extra plugs.

Mouse

The mouse is an accessory. That doesn't make much sense any more, since it's almost impossible to use a modern computer without one. Besides that, a good mouse can cost almost as much as a good keyboard. In some ways, the mouse is almost as complicated as the keyboard, too. Calling the mouse an accessory is just one of those goofy things technical people do, and it's best not to disturb their delicate sensibilities.

Computer or CPU Box

The CPU Box is the part that, well, looks like a box. CPU Box is not really a good name for it, but it will have to do until somebody comes up with a name everybody can agree on. CPU stands for Central Processing Unit, and technical people refer to the box part as the CPU Box because that's where the CPU is, and that's where the real work gets done. Because that's where the computing is really done, the CPU component alone can be called the computer. You might hear people say, "not the screen, but the actual computer" when referring to the CPU box. Some people refer to it as the hard drive, since the hard drive is in there too, but that is confusing. The hard drive is inside the CPU Box all right, but you'll drive technical people bonkers if you call the box a hard drive, because they just don't think that way. Of course it's not logical to say that CPU is an OK name but hard drive isn't, but remember that technical people aren't normal and humor them. A box is a box, but not if it's a CPU Box. CPU Boxes come in two special types, called Desktop and Tower. The Desktop kind is designed to lie down flat, even though you can stand it on its side if you want to. In a desktop CPU Box, you will normally find that the openings for the CD and Floppy disk drives are horizontal when the CPU Box is lying down flat. A lot of people don't keep their CPU Box lying flat on top of their desk any more, but the name has persisted. A Tower CPU Box is designed to stand up tall, so it will be easier to knock it over. Well, no, it's really supposed to stand under your desk and not take up too much floor space. In a Tower CPU Box, you will usually find that the openings for the CD and Floppy disk drives are horizontal when the CPU Box is standing up on what you might think was its side. Another clue is the writing on the box. Usually, the computer manufacturer will put the brand name and maybe the model number somewhere on the outside, usually on the part they think of as the front, and the words will be right side up when the CPU Box is standing or lying the way they had in mind when they designed it. This seems pretty complicated, and you're probably wondering why anybody cares. Unfortunately, sometimes it does matter. The parts inside the CPU Box are often different, depending on whether it was designed as a Desktop or a Tower. If you can identify which kind it is when you need help, a technical person may be able to resolve your problem more easily.

Monitor

The monitor is the part that looks a lot like a TV set. You look at it and see pictures and words. When you type things on the keyboard, the letters and numbers show up on the monitor, or at least you hope they will. The monitor is also called the Display Monitor, or just the Display, or sometimes the Video Display, or maybe just The Screen. The important thing to know about the monitor is that it is not really the computer, it's just the part that makes pictures for you to see. It is pretty common for technical support people to find out, after about half an hour on the phone, that the ordinary human being is turning the monitor on and off instead of the computer, and that's why none of their instructions have had any effect. Don't let that happen to you.

Keyboard

The keyboard is easy. We all know what a keyboard is, right? It's the thing that looks like part of a typewriter. Uh, oh, maybe you've never seen a typewriter? OK, it looks like a cellular phone, except it's bigger and has more buttons. Seriously, the keyboard is the part with the letters and numbers, where you put your hands and type, but you knew that. On most traditional computers, it is a separate device, connected by a cord, but it may be attached to or built into one of the other parts, particularly in a laptop computer.

The Motherboard

he motherboard is the large circuit board inside your computer's case. It's sometimes called the system board, the logic board, the baseboard, or less commonly, the planar board. Everything connected to your computer system, plugs either directly or indirectly into the motherboard.

I'm sure everyone has heard the CPU, or Central Processing Unit, referred to as the 'brain' of your computer. Well, the CPU sits on the motherboard, and if it can be called the brain of your computer then the motherboard is truly the central nervous system. The motherboard contains the CPU, the BIOS ROM chip (Basic Input/Output System), and the CMOS Setup information. It has expansion slots for installing different adapter cards like your video card, sound card, Network Interface Card, and modem. This circuit board provides a connector for the keyboard as well as housing the keyboard controller chip. It has RAM slots for your system's Random Access Memory (SIMMs or DIMMs), and provides the system's chipset, controllers, and underlying circuitry (bus system) to tie everything together.

The motherboard, more or less, is your computer. It defines your computer type, upgradeability, and expansion capability.

Non-Integrated Motherboards
Non-Integrated Motherboards have assemblies such as the I/O Port connectors (serial and parallel ports), hard drive connectors or paddle boards, floppy controllers and connectors, joystick connections, etc. installed as expansion boards. This takes up one or more of the motherboard's expansion slots and reduces the amount of free space inside your computer's case. Hence, the individual motherboard is relatively cheap to produce but, because of the cost of manufacturing, testing, and installing the expansion boards separately, there's an added cost to the computer system. However, if something should go wrong with the individual assemblies, such as a bent or broken pin in a connector, or a defective controller chip etc., you could repair the problem by replacing the individual expansion card at a relatively minor cost.

Most of the older motherboards were Non-Integrated. Some of the later 486 system boards began to integrate some of these assemblies right onto the circuit board.

Integrated Motherboards
Integrated Motherboards have assemblies that are otherwise installed as expansion boards, integrated or built right onto the board. The serial and parallel ports, the IDE and floppy drive, and joystick all connect directly to the motherboard. This is now standard on any late model 486 and above. It tends to free up some space inside the case and allows for better accessibility and air flow. The systems are cheaper to produce because there's less material involved, less installation, and testing can all be done at the same time. They are more expensive to repair because, if you end up with a controller failure or broken pin, it means a new motherboard (and, of course, because of the added assemblies, the motherboard can be more expensive than its non-integrated counterpart). However, these particular integrated assemblies are generally fairly stable and although problems can occur, they tend to be fairly rare.

All in all, the integrated motherboard tends to be a good thing as

Integrated Circuit Chips (IC)

rcuits are made up of different electronic components (capacitors, resistors, transistors, etc.) wired together in a manner that performs a specific electronic function. . These components are hooked together on fiberglass boards called circuit boards. You can see the small thin copper or metal lines (wires) on a circuit board that connect the different components together. These are called traces.

If you could find an old 7 or 10-transistor radio from the sixties, you could open it up and see the peanut sized transistors on the circuit board. They looked like little water towers with their 3 legs going down to the circuit board where they were soldered in place. Now, imagine that circuit board and all of its transistors, shrunk down to about the size of your little fingernail and put into a small plastic case for protection. In other words, the entire circuit would be integrated into that one small chip. Hence the name, Integrated Circuit (IC).

With today's technology, these circuits and transistors can be etched into small wafers of silicon and sandwiched into a small plastic package or chip. There are small metal pins coming out of the chip that are connected to the silicon wafer inside with microscopic wiring. The most amazing thing is, today's small integrated circuits now contain millions of transistors, not just 7 or 10. These IC chips are then placed on a circuit board connecting them to other components and IC chips.

One technology for producing ICs is called TTL (Transistor Transistor Logic). These chips are actually more tolerant of ESD (Electrostatic Discharge) and even faster than the newer technology. However, they're also larger, use more electricity or power, and their resistance causes them to run very hot. As computer technology advanced and the number of transistors in the chips increased, heat and power consumption became a huge problem.

TTL ICs are still used in computers today, but the newer CMOS chips (Complimentary Metal-Oxide Semiconductor) have all but replaced them entirely. These chips have a semi-conductive metal oxide layer that allows for less resistance, reducing the power consumption and the generated heat. Unfortunately, the very technology that makes these CMOS chips more efficient also makes them very susceptible to electrostatic discharge.

The Bus

The CPU has to be able to send various data values, instructions, and information to all the devices and components inside your computer as well as the different peripherals and devices attached. If you look at the bottom of a motherboard you'll see a whole network of lines or electronic pathways that join the different components together. These electronic pathways are nothing more than tiny wires that carry information, data and different signals throughout the computer between the different components. This network of wires or electronic pathways is called the 'Bus'.

That's not that difficult to comprehend, but you've probably heard mention of the internal bus, the external bus, expansion bus, data bus, memory bus, PCI bus, ISA bus, address bus, control bus,… it really can get quite confusing.

A computer's bus can be divided into two different types, Internal and External.

The Internal Bus connects the different components inside the case: The CPU, system memory, and all other components on the motherboard. It's also referred to as the System Bus.

The External Bus connects the different external devices, peripherals, expansion slots, I/O ports and drive connections to the rest of the computer. In other words, the External Bus allows various devices to be added to the computer. It allows for the expansion of the computer's capabilities. It is generally slower than the system bus. Another name for the External Bus, is the Expansion Bus.

So now we know the bus is just a bunch of tiny wires (traces and electronic pathways). One bunch carries info around to the different components on the motherboard, and another bunch of wires connects these components to the various devices attached to the computer.

What kind of stuff travels on the bus? For one thing, data. Data has to be exchanged between devices. Some of the electronic pathways or wires of the Internal Bus or the External Bus are dedicated to moving data. These dedicated pathways are called the Data Bus.

Data is stored, manipulated and processed in system memory. System memory is like a vast sea of information full of fish (data). Your computer has to move information in and out of memory, and it has to keep track of which data is stored where. The computer knows where all the fishes are, but it has to transmit that information to the CPU and other devices. It has to keep a map of the different address locations in memory, and it has to be able to transmit and describe those memory locations to the other components so that they can access the data stored there. The info used to describe the memory locations travels along the address bus. The size, or width of the address bus directly corresponds to the number of address locations that can be accessed. This simply means that the more memory address locations that a processor can address, the more RAM it has the capability of using. It makes sense, right?

A 286 with a 16 bit address bus can access over 16 million locations, or 16 Mb of RAM. A 386 CPU with a 32 bit address bus can access up to 4 GB of RAM. Of course, at the present time, due to space and cost limitations associated with the average home computer, 4GB of RAM is not practical. But, the address bus could handle it if it wanted to! Another name for the address bus is the memory bus.

ESD - Electrostatic Discharge

ESD is simply the discharge of built up static electricity. Electrostatic discharge however, should not be taken lightly when working with computers. ESD has the capability of causing enough damage to the components inside your computer to render it completely inoperable. It's important to use an anti-static strap and/or a grounding mat whenever working on the inside of your computer. Being aware of ESD precautions can significantly reduce the chance of damage to your computer due to static electricity.

Static electricity is a fact of Life. You're producing it and discharging it constantly. Moisture in the air can help it to dissipate, and raising the humidity in your workplace is another very good measure you can take to reduce the chance of damage caused by ESD. ESD is bad for computers and electronic components. If you are opening the case or working inside a computer, always take ESD precautions.

WARNING:
There are capacitors inside a computer's monitor and power supply that store enough electricity to stop your heart, even when they are unplugged. You should not be grounded if you are working inside a monitor or a power supply.
But then, you shouldn't even open a monitor or a power supply unless you know what you're doing.

How Much is Too Much?
Have you ever walked across a carpet on a dry winter day, touched somebody, and heard the snap of electricity as a small blue spark jumped from your fingertip to the edge of your victims ear?

If the discharge was felt, it was probably more than 2000 volts. If you heard it, then it could have been between 3000 - 5000 volts. If you actually saw a small blue spark, it was more than likely in excess of 10,000 volts.

So do I have to walk on a shag rug? Does it have to be a dry winter day? No! Your body is building up and discharging static electricity all the time. Just shifting in your chair can produce up to 150 - 200 volts! You can't completely eliminate this problem but it can be controlled.

ESD and Your Computer
The ICs (Integrated Circuit chips) on the various circuit boards in your computer use between 2 - 5 volts. They can be damaged by less than 200 volts. Some may be damaged by as little as 30 volts. This means that ESD can cause damage to the various components inside your computer and you won't feel it, hear it, or see it. In fact, the discharge voltage could be 1000 times less than what the human body can feel!

When they started replacing the tubes in radios with transistors, people were amazed at how small radios were getting. You could get a transistor radio that boasted 7 transistors and was about the size of a brick! …Today, some of the individual chips on your computer's circuit boards contain millions of transistors. Advancements in technology allowed for entire circuits to be etched onto a single chip. These are called Integrated Circuit Chips.

One technology for producing ICs is called TTL (Transistor Transistor Logic). These chips are actually more tolerant of ESD and even faster than the newer technology. However, they're also larger, use more electricity or power, and their resistance causes them to run very hot. As computer technology advanced and the number of transistors in the chips increased, heat and power consumption became a huge problem.

TTL ICs are still used in computers today, but the newer CMOS chips (Complimentary Metal-Oxide Semiconductor) have all but replaced them entirely. These chips have a semi-conductive metal oxide layer that allows for less resistance, reducing the power consumption and the generated heat. Unfortunately, the very technology that makes these CMOS chips more efficient also makes them very susceptible to electrostatic discharge.

Wait a minute! If you can discharge 10,000 volts of static electricity into somebody's ear, how come it doesn't kill them? Well, the amperage is very low. The human body is very tolerant of voltage, it's the amperage, or a current's strength, that will kill you. The chips inside your computer however, are highly sensitive to any kind of voltage, even at very low amperages. ...By the way, all of today's CPUs (Central Processing Units) and system memory chips on your RAM SIMMs or DIMMs are CMOS chips.

ESD Damage
So what kind of damage can be caused by ESD? Anything from a simple system interruption, causing your computer to reboot with no further problems, to complete destruction of a chip that will make your system unusable until the chip is replaced. Also, you won't find any discoloration, burn mark or physical indication whatsoever as to which chip has been damaged.

There are 2 types of damage that can occur:

Immediate Failures (Direct Failures, Catastrophic Failures) - These are failures that occur immediately after the chip has been damaged and usually aren't recoverable until the chip has been replaced. Say you open your computer (or worse yet, someone else's) to install more memory. Without using an anti-static strap or a grounding mat (you've done it lots of times before and never had a problem), you take a brand new DIMM from its anti-static tube, install it, and now the computer won't even finish its boot sequence. Not a good situation, but you do know the computer worked before you installed the RAM, so you should have an idea where to start looking for the problem.

Latent Failures (Upset Failures, Delayed Failures) - Sometimes a chip can be damaged by electrostatic discharge and the results aren't immediately noticeable. Months later the chip could finally fail completely, or you may end up with intermittent failures that occur sporadically. These are usually difficult to attribute to any specific cause, and very hard to track down.

Disassembling the Computer

It's Not a Race
If you are familiar with the procedure of disassembling a computer, then you can skip this section. If you are a beginner and actually want to learn something, then document well, and learn or re-affirm your knowledge about everything you see inside. Given a screwdriver, a 10-year old could probably have the computer apart in a half-hour or so. If you're using this section as a lab or learning assignment, and you have your computer apart in the same time as a 10-year old, then that's the level you'll be at. But if you take a slow relaxed approach, discuss, question and research each component as it's removed, you'll learn alot. Read the sections on What's Inside and What You See, fall back on your own knowledge, use the Internet, your books and resource material. It's impossible to retain all the information, so one of the most important computer skills you can learn is how to research and use your resources to find what you need. Here's an example of some questions to think about or discuss as you proceed:

• Should I document everything I do or everything I remove?
• Am I taking the best ESD precautions available to me right now.
• When you remove an expansion card what kind of card is it? What kind of expansion slot did it come from? How many bits wide is that slot? What is the bus speed? What does the card do? If there's any wires attached to the card, what's the other end attached to and what are the wires or cables for. What kind of port is on the end of the card?
• When removing a drive, what kind of drive is it? Is there information documented right on the drive itself? What kind of power connector does it use? Are there jumper settings on the drive? What for? Are any drives connected together or do they all have their own cable? Does it matter which cable I hook up when I reassemble? What are some of the things I know about this particular type of drive?
• When removing wires or cables, what are the cables for? Which connectors are actually being used and what could the other ones be for? Are they following the pin-1 rule? Is pin-1 actually designated on the device the cable is attached to? Is it designated in more than one way?
• Am I still taking proper ESD precautions and is my antistatic strap still hooked up?
• Look at the motherboard again when there's not so much in the way. Can you point out the CPU? How about the BIOS chip, the battery, cache RAM, keyboard connector? Is it an AT, Baby AT, or ATX format? Is there a math coprocessor? Where is it? Is the system memory supplied on SIMMs or DIMMs? How many pins on the memory module? How many memory slots are thee for each bank of system memory? Is the CPU installed in a ZIF socket or a friction socket? Are there any jumpers on the motherboard? Is there any information silk-screened on the board itself?

This is just an example of the questions you should be asking yourself. Try to come up with lots more. Even if you are not prepared to actually take your computer apart at this time, just take the cover off and ask yourself these questions as you visualize the various steps involved. Remember, not all questions can be answered by a single resource. Look in your notes, check out your manuals and resource material, ask questions.

ESD
Read the section on Electrostatic Discharge and always take ESD precautions. Remember, if you can feel a static shock its probably close to 3000 volts. Some ICs can be affected by as little as 30 volts.

• Always use an antistatic wrist strap.
• Keep a supply of antistatic bags to place components in as they are removed.
• Leaving the computer plugged in is a recommended procedure. However, make sure it's switched off and remember that the cable going to the remote switch on the front of the case carries AC current at house voltage. Also, ATX motherboards have power to them all the time, even when the switch is off. Before beginning to remove a power supply or an ATX motherboard, always make sure your computer is unplugged.

Before Beginning
You want to make sure you have what you need. Your wrist strap is attached to the computer, you don't want to have to run to the other side of the room or to another room to get something. Forget about the strap and your computer may follow you.

• Have a pen and paper ready. Documentation is real important. After you've changed a few jumpers or removed or replaced a few cables and cards, you probably will have to put some back the way they were. If you have documentation, putting things back together can be a simple reverse process. This is true of software troubleshooting as well.
• Make sure you have the tools you need and they're all close by and handy.
• Be sure to have a container to keep the screws in so you have them when you want to put things back together.
• Make sure you have the resource material, drivers or software that you may need.
• If possible, enter the CMOS setup and record the information available. At least record the floppy and hard drive configuration and any settings that may be different from the default. You want to be careful not to remove the battery and lose these settings, but stuff happens.
• Disassembly is major surgery, do a full backup of the system. Programs that you have the original disks for can always be replaced, but any upgrades for those programs and any programs that have been downloaded from the Internet may or may not still be available. Bookmarks, e-mail addresses, phone and fax numbers, dial-up connections, DNS settings and networking protocols can be a real pain to replace. Even the best technicians cannot guarantee your data, so back it up. Also, in Windows9x, all the IRQ, I/O addresses, and DMA settings can be found (and printed) from the Device Manager in Control Panel. In Windows98 check out Start/Programs/Accessories/System Tools/System Information.
• Close all programs, shut down Windows, and turn off your computer. Then remove the cables from the back of the case.
• One other thing: you have to use a little common sense. Don't necessarily follow this information to the letter, it's only meant to be a guide. If you think it would be easier to remove some expansion cards before removing the drive bay, then do it. If you can better access the data cables after the drive is out, then do it that way. If it would be easier to disconnect the power cables and remove the power supply before accessing DIMMs or SIMMs …..
  I think you get the point.

Reassembling the Computer

Same Way, Only Backwards
When it comes time to put a computer back together, it's usually just a matter of reversing the order in which you took it apart. Again, you're going to visualize each step because there may be an easier way. Is it easier to install the motherboard or the power supply first? Are the RAM DIMMs (or SIMMs) easier to access before, or after the power supply is in. If the motherboard pops out the back on a tray, then install the CPU and the RAM before replacing the tray. If you install the power supply first, then you may be able to clip the power connectors on as you install the motherboard and tray.

Tools
When putting the computer back together (or disassembling it for that matter), there are some basic tools that you'll find handy. Of course, you'll need your ESD protection equipment and a phillips screwdriver (keep a flat-head screwdriver nearby, too. A small flashlight and a magnifying glass may come in handy, as well as needle nose pliers. A useful device for any kind of computer work is a small srewdriver with a phillips head on one end and a flat-head on the other (I have no idea what the proper name is for the tool). Once again, you're not in a race. A good carpenter measures twice, and cuts once. When working on computers you want to double-check everything you install or connect. Before you start, make sure you have taken all your ESD precautions. As you continue, make a conscious effort to remain aware of these precautions.

The Power Supply
A fairly basic installation, just lineup the holes and screw it on. Don't plug it in yet. Remember, the cable going to the remote switch on the front of the case carries 110 volts AC. If you took the wires off the switch, make sure you connect them just as they were before (I hope you documented). A wrong connection here can burn up your PC. After your power supply is installed, do not plug it in, you may not be able to tell if the switch is on or off and you don't want to turn the power supply on without a load.

The RAM
DIP memory modules are the hardest to install. Luckily, it's not done much anymore. SIMMs are inserted at about a 45 degree angle then stoand up until they clip into place. If they don't clip in properly, maybe you have them in backwards. They'll usually have a key cut into one side. DIMMs are keyed on the edge connector side, they can only be inserted one way. Once they are lined up, push them down until the locking tabs on the side come up. You may have to support the motherboard from underneath if it looks as though its going to flex too much. COAST modules are also keyed on the bottom and insert much like an adapter card (Coast On A STick memory is cache SRAM).

The CPU
Luckily, CPU sockets aren't friction fit anymore. If you have a PGA Central Processor (Pentium MMX or Celeron, Cyrix or AMD), it will fit into a ZIFF (zero insertion force) socket. Pin#1 on the chip has to be lined up with pin#1 on the socket. This can be indicated on the socket with an arrow, a #1 silk-screened on the board, or a flattened corner. Usually the CPU will indicate pin#1 with a flattened corner (and, or a dot on top, and, or an arrow on the bottom center of the chip). Unclip and lift the handle, insert the chip, lower the handle and clip it in. If it's a Pentium II or a Pentium III, it will fit into a Slot 1 socket. These are rectangular in shape and have 242 pins in two rows. They're keyed, and the cartridge should only fit in one way. Check any documentation that came with the motherboard or CPU, and refer to your notes.

The Motherboard
Most PC cases will allow you to remove the metal tray that the motherboard attaches to by removing 2 or more screws. If you didn't do that during disassembly then you should familiarize yourself with it now. The plastic standoffs on the motherboard are to keep the solder-side of the board from touching the metal case and shorting out. Usually, it's better to install the RAM and CPU first to avoid the possibility of flexing the board and cracking solder connections or traces. Orient the motherboard properly and either clip in, or slide in the standoffs until the mounting screw holes line up. Insert the screws that hold the board in place. The screws need to be snug, but do not twist them into the motherboard. You may be able to connect the power to the system board as you install the tray. A power supply with a baby-AT form factor will have two motherboard connectors (P8 and P9). These connectors are keyed but can be reversed. Make sure the black wires on the two connectors are beside each other. Clip the keyed edge in at an angle, then straighten the connector up and slide it on. ATX power connectors slide in until the tab clicks. Once the motherboard and tray are secured in place, you can re-install the wires for the front of the case (refer to your documentation).

At this point, you can install the video card. Do a final check on everything installed. Re-check all installations and connections, attach and plug in the monitor. Turn the computer on. Watch for lights on the front panel. How far does the BIOS POST routine get? Are there any error messages? Is this expected?

One thing that I don't like to do is to completely re-assemble a computer and then just turn it on. If it doesn't work at that point, then you have to tear it all apart again to find the problem. Once you've installed the power supply, motherboard, CPU and RAM, install the video card, hook up the keyboard and cables and start your computer. Of course, there's no drives installed and some expansion cards are missing, so you're going to get errors. But if it recognizes your video card, counts the memory and allows access to you CMOS setup program, then you know everything installed to that point is working. Now, turn your computer on after installing each new device. If you get an unexpected error, then it should be due to the last device you installed.

System Memory

ROM (Read Only Memory)
(Pronounced 'rahm')

An acronym for Read Only Memory, ROM is computer memory on which data has been pre-recorded. The programming code and/or data on a ROM chip is written to the chip at the factory. It can be read, but it cannot be erased or removed. It's permanent. ROM retains its data or content even when the computer is turned off, unlike a computer's main memory (RAM), which needs a constant charge of electricity to keep its information. For this reason, ROM is considered to be 'non-volatile' and RAM is 'volatile'.

ROM chips are used in all kinds of electronic devices from calculators to video games. Most personal computers have several applications of ROM memory. These chips often store permanent and critical information and programs that don't need to be changed, or don't need to be written to. Most personal computers have a small amount of ROM that stores the code that starts up or boots the computer. Early computers also used ROM to store the BIOS (Basic Input Output System) which acts as a translator between the PC's hardware and the operating system.

The problem with using these ROM chips for BIOS information is that computer manufacturers had to build their systems around the available fabricated ROMs and their coding. Because the information was written to the chip during the fabrication process, changes to the chip would involve developing new assembly lines and purchasing new equipment. It would cost a small fortune if a single computer or motherboard manufacturer had visions of having the coding on these chips changed to accommodate new developments or enhancements they might want to incorporate into their product.

PROM (Programmable Read Only Memory)
(Pronounced 'prahm')

Remember, all a computer can understand is 1s and 0s. That's how a transistor works. It either allows electricity to pass or it doesn't. There's either an electrical charge, or there isn't. It's on or off (1 or 0). These memory chips are made up of millions of tiny transistors that hold the 1s or 0s. Just like millions of tiny switches or fuses.

With a ROM chip, these switches are permanently set at the factory in their respective on or off (1 or 0) positions. In the case of a PROM (Programmable Read Only Memory), the chip is manufactured with all its little switches or fuses intact, or set to '1' (blank memory). When these chips are programmed, with the use of a device called a PROM programmer (or burner), a high voltage electronic pulse destroys selected switches, burning in the settings that need to be change to a '0'.

So PROM is programmable, but only once. Like ROM, it's permanent, or non-volatile. It cannot be erased.

Cache

You will hear of many different types of caches associated with your computer system. There are caches for your CD-ROM, memory caches and hard drive caches. Your favorite browser even keeps a cache on your hard drive of the most recent websites you've visited. If the site hasn't changed, it can load it quicker from your hard drive than it can over the phone lines. A cache is meant to improve access times and enhance the overall performance of your computer. The type we're concerned with in this section is cache memory.

Everything you do on your computer requires RAM (Random Access Memory). RAM allows for quick access of data and instructions. Access to your ROM chips, drives, CD-ROM etc., is miserably slower than Random Access Memory. Therefore, that's where all the different devices and components get their data from. RAM is where your CPU processes information and instructions. Random Access Memory is the go-between for all your devices. When you start your computer, the operating system, device drivers, any active files and all running programs are loaded into RAM. (Hence the term "loading a program"). This is also why one of the best upgrades you can do for your computer is to increase the amount of memory. If your memory is low, then you have to wait each time new information is swapped into memory from the slower device.

Personal Computers use Dynamic Random Access Memory (DRAM) for system memory or RAM. These chips need a constant electrical 'kick in the but' to remind them what they have stored in their registers. In other words, they need to be constantly 'refreshed'. This constant refreshing takes up a lot of the CPU's time. There's another type of RAM called Static Random Access Memory (SRAM). SRAM will hold its information as long as there's electricity to the chip. It doesn't need constant refreshing and therefore is faster than DRAM. However, SRAM is a lot more expensive than DRAM, and it takes up a lot more space. You wouldn't have enough room on your motherboard for even 16MB of SRAM, and the cost would take it out of the realm of the affordable home computer. This makes DRAM the affordable and practical choice for system memory. But SRAM is used for Cache memory.

So where does cache come in? Cache is the way that most CPUs are matched to RAM. In between the CPU and system memory (DRAM) there is a small cache of the faster SRAM. Circuitry on the motherboard, called a cache controller, decides the content of this cache. The most recently accessed information or instructions can help the controller to guess at what RAM locations may be accessed next and these are stored in the cache. When the CPU needs its next instruction or piece of data, it looks in the cache. If the info is there, it's called a 'cache hit' and is retrieved at a faster speed than it would be from system memory. If it isn't in the cache, then it's called a 'cache miss' and the information is retrieved from system memory (slower DRAM). The cache controller then guesses at the next access and loads it into the cache. The number of cache hits, far outnumber the misses and this speeds up system performance dramatically.

Two of the main factors that affect a cache's performance are size (or amount of cache memory) and level.

Your Computer’s Case

Your computer's case plays a large part in the overall expandability, protection, cooling and lifetime of your system.

Expandability
At one time or another you may decide to add something to your computer. Maybe a CD Player, Zip Drive, a second Hard Drive, Floppy or Tape Drive. These fit on shelves in your computer called Bays. If there are no extra bays, then there’s no place for these devices to go. These bays can be 3.5 inches or 5.25 inches wide. There are internal and external bays. You can notice the external bays by looking at the front of your case.

Devices like floppy drives and CD-ROMs slide into external bays and can be seen (and accessed) from the front of your system case. If you have unused bays, they will have a plastic faceplate over them (or at least they should have). If you see that you have empty external bays, then you know that at least there's room to add another of this type of device. Remember however, that hard drives are sometimes hidden behind these faceplates, so a look inside the case will tell you for sure. Hard drives are generally situated in internal bays because there is no need for physical outside access. You must look inside the case to see if you have any free internal expansion bays.

Any device added to your system requires power, and your power supply has to have enough juice to supply that power. If you have a 150 or 200-watt power supply, your system may be limited to the amount of devices that can be added. Most cases come with a power supply, but make sure.

Getting Your Hands in There
You may at one time or another, have to remove, change or check a device in your computer. You want to be able to access that device without too much difficulty. It would be nice if you didn't have to remove the ribbon cables and expansion cards just to install another DIMM, or replace a battery. Or, if you're the type that likes to just jam his hand in amongst all that stuff, it would be nice to be able to remove it without accidentally loosening or removing other connections.

On some cases you can loosen a single screw, and completely remove the drive bays for easy access to the drives and their mounting screws. Others have access ports for the screws on the back plate of the case. However, I’ve seen some drive bays that are spot-welded in place without access ports, and you just about have to remove the motherboard to access the screws for the drives. Think about what you want to put inside and make sure there's enough room for access.

Protection
A good, rigid, well built case can protect the internal components from dust, vibration, foreign objects, ESD and EMI. Electromagnetic interference is not always an easy one to nail down. My computer has a good solid case, but when you watch TV, you can tell if the computer is on or not.

You can find cheap cases that fit together so poorly, or are so flimsy, that they actually rattle when the computer is turned on. One thing that can be bad for your system are vibrations caused by outside sources; whether it’s a panel or cover that rattles, a desk drawer being closed, or a printer that doesn’t operate the smoothest. The stability of the case makes a big difference.

Some of the case frames are left unfinished, resulting in sharp edges, burrs and metal slivers. Ribbon cables and wires (and your fingers) can be nicked or cut by these sharp edges just by removing or replacing the outside cover. Companies that actually put some research and development into their product will address these issues. You should think about them when you purchase. As with just about everything, you usually get what you pay for.

Longevity
Heat shortens the life of electronic components. The different devices and components inside your case need room to breathe and release heat build-up. Sometimes, things are packed so tightly inside your system case that you can’t even get your hand in to remove a cable or an expansion card, let alone allow for any kind of air flow. Air flow is needed to cool down the components and devices inside the case.

The cooler that your computer operates, the longer it will last. The fan on the power supply is made to help cool the power supply as it operates. The way the case is built should allow the intake of cooler air through vents, draw it over the different components inside to help cool them, and push the heated air out. Some cases come with auxiliary fans that fit over the intake vents to add to the airflow inside.

It’s important not to interrupt this flow. Don’t pack things around the case or obstruct the intake vents. Keep the intake vents clean and clear of grime and dust balls. Also, by leaving the expansion slot inserts off the back of the case, or the faceplate off unused external bays, you could possibly be changing the way the air flows through your case and reducing cooling efficiency.

If you have a system that seems to run fine, then just shuts itself off after a half hour or so, check out the operating temperature of the CPU. See that the fans are working on the power supply and the CPU's heatsink.

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