Computer System Architecture

This document provides an overview of computer system architecture, including the Von Neumann Architecture, input and output units, memory units, CPU, ALU, control unit, operating systems, and different types of operating systems. It also discusses the functions of operating systems and their importance in running a computer system.

Computer System Architecture
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Contents
Aim..................................................................................................................................................4
Computer System Architecture........................................................................................................4
Von Neumann Architecture.............................................................................................................4
Input unit......................................................................................................................................5
Output unit...................................................................................................................................5
Memory unit................................................................................................................................5
Random Access Memory (RAM)............................................................................................6
Read Only Memory (ROM).....................................................................................................6
CPU (Central Processing Unit)....................................................................................................7
Operations of Central Processing Unit....................................................................................7
Arithmetic Logic Unit (ALU)......................................................................................................8
Working of ALU......................................................................................................................8
Control Unit.................................................................................................................................9
Operating System...........................................................................................................................10
Types of operating system.............................................................................................................10
Batch operating system..............................................................................................................10
Time-sharing operating systems................................................................................................11
Distributed Operating System....................................................................................................11
Network Operating System........................................................................................................11
Real-time Operating System......................................................................................................12
Hard real-time systems..........................................................................................................12
Soft real-time systems............................................................................................................13
Operating System Architecture......................................................................................................13
Functions of Operating Systems....................................................................................................15
Program management................................................................................................................15
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Memory management................................................................................................................15
Process Management.................................................................................................................15
I/O Operation.............................................................................................................................15
File System Management..........................................................................................................15
Communication..........................................................................................................................15
Error Handling...........................................................................................................................16
Protection...................................................................................................................................16
Conclusion.....................................................................................................................................16
References......................................................................................................................................17
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Aim
Understand the relationships between hardware components and the subsystems used in a
computer system and key features and services provided by different computer operating systems
and hardware.
Computer System Architecture
Computer systems are governed by some set of rules that define the functioning, organization as
well as the implementation of it. These rules are collectively termed as "computer systems
architecture". (Martin 2009) With time many computer systems architecture has been proposed
but in the end, we follow an architecture that comprises all the rules and systems of computers
and for this reason, Von Neumann Architecture is widely followed across the world.
Von Neumann Architecture
John von Neumann, a physicist and mathematician, in 1945 designed an architecture
machine which is called von Neumann model or Princeton architecture. (Neumann 1945) It was
a design architecture for a digital computer which comprised of Arithmetic Logic Unit (ALU),
Control Unit (CU), Storage Unit, Input Unit and Output Unit.
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Figure - Von Neumann Architecture
Input unit
The very first unit which is at the user end is the input unit. This unit helps to connect the
external environment to the internal one. (Burger et. al. 2010) Whenever the user wants to give
instruction to the system it uses the input devices which make the input unit. These devices
include the keyboard, mouse, joysticks, digital cameras, scanners etc.
Following are the functions of the input unit.
 to accept the instruction and data from the user end.
 to convert the input data into a machine language so that the system understands it.
 to supply the data converted by the input devices to the computer system.
Output unit
Same as the input unit, the output unit is also available at the user end and connects the outer
environment to the internal environment of the system. (Wilson et al. 2018) The output unit
shows the result of the processed data and instruction to the user by using output devices. These
output devices include monitor, printer, speaker, projectors etc.
Functions of the output unit are -
 to accept the processed data from the internal environment.
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 to convert the processed data into a user-friendly language so that the user understands it.
 to show the results to the user using output devices.
Memory unit
Memory unit comprises of nothing but integrated circuits made of tiny memory cells. These cells
store the information during the processing and also for later use. The information is stored in
terms of the binary digits i.e. in the form of 0's and 1's. Bits (BInary + digiTS) are popularly used
to define the memory size. (Keeton et al.1998) Apart from the binary streams, binary coded
information is also used as an alternative. A byte is a group of 8 bits. For bigger units kilobytes,
megabytes and gigabytes are used.
Memory is classified into two categories first is RAM which is a primary memory or volatile
memory and ROM which is a secondary memory or non-volatile memory.
Random Access Memory (RAM)
RAM is the main or primary memory in a computer system. It stores the information for
temporary use during the session on a system. (Rao 2009) When the computer is turned off the
data stored in the memory is erased and that is the reason why it is called volatile memory.
Random Access Memory is further classified into DRAM and SRAM.
Dynamic Random Access Memory
DRAM is made up of very small capacitors that leak electricity. Its processing speed is slower
compared to the DRAM. DRAM is cheaper compared to the SRAM. It uses less power and
hence it leads to a less heat generation. DRAM is used extensively in making main memory.
Static Random Access Memory
SRAM is made up of D flip-flops circuit. SRAM is able to hold the data as long as the power is
provided to it. SRAM is expensive but this comes with its processing speed which is high
compared to the DRAM. SRAM uses more power compared to DRAM and hence it releases
more heat. Last but not the least SRAM is used in cache memory.
Read Only Memory (ROM)
ROM performs read operations only in contrast to RAM which performs both read and write
operations. ROM stores the information which is essential for running the system like boot
information. Read Only Memory retains its data always and hence it is volatile in nature. ROM is
employed in embedded systems, where programming needs to be done for forever for examples
in calculators, toys etc.
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Read Only Memory is further classified into PROM, EPROM, and EEPROM.
PROM (Programmable read-only memory)
These are programmable ROM but once they are programmed, they can not be erased and hence
PROM can be programmed only once.
EPROM (Erasable Programmable read-only memory)
EPROM is programmable in nature along with it, it can be reprogrammed after removing the
data stored in it already. For the removal of data, EPROM is exposed to the ultra violet
radiations.
EEPROM (Electrically erasable programmable read only memory)
These are same in nature as the EPROM but in this type of ROM the data is removed by
applying magnetic field using electrical means. Hence EEPROM is also reprogrammable in
nature.
CPU (Central Processing Unit)
CPU is termed as the brain of a computer. It consists of two important units called Arithmetic
Logic Unit and Control unit. Hence CPU covers all the major functioning of the computer. Its
function mainly rely upon, the clock frequency of the system, address and data buses and
memory both secondary and primary.
The term microprocessor broadly refers to the Central Processing Unit.
Operations of Central Processing Unit
All the CPU irrespective of their design principle follow three basic steps. These steps are
fetching, decoding and executing. These steps are altogether known as the instruction cycle.
Fetching - The very first step in the instruction cycle is fetching the information from the
memory where the required information is already programmed. This is done by retrieving the
data from memory. The location of instruction is stored in the program counter.
Decoding - CPU's action is determined by the information retrieved by fetching. Decoding is
done using a special circuitry called the instruction decoder. It takes into account what is known
as the opcodes.
Execution - The third step in the instruction cycle is execution. This may be simply a single stage
of a group of stages depending upon the complexity level of the CPU design. When action is
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performed in the first stage, the second stage is ready to perform its part and hence the reaming
stages work. The management of stages is done by the clock frequency of the processor.
Below diagram which depicts the Single processor CPU functioning. Here data flow is
represented by the red lines whereas the control flow is sown by the black line. Please note here
the direction of arrow sows the direction of operation to be performed.
Figure - Single processor CPU functioning
Arithmetic Logic Unit (ALU)
Arithmetic Logic Unit is responsible for performing the calculations related to computer
systems, hence it is called the calculator of the computer. ALU is an essential part of the
computer's CPU (Central Processing Unit). Different arithmetic operations like addition,
subtraction, multiplication and division are performed in the Arithmetic Logic Unit. Apart from
this, various logical operations such as AND, OR, NOT, XOR etc. are also performed in the
ALU.
Operand - whatever input is given to the ALU in the form of data on which the operation is to be
performed is called operand.
Another input in ALU is given in the form of code which directs the operation to be performed
on the input data.
Working of ALU
Arithmetic Logic Unit performs operations on the binary numbers. For this, the input
information is converted into the binary format of 0's and 1's. To realize this sort of task
transistor switches are employed. (Eyre et al. 1998) These transistors work on the principle of
current flow. The transistor switch is ON when current flows through it, in binary it is termed 1.
If no current flows through it then transistor switch is in OFF condition and the result is 0 in
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terms of binary digit. To make the process fast an array of these transistors is used which is
called gate. And depending upon the algorithm various gates are realized.
Some important gates are discussed here.
NOT gate - This is the simplest type of gate. Only a transistor is used in NOT gate. There is a
single input in NOT gate and the output is also single. The output is always the opposite of the
input.
OR gate - This gate has a minimum of two inputs. If anyone of the input is 1 the output is going
to be one. The result of the OR gate is OFF when all the inputs are 0.
AND gate - This gate also has a minimum of two inputs. If any one of the input is 0 the output is
going to be LOW. The result of the OR gate is ON when all the inputs are 1.
Control Unit
Control unit, as its name suggests, controls all other units of the system. This is the reason why it
is called the CNS (central nervous system) of the computer. Its main objective is to transfer data
from one unit of the system to the other unit and in this way it controls all the data traveling.
Important functions of the control unit are listed below -
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 to transfer data from one unit to the other unit.
 to manage all the units of the system and coordinate them.
 to communicate the i/o devices in order to transfer data from the memory unit.
Note - A control unit does not store or process data. It is responsible only for the transfer and
management of the data.
Figure - Functions of Control Unit
Operating System
An operating system is what makes the hardware alive. Without a proper operating system, we
cannot make use of the system. An operating system consists of the softwares which are required
in order to run the system. The operating system software is an essential part of system
softwares.
Types of operating system
One thing that has constantly evolved from the time of inception of computers is operating
systems. Various types of operating systems are discussed in detail.
Batch operating system
In batch operating system user does not interact with the system in the first place. In a batch
operating system user is required to submit its desired process in some off-line devices like a
punch-card etc. One question is why it is called a 'batch' operating system? It is simply because
the computer operator, who is not the actual user, groups together the all the submissions similar
in requirement and then run them as a 'batch'. This way by saving the time process has been
performed. In the end, the operator differentiates the results and submit back to the original user
who submitted the query in the first place.
Why the batch operating system is not feasible?
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There are certain reasons why the batch system is not feasible in this internet age.
 As we have noticed the actual user has no control over the process and this lack in user-
system interaction makes the batch operating system less user-friendly.
 The selection of queries is done by the operator and hence the batch which is made for
processing does not take into account the submissions efficiently.
Time-sharing operating systems
This process is better than to its predecessor because the time-sharing operating systems allow
the user to interact with the system. Here the time of processing is divided among the users and
within the range of allowed time they are required to submit their query.
This multitasking approach is better in terms of optimum use of CPU's capabilities because no
idle time for CPU is allowed in this technique.
Advantages of the time-sharing operating system
 First and foremost the user interaction. The user is allowed to interact directly with the
system and not via an operator. In this manner, the time-sharing operating system is more
user-friendly compared to the batch operating system where an operator was the must.
 The time-sharing operating system completely avoids duplication of softwares and hence
it is more efficient than the batch operating system.
Disadvantages of the time-sharing operating system
 On the one hand, it allows the user to directly interact with the system than on the other
hand it takes so many inputs during multitasking. This makes it less reliable.
 In multitasking or multi user security is the prime concern.
 Apart from security data communication is also a problem in the time-sharing operating
system because of the limited amount of time provided to the user.
Distributed Operating System
Distributed Operating System is named so because they implement multiple central processing
units in order to perform multiple real-time operations. Various central processing units interact
with each other using special communication lines made out of buses or telephone lines. It is not
necessary that all the central processing units are identical in nature but they differ in terms of
size and functionality.
Advantages of distributed systems
 First, the multiple central processing units improve speed and reliability.
 The distribution network is designed so that if one site fails to function the remaining are
not affected by this halt.
 Fast communication lines make the data transfer speedy and hence delay is negligible.
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Network Operating System
Network operating systems are installed on the servers. Networking function like managing data
and other things related to security and applications are allowed in network operating systems.
( Harris 2008) In a network, such as LAN (local area network), file sharing and multiple user
printer access is realized using the network operating systems.
Network operating systems examples include Microsoft Windows Server 2003 and 2008, UNIX
and LINUX etc.
Advantages of network operating systems
 A major advantage of network operating systems is in terms of its stability which is due
to its centralized approach.
 As mentioned earlier the network operating systems are installed on the servers and the
security is fully on the shoulders of the server.
 New technologies are easily implemented and upgraded in network operating systems.
 Servers can be accessed remotely making it the best terms of feasibility.
Disadvantages of network operating systems
 Installing network operating systems is a costly affair.
 However, it can be managed remotely but dependency on a central system is necessary in
some cases.
 Not only its installation is a costly affair but regular maintenance, as well as, updates are
also important in network operating systems.
Real-time Operating System
Real-time Operating System is also known as multitasking operating systems. These are used
when a more reliable and dedicated operating system is needed. Real-time operating systems are
used in embedded systems, robotic applications, defense-related weapons systems, in systems
used in air traffic control, in applications of scientific research and things like that. One specialty
of the real-time operating system is its 'response time' which is very less compared to online
processing. A real-time operating system fails to operate if it does not have a well-defined or
fixed time constraints which ultimately define its response time.
Depending upon the response time flexibility the Real-time Operating Systems are classified into
two categories namely, the hard real-time systems and the soft real-time systems.
Hard real-time systems
The hard real-time systems are more robust in terms of their response time. For example, if the
input data is to be processed in 5 seconds then it has to be processed exactly in 5 seconds and not
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in 6 seconds or in 4 seconds. Hence it is used in critical cases when the dire need of time
constraint is to be fulfilled.
Soft real-time systems
The soft real-time systems are not that restrictive in nature compared to the hard real-time
systems. If the time mismatch is observed, it leads to less reliance on the system. The common
applications of soft real-time systems are employed in audio and video systems or various other
multimedia systems.
Operating System Architecture
The operating system architecture is generally referred to as kernel or kernel architecture. It
mainly consists of a process manager, memory manager, Inter-process communication, I/O
manager, User interface. (Silberschatz 2006) Operating systems are organized in Monolithic,
Layered, Microkernel, Distributed and VM based OS architecture.
In the monolithic system architecture, each component of the operating system is covered.
Figure - Monolithic operating system architecture
With time, when the operating systems became complex the components based on similar
operations were grouped together in a layer and then layered architecture came into existence.
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Figure - Layered operating system architecture
In Microkernel, the architecture became more compact and it included very less number of
services in the kernel side. (Deitel et al. 2007)
Figure - Microkernel operating system architecture
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Functions of Operating Systems
The operating system provides various functions to help user apart from creating a suitable
environment for the programs. These functions are discussed below in short detail.
Program management
Operating systems provide us with program execution. first, it loads the program in the memory,
execute it by synchronizing the program.
Memory management
Operating system helps to manage memory and it also looks for the free space and space which
is filled with information. There are various memory management techniques. First one is Single
contiguous memory allocation technique in which is widely used in embedded systems. The
second one is Partitioned memory allocation technique. In this technique, the main memory is
divided into various parts and managed and utilized partly. The next is Paged memory allocation
technique in which the memory is divided into equal portions called page frames.
Process Management
Depending upon the type of OS the process management is done by the process or a thread. The
operating system basically controls the process rather than the applications. An operating system
does perform so by allocating the execution cycle to the CPU.
I/O Operation
Input-output operations with respect to a file or with respect to a program are done with the help
of operating systems by providing access to various input-output devices.
File System Management
Information is stored in terms of files. For future purposes, the file is stored in secondary storage
devices. With respect to file system management, a file can be read or written with the help of
system management. The operating system provides a user interface for this application.
Communication
An operating system provides communication support, where no special devices like bus or
clock frequency are not available.
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Error Handling
Errors may arise due to various reasons in the system or in the processing. The operating system
checks for the errors and tries to rectify them to make a better computing environment.
Protection
In the case of multiple users operating on the same system, the protection of one users' data to
the others is very important. This task is realized with the help of the operating system.
Operating systems provide a facility of password and username to access the respective users.
Conclusion
What type of operating system should be used on desktop, mobile or on the server is a puling
question. But the suitable operating system is an essential need for a system to work properly.
Following is the list of some of the best operating system used for various purposes.
System Operating System
Desktop Microsoft Windows, Mac OS X, and Linux.
Mobile Android OS by Google, iOS by Apple
Server Ubuntu server, centos server, Unix server, Microsoft windows server
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References
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Performance Computer Architecture
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Eyre, J and Bier, J 1998 DSP Processors Hit the Mainstream
Harris, J & Singh J 2008, Operating Systems (SIE), McGraw Hill Education, India
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OLTP Workloads,” Proc. Int’l Symp. Computer Architecture, IEEE CS Press, Los Alamitos,
Calif.
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P. Trancoso et al., 1997, The Memory Performance of DSS Commercial Workloads in Shared-
Memory Multiprocessors, Proc. Third Int’l Symp. High-Performance Computer Architecture,
IEEE CS Press, Los Alamitos, California
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Rao, PVS 2009, Computer System Architecture, Prentice Hall India Learning Private Limited,
India.
Silberschatz, A , Galvin, B G, Gnage G 2006, Wilay publication, India
Wilson, J et al., 2018 Challenges and Trends in Processor Design
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