Computers are classified according to size and level of
power into supercomputers, mainframe computers, minicomputers, and
microcomputers.
General users most commonly deal with the microcomputer,
which uses a chip as its CPU and has three basic hardware components:
keyboard, monitor, and system unit. Monitors can be monochrome or color. The
system unit houses the power supply, the system board, and some storage
devices.
Computers come in a variety of sizes What are
computer systems? Types of computer systemsand shapes and with a variety of pro
cessing capabilities. The earliest computers were very large because of the
technologies used. However, as technological improvements were made in computer
components, the overall size of computers began to shirk and continues to
do so. To provide a basis for comparing their capabilities, computers are
generally grouped into four basic categories
1. supercomputer
2. mainframe computers
3. minicomputers
4. microcomputers
2. mainframe computers
3. minicomputers
4. microcomputers
Supercomputer:
A supercomputer can handle gigantic amounts of
scientific computation it s maintained in a special room or environment. May be
about 50,000 times faster than a microcomputer, and may cost as such as $20
million. As a user in business, you probably would not have contact with a
supercomputer.
Mainframe computer:
Mainframe computer is a larger computer, usually
housed in a controlled environment that can support the processing requirements
of hundreds and often thousands of users and computer professionals. It is
smaller and less powerful than a supercomputer. It may cost from several
hundred thousand dollars up to $10 million. If you go to work for an airline, a
bank, a large insurance company.
Minicomputer:
A minicomputer also known as a mid-size or mid-range
computer, is similar to but smaller and less powerful than a mainframe
computer. It can support 2 to about 50 users and computer professionals.
Minicomputers and mainframe computers can work much faster than microcomputers
and have many more storage locations in main memory.
How Computers Work: The
CPU and Memory
Figure 0 shows the parts of a computer:
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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)
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Figure 1: The Central Processing
Unit
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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 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:
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.
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Storage
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Speed
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Capacity
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Relative Cost ($)
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Permanent?
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Registers
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Fastest
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Lowest
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Highest
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No
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RAM
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Very Fast
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Low/Moderate
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High
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No
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Floppy Disk
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Very Slow
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Low
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Low
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Yes
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Hard Disk
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Moderate
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Very High
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Very Low
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Yes
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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.
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Figure 2: The Machine Cycle
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- 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:
- The control unit fetches (gets) the instruction
from memory.
- 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.
- 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.
- 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.
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Figure 3: The Machine Cycle in
Action
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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?
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Figure 4: Memory Addresses Like Mailboxes
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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.
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.
7. Protocols:
These are written rules used for communications. They are the
languages that computers use to talk to each other on a network
