Operating Systems Theory c1

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OPERATING SYSTEMSTHEORY

LECTURE NOTES

Chapter 1OPERATING SYSTEMS OVERVIEW


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TEXT BOOK

Operating Systems Theory

by Dr G O Ofori-Dwumfuo

Adwinsah Publications, 2007


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Other Literature

Any good operating systems book

Internet

Etc.


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OPERATING SYSTEM OVERVIEW

Computer systems provide for gathering

data, performing computations, storing

information, communicating with other

computer systems, and generating outputreports. Some of those are implemented in

hardware, others in software.

An operating system is the software thatmanages hardware resources, provides

services for accessing those resources, such

as creation of files, directories, processes,

etc.


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Without OS, Computers are just machines

ApplicationPackages

Operating

System

BIOS

Software


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A typical computer system (Fig 1.1):

The hardware does all the actual work and includes thememory, the central processing unit (CPU), and theinput and output (I/O) devices.

The OS provides a set of services to programs.

The user interacts with the operating system indirectly,through the programs.

Systems programs are a set of utility programs suppliedwith an operating system to provide basic services forusers. Eg. window manager for a graphical user

interface, a command interpreter, and programs torename, copy or delete files.

Application programs provide the computer with thefunctionality the users require. Eg. tax preparationsoftware, a financial planner, a word processor,

spreadsheet.


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CPU Organization

The CPU is the heart and brain of a computer system.

It contains a number of special-purpose registers, an

arithmetic logic unit (ALU), and the control unit / logic

necessary to decode and execute instructions

Connected to the CPU by way of a communication

bus are the memory and the I/O devices.

The operation of the CPU is controlled by the

instructions the CPU fetches from memory.

The I/O devices, in turn, are commanded by the CPU.


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CPU operations

The operation of a CPU can be

described in terms of a simple loop

Each time through the loop one

instruction is executed.

This basic process by which instructions

are executed never varies.

It is called the Fetch-and-execute

operation


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Fetch and execute operations

An instruction is fetched from the memory location specifiedby program counter. (All instructions are fetched frommain memory.)

The instruction is placed in a special register called the

instruction register. The program counter is incremented so it points to the nextinstruction to be executed.

The instruction is decoded to determine what action is to beperformed. The action is specified by the instructions op-

code (operation code) bits. Depending on the operation to be performed, the value of

one or more operands is fetched from memory.

The operation specified in the op-code is performed (seebelow).

If required, a value is stored back into main memory.


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Op-code operations

Movement: Move a value from one location to another(registers or memory locations).

Computation: Send one or more operand values to theALU and have a computation performed.

Conditional Branch: If the branch condition is true, reset

the program counter to point to the branch address. Procedure Call: Save the current value of the program

counter. Then reset the program counter to point to thebeginning of a procedure. At the end of the procedure, abranch instruction specifying the saved program counterwill allow the program to return to the current point ofexecution. (The saved program counter may be stored ina register, in memory, or on the stack.)

Input/Output: Transfer information concerning an input oroutput operation between the CPU and the I/O device.


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Traps and Interrupts

These are events that disrupt thenormal sequence of instructionsexecuted by the CPU

A trap is an abnormal conditiondetected by the CPU that indicates anerror; e.g. dividing by zero,

trying to access a memory location that doesnot exist or for which the program does nothave access,

executing an instruction with an undefined op-code, or

trying to access a nonexistent I/O device.


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Traps and Interrupts

An interrupt is a signal sent to the CPUby an external device, typically an I/Odevice; to stop its current activities and

attend to the interrupting devicesneeds, eg, to perform a read operation.

(A CPU will, however, check interruptsonly after it has completed theprocessing of one instruction and beforeit fetches another.)


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Traps and Interrupts.

The CPU responds to traps and interrupts by

saving the current value of the program

counter and resetting the program counter to

a new address. The CPU then returns to executing at the

point the trap or interrupt occurred, after it has

handled the trap or interrupt.

Information saved when a trap or interrupt

occurs is stored in a special register

(program status word, PSW register).


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Traps and Interrupts..

One difference between trap & interrupts:

traps are said to be synchronous and

interrupts are asynchronous.

Given the same machine state and input

data, a trap will occur at the same point

of execution each time the program runs.

The occurrence of an interrupt, however,

depends on the relative timing between

the interrupting device and the CPU.


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Multimode Execution

To provide an OS with privileges not granted to

application programs, the hardware must support

multiple modes of execution. Two modes of execution

are usually supported: supervisor(orkernel) mode

and user mode. A system can enter kernel mode from user mode in

one of three ways.

Supervisor call or a system call (A special

instruction call - similar to a procedure call exceptthat it sets the systems state to kernel mode.

Traps

Interrupts


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Objectives of Operating Systems

To hide details of hardware by creating

abstraction An abstraction is software that hides the lower

level details and provides a set of higher-levelfunctions

To allocate resources to processes

Provide a pleasant and effective userinterface


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Operating System Structure

The OS provides applications with a virtual machine.

The OS supervisor calls expand the instruction set

provided by the raw hardware by supporting

processes, file systems, etc. In addition, the OS manages the underlying

hardware resources. Applications cannot access I/O

devices or execute privileged instructions directly.

The OS performs these tasks on their behalf. In doing so, the OS attempts to efficiently utilize the

resources available to it and protect the integrity of

the applications that must share those resources.


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Operating System Structure.

The tasks performed by an operating system

can be divided into four major areas.

Process Management

Memory Management

File System Management

Device Management


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Process Management

A process is an executing program.

Associated with a process are its code, itsdata, a set of resources allocated to it, and

one or more flows of execution through itscode.

The OS provides supervisor calls formanaging processes and must manage the

allocation of resources to processes. If multiple processes can exist

simultaneously, the OS must provide eachprocess with an appropriate virtual

environment in which to run


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Process Management

Five major activities of an operating system in

regard to process management are:

Creation and deletion of user and system

processes. Suspension and resumption of processes.

A mechanism for process synchronization.

A mechanism for process communication.A mechanism for deadlock handling.


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Memory Management

At a minimum, memory must be shared

by an application program and the OS.

On more sophisticated systems,

memory can be shared by a number of

processes.

The OS must manage the allocation of

memory to processes and control the

hardware that determines which

memory locations a process may

access.


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Memory Management

The major activities of an operating system in

regard to memory-management are:

Keep track of which part of memory are

currently being used and by whom. Decide which processes are loaded into

memory when memory space becomes

available.

Allocate and de-allocate memory space as

needed.


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Computers process information. That

information must be transmitted, processed,

and stored.

A file system object is an abstract entity forstoring or transmitting a collection of

information.

The file system is an organized collection of

file system objects.

The OS must provide primitives to manipulate

those objects


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The five main major activities of an operating

system in regard to file management are:

The creation and deletion of files.

The creation and deletion of directories. The support of primitives for manipulating

files and directories.

The mapping of files onto secondary storage.

The back-up of files on stable storage media.


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Device Management

A computer communicates information

through its input and output devices.

Processes access those devices

through the OS supervisor calls

provided for that purpose.

The OS attempts to manage those

devices in a manner that allows them to

be efficiently shared among the

processes requiring them


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Operating System Services

The following services are provided by OS to

the convenience of the users:

Program Execution

I/O Operations File System Manipulation

Inter-process communication

Error Detection


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Operating System Types

Batch systems

Multiprogrammed batch systems

Time-shared systemsNetwork operating systems

Real-time operating systems

Distributed operating systems


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Batch systems

Earlier, computer input, both programs and data,was prepared on input media; paper tape orpunch cards

Users would hand these in to an operator andreturn later to pick up the outputs. The operatorwould assemble similar jobs into batches andrun the batches through the computer. Each jobhad total control of the machine until it

terminated.A batch OS manages a machine run in this

manner. It provides virtually no functionalitysince there are no complications of sharing

resources with multiple processes.


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Multiprogrammed batch systems

On multiprogrammed batch systems,

jobs are read into a job pool stored on a

disk.

When one job is unable to execute

because it is waiting for an I/O operation

to complete, another job may be allowed

to run.This sharing of computer resources by

concurrently executing processes greatly

increases operating system complexity.


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Time-shared systems

Time-shared OS allow for interaction between userand process.

In batch systems, all data is supplied at the time theprogram is input. This may be fine for a program that

takes payroll information and prints weekly paycheques.

But for programs that must interact with the user, theOS must allow programs to respond to user inputs in areasonable amount of time.

The OS must not only share resources among thevarious processes, but it must create the illusion thatprocesses are running simultaneously.

It does this by shifting execution rapidly among all the

active processes.


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Networkoperating systems

Increasingly, computers exist not as

stand-alone entities but as part of a

network of computers.

Network communications involve

complex protocols

For the reliability of the network, support

for those protocols is built into network

operating systems.


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Real-time operating systems

A real-time OS is designed for execution oftasks within specific time constraints.

Normally, users want the computer systems

to execute their programs as soon aspossible, but no exact timing is required.

In real-time systems, the correctness of aprocessing task is dependent on the time at

which the processing occurred. Eg, a real-time system sensing loss of coolant

to a nuclear reactor may be required to initiatea backup system within a fraction of a

second.


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Real-time operating systems.

In a real-time environment, the OS must

guarantee that the task can be

executed within a specified time

constraint.

Use of real-time systems is most limited

to dedicated applications such as

industrial control systems, weaponsystems and computer-controlled

products.


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Real-time operating systems.

Desirable features in a time-shared system,

(resource sharing and management of I/O

devices by the operating system) are

detriments in a real-time system. Minimization of delays in the completion of

tasks discourages resource sharing and

encourages low-level access to hardware.

For this reason, the design of general-

purpose OS is much different from the design

of real-time systems.


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Distributed operating systems

With a network OS, the resources on each

machine on the network are managed by

that machines OS. OS support facilitates

communications among the machines.With a distributed operating system, the

OS on all the machines work together to

manage the collective network resources.A single collective distributed OS manages

the network resources provided by each

network computer or node.


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The Booting Process

When power is first supplied to a computer system, aprogram in read-only memory (ROM) executes.

After performing some diagnostic checks, a stage-0boot program is executed. The program checks for the

presence of one or more boot devices, eg, floppy disk,a CD-ROM or hard disk.

Once found, the stage-0 program reads the first sector(called boot sector) from the boot device into mainmemory. This boot sectorshould contain a stage-1

boot program. There is however, no guarantee. If the boot sector has never been initialized, undefined

values will be copied into memory. Before transferringcontrol to the next stage in the boot process, the currentboot program will check


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The Booting Process..

On some systems, the stage-1 boot program then

reads in the OS.

On others, a series of bootstrap programs must

be copied into memory and executed. In either case, the OS eventually gets copied into

memory and the final boot program branches to

the OSs initialization entry point.

On a hard disk PC, the boot sector also contains apartition table which contains the starting and

ending location of partitions. One partition is

defined as active.


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The Booting Process..

At the end of the boot process, the OS has been

loaded into memory and the boot program has

branched to the OS.

The machine is now executing in kernel mode. Data

structures not set when the operating system wasloaded, system registers and devices are all initialized

as needed.

Once the OS is ready to service processes, it creates

any processes that provide OS services. Once the system is switched into user mode, the OS

has completed its boot responsibilities and the system

is running.


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The Booting Process over ..

The operating system is now

running and the computer

system is now ready for use.

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Operating Systems Theory c1