This is a very short course about computers. It won't make you into a computer technician, but it will let you talk to one without thinking they are speaking a foreign language. The computers we are interested in are the general-purpose personal computers like we use at our desk, rather than the specialized computer under the hood of your car, or the big mainframe computers that the IRS uses to keep track of us all.

For our purposes, a computer can be defined as a machine for processing and storing information electronically. To be useful, it must have a way for us to get information into the machine, and some way to get it out afterwards so we can see it. Therefore, a computer has four basic functions:

1. Input
2. Processing
3. Storage
4. Output

Every part of a computer system, and everything it does, is connected to one or more of these basic functions. Computers can seem complex because there are many ways of doing each of these functions, and because everything has a new name, often made up of what appear to be nonsense initials like RAM or CPU.

To keep it simple, first look up any word or technical term you don't understand. There is a glossary at the back of this course pack with definitions for all the technical terms we use here or that you are likely to hear in talking to technicians. Second, when you encounter any part of a computer for the first time, find out which of these four functions it is used for.

Input

The most obvious device for getting information (also called data) into a computer system is the keyboard. Another common input device is the mouse. Almost any time we use a computer we use one or both of these to get text data or instructions into the system. To get large amounts of information into the computer we would probably use a CD (compact disc), a floppy diskette, a modem connected to a phone line, or a network card connected to a network.

With the right sound equipment, a microphone or musical instrument can be used to bring in sound, and a digital camera can download visual information, so these are all being used as input.

Processing

If we just put information into the computer and took it out again later, computers could be much simpler. Most of the time though, we want to do something with it or change it in some way. Doing something with it is called processing, or data processing. Most of this takes place in a part called the processor, or Central Processing Unit. This is usually abbreviated to CPU.

The CPU is such an important part of the computer that we often refer to a system by the type of CPU it contains. My office computer is a Pentium III but at home I have an Athlon system. Sometimes you will hear the entire computer cabinet called a CPU.

To help the CPU there is another part that takes over a lot of the arithmetic. It is called the co-processor and also the Numerical Processing Unit or NPU.

The smallest piece of data that a computer can process is called a bit, and each bit will be either a one or a zero. For the sake of efficiency, the computer normally deals with a minimum of 8 bits at a time, and 8 bits together is called a byte. This is an important word to remember, because in working with computers you will hear about bytes frequently, and also megabytes which is one million bytes. Megabyte is usually abbreviated MB.

Storage

Information is stored in a computer in several different ways, but the main two are RAM and hard drive, also called a hard disk.

The hard drive can store huge amounts of information, and it can keep this information when the computer is turned off. The only problem with hard drives is that it takes too long to get the data from them, because the system must wait as the disk spins until the right part of the disk surface comes under the read heads, and then the drive transfers a block of data in sequence.

For information being used at the moment, the system needs to be able to get to any part of that information very quickly. The storage for this information is RAM, which stands for Random Access Memory. Random access means the system can ask for any piece of stored data at random and get it immediately, without waiting for to come up in sequence.

Random access is faster than sequential access. The main reason that computers don't just store everything in RAM is that information in RAM is lost whenever the power is turned off. So, the computer needs both RAM and disk storage. Everything stored long-term is on the disk, and whatever is needed at the moment is copied into RAM. If changes are made to the data in RAM, the changes must be copied back to the disk before that computer is turned off so those changes are not lost.

When people talk about the memory of a computer, they almost always mean RAM, and not the hard drive or other forms of storage that we haven't covered yet. Sometimes they will say 'main memory' just to be perfectly clear about it.

Output

There are a number of ways to get data back out of the computer. One of them is right there in front of your face, and that's the monitor. It has a screen that shows you information from the computer, so it is an output device. The monitor and the parts of the computer that run it are known as video. Another common output device is a printer, which of course puts the computer's output on paper for you.

Some devices are both input and output, like the floppy diskette mentioned earlier. If you have a modem or network connection, information can go both ways over the line so it's both input and output. In many cases it is practical to consider input and output together, in which case they are called Input/Output and abbreviated as I/O.

Software

The physical parts of as computer are called hardware. These are things you can see and touch, and yes, they are hard.

These same pieces of hardware are used whether you want to write a letter, balance your checkbook or play a game of computer solitaire, so how does this hardware know what to do, and when to do it? Actually, the hardware parts don't have a clue. They need instructions to tell them what to do, so some of that information we're storing and processing must be the instructions telling the computer what to do.

These instructions are called software, and they are organized into sets of instructions called programs. If you want to play a game on the computer, the computer must have a program with the instructions for that particular game. If you want to send e-mail, there must be an e-mail program.

One program called the operating system contains the instructions for actually running the computer. Certain things need to be done no matter what you are using the computer for. These are things like getting data on and off the disk, getting information to the monitor screen, and so forth. A program called Windows is the most common operating systems for PCs, and the one used in this office. It comes from a little company called Microsoft. Perhaps you have heard of it.

Anyway, the operating system takes care of the basic details of running the computer, and allows other programs to concentrate on their particular tasks. These other programs are called applications, because their job is to apply the computer to a specific task. When we say software, that can mean the applications, the operating system, or both.

There are other little chunks of software that attach to the operating system and tell it how to run particular hardware components or applications. These little chunks are called drivers, and anytime you add a piece of hardware to a system or upgrade to a new version of a device, you will need to have the right driver for that new piece of hardware.

Circuits

The word 'circuit' means a path something follows that goes back to where it started. In an electrical or electronic circuit, the thing following the path is of course electricity. Whatever work it does and whatever components it must go through on the way, the 'circuit' just means there is a complete path back to the starting point. The word 'circuitry' means the components, and the connections between them, that make up the path. It's as simple as that.

In a computer, this word 'circuit' shows up in a couple of places. Most of the components will be found soldered to a 'circuit board', also called a printed circuit board. This is a thin board of fiberglass, with the connections between components (called traces) printed right on the board in copper.

The components themselves are often something called an Integrated Circuit, abbreviated IC. To integrate means to combine things together, so an integrated circuit is one that combines many circuits into the same part. These are formed on a thin wafer or 'chip' of silicon, and you will sometimes hear an IC being called a 'chip'. This is also where we get the name 'Silicon Valley', for the area in Central California where many of them are made.

This chip of silicon is put into a case with metal legs, and tiny wires between the wafer and the legs connect those circuits with the outside world. There are now ICs containing millions of transistors and other components, all in a package that will fit in your palm with lots of room to spare.

The Motherboard

The biggest and most important circuit board in a computer is the motherboard. It's called the motherboard because every other part of the computer is connected to it. The CPU plugs into the motherboard, and so does the memory. And all the components that plug into the back of the case, like the keyboard and monitor, are connected from there to the motherboard. It is also called the system board.

There is a set of slots near the back edge of the board for plugging in other circuit boards, called adapter cards. These plug-in slots are for things like modems, sound cards, network cards and just about any other add-on feature you can think of. There is a set of traces connecting these slots to each other. A trace goes from Pin 1 of the first slot to Pin 1 of each of the other slots, and other traces connect each of the Pin 2s, each of the Pin 3s, etc. These traces then go on to connect to the memory, CPU, disk drives and other parts of the computer.

The Bus

These traces are called a bus, and they provide a way for each part of the system to exchange information with every other part. There are different types of bus that have been developed over the years, and your computer may have more than one. The most common one, found in every PC sold today, is called the PCI bus. An older version, still seen in some new computers, is the ISA bus. These are fairly easy to tell apart, because the connector that fits in the PCI slot has smaller pins and more of them.

One of the differences between PCI and ISA is that PCI supports a feature called Plug-and-Play, which allows you to add new hardware and have the system detect and configure it automatically. As anyone knows who has used the old method, plug-and-play is a great convenience.

Resources

Since just about everything in the computer uses the bus to exchange data, there must be a way to decide whose turn it is. This is done with something called an Interrupt Request, abbreviated IRQ. Each component or device that will need access to the bus is assigned an IRQ level, from IRQ 0 to IRQ 15, and there are lines on the bus that correspond to these levels. When a device needs to transfer data on the bus, it tugs on the appropriate Interrupt Request line and waits until the CPU grants an interrupt for that level. It's all very nice and orderly. IRQs are an example of a system resource, which is a feature available in limited quantity that must be assigned to specific components.

There are other resources too. Each device must have a unique memory location where their data is stored as it is transferred to and from the bus. These locations are called I/O Ports. For the BIOS and any other devices that use ROM, there must be a range of memory set aside, so that ROM and RAM are not trying to use the same addresses. These ROM addresses are a resource just like the IRQs and I/O Ports. Some devices need to transfer large amounts of data directly to and from memory without using an interrupt for each little piece of data. These devices, such as disk drives, are assigned a resource called DMA, for Direct Memory Access. Like IRQs, there are 16 DMA levels.

Ports

In addition to the I/O ports mentioned in the last section which are really addresses, there are physical connections on the back of the computer that are also called ports, and various devices connect to the computer through cables attached to these ports. One of these connections is called the Serial Port because data goes over a single signal line as a series of bits, one right after the other. Serial port connectors have either 9 or 25 pins, with the male connector on the back of the computer and a female connector on the cable. The 9-pin version is more common, and is often used to connect a mouse if there is not a separate mouse connection.

Another connection is called the Parallel Port because its cable has 8 parallel signal lines to transmit 8 bits at a time. The computer has a 25-pin female connector on the back, so it won’t be confused with the 25-pin male serial connector. The parallel port is most often used to connect a printer. Computers sold in the last few years will probably also have a connection called USB, for Universal Serial Bus. It is a bus because several devices can be connected on the same cable, but it’s also a port because data goes into and out of the computer cabinet through its connector. There is USB1 and USB2. USB2 is newer and much faster.

The BIOS

As we mentioned earlier, the computer knows what to do by taking instructions from programs stored in RAM. The main instructions come from a program called the operating system, and those instructions direct traffic for other programs called applications.

When the computer is turned off, all the instructions copied into the RAM are gone. When the system is turned on again, it needs to go out to the disk, get the operating system and load it into RAM, but there are no instructions in the RAM to tell it how to do this. The solution to this problem is a set of instructions that stay in memory and don’t get lost when the computer is turned off.

This set of instructions is called the BIOS, for Basic Input Output System. Since the instructions don’t need to change, they can be stored in a different kind of chip than we use for RAM. It’s called ROM, for Read Only Memory. We say that the instructions in the BIOS are hard-wired, and instead of software they are called firmware.

The computer goes through a process called booting up when it is first turned on. This involves executing the BIOS instructions, loading the operating system from disk into RAM, and then turning control of the computer over to the operating system after everything checks out OK. The term refers to somebody pulling themselves up by their own bootstraps (without outside help, in other words). Any computer term that includes ‘boot’ will have something to do with this start-up process.

CMOS and RTC

There is other start-up information that normally stays the same but that we might want to change once in a while. This includes info about the various pieces of hardware connected to the system, which disk drive to check first for the operating system and that sort of thing. This data can’t be stored on the hard drive because we need it to boot up. It can’t be stored in RAM because it will be lost at power-off, and it can’t be stored in the BIOS because we might need to change it.

The problem is solved by a type of RAM chip that uses very low power, and it is connected to a battery. This type of low-power memory chip is called CMOS. It stands for the type of technology used in the chip, which is Complementary Metal Oxide Substrate. This is probably more than you need to know, but I’m a fanatic about defining things. By the way, since batteries don’t last forever, if you leave your computer unplugged for about 5 years you’ll find it needs a bit of trickery to get it to boot again, because the CMOS information will be gone.

There is another feature in the computer that has the same requirements as CMOS, and that is the date and time function. This obviously needs to change very minute, but we don’t want to lose track when the computer is turned off. The circuitry for this is called the RTC or Real Time Clock, and for convenience it is usually included in the same chip with the CMOS. A little trickle of juice from the CMOS battery keeps the clock running, and when you turn the computer on again it knows exactly what time and day it is. Convenient, isn’t it?

More About Video

The monitor is a passive device that just displays the video output from the system. However, so much data is needed for the constantly changing screen display that special provisions are made for it.

The video card (or video circuitry on the motherboard) has its own RAM memory just to hold the display information, and its own ROM BIOS to control the output. Some motherboards even have a special high-speed connection between the CPU and the video. It’s called the AGP, or Accelerated Graphics Port.

The important numbers in evaluating a video display are how many distinct colors can be displayed and also the resolution, which is how many pixels the image contains across and from top to bottom. Each dot of color making up the image is one pixel. As video technology evolved there have been a number of standards, and each one has its own set of initials like EGA, CGA or VGA. A common one is SVGA, which stands for Super Video Graphics Array and has a resolution of 800x600 (that’s 800 pixels across and 600 down). Some high-performance monitors use SXGA (1280x1024) or even UXGA with a resolution of 1600x1200.

More About Disk Drives

Floppies – Although floppy drives are being phased out in some new computers, there are still millions of them out there and you should know something about them. The floppy drive has a little slot on the face of the computer cabinet, and into this slot you can slide a floppy diskette like the one shown here. One of the reasons floppy drives are still around is that it is very easy to take a floppy diskette from one system to another.

Inside the floppy diskette is a round flat disk coated with iron oxide on each side so that data can be stored on it magnetically. This disk is called a platter, and it spins underneath an electro-magnet called the write head that puts data onto the platter surface. There is another head called the read head that copies data from the platter.

Once the disk has made one complete revolution, data is written all the way around. That is called a track. The head then moves a bit and writes another circle of data to create a second track. Altogether, there are 80 tracks on each side, for a total of 160. Altogether, the floppy can hold 1.44 MB (megabytes) of data.

If we are looking for just a few bytes out of 1.44 million, it’s not enough to know which track it is in. To help narrow the search, the track is divided into 18 pieces, called sectors, which look much like a slice of pie. Each sector holds 512 bytes of data, so if we know the track and sector number of the data we want it won’t be hard to find.

Hard Drives – On a hard drive, data is also organized into tracks and sectors. While each sector still holds 512 bytes, there can be many more tracks and sectors on a platter. There are also multiple platters, one on top of the other like a stack of pancakes. Hard drives can hold much more data than floppies, sometimes into the billions of bytes, called gigabytes (GB).

Multiple platters require multiple read and write heads, all attached to the same arm so they move together. It’s called an actuator arm. When we are reading track number 10 on the top platter, the other heads are also positioned over track 10 of the other platters, and together all of these track 10s make up a cylinder. To specify the location of data on a hard drive it is necessary to say what cylinder, then the track and sector. Moving the heads from one cylinder to another is called a seek, and the amount of time this takes is the average seek time.

Although hard drives can hold much more data than floppies, the platters are sealed into a metal case that is fastened inside the computer cabinet, so it’s not an easy matter to move from one system to another like you can with floppies. A hard drive is sometimes called a fixed disk for this reason.

Operating systems use a couple of different methods to keep track of what data is stored where on a drive. One common method uses a table called a File Allocation Table or FAT, which is a section of the disk with pointers to data locations. There are two versions, called FAT16 and FAT32. Windows NT, XP and 2000 use a similar method called NTFS.

There are two different interfaces commonly by hard drives to talk to the rest of the system. These are called IDE for Integrated Drive Electronics, and SCSI for Small Computer System Interconnect. The technical differences are not important at this point, but you should know about the two types because they are not interchangeable.

Figuring out where the heads should go next and then moving them there is the job of some electronic circuitry called the disk controller. Every disk drive has its own controller, which may be on the motherboard or inside the drive itself, depending on the type of drive.

There are a few more things you should know about disk drives before we leave the subject. The first sector of Cylinder 0, Track 0 is called the boot sector, and it contains a Master Boot Record (MBR) that shows whether the disk contains an operating system and the location of the code. If there is more than one operating system, the drive must be divided into multiple partitions. If not, then the whole drive will be a single partition. All of the disk space assigned to a partition is called a volume.

Another term you will encounter is a disk format. There is a high-level format, which creates a new file allocation table and is done with a FORMAT command. There is also a low-level format that creates a new pattern of sectors. A low-level format must be followed by an FDISK command to create a new Master Boot Record and partitions.

Last, we have the word media. This refers to the actual surface holding the data, which is the platter in the case of a disk drive. Because the floppy platter can be taken out of the drive, it is called removable media, while a hard drive is called fixed media.

Other Drives – Most systems today, especially home systems, have additional storage drives that use CD or DVD discs. The technology for both is similar but DVDs hold much more data. These drives do not store data magnetically but use optical markings that are read with a laser. They are mostly used just to read data and not to write it. The full name for CD in fact is CD-ROM, which stands for Compact Disc - Read Only Memory. However, there are versions that can be used to write also, and these are called CD-RW and DVD-RW. Even so they are mostly used to write just once for permanent storage, and are not practical for constantly changing data.

Like hard drives, CD-ROM drives can use either an IDE or SCSI interface. The version of IDE for CD-ROM drives is called ATAPI, and for SCSI the CD-ROM version is ASPI.

Because the discs can be removed, CD-ROM and DVD are considered removable media. There are other types of removable media also that are not as common, such as tape drives and Zip disks, which are similar to floppies but with a storage capacity of 100 or 250 MB. Zip disks and tape drives also use the ATAPI interface.

More About Memory

RAM memory is installed in the system on little circuit boards called modules that plug into the motherboard. This makes it very convenient to add more memory by plugging in another module. Older computers used a SIMM, which stands for Single Inline Memory Module, and later ones use a DIMM or Dual Inline Memory Module. The difference is that DIMMs have different signals on each side of the module connector, and the additional signals allow more memory addresses and more data lines going in and out.

The amount of memory is always is some multiple of two bytes. The most common values are 32, 64, 128, 256 or 512 megabytes.

As it is used by the system, RAM is divided into base memory and extended memory. Base memory is the first megabyte, and extended memory is everything over that. The very first PCs had only base memory, and then a scheme was developed called expanded memory that tricked the computer into allowing more than 1 MB of memory. Once extended memory was developed though, expanded memory quickly became obsolete and will only be found in older computers.

Another memory term you will encounter is cache memory. This refers to memory that is not on DIMMs but is either built into the CPU or connected directly to the motherboard near the CPU. It provides even faster access than RAM for the data that the CPU expects to need next. The contents of the cache will change constantly depending on what the CPU is doing.