Fundamentals of Computers I-13
comprises four components, namely, central processing unit (CPU), input unit, output unit and storage
(or memory) unit. The block diagram of a digital computer is shown in Figure 1.1.
CPU
CPU, also known as ‘processor’, is the brain of the computer system that processes data (input) and
converts it into meaningful information (output). It is a highly complex, extensive set of electronic
circuitry, which executes stored program instructions. A CPU controls all internal and external devices,
performs arithmetic and logic operations, and operates only on binary data, that is, data composed of
1s and 0s. In addition, it also controls the usage of main memory to store data and instructions and
controls the sequence of operations. The CPU consists of three main subsystems, which are described
as follows:
• Arithmetic logic unit (ALU): This unit performs the arithmetic (add, subtract) and logical opera-
tions (and, or) on the data made available to it. Whenever an arithmetic or logical operation is
to be performed, the required data is transferred from the memory unit to ALU, the operation is
performed and the result is returned to memory unit. Before the completion of the processing, data
may need to be transferred back and forth several times between these two sections. Subsequently,
the results are transferred from internal storage to an output device.
• Control unit (CU): This unit checks the correctness of the sequence of operations. It fetches the
program instructions from the memory unit, interprets them and ensures correct execution of the
program. It also controls the input/output devices and directs the overall functioning of the other
units of the computer.
• Registers: These are the special-purpose; high-speed temporary memory units that can hold varied
information such as data, instructions, addresses and intermediate results of calculations. Essen-
tially, they hold the information that the CPU is currently working on. Registers can be considered
as the CPU’s working memory, a special additional storage location that provides the advantage
of speed.
Memory
unit
Registers
Control
unit
Arithmetic /
logic unit
Input
unit
Output
unit
Instruction flow
Data flow
Central processing unit
Figure 1.1 Block Diagram of Digital Computer
M01_ITL-ESL4791_01_SE_C01.indd 13 12/22/2012 4:52:21 PM
I-14 Computer Fundamentals
Input and Output Unit
The user must enter instructions and data into the computer system before any operation can be
performed on the given data. Similarly, after processing the data, the information must go out from the
computer system to the user. For this, every computer system incorporates input and output units that
serve as a communication media between the computer system and the user.
An ‘input unit’ accepts instructions and data from the user with the help of input devices such as
keyboard, mouse, light pen and so on. Since the data and instructions entered through different input
devices will be in different form, the input unit converts them into the form that the computer can under-
stand. After this, the input unit supplies the converted instructions and data to the computer for further
processing.
The ‘output unit’ performs just opposite to that of input unit. It accepts the outputs (which are in
machine-coded form) produced by the computer, converts them into the user understandable form and
supplies the converted results to the user with the help of an output devices such as printer, monitor and
plotter.
Storage Unit
A computer system incorporates ‘storage unit’ to store the input entered through input unit before
processing starts and to store the results produced by the computer before supplying them to the output
unit. The storage unit of a computer comprises two types of memory/storage, namely, ‘primary’ and
‘secondary’.
The primary memory (also called ‘main memory’) is the part of a computer that holds the instructions
and data currently being processed by the CPU, the intermediate results produced during the course of
calculations and the recently processed data. While the instructions and data remain in main memory, the
CPU can access them directly and quickly. However, primary memory is quite expensive and has a limited
storage capacity. Due to limited size of primary memory, a computer employs ‘secondary memory’, which
is extensively used for storing data and instructions. It supplies the stored information to the other units
of computer as and when required. It is less expensive and has higher storage capacity than the primary
memory. Some commonly used secondary storage devices are floppy disks, hard disks and tape drives.
11. Explain Von Neumann architecture.
Ans: The earliest computers such as ENIAC and Colossus were program-controlled machines in
which writing a new program or changing some existing program required re-wiring, restructuring or
redesigning the machines. Thus, programming in these computers was a quite tedious task. To ease the
programming process, an idea known as ‘stored program concept’ was proposed by a mathematician
and an early computer scientist named John Von Neumann. According to this concept, both data and
instructions are kept in the main memory. Thus, a computer could get its instructions from the memory
and the programs could be setup or changed from within the memory as well, without having to redesign
the computer. All the computers based on stored program concept are known as Von Neumann systems
employing the ‘Von Neumann architecture’.
Most of the conventional and today’s modern computers that are based on Von Neumann architecture
consists of four hardware components: a CPU with control unit, ALU, main memory and an I/O sys-
tem (see Figure 1.2). In addition, these computers can process the instructions in a sequential manner.
The architecture shown in Figure 1.2 depicts the working of a Von Neumann computer. Here, the I/O
instructions are passed through ALU to the control unit. All the programs are executed by following the
M01_ITL-ESL4791_01_SE_C01.indd 14 12/22/2012 4:52:21 PM
Fundamentals of Computers I-15
‘Von Neumann execution cycle’ or ‘fetch-decode-execute cycle’, which describes the working of the
machine. One iteration of this cycle involves the following steps:
1. The control unit determines the location of the next program instruction to be executed using the
program counter and fetches it from the main memory.
2. The instruction is then decoded into the language understandable by the ALU.
3. The required data operands for executing the instructions are fetched from main memory and
stored into the CPU registers.
4. The instructions are executed by ALU and the results are placed in registers or memory.
12. What are the phases of information processing cycle? Explain briefly.
Ans: A task is assigned to computer in a set of step-by-step instructions, which is known
as ‘ program’. These instructions tell the computer what to do with input in order to produce the
required output. To complete any task, a computer performs three basic steps which are as follows
(see Figure 1.3):
ALU
PC
Control unit
Registers
Central processing unit
Main
memory
Input/
output
system
Figure 1.2 Von Neumann Architecture
Storage
Input
Processing
Output
Figure 1.3 Basic Computer Operations
M01_ITL-ESL4791_01_SE_C01.indd 15 12/22/2012 4:52:22 PM
I-16 Computer Fundamentals
1. Accepts input: Computer input is whatever is entered or fed into a computer system. Input can
be supplied by the user (such as by using a keyboard) or by another computer or device (such as
a diskette or CD-ROM). Some examples of input include the words and symbols in a document,
numbers for a calculation and instructions for completing a process, and so on.
2. Processes the data: During this stage, the computer follows the instructions using the data that
have been input. Examples of processing include calculations, sorting lists of words or numbers
and modifying documents according to user instructions.
3. Produces output: Computer output is the information that has been produced by a computer.
Some examples of computer output include reports, documents and graphs. Output can be in sev-
eral different formats, such as printouts, or displayed on the screen.
13. Discuss some limitations of computers.
Ans: Although computers are widely used in various fields, but still they have some limitations
which are as follows:
• The computers are faster, more diligent, accurate and versatile than human beings but they cannot
replace them. Thus, one can say that they do not have any intelligence.
• They can only perform what they are programmed to do. They work only on stored procedures and
cannot think for themselves. Thus, they cannot make any decisions of their own.
• They need well-defined instructions to perform any operation. Therefore, they are unable to give
any conclusion without going through intermediate steps and thus, cannot generate information
on their own.
• Their use is limited in areas where qualitative considerations are important. For instance, they can make
plans based on situations and information but they cannot foresee whether they will succeed or not.
• They cannot correct wrong instructions. If the instructions are not accurate then the working of
the computer will not be accurate. Therefore, computer will give wrong information if feed with
wrong data.
• They need to be installed in a dust-free place. As some parts of computers get heated up due to
heavy processing, therefore, the ambient temperature of system should be maintained.
• Computer parts require regular checking and maintenance in order to give correct result. This is
because sometimes a program which was running correctly for some period of time might produce
an error when re-run later.
14. What are the functions of a CPU? How does the CPU perform them?
Ans: The central processing unit (CPU) is referred to as the brain of a computer system as it converts
data (input) into meaningful information (output). Its functions can be summed up as follows:
• Carrying out arithmetic and logic functions.
• Controlling the use of main storage (memory) to store data and instructions.
• Controlling the sequence of operations.
• Controlling all the parts of the computer system
The CPU accomplishes its functions with the help of its three main subsystems (see Figure 1.4): the
arithmetic and logic unit (ALU), the control unit (CU) and the registers, which help the processor in
performing its functions correctly. These are described as follows:
• ALU: It contains the electronic circuitry that executes all arithmetic and logical operations on the
data made available to it. The data required to perform the arithmetic and logical functions are
input from the designated registers. ALU comprises two units which are as follows:
M01_ITL-ESL4791_01_SE_C01.indd 16 12/22/2012 4:52:22 PM
Fundamentals of Computers I-17
Arithmetic unit (AU): It contains the circuitry that is responsible for performing the actual
computing and carrying out the arithmetic calculations, such as addition, subtraction,
multiplication and division. It can perform these operations at a very high speed.
Logic unit (LU): It enables the CPU to make logical operations based on the instructions
provided to it. These operations are logical comparison between data items. The LU can com-
pare numbers, letters or special characters and can then take action based on the result of
the comparison. Logical operations of the LU test for three conditions: equal-to condition,
less-than condition and greater-than condition. In a test for ‘equal-to condition’, the ALU
compares two values to determine if they are equal. For example, if the number of tickets
sold equals the number of seats in a cinema hall, then houseful is declared. To test ‘less-than
condition’, the ALU compares values to determine if one is less than another. For example, if a
person purchases less than two tickets, then no discount rate is given. In a test for ‘greater-than
condition’, the ALU determines if one value is greater than another. For example, if a person
purchases greater than two tickets, then discount rate is 5%.
• CU: It contains circuitry that uses electrical signals to direct the entire computer system to carry
out, or execute, stored program instructions. This resembles orchestra leader who himself does not
play a musical instrument but directs other people to play the musical instrument in a harmonized
manner. It also does not execute program instructions; rather, it directs other parts of the system
to do so by communicating with both ALU and memory. It controls the I/O devices and transfer
of data to and from the primary storage. An analogy can be considered between the CU and the
traffic police. The CU decides which action will occur just as the traffic police takes decisions
on which lanes traffic will move or stop. It also determines what data are needed, where they are
stored and where to store the results of the operation as well as sends the control signals to the
devices involved in the execution of the instructions. It administers the movement of large amount
of instructions and data used by the computer. In order to maintain the proper sequence of events
required for any processing task, the CU uses clock inputs.
Secondary storage
Stores data and programs
Sends data to
the central
processing unit
Input
Makes
processed
information
available
Output
Processing
Central processing unit
Registers
Control
unit
Arithmetic/
logic unit
Figure 1.4 CPU
M01_ITL-ESL4791_01_SE_C01.indd 17 12/22/2012 4:52:22 PM
..................Content has been hidden....................
You can't read the all page of ebook, please click here login for view all page.