CNT_Lecture 8.pptx operating system computer

COMPUTER AND NETWORK
TECHNOLOGY
Chapter 2: Introduction to Operating
Systems and System Software
Lecture 8

Contents
• The human interface; the operating principles and
characteristics of printers, display devices, input
devices, biometric devices, etc.
• Multitasking: interrupts, concurrency, scheduling,
memory management and virtual memory
• System software: Internet browsers, email systems,
security products (e.g. anti virus software)
• System performance and its evaluation: definition,
measurement and benchmark

Multitasking - 1
• In many current systems, the CPU contains multiple
cores, which means it can run several processes at the
same time.
• In addition, each core is capable of “multitasking”,
which means it can switch from one process to another
quickly, creating the illusion that many processes are
running at the same time.
• The part of the operating system that implements
multitasking is the “kernel”.

Multitasking - 2
• In an operating system, the kernel is the lowest level of
software, surrounded by several other layers, including
an interface called a “shell.”
• At its most basic, the kernel’s job is to handle
interrupts.
• An “interrupt” is an event that stops the normal
instruction cycle and causes the flow of execution to
jump to a special section of code called an “interrupt
handler”.

Multitasking - 3
• A hardware interrupt is caused when a device sends a
signal to the CPU for e.g., a network interface might cause
an interrupt when a packet of data arrives, or a disk drive
might cause an interrupt when a data transfer is complete.
• Most systems also have timers that cause interrupts at
regular intervals, or after an elapsed time.
• A software interrupt is caused by a running program for
e.g., if an instruction cannot complete for some reason, it
might trigger an interrupt so the condition can be handled
by the operating system.
• Some floating-point errors, like division by zero, are
handled using interrupts

Multitasking - 4
• When a program needs to access a hardware device, it
makes a system call, which is similar to a function call,
except that instead of jumping to the beginning of the
function, it executes a special instruction that triggers
an interrupt, causing the flow of execution to jump to
the kernel.
• The kernel reads the parameters of the system call,
performs the requested operation, and then resumes
the interrupted process.

Context Switching - 1
• Context Switching defines the characteristics of a
multitasking operating system in which, multiple
processes shared the same CPU to perform multiple
tasks without the need for additional processors in the
system
• It is a technique or method used by the operating
system to switch a process from one state to another
to execute its function using CPUs in the system.
• When switching perform in the system, it stores the old
running process's status in the form of registers and
assigns the CPU to a new process to execute its tasks.

Context Switching - 2
• While a new process is running in the system, the
previous process must wait in a ready queue.
• The execution of the old process starts at that point
where another process stopped it.

Need for Context Switching - 1
• Following are the reasons that describe the need for
context switching in the Operating system:
1. The switching of one process to another process is
not directly in the system. A context switching
helps the operating system that switches between
the multiple processes to use the CPU's resource to
accomplish its tasks and store its context. It can
resume the service of the process at the same
point later. If the currently running process's data
or context is not stored, the stored data may be
lost while switching between processes.

2. If a high priority process falls into the ready queue, the
currently running process will be shut down or stopped
by a high priority process to complete its tasks in the
system.
3. If any running process requires I/O resources in the
system, the current process will be switched by
another process to use the CPUs. And when the I/O
requirement is met, the old process goes into a ready
state to wait for its execution in the CPU. Context
switching stores the state of the process to resume its
tasks in an operating system. Otherwise, the process
needs to restart its execution from the initials level.

4. If any interrupts occur while running a process in
the operating system, the process status is saved as
registers using context switching. After resolving
the interrupts, the process switches from a wait
state to a ready state to resume its execution at the
same point later, where the operating system
interrupt occured.
5. A context switching allows a single CPU to handle
multiple process requests simultaneously without
the need for any additional processors.

What is a PCB? - 1
• A PCB (Process Control Block) is a data structure used
in the operating system that contains information that
are related to the process - for example, when a
process is created in the operating system, updated
information of the process, switching information of
the process etc.
• The PCB is allocated by the operating system at the
time a process is created.

What is a PCB? - 2
• Among other things, the PCB keeps track of the process
state, which is one of the below:
– Running: if the process is currently running on a core.
– Ready: if the process could be running, but isn’t,
usually because there are more runnable processes.
– Blocked: if the process cannot run because it is
waiting for a future event like network communication
or a disk read.
– Done: if the process has completed, but has exit
status information that has not been read yet.

Example of Context Switching - 1
• Suppose that multiple processes are stored in a Process
Control Block (PCB).
• One process (Process A) is in running state to execute
its task with the use of CPUs.
• As Process A is running, another process (Process B)
arrives in the ready queue, which has a high priority of
completing its task using CPU.
• Here we used context switching that switches the
current process (Process A) with the new process
(Process B) which requiring the CPU to finish its tasks.

Example of Context Switching - 2
• While switching the process, a context switch saves the
status of the old process (Process A) in registers.
• When Process A reloads into the CPU, it starts the
execution of the process where the new process
(Process B) has stopped the old process (Process A).
• If we do not save the state of the process, we have to
start its execution at the initial level.
• In this way, context switching helps the operating
system to switch between the processes, store or
reload the process when it requires executing its tasks.

Steps for Context Switching - 1
• There are several steps involved in context switching of the
processes.
• The following diagram represents the context switching of
two processes, P1 to P2, when an interrupt, I/O needs, or
priority-based process occurs in the ready queue of PCB.

• As can be seen in the diagram, initially, the P1 process
is running on the CPU to execute its task, and at the
same time, another process, P2, is in the ready state.
• If an error or interruption has occurred or the process
requires input/output, the P1 process switches its state
from running to the waiting state.
• Before changing the state of the process P1, context
switching saves the context of the process P1 in the
form of registers and the program counter to the PCB1.
• After that, it loads the state of the P2 process from the
ready state of the PCB2 to the running state.

• The following steps are taken when switching Process P1 to
Process 2:
1. First, the context switching needs to save the state of
process P1 in the form of the program counter and the
registers to the PCB, which is in the running state.
2. Now update PCB1 to process P1 and moves the
process to the appropriate queue, such as the ready
queue, I/O queue and waiting queue.
3. After that, another process gets into the running state,
or we can select a new process from the ready state,
which is to be executed, or the process has a high
priority to execute its task.

4. Now, we have to update the PCB for the selected
process P2. It includes switching the process state
from ready to running state or from another state
like blocked, exit, or suspend.
5. If the CPU already executes process P2, we need to
get the status of process P1 to resume its execution
at the same point where the system interrupt
occurred.
6. Process P1 is reloaded from PCB1 to the running
state to resume its task at the same point.
• Otherwise, the information is lost, and when the process
is executed again, it starts execution at the initial level.

Scheduling - 1
• There might be hundreds of processes on a computer,
but usually most of them are blocked.
• Most of the time, there are only a few processes that
are ready or running.
• When an interrupt occurs, the scheduler decides which
process to start or resume.
• On a workstation or laptop, the primary goal of the
scheduler is to minimize response time; that is, the
computer should respond quickly to user actions.

Scheduling - 2
• As an analogy, suppose you are making an apple pie.
The crust takes 5 minutes to prepare, but then it has to
chill for half an hour. It takes 20 minutes to prepare the
filling. If you prepare the crust first, you can prepare
the filling while the crust is chilling, and you can finish
the pie in 35 minutes. If you prepare the filling first, the
process takes 55 minutes.
• Most schedulers use some form of priority-based
scheduling, where each process has a priority that can
be adjusted up or down over time.
• When the scheduler runs, it chooses the runnable
process with the highest priority.

Scheduling - 3
• Here are some of the factors that determine a
process’s priority:
– A process usually starts with a relatively high priority
so it starts running quickly.
– If a process makes a request and blocks before its
time slice is complete, it is more likely to be
interactive or I/O-bound, so its priority should go up.
– If a process runs for an entire time slice, it is more
likely to be long-running and CPU-bound, so its
priority should go down.

Scheduling - 4
– If a task blocks for a long time and then becomes
ready, it should get a priority boost so it can respond
to whatever it was waiting for.

Concurrency
• Concurrency is the execution of a set of multiple
instruction sequences at the same time.
• This occurs when there are several process threads
running in parallel.
• These threads communicate with the other
threads/processes through a concept of shared
memory or through message passing.
• Because concurrency results in the sharing of system
resources - instructions, memory, files - problems can
occur like deadlocks and resources starvation.

Principles of Concurrency - 1
• With current technology such as multi core processors,
and parallel processing, which allow for multiple
processes/threads to be executed concurrently (i.e. at
the same time), it is possible to have more than a
single process/thread accessing the same space in
memory, the same declared variable in the code, or
even attempting to read/write to the same file.
• The amount of time it takes for a process to execute is
not easily calculated – thus it is difficult to predict
which process will complete first.

Principles of Concurrency - 2
• The amount of time a process takes to complete
depends on the following:
– The activities of other processes
– The way operating system handles interrupts
– The scheduling policies of the operating system

Advantages of Concurrency - 1
Running of multiple applications
• Having concurrency allows the operating system to run
multiple applications at the same time.
Better resource utilization
• Having concurrency allows the resources that are NOT
being used by one application can be used for other
applications.
Better average response time
• Without concurrency, each application has to be run to
completion before the next one can be run.

Advantages of Concurrency - 2
Better performance
• Concurrency provides better performance by the
operating system.
• When one application uses only the processor and
another application uses only the disk drive then the
time to concurrently run both applications to
completion will be shorter than the time to run each
application consecutively.

Memory Management
• Memory Management is the process of controlling and
coordinating computer memory, assigning portions
known as blocks to various running programs to
optimize the overall performance of the system.
• It is the most important function of an operating
system that manages primary memory.
• It helps processes to move back and forward between
the main memory and execution disk.
• It helps OS to keep track of every memory location,
irrespective of whether it is allocated to some process
or it remains free.

Why use Memory Management?
• It allows you to check how much memory needs to be
allocated to processes that decide which processor
should get memory at what time.
• Tracks whenever inventory gets freed or unallocated.
According to it will update the status.
• It allocates the space to application routines.
• It also make sure that these applications do not
interfere with each other.
• Helps protect different processes from each other
• It places the programs in memory so that memory is
utilized to its full extent.

Memory Management Techniques - 1
Single Contiguous Allocation
• It is the easiest memory management technique.
• In this method, all types of computer's memory except
a small portion which is reserved for the OS is available
for one application.
• For example, MS-DOS operating system allocates
memory in this way.

Partitioned Allocation
• It divides primary memory into various memory
partitions, which is mostly contiguous areas of
memory.
• Every partition stores all the information for a specific
task or job.
• This method consists of allotting a partition to a job
when it starts & de-allocate when it ends.

Paged Memory Management
• This method divides the computer's main memory into
fixed-size units known as page frames.
• This hardware memory management unit maps pages
into frames which should be allocated on a page basis.

Segmented Memory Management
• Segmented memory is the only memory management
method that does not provide the user's program with
a linear and contiguous address space.
• Segments need hardware support in the form of a
segment table.
• It contains the physical address of the section in
memory, size, and other data like access protection bits
and status.

Methods involved in Memory
Management

Management - 1
1. What is Swapping?
• Swapping is a method in which the process should be
swapped temporarily from the main memory to the
backing store.
• It will be later brought back into the memory to
continue execution.
• Backing store is a hard disk or some other secondary
storage device that should be big enough in order to
accommodate copies of all memory images for all users.
• It is also capable of offering direct access to these
memory images.

Management - 2

Management - 3
Benefits of Swapping
• It offers a higher degree of multiprogramming.
• Allows dynamic relocation. For example, if address
binding at execution time is being used, then processes
can be swap in different locations. Else in case of
compile and load time bindings, processes should be
moved to the same location.
• It helps to get better utilization of memory.
• Minimum wastage of CPU time on completion so it can
easily be applied to a priority-based scheduling method
to improve its performance.

Management - 4
2. What is Memory allocation?
• Memory allocation is a process by which computer
programs are assigned memory or space.
• The main memory is divided into two types of
partitions
1. Low Memory - Operating system resides in this
type of memory.
2. High Memory- User processes are held in high
memory.

Management - 5
3. Partition Allocation
• Memory is divided into different blocks or partitions.
• Each process is allocated according to the
requirement.
• Partition allocation is an ideal method to avoid
internal fragmentation.

Management - 6
Below are the various partition allocation schemes:
1. First Fit: In this type fit, the partition is allocated, which
is the first sufficient block from the beginning of the
main memory.
2. Best Fit: It allocates the process to the partition that is
the first smallest partition among the free partitions.
3. Worst Fit: It allocates the process to the partition, which
is the largest sufficient freely available partition in the
main memory.
4. Next Fit: It is mostly similar to the first Fit, but this Fit,
searches for the first sufficient partition from the last
allocation point.

Management - 7
4. What is Paging?
• Paging is a storage mechanism that allows OS to retrieve
processes from the secondary storage into the main
memory in the form of pages.
• In the Paging method, the main memory is divided into
small fixed-size blocks of physical memory, which is called
frames.
• The size of a frame should be kept the same as that of a
page to have maximum utilization of the main memory
and to avoid external fragmentation.
• Paging is used for faster access to data, and it is a logical
concept.

Management - 8
5. What is Fragmentation?
• Processes are stored and removed from memory.
• This creates free memory space, which are too small
to use by other processes.
• After sometimes, that processes not able to allocate
to memory blocks because its small size and memory
blocks always remain unused is called fragmentation.
• This type of problem happens during a dynamic
memory allocation system when free blocks are quite
small, so it is not able to fulfill any request.

Management - 9
Two types of Fragmentation methods are:
1. External fragmentation can be reduced by
rearranging memory contents to place all free
memory together in a single block.
2. The internal fragmentation can be reduced by
assigning the smallest partition, which is still good
enough to carry the entire process.

Management - 10
What is Segmentation?
• Segmentation method works almost similarly to paging.
• The only difference between the two is that segments are
of variable-length, whereas, in the paging method, pages
are always of fixed size.
• A program segment includes the program's main function,
data structures, utility functions, etc.
• The OS maintains a segment map table for all the
processes.
• It also includes a list of free memory blocks along with its
size, segment numbers, and its memory locations in the
main memory or virtual memory.

What is Virtual Memory? - 1
• Virtual memory is a memory management technique
where secondary memory can be used as if it were a
part of the main memory.
• Virtual memory is a common technique used in a
computer's operating system (OS).
• Virtual memory uses both hardware and software to
enable a computer to compensate for physical
memory shortages, temporarily transferring data from
random access memory (RAM) to disk storage.
• Mapping chunks of memory to disk files enables a
computer to treat secondary memory as though it
were main memory.

• Today, most personal computers (PCs) come with at
least 8 GB (gigabytes) of RAM.
• However, sometimes, this is not enough to run several
programs at one time and this is where virtual
memory comes in.
• Virtual memory frees up RAM by swapping data that
has not been used recently over to a storage device,
such as a hard drive or solid-state drive (SSD).
• Virtual memory is important for improving system
performance, multitasking and using large programs.

• However, users should not overly rely on virtual
memory, since it is considerably slower than RAM.
• If the OS has to swap data between virtual memory
and RAM too often, the computer will begin to slow
down and this is called thrashing.
• Virtual memory was developed at a time when
physical memory -- also referenced as RAM -- was
expensive.
• Computers have a finite amount of RAM, so memory
will eventually run out when multiple programs run at
the same time.

• A system using virtual memory uses a section of the
hard drive to emulate RAM.
• With virtual memory, a system can load larger or
multiple programs running at the same time, enabling
each one to operate as if it has more space, without
having to purchase more RAM.

How does the Virtual Memory works?
- 1
• Virtual memory uses both hardware and software to
operate.
• When an application is in use, data from that program
is stored in a physical address using RAM.
• If, at any point, the RAM space is needed for
something more urgent, data can be swapped out of
RAM and into virtual memory.
• The computer's memory manager is in charge of
keeping track of the shifts between physical and virtual
memory.

- 2
• If that data is needed again, the computer's Memory
Management Unit will use a context switch to resume
execution.
• While copying virtual memory into physical memory,
the OS divides memory with a fixed number of
addresses into either page files or swap files.
• Each page is stored on a disk, and when the page is
needed, the OS copies it from the disk to main
memory and translates the virtual addresses into real
addresses.

- 3
• However, the process of swapping virtual memory to
physical is rather slow.
• This means using virtual memory generally causes a
noticeable reduction in performance.
• Because of swapping, computers with more RAM are
considered to have better performance.

Advantages of Virtual Memory
• Virtual Memory allows you to run more applications at a
time.
• It allows you to easily fit many large programs into smaller
programs.
• It helps to easily implement a multiprogramming
environment.
• Common data can be shared easily between memory.
• With the help of virtual memory, speed is gained when
only a particular segment of the program is required for
the execution of the program.
• The process may even become larger than all of the
physical memory.

Disadvantages of Virtual Memory
• Virtual memory reduces the stability of the system.
• The performance of Virtual memory is not as good as
that of RAM.
• If a system is using virtual memory then applications
may run slower.
• Virtual memory negatively affects the overall
performance of a system.
• Virtual memory occupies the storage space, which
might be otherwise used for long term data storage.
• This memory takes more time in order to switch
between applications.

CNT_Lecture 8.pptx operating system computer

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