What is a Computer System???

What is computer system???

Computer

                                                                                                                                                                Computer system in contemporary usage refers to a desktop system, including the computer itself (aka "The CPU" or "The Box") and all the peripheral devices needed to operate it, usually including:

Monitor

• The computer, usually mounted in a tower (vertical) case, including components: the CPU or central processing unit; a mother board or mainboard, including a BIOS; A graphics card; a sound card (often build onboard, ie. into the mainboard), a variety of disk ports to serve hard drives, disk drives and optical drives, a variety of peripheral ports, including USB ports, legacy serial and parallel ports, legacy psx, and SCSI; and LAN ports, either ethernet or firewire. A hard drive (sometimes more) is included for long-term data storage. The case also contains a transformer power supply and a number of fans to keep the internal components cool while operating. 

• A monitor, typically either a CRT or an LCD flatscreen.

• A set of speakers to hear sound.

• A keyboard, typically a standard 104-key QWERTY keyboard.

• A mouse, typically a three button mouse with scroll wheel.

Other peripherals can be added to the system to augment its functionality. Common ones include: 

• A wi-fi adapter to connect to wireless networks.

• A bluetooth adapter to connect to wireless peripherals.

• A modem for a direct connection to an internet service provider.

• A printer to produce (paper) hardcopy

• A scanner to input data from hardcopy

all these pieces, pulled together comprise a personal computer system

                                            

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How Computers Work: The CPU and Memory

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Figure 0 shows the parts of a computer:

The Central Processing Unit: (CPU), Buses, Ports and controllers, ROM; Main Memory (RAM); Input Devices; Output Devices; Secondary Storage; floppy disks, hard disk, CD-ROM

Figure 0: Inside The Computer

This part of the reading will examine the CPU, Buses, Controllers, and Main Memory. Other sections will examine input devices, output devices, and secondary memory.

The Central Processing Unit (CPU)

Figure 1: The Central Processing Unit

The computer does its primary work in a part of the machine we cannot see, a control center that converts data input to information output. This control center, called the central processing unit (CPU), is a highly complex, extensive set of electronic circuitry that executes stored program instructions. All computers, large and small, must have a central processing unit. As Figure 1 shows, the central processing unit consists of two parts: The control unit and the arithmetic/logic unit. Each part has a specific function.

Before we discuss the control unit and the arithmetic/logic unit in detail, we need to consider data storage and its relationship to the central processing unit. Computers use two types of storage: Primary storage and secondary storage. The CPU interacts closely with primary storage, or main memory, referring to it for both instructions and data. For this reason this part of the reading will discuss memory in the context of the central processing unit. Technically, however, memory is not part of the CPU.

Recall that a computer's memory holds data only temporarily, at the time the computer is executing a program. Secondary storage holds permanent or semi-permanent data on some external magnetic or optical medium. The diskettes and CD-ROM disks that you have seen with personal computers are secondary storage devices, as are hard disks. Since the physical attributes of secondary storage devices determine the way data is organized on them, we will discuss secondary storage and data organization together in another part of our on-line readings.

Now let us consider the components of the central processing unit.

The Control Unit

The control unit of the CPU contains circuitry that uses electrical signals to direct the entire computer system to carry out, or execute, stored program instructions. Like an orchestra leader, the control unit does not execute program instructions; rather, it directs other parts of the system to do so. The control unit must communicate with both the arithmetic/logic unit and memory.

The Arithmetic/Logic Unit

The arithmetic/logic unit (ALU) contains the electronic circuitry that executes all arithmetic and logical operations.

The arithmetic/logic unit can perform four kinds of arithmetic operations, or mathematical calculations: addition, subtraction, multiplication, and division. As its name implies, the arithmetic/logic unit also performs logical operations. A logical operation is usually a comparison. The unit can compare numbers, letters, or special characters. The computer can then take action based on the result of the comparison. This is a very important capability. It is by comparing that a computer is able to tell, for instance, whether there are unfilled seats on airplanes, whether charge- card customers have exceeded their credit limits, and whether one candidate for Congress has more votes than another.

Logical operations can test for three conditions:

• Equal-to condition. In a test for this condition, the arithmetic/logic unit compares two values to determine if they are equal. For example: If the number of tickets sold equals the number of seats in the auditorium, then the concert is declared sold out. 
• Less-than condition. To test for this condition, the computer compares values to determine if one is less than another. For example: If the number of speeding tickets on a driver's record is less than three, then insurance rates are $425; otherwise, the rates are $500. 
• Greater-than condition. In this type of comparison, the computer determines if one value is greater than another. For example: If the hours a person worked this week are greater than 40, then multiply every extra hour by 1.5 times the usual hourly wage to compute overtime pay.

A computer can simultaneously test for more than one condition. In fact, a logic unit can usually discern six logical relationships: equal to, less than, greater than, less than or equal to, greater than or equal to, and not equal.

The symbols that let you define the type of comparison you want the computer to perform are called relational operators. The most common relational operators are the equal sign(=), the less-than symbol(<), and the greater-than symbol(>).

• Registers: Temporary Storage Areas
  Registers are temporary storage areas for instructions or data. They are not a part of memory; rather they are special additional storage locations that offer the advantage of speed. Registers work under the direction of the control unit to accept, hold, and transfer instructions or data and perform arithmetic or logical comparisons at high speed. The control unit uses a data storage register the way a store owner uses a cash register-as a temporary, convenient place to store what is used in transactions.
  
  
  Computers usually assign special roles to certain registers, including these registers:
  • An accumulator, which collects the result of computations.
  • An address register, which keeps track of where a given instruction or piece of data is stored in memory. Each storage location in memory is identified by an address, just as each house on a street has an address.
  • A storage register, which temporarily holds data taken from or about to be sent to memory.
  • A general-purpose register, which is used for several functions.
• Memory and Storage 
  Memory is also known as primary storage, primary memory, main storage, internal storage, main memory, and RAM (Random Access Memory); all these terms are used interchangeably by people in computer circles. Memory is the part of the computer that holds data and instructions for processing. Although closely associated with the central processing unit, memory is separate from it. Memory stores program instructions or data for only as long as the program they pertain to is in operation. Keeping these items in memory when the program is not running is not feasible for three reasons:
  • Most types of memory only store items while the computer is turned on; data is destroyed when the machine is turned off.
  • If more than one program is running at once (often the case on large computers and sometimes on small computers), a single program can not lay exclusive claim to memory.
  • There may not be room in memory to hold the processed data.
  
  
  How do data and instructions get from an input device into memory? The control unit sends them. Likewise, when the time is right, the control unit sends these items from memory to the arithmetic/logic unit, where an arithmetic operation or logical operation is performed. After being processed, the information is sent to memory, where it is hold until it is ready to he released to an output unit.
  
  
  The chief characteristic of memory is that it allows very fast access to instructions and data, no matter where the items are within it. We will discuss the physical components of memory-memory chips-later in this chapter.
  To see how registers, memory, and second storage all work together, let us use the analogy of making a salad. In our kitchen we have:
  • a refrigerator where we store our vegetables for the salad;
  • a counter where we place all of our veggies before putting them on the cutting board for chopping;
  • a cutting board on the counter where we chop the vegetables;
  • a recipe that details what veggies to chop;
  • the corners of the cutting board are kept free for partially chopped piles of veggies that we intend to chop more or to mix with other partially chopped veggies.
  • a bowl on the counter where we mix and store the salad;
  • space in the refrigerator to put the mixed salad after it is made.
  The process of making the salad is then: bring the veggies from the fridge to the counter top; place some veggies on the chopping board according to the recipe; chop the veggies, possibly storing some partially chopped veggies temporarily on the corners of the cutting board; place all the veggies in the bowl to either put back in the fridge or put directly on the dinner table.The refrigerator is the equivalent of secondary (disk) storage. It can store high volumes of veggies for long periods of time. The counter top is the equivalent of the computer's motherboard - everything is done on the counter (inside the computer). The cutting board is the ALU - the work gets done there. The recipe is the control unit - it tells you what to do on the cutting board (ALU). Space on the counter top is the equivalent of RAM memory - all veggies must be brought from the fridge and placed on the counter top for fast access. Note that the counter top (RAM) is faster to access than the fridge (disk), but can not hold as much, and can not hold it for long periods of time. The corners of the cutting board where we temporarily store partially chopped veggies are equivalent to the registers. The corners of the cutting board are very fast to access for chopping, but can not hold much. The salad bowl is like a temporary register, it is for storing the salad waiting to take back to the fridge (putting data back on a disk) or for taking to the dinner table (outputting the data to an output device).
  
  
  Now for a more technical example. let us look at how a payroll program uses all three types of storage. Suppose the program calculates the salary of an employee. The data representing the hours worked and the data for the rate of pay are ready in their respective registers. Other data related to the salary calculation-overtime hours, bonuses, deductions, and so forth-is waiting nearby in memory. The data for other employees is available in secondary storage. As the CPU finishes calculations about one employee, the data about the next employee is brought from secondary storage into memory and eventually into the registers.
  The following table summarizes the characteristics of the various kinds of data storage in the storage hierarchy.

• Storage	Speed	Capacity	Relative Cost ($)	Permanent?
  Registers	Fastest	Lowest	Highest	No
  RAM	Very Fast	Low/Moderate	High	No
  Floppy Disk	Very Slow	Low	Low	Yes
  Hard Disk	Moderate	Very High	Very Low	Yes

  Modern computers are designed with this hierarchy due to the characteristics listed in the table. It has been the cheapest way to get the functionality. However, as RAM becomes cheaper, faster, and even permanent, we may see disks disappear as an internal storage device. Removable disks, like Zip disks or CDs (we describe these in detail in the online reading on storage devices) will probably remain in use longer as a means to physically transfer large volumes of data into the computer. However, even this use of disks will probably be supplanted by the Internet as the major (and eventually only) way of transferring data. Floppy disks drives are already disappearing: the new IMac Macintosh from Apple does not come with one. Within the next five years most new computer designs will only include floppy drives as an extra for people with old floppy disks that they must use.For more detail on the computer's memory hierarchy, see the How Stuff Works pages on computer memory.. This is optional reading.
• How the CPU Executes Program Instructions
  Let us examine the way the central processing unit, in association with memory, executes a computer program. We will be looking at how just one instruction in the program is executed. In fact, most computers today can execute only one instruction at a time, though they execute it very quickly. Many personal computers can execute instructions in less than one-millionth of a second, whereas those speed demons known as supercomputers can execute instructions in less than one-billionth of a second.
  
  

  Figure 2: The Machine Cycle

  Before an instruction can be executed, program instructions and data must be placed into memory from an input device or a secondary storage device (the process is further complicated by the fact that, as we noted earlier, the data will probably make a temporary stop in a register). As Figure 2 shows, once the necessary data and instruction are in memory, the central processing unit performs the following four steps for each instruction:
  1. The control unit fetches (gets) the instruction from memory.
  2. The control unit decodes the instruction (decides what it means) and directs that the necessary data be moved from memory to the arithmetic/logic unit. These first two steps together are called instruction time, or I-time.
  3. The arithmetic/logic unit executes the arithmetic or logical instruction. That is, the ALU is given control and performs the actual operation on the data.
  4. Thc arithmetic/logic unit stores the result of this operation in memory or in a register. Steps 3 and 4 together are called execution time, or E-time.
  
  
  The control unit eventually directs memory to release the result to an output device or a secondary storage device. The combination of I-time and E-time is called the machine cycle. Figure 3 shows an instruction going through the machine cycle.
  
  
  Each central processing unit has an internal clock that produces pulses at a fixed rate to synchronize all computer operations. A single machine-cycle instruction may be made up of a substantial number of sub-instructions, each of which must take at least one clock cycle. Each type of central processing unit is designed to understand a specific group of instructions called the instruction set. Just as there are many different languages that people understand, so each different type of CPU has an instruction set it understands. Therefore, one CPU-such as the one for a Compaq personal computer-cannot understand the instruction set from another CPU-say, for a Macintosh.

  Figure 3: The Machine Cycle in Action

  It is one thing to have instructions and data somewhere in memory and quite another for the control unit to be able to find them. How does it do this?
  
  

  Figure 4: Memory Addresses Like Mailboxes

  The location in memory for each instruction and each piece of data is identified by an address. That is, each location has an address number, like the mailboxes in front of an apartment house. And, like the mailboxes, the address numbers of the locations remain the same, but the contents (instructions and data) of the locations may change. That is, new instructions or new data may be placed in the locations when the old contents no longer need to be stored in memory. Unlike a mailbox, however, a memory location can hold only a fixed amount of data; an address can hold only a fixed number of bytes - often two bytes in a modern computer.
  
  
  Figure 4 shows how a program manipulates data in memory. A payroll program, for example, may give instructions to put the rate of pay in location 3 and the number of hours worked in location 6. To compute the employee's salary, then, instructions tell the computer to multiply the data in location 3 by the data in location 6 and move the result to location 8. The choice of locations is arbitrary - any locations that are not already spoken for can be used. Programmers using programming languages, however, do not have to worry about the actual address numbers, because each data address is referred to by a name. The name is called a symbolic address. In this example, the symbolic address names are Rate, Hours, and Salary.
  
  

Computer Network

COMPUTER NETWORK

Two or more computers connected together through a communication media form a computer network.

The computers are connected in a network to exchange information and data. The computers connected in a network can also use resources of other computers.

Computer Network Components

There are different components of a network. Following are the basic components of network.

1. Server:

Powerful computers that provides services to the other computers on the network.

2. Client:

Computer that uses the services that a server provides. The client is less powerful than server.

3. Media:

A physical connection between the devices on a network.

4. Network Adopter:

Network adopter or network interface card (NIC) is a circuit board with the components necessary for sending and receiving data. It is plugged into one of the available slots on the Pc and transmission cable is attached to the connector on the NIC.

5. Resources:

Any thing available to a client on the network is considered a resource .Printers, data, fax devices and other network devices and information are resources.

6. User:

Any person that uses a client to access resources on the network.

8. Protocols:

These are written rules used for communications. They are the languages that computers use to talk to each other on a network

The History of the Computer Generations

Punched Card System

Source: Wikipedia

It is very hard to believe but computer's era begins in ancient times. The first mechanical devices that somewhat can be called a limited function computers appeared in 2500 BC. Well, Sumerian abacus cannot be called a real computer, but it's goal was helping in various calculations. The slide rule, that can be called a mechanical analog computer appeared in 1620. Looking deeply in the ancient history many interesting and useful machines, that can be referred as primitive computers can be found. There were machines which calculated the astronomical positions of stellar bodies. Various devices, such as astolabe also can be called computers.

The main idea is that the human is such a creature, whose aim is to create some devices to help him solving various problems. The main aim of every computer is to help us in calculations. The definition of the "computer" says that a computer is a programmable machine designed to automatically carry out a sequence of arithmetic or logical operations. The history of computer generations is very interesting field to explore, because since the 1946, when first real computers were created, they evolve very rapidly. In the short period computer becomes more powerful in enormous steps.

ENIAC

Source: Wikipedia

The First Generation: Vacuum Tubes

The first computer generation appeared in 1946-1958. It is the era of vacuum tubes. The computers of this era were huge, very expensive, slow and had limited application capabilities. The first computer that had vacuum tubes was constructed in 1946 by J. P. Eckert and J. Mauchly. Their machine was called ENIAC (Electronc Numerical Integrator and Computer). ENIAC had thousands of vacuum tubes.

IBM 701

Source: Wikipedia

 The Second Generation: Transistors

The second generation of computers lasted from 1959-1964. It was a very short era. The vacuum tubes were replaced by transistors. Actually, the invention of a transistor had a huge influence on the development of electronics.

The invention of transistor allowed computer to became smaller, faster and cheaper. These computer also were more efficient and more reliable that the computers of the first generation. The first symbolic (assembly) programming languages appeared in this generation. Also, the backgrounds of high level programing languages were developed. The first versions of COBOL and FORTRAN appeared.

The computer instructions storage also changed. The step from magnetic drum to magnetic core technology was made.

Integrated Circuit - 8 bit microcontroller

Source: Wikipedia

The Third Generation: Integrated Circuits

The invention of integrated circuits were another great step in developing semiconductors technologies. The third computer generations appeared. The third generation lasted from 1964 to 1971.

The appearance of integrated circuits allowed to place miniaturized transistors on silicon chips. This technological invention allowed computers to increase speed and to become more efficient.

The human interaction with the computer had changed much in this generation. The keyboards and monitors appeared instead of punched cards and printouts. In this generation computers had first operating systems that allowed to run multiple applications on the computer.It was the first time when computer became accessible to a mass audience. they become much smaller and more cheaper, so the usual customer can afford them.

Intel 4004 microprocessor

Source: Wikipedia

The Fourth Generation: Microprocessors

The appearance of microprocessor allowed the fourth generation of computers to appear. This happed in 1971. At the moment we still have fourth generation computers. Microprocessor is a device that have thousand integrated circuits (millions of transistors) built in one silicon chip. The fourth generation was started by Intel 4004 chip that was developed in 1971. The greatest thing in computer evolution was that the whole computer that in first generation filled the whole room, now was integrated into a small chip.

4004 had all computer components integrated on one chip. This chip contained CPU, memory and input/output controls.

Ten years later (in 1981), IBM introduced the first computer that was dedicated to home user. Tree years later Apple introduced Macintosh. The era of mass usage of computers began. The use of microprocessors expanded and in our days, mostly all electronic devices use microprocessors or micro-controllers.

In the fourth generation the development of computer networks began. This lead to invention of the Internet. Many other great thing were developed during these years. Now it is very hard to imagine using computer without mouse, GUI or other features.

The fourth computer generation is a generation of computers as we know them now. Off course, during these years computers became more powerful, smaller, cheaper and so on. But the main idea is the same - microprocessor.

The Fifth Generation

lIntelligent robots that could ‘see’ their environment (visual input  -  e.g. a video camera) and could be programmed to carry out certain tasks and  should be able to decide for itself how the task should be accomplished, based on the observations it made of its environment.

The Fifth Generation: Virtual Reality

Engaged a user in a computer-created environment:

-User physically interacts with computer-created environment.

Classification of computers

Computer is an electronic device which is used for calculation or computing any data.

Computers are classified according to their size and techniques. Computers are classified in to different catagories. These catagories are according to purpose, technology and the storage and size capacity. There are two types of computers one is special purpose and general purpose computers.Special purpose computer are used for particular job. Like statellite launching and weather forecasting etc. General purpose computer are used to perform not specific purpose but they are used to work with different applications. Like Computers are used in offices, schools and institutes etc. These are not permanently stored.

Technical catagories of computers are

a. Digital Computers- Digital computers store the data and information in the digital form means numbers, letters and used some special characters. Arithmetical calculations are done by digital computers like calculate the distance, time and speed etc. Data are stored in digital form that is binary form of 0 and 1. Here 0 and 1 means on and off respectively. Storage capacity is very large and have tough programming. Digital computers are costly also. 

b. Analog Computers- Analog computer falls in special purpose computer and they store information in physical quantity form such as voltage, current pressure, temperature etc which changes with time. Thermometer is an example of analog computer. Analog computers 

c. Hybrid computers- As the name suggest hybrid means it is the combination of both digital as well as analog computer. It can perform both operation of analog and digital computers. Analog signals are converted into digital signals to obtain specific information. For example it is used in artificial intelligence and computer aided manufacturing.

Size and storage

a. Personal computers- This is the normal computer system and less expensive than any other computer listed below. It is easy to install in single room. Size is small and perform many calculations. It has enough memory for storing you data.

b. Micro computers- Micro computer stores upto 64 Mb data and usually used by only one person at a time. It consists one or two microprocessor. IBM system 123 is an example of micro computer.

c. Mini Computers- Mini computers are allow to serve more than one user and have multitasking and networking abilities. The speed of processing is high than micro computers. It can support upto 115 terminals. Storage capacity of mini computers are high. Memory of these computers are lies between 8-1024 mega byte(MB). IBM is an example of mini computers.

d. Mainframe computers-

Main frame computers are used in business as well as scientific applications and having large storage capacity. It can support peripheral like hard disk vdu, magnetic tape etc. Banking applications is an example of main frame computers. It can support 100 terminals and multi user computer. Main frame support 400 to 500 users at a time.

e. Super computer- Super computers are used as special purpose. These computers have very high speed and large memory size. Super computers are very large sized and having very high processing speed. These are very costly and powerful computers. These are mult user and thousands of user can used it at a time. Millions of instructions can be executed per second. Super computers perform very large calculations. PARAM is an example of super computer. Super computers are used by government only for their bulky jobs. Super computer are used in nuclear energy research, weather forecasting and defence services etc.

Computer Components

Computers are made up of many parts. These different parts perform one or more functions including input, output, processing, or storage. OUTPUT DEVICES Output devices send information from your computer to you. This information is usually in the form of sound and sight, but some devices can send information as touch and even as smell! Some common output devices are monitors, printers, and speakers.        INPUT DEVICES Input devices are the parts that let you enter and manipulate information on a computer. These devices range from the standard keyboard and mouse, to scanners, microphones, joysticks, and light pens.        There are some devices that can input and output. Some examples would touch screen monitors (input by touch, output by sight) and force feedback joysticks. ON THE INSIDE The inside of your computer has many parts that all work together. These parts are generally found within your computer case - this is usually the big "box" that probably sits under your desk or below your monitor. If you're using an iMac, many of the computer parts are built into the monitor case. The motherboard [shown at right], or mainboard, is the backbone of the computer. All the individual pieces connect to the motherboard in some way. The motherboard is home the processor chip, pci slots, and memory. Processor - This is the chip that does the "thinking" of the computer. These are the "Pentium" and "AMD" chips you hear about. Processor speed is measured in MegaHertz(Mhz) and GigaHertz(Ghz). 1 Ghz = 1000 Mhz Memory - This is where information is temporarily stored for the processor to use and manipulate before storing on the HARD DRIVE. Also known as RAM (Random access memory). Information is stored in memory only when the computer is turned on. Ram is measured in Megabytes(Mb), which is storage capacity, not to be confused with MegaHertz, which is speed. PCI Slot - These are outlets in the motherboard that allow you to install extra components like sound cards, modems, video cards, and other devices. The images below show different PCI card components.        Hard Drive - This is the part of your computer where information is stored for later retrieval. All the information you access on your computer, all your documents, pictures, email messages, and programs are here. Unlike memory, the hard drive stores information even after the power is turned off. The image to the right shows the inside of a hard drive. Floppy Drive - This is the slot in the front of your computer where you insert a disk to store data and move it to another computer. If your computer is an iMac, you will not have a Floppy Drive. Floppy disks are 3¼ inches in size, and hold 1.44Mb of data. The images below show a floppy drive, and some floppy disks.     CD ROM or DVD ROM Drives - This plays your music and data cd's, or if you have a DVD drive, it will also play DVD movies. Data CD's hold up to 700Mb of information. If you have a CD-R or CD-RW drive, you can store your own information on CDs.