Operating System introduction topics for beginners

Operating System
• An operating system acts as an intermediary between
the user of a computer and the computer hardware. The
purpose of an operating system is to provide an
environment in which a user can execute programs in a
convenient and efficient manner
• An operating system is software that manages the
computer hardware. The hardware must provide
appropriate mechanisms to ensure the correct operation
of the computer system and to prevent user programs
from interfering with the proper operation of the system

Computer System Overview
• A computer consists of processor, memory, and I/O
components, with one or more modules of each type. These
components are interconnected in some fashion to achieve the
main function of the computer
• Processor: Controls the operation of the computer and
performs its data processing functions. When there is only
one processor, it is often referred to as the central processing
unit (CPU)
• Main memory: Stores data and programs. This memory is
typically volatile; that is, when the computer is shut down,
the contents of the memory are lost.
• In contrast, the contents of disk memory are retained even
when the computer system is shut down. Main memory is also
referred to as real memory or primary memory.

Computer System Overview
• I/O modules: Move data between the
computer and its external environment. The
external environment consists of a variety of
devices, including secondary memory devices
(e.g., disks), communications equipment, and
terminals.
• System bus: Provides for communication
among processors, main memory, and I/O
modules.

INSTRUCTION EXECUTION
• Instruction processing consists of two steps: The processor reads
( fetches ) instructions from memory one at a time and executes
each instruction.
• Instruction execution may involve several operations and
depends on the nature of the instruction.
• Processor-memory: Data may be transferred from processor
to memory or from memory to processor.
• Processor-I/O: Data may be transferred to or from a
peripheral device by transferring between the processor and an
I/O module.
• Data processing: The processor may perform some
arithmetic or logic operation on data.
• Control: An instruction may specify that the sequence of
execution be altered.

INTERRUPTS
• Virtually all computers provide a mechanism by which
other modules (I/O, memory) may interrupt the
normal sequencing of the processor.
• Suppose that the processor is transferring data to a
printer using the instruction cycle, After each write
operation, the processor must pause and remain idle
until the printer catches up.
• The length of this pause may be on the order of many
thousands or even millions of instruction cycles.
Clearly, this is a very wasteful use of the processor.

Multiple Interrupts
• one or more interrupts can occur while an
interrupt is being processed. For example, a
program may be receiving data from a
communications line and printing results at the
same time.
• The printer will generate an interrupt every
time that it completes a print operation. The
communication line controller will generate an
interrupt every time a unit of data arrives

Memory Hierarchy
• The design constraints on a computer’s memory
can be summed up by three questions: How
much? How fast? How expensive?
• As might be expected, there is a trade-off among
the three key characteristics of memory: namely,
capacity, access time, and cost.
• Faster access time, greater cost per bit
• Greater capacity, smaller cost per bit
• Greater capacity, slower access speed

CACHE MEMORY
• All instruction cycles, the processor accesses
memory at least once, to fetch the instruction,
and often one or more additional times, to
fetch operands and/ or store results.
• The principle of locality by providing a small,
fast memory between the processor and main
memory, namely the cache.

CACHE MEMORY
• The cache contains a copy of a portion of main
memory. When the processor attempts to read
a byte or word of memory, a check is made to
determine if the byte or word is in the cache. If
so, the byte or word is delivered to the
processor. If not, a block of main memory,
consisting of some fixed number of bytes, is
read into the cache and then the byte or word
is delivered to the processor.

CACHE MEMORY
• Cache size . It turns out that reasonably small caches can have a
significant impact on performance
• Block size : the unit of data exchanged between cache and main
memory.
• Mapping function determines which cache location the block will
occupy
• Replacement algorithm chooses, within the constraints of the mapping
function, which block to replace when a new block is to be loaded into
the cache and the cache already has all slots filled with other blocks.
• The write policy dictates when the memory write operation takes place.
At one extreme, the writing can occur every time that the block is
updated. At the other extreme, the writing occurs only when the block
is replaced.

DIRECT MEMORY ACCESS
• (DMA):The DMA function can be performed by a separate module on
the system bus or it can be incorporated into an I/O module.
• In either case, the technique works as follows. When the processor
wishes to read or write a block of data, it issues a command to the DMA
module, by sending to the DMA module the following information.
– Whether a read or write is requested
– The address of the I/O device involved
– The starting location in memory to read data from or write data to
– The number of words to be read or written
• The processor then continues with other work. It has delegated this I/O
operation to the DMA module, and that module will take care of it. The
DMA module transfers the entire block of data, one word at a time,
directly to or from memory without going through the processor.

• Types of Direct Memory Access (DMA)
• There are four popular types of DMA.
• Single-Ended DMA
• Dual-Ended DMA
• Arbitrated-Ended DMA
• Interleaved DMA

• Single-Ended DMA: Single-Ended DMA Controllers operate
by reading and writing from a single memory address.
They are the simplest DMA.
• Dual-Ended DMA: Dual-Ended DMA controllers can read
and write from two memory addresses. Dual-ended DMA is
more advanced than single-ended DMA.
• Arbitrated-Ended DMA: Arbitrated-Ended DMA works by
reading and writing to several memory addresses. It is
more advanced than Dual-Ended DMA.
• Interleaved DMA: Interleaved DMA are those DMA that
read from one memory address and write from another
memory address.

• Advantages of DMA Controller
• Data Memory Access speeds up memory
operations and data transfer.
• CPU is not involved while transferring data.
• DMA requires very few clock cycles while
transferring data.
• DMA distributes workload very appropriately.
• DMA helps the CPU in decreasing its load.

• Disadvantages of DMA Controller
• Direct Memory Access is a costly operation
because of additional operations.
• DMA suffers from Cache-Coherence Problems.
• DMA Controller increases the overall cost of
the system.
• DMA Controller increases the complexity of
the software.

MULTIPROCESSOR AND MULTICORE
ORGANIZATION
• Symmetric Multiprocessors
• There are two or more similar processors of comparable capability.
• These processors share the same main memory and I/O facilities and are
interconnected by a bus or other internal connection scheme, such that
memory access time is approximately the same for each processor.
• All processors share access to I/O devices, either through the same
channels or through different channels that provide paths to the same
device.
• All processors can perform the same functions(hence the term
symmetric).
• The system is controlled by an integrated operating system that provides
interaction between processors and their programs at the job, task, file,
and data element levels.

ORGANIZATION
• An SMP organization has a number of potential advantages over a
uniprocessor organization
• Performance: If the work to be done by a computer can be organized so that
some portions of the work can be done in parallel, then a system with multiple
processors will yield greater performance than one with a single processor
• Availability: In a symmetric multiprocessor, because all processors can
perform the same functions, the failure of a single processor does not halt the
machine. Instead, the system can continue to function at reduced performance
• Incremental growth: A user can enhance the performance of a system by
adding an additional processor.
• Scaling: Vendors can offer a range of products with different price and
performance characteristics based on the number of processors configured in
the system.

ORGANIZATION

Multicore Computers
• A multicore computer, also known as a chip
multiprocessor , combines two or more processors
(called cores) on a single piece of silicon (called a die).
• Typically, each core consists of all of the components
of an independent processor, such as registers, ALU,
pipeline hardware, and control unit, plus L1
instruction and data caches.
• In addition to the multiple cores, contemporary
multicore chips also include L2 cache and, in some
cases, L3 cache.

Operating System Objectives and Functions
• Convenience: An OS makes a computer more convenient to use.
• Efficiency: An OS allows the computer system resources to be used in
an efficient manner.
• Ability to evolve: An OS should be constructed in such a way as to
permit the effective development, testing, and introduction of new
system functions without interfering with service
• If one were to develop an application program as a set of machine
instructions that is completely responsible for controlling the computer
hardware.
• Some of these programs are referred to as utilities, or library programs.
These implement frequently used functions that assist in program
creation, the management of files, and the control of I/O devices

• OS typically provides services in the following areas
• Program development: The OS provides a variety of facilities and
services, such as editors and debuggers, to assist the programmer in
creating programs.
• Program execution: A number of steps need to be performed to
execute a program. Instructions and data must be loaded into main
memory, I/O devices and files must be initialized, and other resources
must be prepared. The OS handles these scheduling duties for the user.
• Access to I/O devices: Each I/O device requires its own peculiar set
of instructions or control signals for operation. The OS provides a
uniform interface that hides these details so that programmers can
access such devices using simple reads and writes.

• Controlled access to files: For file access, the OS must reflect a detailed
understanding of not only the nature of the I/O device (disk drive, tape drive)
but also the structure of the data contained in the files on the storage medium.
• System access: For shared or public systems, the OS controls access to the
system as a whole and to specific system resources. The access function must
provide protection of resources and data from unauthorized users and must
resolve conflicts for resource contention.
• Error detection and response: A variety of errors can occur while a computer
system is running. These include internal and external hardware errors, such as
a memory error, or a device failure or malfunction; and various software
errors, such as division by zero, attempt to access forbidden memory location
• Accounting: A good OS will collect usage statistics for various resources and
monitor performance parameters such as response time.
• Instruction set architecture (ISA) :This interface is the boundary between
hardware and software. Note that both application programs and utilities may
access the ISA directly.

• Application binary interface (ABI): The ABI
defines the system call interface to the
operating system and the hardware resources
and services available in a system through the
user ISA.
• Application programming interface (API) :
The API gives a program access to the
hardware resources and services available in
system through the user

THE EVOLUTION OF OPERATING SYSTEMS
• Serial Processing:With the earliest computers, from the late 1940s to the mid-1950s,
the programmer interacted directly with the computer hardware; there was no OS.
• These early systems presented two main problems:
• Scheduling: Most installations used a hardcopy sign-up sheet to reserve computer
time. Typically, a user could sign up for a block of time in multiples of a half hour or
so. A user might sign up for an hour and finish in 45 minutes; this would result in
wasted computer processing time. On the other hand, the user might run into
problems, not finish in the allotted time, and be forced to stop before resolving the
problem.
• Setup time: A single program, called a job , could involve loading the compiler plus
the high-level language program (source program) into memory, saving the compiled
program (object program) and then loading and linking together the object program
and common functions. Each of these steps could involve mounting or dismounting
tapes or setting up card decks. If an error occurred, the hapless user typically had to
go back to the beginning of the setup sequence.

Multi-programmed Batch Systems
• we have a single program, referred to as uniprogramming.
The processor spends a certain amount of time executing,
until it reaches an I/O instruction. It must then wait until
that I/O instruction concludes before proceeding.
• When one job needs to wait for I/O, the processor can
switch to the other job, which is likely not waiting for
I/O . Furthermore, we might expand memory to hold
three, four, or more programs and switch among all of
them. The approach is known as multiprogramming , or
multitasking .

Multi-programmed Batch Systems

Time-Sharing Systems
• Just as multiprogramming allows the processor to handle multiple
batch jobs at a time multiprogramming can also be used to handle
multiple interactive jobs.
• In this latter case, the technique is referred to as time sharing ,
because processor time is shared among multiple users.
• In a time-sharing system, multiple users simultaneously access the
system through terminals,
• If there are n users actively requesting service at one time, each user
will only see on the average 1/ n of the effective computer capacity
• A system clock generated interrupts at a rate of approximately one
every 0.2 seconds. At each clock interrupt, the OS regained control and
could assign the processor to another user. This technique is known as
time slicing

• MAJOR ACHIEVEMENTS: Process related
• Improper synchronization: It is often the case that a routine must
be suspended awaiting an event elsewhere in the system.
• Failed mutual exclusion: It is often the case that more than one
user or program will attempt to make use of a shared resource at
the same time.
• Nondeterminate program operation: The results of a particular
program normally should depend only on the input to that program
and not on the activities of other programs in a shared system.
• Deadlocks: It is possible for two or more programs to be hung up
waiting for each other. For example, two programs may each
require two I/O devices to perform some operation

• MAJOR ACHIEVEMENTS: - Memory Management
• Process isolation: The OS must prevent independent processes from interfering
with each other’s memory, both data and instructions.
• Automatic allocation and management: Programs should be dynamically
allocated across the memory hierarchy as required. Allocation should be
transparent to the programmer.
• Support of modular programming: Programmers should be able to define
program modules, and to create, destroy, and alter the size of modules dynamically.
• Protection and access control: Sharing of memory, at any level of the memory
hierarchy, creates the potential for one program to address the memory space of
another.
• Long-term storage: Many application programs require means for storing
information for extended periods of time, after the computer has been powered
down.

• Information Protection and Security:
• Availability: Concerned with protecting the system
against interruption.
• Confidentiality: Assures that users cannot read data
for which access is unauthorized.
• Data integrity: Protection of data from unauthorized
modification.
• Authenticity: Concerned with the proper verification
of the identity of users and the validity of messages or
data.

DEVELOPMENTS LEADING TO MODERN OPERATING
SYSTEMS
Microkernel architecture assigns only a few essential
functions to the kernel, including address spaces, inter-
process communication (IPC), and basic scheduling.
Other OS services are provided by kernel modules
• Multithreading:
• Thread: A thread executes sequentially and is
interruptible so that the processor can turn to another
thread.
• Process: A collection of one or more threads and
associated system resources

DEVELOPMENTS LEADING TO MODERN OPERATING
SYSTEMS
• Symmetric multiprocessing (SMP):
• Performance
• Availability
• Incremental growth
• Scaling

MICROSOFT WINDOWS OVERVIEW
• Windows Objects:
• Encapsulation: An object consists of one or more items of data, called
attributes, and one or more procedures that may be performed on those
data, called services.
• Object class and instance: An object class is a template that lists the
attributes and services of an object and defines certain object
characteristics.
• Inheritance: Although the implementation is hand coded, the
Executive uses inheritance to extend object classes by adding new
features.
• Polymorphism: Internally, Windows uses a common set of API
functions to manipulate objects of any type; this is a feature of
polymorphism

New in Windows
• Engineering improvements: The performance of hundreds of key
scenarios, such as opening a file from the GUI, are tracked and
continuously characterized to identify and fix problems.
• Performance improvements: The amount of memory required has
been reduced, both for clients and servers.
• Reliability improvements: The user-mode heap is more tolerant of
memory allocation errors by programmers, such as continuing to use
memory after it is freed
• Energy efficiency: Many improvements have been made to the energy
efficiency of Windows. On servers, unused processors can be “parked,”
reducing their energy use.
• Security: The security features in Windows keeps updating in every
versions

LINUX
• Linux started out as a UNIX variant for the IBM PC (Intel 80386)
architecture.
• Key to the success of Linux has been the availability of free software
packages under the auspices of the Free Software Foundation (FSF).
FSF’s goal is stable, platform-independent software that is free, high
quality, and embraced by the user community.
• Dynamic linking: A kernel module can be loaded and linked into the
kernel while the kernel is already in memory and executing. A module
can also be unlinked and removed from memory at any time.
• Stackable modules: The modules are arranged in a hierarchy.
Individual modules serve as libraries when they are referenced by
client modules higher up in the hierarchy, and as clients when they
reference modules further down

Operating-System Structure
• An operating system provides an environment
for the execution of programs. It provides
certain services to programs and to the users
of those programs.

System Calls
• System calls provide an interface to the services made available
by an operating system. Frequently, systems execute thousands
of calls per second.
• The API specifies a set of functions that are available to an
application programmer, including the parameters that are
passed to each function and the return values the programmer
can expect.
• Three of the most common APIs available to application
programmers are the Windows API for Windows systems, the
POSIX API for POSIX-based systems (which include virtually
all versions of UNIX, Linux, and Mac OSX), and the Java API
for programs that run on the Java virtual machine.

Types of System Calls
• System calls can be grouped roughly into six
major categories: process control, file
manipulation, device manipulation,
information maintenance, communications,
and protection.

System Programs
• System programs, also known as system utilities, provide a convenient
environment for program development and execution.
• File management. These programs create, delete, copy, rename, print, dump,
list, and generally manipulate files and directories.
• Status information. Some programs simply ask the system for the date, time,
amount of available memory or disk space, number of users, or similar status
information.
• File modification. Several text editors may be available to create and modify the
content of files stored on disk or other storage devices. There may also be
special commands to search contents of files or perform transformations of the
text.
• Programming-language support. Compilers, assemblers, debuggers, and
interpreters for common programming languages (such as C, C++, Java, and
PERL) are often provided with the operating system or available as a separate
download.

System Programs
• Program loading and execution. Once a program is assembled or
compiled, it must be loaded into memory to be executed. The system may
provide absolute loaders, relocatable loaders, linkage editors, and overlay
loaders.
• Communications. These programs provide the mechanism for creating
virtual connections among processes, users, and computer systems. They
allow users to send messages to one another’s screens, to browse Web
pages, to send e-mail messages, to log in remotely, or to transfer files from
one machine to another.
• Background services. All general-purpose systems have methods for
launching certain system-program processes at boot time. Some of these
processes terminate after completing their tasks, while others continue to
run until the system is halted. Constantly running system-program
processes are known as services, subsystems, or daemons.

• Cold/Hard Booting: When the computer starts for the first
time or is in a shut-down state and switch on the power
button to start the system, this type of process to start the
computer is called cold booting. During cold booting, the
system will read all the instructions from the ROM (BIOS)
and the Operating System will be automatically get loaded
into the system. This booting takes more time than Hot or
Warm Booting.
• Warm/Soft Booting: Warm or Hot Booting process is when
computer systems come to no response or hang state, and
then the system is allowed to restart during on condition. It
is also referred to as rebooting. There are many reasons for
this state, and the only solution is to reboot the computer.

System Booting process
1. Startup: Turning the computer on
2. BIOS: The BIOS performs a power-on self-test (POST) of the
hardware components
3. Boot loader: The boot loader loads the kernel, which is the
core code of the operating system
4. Operating system: The operating system is loaded into the
main memory
5. System configuration: Drivers and settings are loaded
6. System utilities: System utilities are loaded
7. User authentication: The user authenticates before being
able to use the computer

System Boot
• The procedure of starting a computer by loading the kernel is known as
booting the system. On most computer systems, a small piece of code
known as the bootstrap program or bootstrap loader locates the kernel,
loads it into main memory, and starts its execution.
• This program is in the form of read-only memory (ROM), because the
RAM is in an unknown state at system startup. ROM is convenient
because it needs no initialization and cannot easily be infected by a
computer virus.
• A problem with this approach is that changing the bootstrap code requires
changing the ROM hardware chips. Some systems resolve this problem
by using erasable programmable read-only memory (EPROM), which
is read only except when explicitly given a command to become writable.
• All of the disk-bound bootstrap, and the operating system itself, can be
easily changed by writing new versions to disk. A disk that has a boot
partition is called a boot disk or system disk.

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