Operating Systems Notes Ndzo
Operating Systems Notes Ndzo
Operating Systems Notes Ndzo
An Operating System (OS) is an interface between a computer user and computer hardware. An
operating system is a software which performs all the basic tasks like file management, memory
management, process management, handling input and output, and controlling peripheral devices
such as disk drives and printers.
Some popular Operating Systems include Linux Operating System, Windows Operating System,
VMS, OS/400, AIX, z/OS, etc.
Definition
An operating system is a program that acts as an interface between the user and the computer
hardware and controls the execution of all kinds of programs.
Memory Management
Memory management refers to management of Primary Memory or Main Memory. Main memory is a
large array of words or bytes where each word or byte has its own address.
Main memory provides a fast storage that can be accessed directly by the CPU. For a program to be
executed, it must in the main memory. An Operating System does the following activities for memory
management −
Advantages
Batch processing takes much of the work of the operator to the computer.
Increased performance as a new job get started as soon as the previous job is finished,
without any manual intervention.
Disadvantages
Difficult to debug program.
A job could enter an infinite loop.
Due to lack of protection scheme, one batch job can affect pending jobs.
6|Page ndzoroland@gmail.com 677 20 00 53
Multitasking
Multitasking is when multiple jobs are executed by the CPU simultaneously by switching between
them. Switches occur so frequently that the users may interact with each program while it is running.
An OS does the following activities related to multitasking −
The user gives instructions to the operating system or to a program directly, and receives an
immediate response.
The OS handles multitasking in the way that it can handle multiple operations/executes
multiple programs at a time.
Multitasking Operating Systems are also known as Time-sharing systems.
These Operating Systems were developed to provide interactive use of a computer system at
a reasonable cost.
A time-shared operating system uses the concept of CPU scheduling and multiprogramming
to provide each user with a small portion of a time-shared CPU.
Each user has at least one separate program in memory.
A program that is loaded into memory and is executing is commonly referred to as a process.
When a process executes, it typically executes for only a very short time before it either
finishes or needs to perform I/O.
Since interactive I/O typically runs at slower speeds, it may take a long time to complete.
During this time, a CPU can be utilized by another process.
The operating system allows the users to share the computer simultaneously. Since each
action or command in a time-shared system tends to be short, only a little CPU time is needed
for each user.
As the system switches CPU rapidly from one user/program to the next, each user is given the
impression that he/she has his/her own CPU, whereas actually one CPU is being shared
among many users.
Multiprogramming
Sharing the processor, when two or more programs reside in memory at the same time, is referred
as multiprogramming. Multiprogramming assumes a single shared processor. Multiprogramming
increases CPU utilization by organizing jobs so that the CPU always has one to execute.
The following figure shows the memory layout for a multiprogramming system.
Advantages
The spooling operation uses a disk as a very large buffer.
Spooling is capable of overlapping I/O operation for one job with processor operations for another job.
Process
A process is basically a program in execution. The execution of a process must progress in a
sequential fashion.
A process is defined as an entity which represents the basic unit of work to be implemented in the system.
To put it in simple terms, we write our computer programs in a text file and when we execute this
program, it becomes a process which performs all the tasks mentioned in the program.
When a program is loaded into the memory and it becomes a process, it can be divided into four
sections ─ stack, heap, text and data. The following image shows a simplified layout of a process
inside main memory −
1 Stack
The process Stack contains the temporary data such as method/function
parameters, return address and local variables.
2 Heap
This is dynamically allocated memory to a process during its run time.
3 Text
This includes the current activity represented by the value of Program Counter and
the contents of the processor's registers.
4 Data
This section contains the global and static variables.
Program
A program is a piece of code which may be a single line or millions of lines. A computer program is
usually written by a computer programmer in a programming language. For example, here is a
simple program written in C programming language −
#include <stdio.h>
int main() {
printf("Hello, World! \n");
return 0;
}
A computer program is a collection of instructions that performs a specific task when executed by a
computer. When we compare a program with a process, we can conclude that a process is a
dynamic instance of a computer program.
A part of a computer program that performs a well-defined task is known as an algorithm. A
collection of computer programs, libraries and related data are referred to as a software.
10 | P a g e ndzoroland@gmail.com 677 20 00 53
Process Life Cycle
When a process executes, it passes through different states. These stages may differ in different
operating systems, and the names of these states are also not standardized.
In general, a process can have one of the following five states at a time.
S.N. State & Description
1 Start
This is the initial state when a process is first started/created.
2 Ready
The process is waiting to be assigned to a processor. Ready processes are waiting
to have the processor allocated to them by the operating system so that they can
run. Process may come into this state after Start state or while running it by but
interrupted by the scheduler to assign CPU to some other process.
3 Running
Once the process has been assigned to a processor by the OS scheduler, the
process state is set to running and the processor executes its instructions.
4 Waiting
Process moves into the waiting state if it needs to wait for a resource, such as
waiting for user input, or waiting for a file to become available.
5 Terminated or Exit
Once the process finishes its execution, or it is terminated by the operating system,
it is moved to the terminated state where it waits to be removed from main memory.
1 Process State
The current state of the process i.e., whether it is ready, running, waiting, or
whatever.
11 | P a g e ndzoroland@gmail.com 677 20 00 53
2 Process privileges
This is required to allow/disallow access to system resources.
3 Process ID
Unique identification for each of the process in the operating system.
4 Pointer
A pointer to parent process.
5 Program Counter
Program Counter is a pointer to the address of the next instruction to be executed
for this process.
6 CPU registers
Various CPU registers where process need to be stored for execution for running
state.
9 Accounting information
This includes the amount of CPU used for process execution, time limits, execution
ID etc.
10 IO status information
This includes a list of I/O devices allocated to the process.
The architecture of a PCB is completely dependent on Operating System and may contain different
information in different operating systems. Here is a simplified diagram of a PCB −
12 | P a g e ndzoroland@gmail.com 677 20 00 53
The PCB is maintained for a process throughout its lifetime, and is deleted once the process
terminates.
Definition
The process scheduling is the activity of the process manager that handles the removal of the
running process from the CPU and the selection of another process on the basis of a particular
strategy.
Process scheduling is an essential part of a Multiprogramming operating systems. Such operating
systems allow more than one process to be loaded into the executable memory at a time and the
loaded process shares the CPU using time multiplexing.
Process Scheduling Queues
The OS maintains all PCBs in Process Scheduling Queues. The OS maintains a separate queue for
each of the process states and PCBs of all processes in the same execution state are placed in the
same queue. When the state of a process is changed, its PCB is unlinked from its current queue and
moved to its new state queue.
The Operating System maintains the following important process scheduling queues −
Job queue − This queue keeps all the processes in the system.
Ready queue − This queue keeps a set of all processes residing in main memory, ready and
waiting to execute. A new process is always put in this queue.
Device queues − The processes which are blocked due to unavailability of an I/O device
constitute this queue.
The OS can use different policies to manage each queue (FIFO, Round Robin, Priority, etc.). The
OS scheduler determines how to move processes between the ready and run queues which can
only have one entry per processor core on the system; in the above diagram, it has been merged
with the CPU.
Two-State Process Model
Two-state process model refers to running and non-running states which are described below −
S.N. State & Description
1 Running
When a new process is created, it enters into the system as in the running state.
2 Not Running
Processes that are not running are kept in queue, waiting for their turn to execute. Each
entry in the queue is a pointer to a particular process. Queue is implemented by using linked
list. Use of dispatcher is as follows. When a process is interrupted, that process is
transferred in the waiting queue. If the process has completed or aborted, the process is
discarded. In either case, the dispatcher then selects a process from the queue to execute.
13 | P a g e ndzoroland@gmail.com 677 20 00 53
Schedulers
Schedulers are special system software which handle process scheduling in various ways. Their
main task is to select the jobs to be submitted into the system and to decide which process to run.
Schedulers are of three types −
Long-Term Scheduler
Short-Term Scheduler
Medium-Term Scheduler
Long Term Scheduler
It is also called a job scheduler. A long-term scheduler determines which programs are admitted to
the system for processing. It selects processes from the queue and loads them into memory for
execution. Process loads into the memory for CPU scheduling.
The primary objective of the job scheduler is to provide a balanced mix of jobs, such as I/O bound
and processor bound. It also controls the degree of multiprogramming. If the degree of
multiprogramming is stable, then the average rate of process creation must be equal to the average
departure rate of processes leaving the system.
On some systems, the long-term scheduler may not be available or minimal. Time-sharing operating
systems have no long term scheduler. When a process changes the state from new to ready, then
there is use of long-term scheduler.
Short Term Scheduler
It is also called as CPU scheduler. Its main objective is to increase system performance in
accordance with the chosen set of criteria. It is the change of ready state to running state of the
process. CPU scheduler selects a process among the processes that are ready to execute and
allocates CPU to one of them.
Short-term schedulers, also known as dispatchers, make the decision of which process to execute
next. Short-term schedulers are faster than long-term schedulers.
Medium Term Scheduler
Medium-term scheduling is a part of swapping. It removes the processes from the memory. It
reduces the degree of multiprogramming. The medium-term scheduler is in-charge of handling the
swapped out-processes.
A running process may become suspended if it makes an I/O request. A suspended processes
cannot make any progress towards completion. In this condition, to remove the process from
memory and make space for other processes, the suspended process is moved to the secondary
storage. This process is called swapping, and the process is said to be swapped out or rolled out.
Swapping may be necessary to improve the process mix.
Comparison among Scheduler
S.N. Long-Term Scheduler Short-Term Scheduler Medium-Term Scheduler
2 Speed is lesser than short Speed is fastest among Speed is in between both short and
term scheduler other two long term scheduler.
14 | P a g e ndzoroland@gmail.com 677 20 00 53
3 It controls the degree of It provides lesser control It reduces the degree of
multiprogramming over degree of multiprogramming.
multiprogramming
5 It selects processes from It selects those It can re-introduce the process into
pool and loads them into processes which are memory and execution can be
memory for execution ready to execute continued.
Context Switch
A context switch is the mechanism to store and restore the state or context of a CPU in Process
Control block so that a process execution can be resumed from the same point at a later time. Using
this technique, a context switcher enables multiple processes to share a single CPU. Context
switching is an essential part of a multitasking operating system features.
When the scheduler switches the CPU from executing one process to execute another, the state
from the current running process is stored into the process control block. After this, the state for the
process to run next is loaded from its own PCB and used to set the PC, registers, etc. At that point,
the second process can start executing.
15 | P a g e ndzoroland@gmail.com 677 20 00 53
Context switches are computationally intensive since register and memory state must be saved and
restored. To avoid the amount of context switching time, some hardware systems employ two or
more sets of processor registers. When the process is switched, the following information is stored
for later use.
Program Counter
Scheduling information
Base and limit register value
Currently used register
Changed State
I/O State information
Accounting information
A Process Scheduler schedules different processes to be assigned to the CPU based on particular
scheduling algorithms. There are six popular process scheduling algorithms which we are going to
discuss in this chapter −
First-Come, First-Served (FCFS) Scheduling
Shortest-Job-Next (SJN) Scheduling
Priority Scheduling
Shortest Remaining Time
16 | P a g e ndzoroland@gmail.com 677 20 00 53
Round Robin(RR) Scheduling
Multiple-Level Queues Scheduling
These algorithms are either non-preemptive or preemptive. Non-preemptive algorithms are
designed so that once a process enters the running state, it cannot be preempted until it completes
its allotted time, whereas the preemptive scheduling is based on priority where a scheduler may
preempt a low priority running process anytime when a high priority process enters into a ready
state.
First Come First Serve (FCFS)
Jobs are executed on first come, first serve basis.
It is a non-preemptive, pre-emptive scheduling algorithm.
Easy to understand and implement.
Its implementation is based on FIFO queue.
Poor in performance as average wait time is high.
P0 0-0=0
P1 5-1=4
P2 8-2=6
P3 16 - 3 = 13
Average Wait Time: (0+4+6+13) / 4 = 5.75
Shortest Job Next (SJN)
This is also known as shortest job first, or SJF
This is a non-preemptive, pre-emptive scheduling algorithm.
Best approach to minimize waiting time.
Easy to implement in Batch systems where required CPU time is known in advance.
17 | P a g e ndzoroland@gmail.com 677 20 00 53
Impossible to implement in interactive systems where required CPU time is not known.
The processer should know in advance how much time process will take.
Given: Table of processes, and their Arrival time, Execution time
Process Arrival Time Execution Time Service Time
P0 0 5 0
P1 1 3 5
P2 2 8 14
P3 3 6 8
P0 0-0=0
P1 5-1=4
P2 14 - 2 = 12
P3 8-3=5
Average Wait Time: (0 + 4 + 12 + 5)/4 = 21 / 4 = 5.25
Priority Based Scheduling
Priority scheduling is a non-preemptive algorithm and one of the most common scheduling
algorithms in batch systems.
Each process is assigned a priority. Process with highest priority is to be executed first and so
on.
Processes with same priority are executed on first come first served basis.
Priority can be decided based on memory requirements, time requirements or any other
resource requirement.
Given: Table of processes, and their Arrival time, Execution time, and priority. Here we are
considering 1 is the lowest priority.
Process Arrival Time Execution Time Priority Service Time
P0 0 5 1 0
P1 1 3 2 11
P2 2 8 1 14
P3 3 6 3 5
18 | P a g e ndzoroland@gmail.com 677 20 00 53
Process Waiting Time
P0 0-0=0
P1 11 - 1 = 10
P2 14 - 2 = 12
P3 5-3=2
Average Wait Time: (0 + 10 + 12 + 2)/4 = 24 / 4 = 6
Shortest Remaining Time
Shortest remaining time (SRT) is the preemptive version of the SJN algorithm.
The processor is allocated to the job closest to completion but it can be preempted by a newer
ready job with shorter time to completion.
Impossible to implement in interactive systems where required CPU time is not known.
It is often used in batch environments where short jobs need to give preference.
Round Robin Scheduling
Round Robin is the preemptive process scheduling algorithm.
Each process is provided a fix time to execute, it is called a quantum.
Once a process is executed for a given time period, it is preempted and other process
executes for a given time period.
Context switching is used to save states of preempted processes.
P0 (0 - 0) + (12 - 3) = 9
P1 (3 - 1) = 2
P3 (9 - 3) + (17 - 12) = 11
Average Wait Time: (9+2+12+11) / 4 = 8.5
Multiple-Level Queues Scheduling
Multiple-level queues are not an independent scheduling algorithm. They make use of other existing
algorithms to group and schedule jobs with common characteristics.
Multiple queues are maintained for processes with common characteristics.
19 | P a g e ndzoroland@gmail.com 677 20 00 53
Each queue can have its own scheduling algorithms.
Priorities are assigned to each queue.
For example, CPU-bound jobs can be scheduled in one queue and all I/O-bound jobs in another
queue. The Process Scheduler then alternately selects jobs from each queue and assigns them to
the CPU based on the algorithm assigned to the queue.
What is Thread?
A thread is a flow of execution through the process code, with its own program counter that keeps
track of which instruction to execute next, system registers which hold its current working variables,
and a stack which contains the execution history.
A thread shares with its peer threads few information like code segment, data segment and open
files. When one thread alters a code segment memory item, all other threads see that.
A thread is also called a lightweight process. Threads provide a way to improve application
performance through parallelism. Threads represent a software approach to improving performance
of operating system by reducing the overhead thread is equivalent to a classical process.
Each thread belongs to exactly one process and no thread can exist outside a process. Each thread
represents a separate flow of control. Threads have been successfully used in implementing network
servers and web server. They also provide a suitable foundation for parallel execution of applications
on shared memory multiprocessors. The following figure shows the working of a single-threaded and
a multithreaded process.
1 Process is heavy weight or resource Thread is light weight, taking lesser resources than a
intensive. process.
2 Process switching needs interaction with Thread switching does not need to interact with
operating system. operating system.
3 In multiple processing environments, each All threads can share same set of open files, child
process executes the same code but has its processes.
own memory and file resources.
4 If one process is blocked, then no other While one thread is blocked and waiting, a second
process can execute until the first process is thread in the same task can run.
20 | P a g e ndzoroland@gmail.com 677 20 00 53
unblocked.
5 Multiple processes without using threads use Multiple threaded processes use fewer resources.
more resources.
6 In multiple processes each process operates One thread can read, write or change another thread's
independently of the others. data.
Advantages of Thread
Threads minimize the context switching time.
Use of threads provides concurrency within a process.
Efficient communication.
It is more economical to create and context switch threads.
Threads allow utilization of multiprocessor architectures to a greater scale and efficiency.
Types of Thread
Threads are implemented in following two ways −
User Level Threads − User managed threads.
Kernel Level Threads − Operating System managed threads acting on kernel, an operating
system core.
User Level Threads
In this case, the thread management kernel is not aware of the existence of threads. The thread
library contains code for creating and destroying threads, for passing message and data between
threads, for scheduling thread execution and for saving and restoring thread contexts. The
application starts with a single thread.
Advantages
Thread switching does not require Kernel mode privileges.
User level thread can run on any operating system.
Scheduling can be application specific in the user level thread.
User level threads are fast to create and manage.
Disadvantages
In a typical operating system, most system calls are blocking.
Multithreaded application cannot take advantage of multiprocessing.
Kernel Level Threads
In this case, thread management is done by the Kernel. There is no thread management code in the
application area. Kernel threads are supported directly by the operating system. Any application can
be programmed to be multithreaded. All of the threads within an application are supported within a
single process.
The Kernel maintains context information for the process as a whole and for individuals threads
within the process. Scheduling by the Kernel is done on a thread basis. The Kernel performs thread
21 | P a g e ndzoroland@gmail.com 677 20 00 53
creation, scheduling and management in Kernel space. Kernel threads are generally slower to create
and manage than the user threads.
Advantages
Kernel can simultaneously schedule multiple threads from the same process on multiple processes.
If one thread in a process is blocked, the Kernel can schedule another thread of the same process.
Kernel routines themselves can be multithreaded.
Disadvantages
Kernel threads are generally slower to create and manage than the user threads.
Transfer of control from one thread to another within the same process requires a mode switch to the
Kernel.
Multithreading Models
Some operating system provide a combined user level thread and Kernel level thread facility. Solaris
is a good example of this combined approach. In a combined system, multiple threads within the
same application can run in parallel on multiple processors and a blocking system call need not
block the entire process. Multithreading models are three types
Many to many relationship.
Many to one relationship.
One to one relationship.
Many to Many Model
The many-to-many model multiplexes any number of user threads onto an equal or smaller number
of kernel threads.
The following diagram shows the many-to-many threading model where 6 user level threads are
multiplexing with 6 kernel level threads. In this model, developers can create as many user threads
as necessary and the corresponding Kernel threads can run in parallel on a multiprocessor machine.
This model provides the best accuracy on concurrency and when a thread performs a blocking
system call, the kernel can schedule another thread for execution.
22 | P a g e ndzoroland@gmail.com 677 20 00 53
If the user-level thread libraries are implemented in the operating system in such a way that the
system does not support them, then the Kernel threads use the many-to-one relationship modes.
1 User-level threads are faster to create and Kernel-level threads are slower to
manage. create and manage.
23 | P a g e ndzoroland@gmail.com 677 20 00 53
3 User-level thread is generic and can run on any Kernel-level thread is specific to the
operating system. operating system.
1 Symbolic addresses
The addresses used in a source code. The variable names, constants, and
instruction labels are the basic elements of the symbolic address space.
2 Relative addresses
At the time of compilation, a compiler converts symbolic addresses into relative
addresses.
3 Physical addresses
The loader generates these addresses at the time when a program is loaded into
main memory.
Virtual and physical addresses are the same in compile-time and load-time address-binding
schemes. Virtual and physical addresses differ in execution-time address-binding scheme.
The set of all logical addresses generated by a program is referred to as a logical address space.
The set of all physical addresses corresponding to these logical addresses is referred to as
a physical address space.
The runtime mapping from virtual to physical address is done by the memory management unit
(MMU) which is a hardware device. MMU uses following mechanism to convert virtual address to
physical address.
The value in the base register is added to every address generated by a user process, which
is treated as offset at the time it is sent to memory. For example, if the base register value is
10000, then an attempt by the user to use address location 100 will be dynamically
reallocated to location 10100.
24 | P a g e ndzoroland@gmail.com 677 20 00 53
The user program deals with virtual addresses; it never sees the real physical addresses.
Static vs Dynamic Loading
The choice between Static or Dynamic Loading is to be made at the time of computer program being
developed. If you have to load your program statically, then at the time of compilation, the complete
programs will be compiled and linked without leaving any external program or module dependency.
The linker combines the object program with other necessary object modules into an absolute
program, which also includes logical addresses.
If you are writing a Dynamically loaded program, then your compiler will compile the program and for
all the modules which you want to include dynamically, only references will be provided and rest of
the work will be done at the time of execution.
At the time of loading, with static loading, the absolute program (and data) is loaded into memory in
order for execution to start.
If you are using dynamic loading, dynamic routines of the library are stored on a disk in relocatable
form and are loaded into memory only when they are needed by the program.
Static vs Dynamic Linking
As explained above, when static linking is used, the linker combines all other modules needed by a
program into a single executable program to avoid any runtime dependency.
When dynamic linking is used, it is not required to link the actual module or library with the program,
rather a reference to the dynamic module is provided at the time of compilation and linking. Dynamic
Link Libraries (DLL) in Windows and Shared Objects in Unix are good examples of dynamic libraries.
Swapping
Swapping is a mechanism in which a process can be swapped temporarily out of main memory (or
move) to secondary storage (disk) and make that memory available to other processes. At some
later time, the system swaps back the process from the secondary storage to main memory.
Though performance is usually affected by swapping process but it helps in running multiple and big
processes in parallel and that's the reason Swapping is also known as a technique for memory
compaction.
25 | P a g e ndzoroland@gmail.com 677 20 00 53
The total time taken by swapping process includes the time it takes to move the entire process to a
secondary disk and then to copy the process back to memory, as well as the time the process takes
to regain main memory.
Let us assume that the user process is of size 2048KB and on a standard hard disk where swapping
will take place has a data transfer rate around 1 MB per second. The actual transfer of the 1000K
process to or from memory will take
2048KB / 1024KB per second
= 2 seconds
= 2000 milliseconds
Now considering in and out time, it will take complete 4000 milliseconds plus other overhead where
the process competes to regain main memory.
Memory Allocation
Main memory usually has two partitions −
Low Memory − Operating system resides in this memory.
High Memory − User processes are held in high memory.
1 Single-partition allocation
In this type of allocation, relocation-register scheme is used to protect user
26 | P a g e ndzoroland@gmail.com 677 20 00 53
processes from each other, and from changing operating-system code and data.
Relocation register contains value of smallest physical address whereas limit
register contains range of logical addresses. Each logical address must be less than
the limit register.
2 Multiple-partition allocation
In this type of allocation, main memory is divided into a number of fixed-sized
partitions where each partition should contain only one process. When a partition is
free, a process is selected from the input queue and is loaded into the free partition.
When the process terminates, the partition becomes available for another process.
Fragmentation
As processes are loaded and removed from memory, the free memory space is broken into little
pieces. It happens after sometimes that processes cannot be allocated to memory blocks
considering their small size and memory blocks remains unused. This problem is known as
Fragmentation.
Fragmentation is of two types −
S.N. Fragmentation & Description
1 External fragmentation
Total memory space is enough to satisfy a request or to reside a process in it, but it
is not contiguous, so it cannot be used.
2 Internal fragmentation
Memory block assigned to process is bigger. Some portion of memory is left
unused, as it cannot be used by another process.
The following diagram shows how fragmentation can cause waste of memory and a compaction
technique can be used to create more free memory out of fragmented memory −
External fragmentation can be reduced by compaction or shuffle memory contents to place all free
memory together in one large block. To make compaction feasible, relocation should be dynamic.
27 | P a g e ndzoroland@gmail.com 677 20 00 53
The internal fragmentation can be reduced by effectively assigning the smallest partition but large
enough for the process.
Paging
A computer can address more memory than the amount physically installed on the system. This
extra memory is actually called virtual memory and it is a section of a hard that's set up to emulate
the computer's RAM. Paging technique plays an important role in implementing virtual memory.
Paging is a memory management technique in which process address space is broken into blocks of
the same size called pages (size is power of 2, between 512 bytes and 8192 bytes). The size of the
process is measured in the number of pages.
Similarly, main memory is divided into small fixed-sized blocks of (physical) memory
called frames and the size of a frame is kept the same as that of a page to have optimum utilization
of the main memory and to avoid external fragmentation.
Address Translation
Page address is called logical address and represented by page number and the offset.
Logical Address = Page number + page offset
Frame address is called physical address and represented by a frame number and the offset.
Physical Address = Frame number + page offset
A data structure called page map table is used to keep track of the relation between a page of a
process to a frame in physical memory.
28 | P a g e ndzoroland@gmail.com 677 20 00 53
When the system allocates a frame to any page, it translates this logical address into a physical
address and create entry into the page table to be used throughout execution of the program.
When a process is to be executed, its corresponding pages are loaded into any available memory
frames. Suppose you have a program of 8Kb but your memory can accommodate only 5Kb at a
given point in time, then the paging concept will come into picture. When a computer runs out of
RAM, the operating system (OS) will move idle or unwanted pages of memory to secondary memory
to free up RAM for other processes and brings them back when needed by the program.
This process continues during the whole execution of the program where the OS keeps removing
idle pages from the main memory and write them onto the secondary memory and bring them back
when required by the program.
Advantages and Disadvantages of Paging
Here is a list of advantages and disadvantages of paging −
Paging reduces external fragmentation, but still suffer from internal fragmentation.
Paging is simple to implement and assumed as an efficient memory management technique.
Due to equal size of the pages and frames, swapping becomes very easy.
Page table requires extra memory space, so may not be good for a system having small RAM.
Segmentation
Segmentation is a memory management technique in which each job is divided into several
segments of different sizes, one for each module that contains pieces that perform related functions.
Each segment is actually a different logical address space of the program.
When a process is to be executed, its corresponding segmentation are loaded into non-contiguous
memory though every segment is loaded into a contiguous block of available memory.
Segmentation memory management works very similar to paging but here segments are of variable-
length where as in paging pages are of fixed size.
A program segment contains the program's main function, utility functions, data structures, and so
on. The operating system maintains a segment map table for every process and a list of free
memory blocks along with segment numbers, their size and corresponding memory locations in main
memory. For each segment, the table stores the starting address of the segment and the length of
29 | P a g e ndzoroland@gmail.com 677 20 00 53
the segment. A reference to a memory location includes a value that identifies a segment and an
offset.
A computer can address more memory than the amount physically installed on the system. This
extra memory is actually called virtual memory and it is a section of a hard disk that's set up to
emulate the computer's RAM.
The main visible advantage of this scheme is that programs can be larger than physical memory.
Virtual memory serves two purposes. First, it allows us to extend the use of physical memory by
using disk. Second, it allows us to have memory protection, because each virtual address is
translated to a physical address.
Following are the situations, when entire program is not required to be loaded fully in main memory.
User written error handling routines are used only when an error occurred in the data or
computation.
Certain options and features of a program may be used rarely.
Many tables are assigned a fixed amount of address space even though only a small amount
of the table is actually used.
The ability to execute a program that is only partially in memory would counter many benefits.
Less number of I/O would be needed to load or swap each user program into memory.
A program would no longer be constrained by the amount of physical memory that is
available.
Each user program could take less physical memory, more programs could be run the same
time, with a corresponding increase in CPU utilization and throughput.
Modern microprocessors intended for general-purpose use, a memory management unit, or MMU, is
built into the hardware. The MMU's job is to translate virtual addresses into physical addresses. A
basic example is given below −
30 | P a g e ndzoroland@gmail.com 677 20 00 53
Virtual memory is commonly implemented by demand paging. It can also be implemented in a
segmentation system. Demand segmentation can also be used to provide virtual memory.
Demand Paging
A demand paging system is quite similar to a paging system with swapping where processes reside
in secondary memory and pages are loaded only on demand, not in advance. When a context switch
occurs, the operating system does not copy any of the old program’s pages out to the disk or any of
the new program’s pages into the main memory Instead, it just begins executing the new program
after loading the first page and fetches that program’s pages as they are referenced.
While executing a program, if the program references a page which is not available in the main
memory because it was swapped out a little ago, the processor treats this invalid memory reference
as a page fault and transfers control from the program to the operating system to demand the page
back into the memory.
Advantages
Following are the advantages of Demand Paging −
Large virtual memory.
More efficient use of memory.
31 | P a g e ndzoroland@gmail.com 677 20 00 53
There is no limit on degree of multiprogramming.
Disadvantages
Number of tables and the amount of processor overhead for handling page interrupts are
greater than in the case of the simple paged management techniques.
Page Replacement Algorithm
Page replacement algorithms are the techniques using which an Operating System decides which
memory pages to swap out, write to disk when a page of memory needs to be allocated. Paging
happens whenever a page fault occurs and a free page cannot be used for allocation purpose
accounting to reason that pages are not available or the number of free pages is lower than required
pages.
When the page that was selected for replacement and was paged out, is referenced again, it has to
read in from disk, and this requires for I/O completion. This process determines the quality of the
page replacement algorithm: the lesser the time waiting for page-ins, the better is the algorithm.
A page replacement algorithm looks at the limited information about accessing the pages provided
by hardware, and tries to select which pages should be replaced to minimize the total number of
page misses, while balancing it with the costs of primary storage and processor time of the algorithm
itself. There are many different page replacement algorithms. We evaluate an algorithm by running it
on a particular string of memory reference and computing the number of page faults,
Reference String
The string of memory references is called reference string. Reference strings are generated
artificially or by tracing a given system and recording the address of each memory reference. The
latter choice produces a large number of data, where we note two things.
For a given page size, we need to consider only the page number, not the entire address.
If we have a reference to a page p, then any immediately following references to page p will
never cause a page fault. Page p will be in memory after the first reference; the immediately
following references will not fault.
For example, consider the following sequence of addresses − 123,215,600,1234,76,96
If page size is 100, then the reference string is 1,2,6,12,0,0
First In First Out (FIFO) algorithm
Oldest page in main memory is the one which will be selected for replacement.
Easy to implement, keep a list, replace pages from the tail and add new pages at the head.
33 | P a g e ndzoroland@gmail.com 677 20 00 53
One of the important jobs of an Operating System is to manage various I/O devices including mouse,
keyboards, touch pad, disk drives, display adapters, USB devices, Bit-mapped screen, LED, Analog-
to-digital converter, On/off switch, network connections, audio I/O, printers etc.
An I/O system is required to take an application I/O request and send it to the physical device, then
take whatever response comes back from the device and send it to the application. I/O devices can
be divided into two categories −
Block devices − A block device is one with which the driver communicates by sending entire
blocks of data. For example, Hard disks, USB cameras, Disk-On-Key etc.
Character devices − A character device is one with which the driver communicates by
sending and receiving single characters (bytes, octets). For example, serial ports, parallel
ports, sounds cards etc
Device Controllers
Device drivers are software modules that can be plugged into an OS to handle a particular device.
Operating System takes help from device drivers to handle all I/O devices.
The Device Controller works like an interface between a device and a device driver. I/O units
(Keyboard, mouse, printer, etc.) typically consist of a mechanical component and an electronic
component where electronic component is called the device controller.
There is always a device controller and a device driver for each device to communicate with the
Operating Systems. A device controller may be able to handle multiple devices. As an interface its
main task is to convert serial bit stream to block of bytes, perform error correction as necessary.
Any device connected to the computer is connected by a plug and socket, and the socket is
connected to a device controller. Following is a model for connecting the CPU, memory, controllers,
and I/O devices where CPU and device controllers all use a common bus for communication.
34 | P a g e ndzoroland@gmail.com 677 20 00 53
Memory-mapped I/O
When using memory-mapped I/O, the same address space is shared by memory and I/O devices.
The device is connected directly to certain main memory locations so that I/O device can transfer
block of data to/from memory without going through CPU.
While using memory mapped IO, OS allocates buffer in memory and informs I/O device to use that
buffer to send data to the CPU. I/O device operates asynchronously with CPU, interrupts CPU when
finished.
The advantage to this method is that every instruction which can access memory can be used to
manipulate an I/O device. Memory mapped IO is used for most high-speed I/O devices like disks,
communication interfaces.
Direct Memory Access (DMA)
Slow devices like keyboards will generate an interrupt to the main CPU after each byte is
transferred. If a fast device such as a disk generated an interrupt for each byte, the operating system
would spend most of its time handling these interrupts. So a typical computer uses direct memory
access (DMA) hardware to reduce this overhead.
Direct Memory Access (DMA) means CPU grants I/O module authority to read from or write to
memory without involvement. DMA module itself controls exchange of data between main memory
and the I/O device. CPU is only involved at the beginning and end of the transfer and interrupted
only after entire block has been transferred.
Direct Memory Access needs a special hardware called DMA controller (DMAC) that manages the
data transfers and arbitrates access to the system bus. The controllers are programmed with source
and destination pointers (where to read/write the data), counters to track the number of transferred
bytes, and settings, which includes I/O and memory types, interrupts and states for the CPU cycles.
35 | P a g e ndzoroland@gmail.com 677 20 00 53
Step Description
5 DMA controller transfers bytes to buffer, increases the memory address, decreases the
counter C until C becomes zero.
36 | P a g e ndzoroland@gmail.com 677 20 00 53
disk, on a hard disk, or on a CD-ROM, without having to modify the program for each different
device.
Device Drivers
Device drivers are software modules that can be plugged into an OS to handle a particular device.
Operating System takes help from device drivers to handle all I/O devices. Device drivers
encapsulate device-dependent code and implement a standard interface in such a way that code
contains device-specific register reads/writes. Device driver, is generally written by the device's
manufacturer and delivered along with the device on a CD-ROM.
A device driver performs the following jobs −
To accept request from the device independent software above to it.
Interact with the device controller to take and give I/O and perform required error handling
Making sure that the request is executed successfully
How a device driver handles a request is as follows: Suppose a request comes to read a block N. If
the driver is idle at the time a request arrives, it starts carrying out the request immediately.
Otherwise, if the driver is already busy with some other request, it places the new request in the
queue of pending requests.
Interrupt handlers
An interrupt handler, also known as an interrupt service routine or ISR, is a piece of software or more
specifically a callback function in an operating system or more specifically in a device driver, whose
execution is triggered by the reception of an interrupt.
When the interrupt happens, the interrupt procedure does whatever it has to in order to handle the
interrupt, updates data structures and wakes up process that was waiting for an interrupt to happen.
The interrupt mechanism accepts an address ─ a number that selects a specific interrupt handling
routine/function from a small set. In most architectures, this address is an offset stored in a table
called the interrupt vector table. This vector contains the memory addresses of specialized interrupt
handlers.
Device-Independent I/O Software
The basic function of the device-independent software is to perform the I/O functions that are
common to all devices and to provide a uniform interface to the user-level software. Though it is
difficult to write completely device independent software but we can write some modules which are
common among all the devices. Following is a list of functions of device-independent I/O Software −
Uniform interfacing for device drivers
Device naming - Mnemonic names mapped to Major and Minor device numbers
37 | P a g e ndzoroland@gmail.com 677 20 00 53
Device protection
Providing a device-independent block size
Buffering because data coming off a device cannot be stored in final destination.
Storage allocation on block devices
Allocation and releasing dedicated devices
Error Reporting
User-Space I/O Software
These are the libraries which provide richer and simplified interface to access the functionality of the
kernel or ultimately interactive with the device drivers. Most of the user-level I/O software consists of
library procedures with some exception like spooling system which is a way of dealing with dedicated
I/O devices in a multiprogramming system.
I/O Libraries (e.g., stdio) are in user-space to provide an interface to the OS resident device-
independent I/O SW. For example putchar(), getchar(), printf() and scanf() are example of user level
I/O library stdio available in C programming.
Kernel I/O Subsystem
Kernel I/O Subsystem is responsible to provide many services related to I/O. Following are some of
the services provided.
Scheduling − Kernel schedules a set of I/O requests to determine a good order in which to
execute them. When an application issues a blocking I/O system call, the request is placed on
the queue for that device. The Kernel I/O scheduler rearranges the order of the queue to
improve the overall system efficiency and the average response time experienced by the
applications.
Buffering − Kernel I/O Subsystem maintains a memory area known as buffer that stores data
while they are transferred between two devices or between a device with an application
operation. Buffering is done to cope with a speed mismatch between the producer and
consumer of a data stream or to adapt between devices that have different data transfer
sizes.
Caching − Kernel maintains cache memory which is region of fast memory that holds copies
of data. Access to the cached copy is more efficient than access to the original.
Spooling and Device Reservation − A spool is a buffer that holds output for a device, such
as a printer, that cannot accept interleaved data streams. The spooling system copies the
queued spool files to the printer one at a time. In some operating systems, spooling is
managed by a system daemon process. In other operating systems, it is handled by an in
kernel thread.
Error Handling − An operating system that uses protected memory can guard against many
kinds of hardware and application errors.
File
A file is a named collection of related information that is recorded on secondary storage such as
magnetic disks, magnetic tapes and optical disks. In general, a file is a sequence of bits, bytes, lines
or records whose meaning is defined by the files creator and user.
File Structure
A File Structure should be according to a required format that the operating system can understand.
A file has a certain defined structure according to its type.
A text file is a sequence of characters organized into lines.
A source file is a sequence of procedures and functions.
An object file is a sequence of bytes organized into blocks that are understandable by the
machine.
38 | P a g e ndzoroland@gmail.com 677 20 00 53
When operating system defines different file structures, it also contains the code to support
these file structure. Unix, MS-DOS support minimum number of file structure.
File Type
File type refers to the ability of the operating system to distinguish different types of file such as text
files source files and binary files etc. Many operating systems support many types of files. Operating
system like MS-DOS and UNIX have the following types of files −
Ordinary files
These are the files that contain user information.
These may have text, databases or executable program.
The user can apply various operations on such files like add, modify, delete or even remove the entire
file.
Directory files
These files contain list of file names and other information related to these files.
Special files
These files are also known as device files.
These files represent physical device like disks, terminals, printers, networks, tape drive etc.
These files are of two types −
Character special files − data is handled character by character as in case of terminals or
printers.
Block special files − data is handled in blocks as in the case of disks and tapes.
File Access Mechanisms
File access mechanism refers to the manner in which the records of a file may be accessed. There
are several ways to access files −
Sequential access
Direct/Random access
Indexed sequential access
Sequential access
A sequential access is that in which the records are accessed in some sequence, i.e., the
information in the file is processed in order, one record after the other. This access method is the
most primitive one. Example: Compilers usually access files in this fashion.
Direct/Random access
Random access file organization provides, accessing the records directly.
Each record has its own address on the file with by the help of which it can be directly
accessed for reading or writing.
The records need not be in any sequence within the file and they need not be in adjacent
locations on the storage medium.
Indexed sequential access
This mechanism is built up on base of sequential access.
An index is created for each file which contains pointers to various blocks.
Index is searched sequentially and its pointer is used to access the file directly.
Space Allocation
Files are allocated disk spaces by operating system. Operating systems deploy following three main
ways to allocate disk space to files.
Contiguous Allocation
Linked Allocation
Indexed Allocation
Contiguous Allocation
Each file occupies a contiguous address space on disk.
Assigned disk address is in linear order.
39 | P a g e ndzoroland@gmail.com 677 20 00 53
Easy to implement.
External fragmentation is a major issue with this type of allocation technique.
Linked Allocation
Each file carries a list of links to disk blocks.
Directory contains link / pointer to first block of a file.
No external fragmentation
Effectively used in sequential access file.
Inefficient in case of direct access file.
Indexed Allocation
Provides solutions to problems of contiguous and linked allocation.
A index block is created having all pointers to files.
Each file has its own index block which stores the addresses of disk space occupied by the file.
Directory contains the addresses of index blocks of files.
Security refers to providing a protection system to computer system resources such as CPU, memory, disk,
software programs and most importantly data/information stored in the computer system. If a computer
program is run by an unauthorized user, then he/she may cause severe damage to computer or data stored in
it. So a computer system must be protected against unauthorized access, malicious access to system
memory, viruses, worms etc. We're going to discuss following topics in this chapter.
Authentication
One Time passwords
Program Threats
System Threats
Computer Security Classifications
Authentication
Authentication refers to identifying each user of the system and associating the executing programs
with those users. It is the responsibility of the Operating System to create a protection system which
ensures that a user who is running a particular program is authentic. Operating Systems generally
identifies/authenticates users using following three ways −
Username / Password − User need to enter a registered username and password with
Operating system to login into the system.
User card/key − User need to punch card in card slot, or enter key generated by key
generator in option provided by operating system to login into the system.
User attribute - fingerprint/ eye retina pattern/ signature − User need to pass his/her
attribute via designated input device used by operating system to login into the system.
One Time passwords
One-time passwords provide additional security along with normal authentication. In One-Time
Password system, a unique password is required every time user tries to login into the system. Once
a one-time password is used, then it cannot be used again. One-time password are implemented in
various ways.
Random numbers − Users are provided cards having numbers printed along with
corresponding alphabets. System asks for numbers corresponding to few alphabets randomly
chosen.
Secret key − User are provided a hardware device which can create a secret id mapped with
user id. System asks for such secret id which is to be generated every time prior to login.
Network password − Some commercial applications send one-time passwords to user on
registered mobile/ email which is required to be entered prior to login.
Program Threats
Operating system's processes and kernel do the designated task as instructed. If a user program made these process do
malicious tasks, then it is known as Program Threats. One of the common example of program threat is a program
installed in a computer which can store and send user credentials via network to some hacker. Following is the list of
some well-known program threats.
40 | P a g e ndzoroland@gmail.com 677 20 00 53
Trojan Horse − Such program traps user login credentials and stores them to send to malicious user who can
later on login to computer and can access system resources.
Trap Door − If a program which is designed to work as required, have a security hole in its code and perform
illegal action without knowledge of user then it is called to have a trap door.
Logic Bomb − Logic bomb is a situation when a program misbehaves only when certain conditions met
otherwise it works as a genuine program. It is harder to detect.
Virus − Virus as name suggest can replicate themselves on computer system. They are highly dangerous and
can modify/delete user files, crash systems. A virus is generatlly a small code embedded in a program. As user
accesses the program, the virus starts getting embedded in other files/ programs and can make system
unusable for user
System Threats
System threats refers to misuse of system services and network connections to put user in trouble. System threats can
be used to launch program threats on a complete network called as program attack. System threats creates such an
environment that operating system resources/ user files are misused. Following is the list of some well-known system
threats.
Worm − Worm is a process which can choked down a system performance by using system resources to
extreme levels. A Worm process generates its multiple copies where each copy uses system resources,
prevents all other processes to get required resources. Worms processes can even shut down an entire network.
Port Scanning − Port scanning is a mechanism or means by which a hacker can detects system vulnerabilities
to make an attack on the system.
Denial of Service − Denial of service attacks normally prevents user to make legitimate use of the system. For
example, a user may not be able to use internet if denial of service attacks browser's content settings.
Computer Security Classifications
As per the U.S. Department of Defense Trusted Computer System's Evaluation Criteria there are four security
classifications in computer systems: A, B, C, and D. This is widely used specifications to determine and model
the security of systems and of security solutions. Following is the brief description of each classification.
S.N. Classification Type & Description
1 Type A
Highest Level. Uses formal design specifications and verification techniques. Grants a high degree of
assurance of process security.
2 Type B
Provides mandatory protection system. Have all the properties of a class C2 system. Attaches a
sensitivity label to each object. It is of three types.
B1 − Maintains the security label of each object in the system. Label is used for making
decisions to access control.
B2 − Extends the sensitivity labels to each system resource, such as storage objects, supports
covert channels and auditing of events.
B3 − Allows creating lists or user groups for access-control to grant access or revoke access to
a given named object.
3 Type C
Provides protection and user accountability using audit capabilities. It is of two types.
C1 − Incorporates controls so that users can protect their private information and keep other
users from accidentally reading / deleting their data. UNIX versions are mostly Cl class.
C2 − Adds an individual-level access control to the capabilities of a Cl level system.
4 Type D
Lowest level. Minimum protection. MS-DOS, Window 3.1 fall in this category.
41 | P a g e ndzoroland@gmail.com 677 20 00 53
Linux is one of popular version of UNIX operating System. It is open source as its source code is freely
available. It is free to use. Linux was designed considering UNIX compatibility. Its functionality list is quite
similar to that of UNIX.
Components of Linux System
Linux Operating System has primarily three components
Kernel − Kernel is the core part of Linux. It is responsible for all major activities of this operating
system. It consists of various modules and it interacts directly with the underlying hardware. Kernel
provides the required abstraction to hide low level hardware details to system or application programs.
System Library − System libraries are special functions or programs using which application
programs or system utilities accesses Kernel's features. These libraries implement most of the
functionalities of the operating system and do not requires kernel module's code access rights.
System Utility − System Utility programs are responsible to do specialized, individual level tasks.
42 | P a g e ndzoroland@gmail.com 677 20 00 53
The architecture of a Linux System consists of the following layers −
Hardware layer − Hardware consists of all peripheral devices (RAM/ HDD/ CPU etc).
Kernel − It is the core component of Operating System, interacts directly with hardware, provides low
level services to upper layer components.
Shell − An interface to kernel, hiding complexity of kernel's functions from users. The shell takes
commands from the user and executes kernel's functions.
Utilities − Utility programs that provide the user most of the functionalities of an operating systems.
43 | P a g e ndzoroland@gmail.com 677 20 00 53