General VIrus types

BITS

DECIMAL NUMBERS

The easiest way to understand bits is to compare them to something you know: digits. A digit is a single place that can hold numerical values between 0 and 9. Digits are normally combined together in groups to create larger numbers. For example, 6357 has 4 digits. It is understood that in the number 6357 that the 7 is filling the "1s place", while the 5 is filling the 10s place, the 3 is filling the 100s place and the 6 is filling the 1000s place. So you could express things this way if you wanted to be explicit:

(6 * 1000) + (3 * 100) + (5 * 10) + (7 * 1) = 6000 + 300 + 50 + 7 = 6357

Another way to express it would be to use powers of 10. Assuming that we are going to represent the concept of "raised to the power of" with the "^" symbol (so "10 squared" is written as "10^2"), another way to express it is like this:

(6 * 10^3) + (3 * 10^2) + (5 * 10^1) + (7 * 10^0) = 6000 + 300 + 50 + 7 = 6357

What you can see from this expression is that each digit is a placeholder for the next higher power of 10, starting in the first digit with 10 raised to the power of zero.

Computers happen to operate using the base-2 number system, also known as the binary number system (just like the base-10 number system is known as the decimal number system). The reason computers use the base-2 system is because it makes it a lot easier to implement them with current electronic technology. You could wire up and build computers that operate in base-10, but they would be fiendishly expensive right now. On the other hand, base-2 computers are dirt cheap.

So computers use binary numbers, and therefore use binary digits in place of decimal digits. The word bit is a shortening of the words "Binary digIT". Where decimal digits have 10 possible values ranging from 0 to 9, bits have only 2 possible values: 0 and 1. Therefore a binary number is composed of only 0s and 1s, like this: 1011. How do you figure out what the value of the binary number 1011 is? You do it in the same way we did it above for 6357, but you use a base of 2 instead of a base of 10. So:

(1 * 2^3) + (0 * 2^2) + (1 * 2^1) + (1 * 2^0) = 8 + 0 + 2 + 1 = 11

You can see that in binary numbers, each bit holds the value of increasing powers of 2. That makes counting in binary pretty easy. Starting at zero and going though 20, counting in decimal and binary look like this

0 = 0 1 = 1 2 = 10 3 = 11 4 = 100 5 = 101 6 = 110 7 = 111 8 = 1000 9 = 100110 = 101011 = 101112 = 110013 = 110114 = 111015 = 111116 = 1000017 = 1000118 = 1001019 = 1001120 = 10100

When you look at this sequence, 0 and 1 are the same for decimal and binary number systems. At the number 2 you see carrying first take place in the binary system. If a bit is 1, and you add 1 to it, the bit becomes zero and the next bit becomes 1. In the transition from 15 to 16 this effect roles over through 4 bits, turning 1111 into 10000.

SYSTEM CASE

The case houses all of the vital working parts of your PC, keeping them shielded from the "dangers" of the outside world (dust, dirt, static electricity, etc…). There are also some "functioning" bits built into the case.

The power supply, that delivers and then divides the electricity going to your PC and all of the internal components, is attached to the inside of your PC case. The power supply is generally, but not always, included when purchasing a PC case.
The other major component that is part of the case is your PC's cooling system. Most of the components inside your PC (Your CPU, RAM, graphics adapter, etc…) produce a fair amount of heat as electrical current passes through their tiny circuits (especially when they're heavily loaded). This heat can not be allowed to build up or it can effect the PC's performance and, in extreme cases, heat can sometimes actually melt the plastic components. There is a large fan on the front of the case (near the bottom) that pulls room temperature air into the case. Then, the fan inside the power supply (at the upper rear)

exhausts the warm air through the rear of the case.

The power and reset buttons, as well as the lights that indicate when the PC is turned and when the hard drive is either

reading or writing, are also built into the case.

The case has a few different sizes of drive bays built into it. There are bays for floppy drives, hard drives and large

capacity removable media drives (CD-ROM, DVD, CD-RW).

At the back of the case there are slots with removable covers. These slots provide access to the external features (ports, jacks etc…) of any expansion cards installed in your PC.

HOW PCI WORKS

Let's say that you have just added a new PCI-based sound card to your Windows 98 computer. Here's an example of how it would work.

1.You open up your computer's case and plug the sound card into an empty PCI slot on the motherboard.

This motherboard has four PCI slots.

2.You close the computer's case and power up the computer.

3.The system BIOS initiates the PnP BIOS.

4.The PnP BIOS scans the PCI bus for hardware. It does this by sending out a signal to any device connected to the bus, asking the device who it is.

5.The sound card responds by identifying itself. The device ID is sent back across the bus to the BIOS.

6.The PnP BIOS checks the ESCD to see if the configuration data for the sound card is already present. Since the sound card was just installed, there is no existing ESCD record for it.

7.The PnP BIOS assigns IRQ, DMA, memory address and I/O settings to the sound card and saves the data in the ESCD.

8.Windows 98 boots up. It checks the ESCD and the PCI bus. The operating system detects that the sound card is a new device and displays a small window telling you that Windows has found new hardware and is determining what it is.

9.If it is able to determine what the device is, it displays the name of the device and attempts to install the driver (the software that enables the device to communicate with the operating system). You may be asked to insert a disk with the driver on it or tell Windows where to find the driver software. If Windows cannot determine what the device is, it provides a dialog window so that you can specify what type of device it is and load a driver to run it.

10. Once the driver is installed, the device should be ready for use. Some devices may require that you restart the computer before you can use them. In our example, the sound card is immediately ready for use.

11. You want to capture some audio from an external tape deck that you have plugged into the sound card. You set up the recording software that came with the sound card and begin to record.

12. The audio comes into the sound card via an external audio connector. The sound card converts the analog signal to a digital signal.

13. The digital audio data from the sound card is carried across the PCI bus to the bus controller. The controller determines which device on the PCI device has priority to send data to the CPU. It also checks to see if data is going directly to the CPU or to system memory.

14. Since the sound card is in record mode, the bus controller assigns a high priority to the data coming from it and sends the sound card's data over the bus bridge to the system bus.

15. The system bus saves the data in system memory. Once the recording is complete, you can decide whether the data from the sound card is saved to a hard drive or retained in memory for additional processing.

MOTHERBOARD AND SYSTEM DEVICES

The motherboard is the base of the modern computer system. Here are the main parts of the motherboard and its related devices:

· Motherboard: The motherboard is the main circuit board in the computer where everything comes together. This is where you plug in your processor, memory, cache, video card and other cards. It is also where you connect your peripherals.

· System Chipset and Controllers: Their job is to direct traffic and control the flow of information inside the computer. These circuits control the processor's access to memory, the flow of data to and from peripheral devices and communications lines, and much more. The chipset is a critical part of any computer, because it plays a big role in determining what sorts of features the computer can support. For example, which processors you can use, which types of memory, how fast you can run the machine, and what kind of system buses your PC can use, are all tied in to the type of chipset the motherboard uses.

· System Buses: The system buses are the electrical channels through which various parts of the computer communicate. The physical part of these buses, the part you see, is the set of slots in the back of the machine into which you put your video card, sound card and other cards. It is over the system buses that your video card gets information from the processor, the processor saves data to your hard disk, etc.

· BIOS: The system BIOS (which stands for Basic Input/Output System) 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.

· 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 dictate how your PC organizes its access to various memory areas and devices. 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): 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. 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. 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. 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.

GRAPHICS ADAPTER

A graphics adapter, also known as a graphics accelerator or video card, is an expansion card that processes data and

then passes it on to your monitor for display.
Unless your PC has onboard video, your monitor will be plugged into the part of the graphics card that is accessible through the back of the case.

Graphics adapters are now available to fit into three different types of expansion slots on your PC's motherboard. A PCI graphics adapter plugs into a PCI (Peripheral Component Interface) slot and the newer types plug into either an AGP (Advanced Graphics Port) or AGP Pro slot. The newer AGP and AGP Pro slots were developed specifically for graphics adapters and have the ability to transfer certain types of data at much faster rates than the older PCI slot is capable of. The trade off with AGP and AGP Pro slots is that they are not multi-use as PCI slots are. They are designed specifically for graphics

adapters only.

There are many different types and brands of graphics adapters available today and there are many different combinations of capabilities and features that may be included in each. All will have their own built in graphics chipset designed to handle graphical data and all will have their own built in memory (RAM). Some even have their own built in processor to handle some of the calculations that would normally be performed by your system's CPU.

Some are excellent at presenting 2D (two dimensional) graphics such as office applications, the Windows® GUI (graphical user interface), etc, others are excellent at presenting 3D (three dimensional) graphics for 3D games, 3D design programs, etc. Others might excel in the presentation of video such as streaming web content, DVD movies, home movies shot with a digital video camera, etc. Then there are some that have combinations of two or more of these capabilities all wrapped up in a single card.

Choosing a graphics adapter can be a tricky decision to make, especially with all these features to take into consideration. The graphics adapter that is right for may be decided upon by either price or included features, or a combination of the two, depending on your specific needs or the amount of money you are prepared to spend. The prices can range from only a hundred dollars or so for a basic 2D/3D model, to several hundred for a high-end 3D adapter and all the way up to several thousand for a top of the line professional workstation model.

CPU

A computer is only as good as its brainpower. The key to that brainpower is the Central Processing Unit (CPU), also known as the microprocessor, or simply, the processor.

The miracle of silicon allows chipmakers to create a brain for your personal computer that is more powerful than many early computers that took up whole rooms. It keeps getting smaller - and smarter - than early computer makers ever dreamed possible. The CPU is what makes it possible for you to join me here on the internet via a computer on your desk at your home or office.

HOW IT WORKS

The CPU in your PC actually contains three parts:

· Input/Output Unit (I/O Unit)

· Arithmetic/Logic Unit (ALU)

· Control Unit

The I/O unit controls interaction between the CPU and all the other parts of your computer. The ALU handles logic and math functions. The control unit basically tells the other parts of the CPU what to do.

Like all things computer, the CPU understands instructions only in terms of ones and zeroes at the core. A change in the electrical signal through a given circuit changes that circuit from a one to a zero or vice versa. When a programmer compiles some code s/he has written, that code is translated into machine language that the CPU can understand.

SYSTEM CACHE

The system cache is responsible for a great deal of the system performance improvement of today's PCs. The cache is a buffer of sorts between the very fast processor and the relatively slow memory that serves it. (The memory is not really slow, it's just that the processor is much faster.) The presence of the cache allows the processor to do its work while waiting for memory far less often than it otherwise would.

There are in fact several different "layers" of cache in a modern PC, each acting as a buffer for recently-used information to improve performance, but when "the cache" is mentioned without qualifiers, it normally refers to the "secondary" or "level 2" cache that is placed between the processor and system RAM.

“LAYERS” OF CACHE

There are in fact many layers of cache in a modern PC. This does not even include looking at caches included on some peripherals, such as hard disks. Each layer is closer to the processor and faster than the layer below it. Each layer also caches the layers below it, due to its increased speed relative to the lower levels:

Level	Devices Cached
Level 1 Cache	Level 2 Cache, System RAM, Hard Disk / CD-ROM
Level 2 Cache	System RAM, Hard Disk / CD-ROM
System RAM	Hard Disk / CD-ROM
Hard Disk / CD-ROM	--

What happens in general terms is this. The processor requests a piece of information. The first place it looks is in the level 1 cache, since it is the fastest. If it finds it there (called a hit on the cache), great; it uses it with no performance delay. If not, it's a miss and the level 2 cache is searched. If it finds it there (level 2 "hit"), it is able to carry on with relatively little delay. Otherwise, it must issue a request to read it from the system RAM. The system RAM may in turn either have the information available or have to get it from the still slower hard disk or CD-ROM.

It is important to realize just how slow some of these devices are compared to the processor. Even the fastest hard disks have an access time measuring around 10 milliseconds. If it has to wait 10 milliseconds, a 200 MHz processor will waste 2 million clock cycles! And CD-ROMs are generally at least 10 times slower. This is why using caches to avoid accesses to these slow devices is so crucial.

Caching actually goes even beyond the level of the hardware. For example, your web browser uses caching itself, in fact, two levels of caching! Since loading a web page over the Internet is very slow for most people, the browser will hold recently-accessed pages to save it having to re-access them. It checks first in its memory cache and then in its disk cache to see if it already has a copy of the page you want. Only if it does not find the page will it actually go to the Internet to retrieve it.

ETHERNET

Ethernet has been a relatively inexpensive, reasonably fast, and very popular LAN technology for several decades. Two individuals at Xerox PARC -- Bob Metcalfe and D.R. Boggs -- developed Ethernet beginning in 1972 and specifications based on this work appeared in IEEE 802.3 in 1980. Ethernet specifications define low-level data transmission protocols and the technology needed to support them. In the OSI model, Ethernet technology exists at the physical and data link layers (layers 1 and 2).

A low-level network technology, Ethernet supports IP and most other higher-level protocols. Traditional Ethernet supports data transfers at the rate of 10 Megabits per second (Mbps). Over time, as the performance needs of LANs have increased, related technologies like Fast Ethernet and Gigabit Ethernet have been developed that extend traditional Ethernet to 100 Mbps and 1000 Mbps speeds, respectively.

TRADITIONAL ETHERNET

Often referred to as "Thicknet," 10Base5 technology was the first incarnation of Ethernet. It was used in the 1980s until 10Base2 "Thinnet" with more flexible cabling appeared. The most common form of traditional Ethernet, however, is 10Base-T due to the inherent advantages of unshielded twised pair (UTP) over coaxial cabling and its low cost compared to alternatives like fiber.

The following table lists these well-known forms of Ethernet technology. Besides the type of cable involved, another important factor in Ethernet networking is the segment length. A single uninterrupted network cable can only span a certain physical distance before its electrical characteristics are critically affected by factors such as line noise or reduced signal strength.

Name	Segment Length (Max.)	Cable
10Base5	500m / 1640ft.	RG-8 or RG-11 coaxial
10Base2	185m / 606ft.	RG 58 A/U or RG 58 C/U coaxial
10Base-T	100m / 328ft.	Category 3 or better unshielded twisted pair

Several other less well-known Ethernet standards exist, including 10Base-FL, 10Base-FB, and 10Base-FP for fiber optic networks and 10Broad36 for broadband (CATV) cabling.

FAST ETHERNET

In the mid-1990s, Fast Ethernet achieved its design goal of increasing the performance of traditional Ethernet while avoiding the need to completely re-cable existing networks. Fast Ethernet comes in two major varieties:

· 100Base-T (using unshielded twisted pair cable)

· 100Base-FX (using fiber optic cable)

By far the most popular of these is 100Base-T, a standard that includes 100Base-TX (Category 5 UTP), 100Base-T2 (Category 3 or better UTP), and 100Base-T4 (100Base-T2 cabling modified to include two additional wire pairs).

GIGABIT ETHERNET

Wheareas Fast Ethernet improved traditional Ethernet from 10 Megabit to 100 Megabit speed, Gigabit Ethernet offers the same order-of-magnitude improvement over Fast Ethernet by offering speeds of 1000 Megabits (1 Gigabit). Gigabit Ethernet was first made to travel over optical and copper cabling, but the 1000Base-T standard successfully supports it as well. 1000Base-T uses Category 5 cabling similar to 100 Mbps Ethernet, although achieving gigabit speed requires the use of additional wire pairs.

HOW A COMPUTER VIRUS WORKS

Virus isa self-replicating piece of computer code that can partially or fully attach itself to files or applications, and can cause your computer to do something you don't want it to do.

Computer viruses can spread swiftly across open networks such as the Internet, causing billions of dollars worth of damage in a short amount of time. Five years ago, the chance you'd receive a virus over a 12-month period was about 1 in 1000; today, your chances have dropped to about 1 in 10. The vital statistics:

· Viruses enter your system via e-mail, downloads, infected floppy disks, or (occasionally) hacking.

· By definition, a virus must be able to self-replicate (make copies of itself) to spread.

· Thousands of viruses exist, but few are found "in the wild" (roaming, unchecked, across networks) because most known viruses are laboratory-made, never released variations of common "wild" viruses.

· Virus behavior can range from annoying to destructive, but even relatively benign viruses tend to be destructive due to bugs introduced by sloppy programming.

· Antivirus software can detect nearly all types of known viruses, but it must be updated regularly to maintain effectiveness.

A virus is just a computer program. Like any other program, it contains instructions that tell your computer what to do. But unlike an application, a virus usually tells your computer to do something you don't want it to do, and it can usually spread itself to other files on your computer -- and other people's computers.

While there are thousands of variations of viruses, most fall into one of the following six general categories, each of which works its magic slightly differently:

Boot Sector Virus: replaces or implants itself in the boot sector---an area of the hard drive (or any other disk) accessed when you first turn on your computer. This kind of virus can prevent you from being able to boot your hard disk.

File Virus: infects applications. These executables then spread the virus by infecting associated documents and other applications whenever they're opened or run.

Macro Virus: Written using a simplified macro programming language, these viruses affect Microsoft Office applications, such as Word and Excel, and account for about 75 percent of viruses found in the wild. A document infected with a macro virus generally modifies a pre-existing, commonly used command (such as Save) to trigger its payload upon execution of that command.

Multipartite Virus: infects both files and the boot sector--a kind of viruses that can reinfect your system dozens of times before it's caught.

Polymorphic Virus: changes code whenever it passes to another machine; in theory these viruses should be more difficult for antivirus scanners to detect, but in practice they're usually not that well written.

Stealth Virus: hides its presence by making an infected file not appear infected, but doesn't usually stand up to antivirus software.

ALL MALIICOUS CODES AREN’T VIRUSES

A common misconception is that other kinds of electronic nasties, such as worms and Trojan horse applications, are viruses. They aren't. Worms, Trojan horses, and viruses are in a broader category analysts call "malicious code."

A worm program replicates itself and slithers through network connections to infect any machine on the network and replicate within it, eating up storage space and slowing down the computer. But worms don't alter or delete files.

A Trojan horse doesn't replicate itself, but it is a malicious program disguised as something benign such as a screen saver. When loaded onto your machine, a Trojan horse can capture information from your system -- such as user names and passwords--or could allow a malicious hacker to remotely control your computer.

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Дата добавления: 2014-12-27