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Chapter 3

Operating SystemStructures

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Operating-System StructuresTopics

System Components

Operating System Services

System Calls

System Programs

System Structure

Virtual Machines

System Design and Implementation

System Generation

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System Components

Process Management

Main Memory Management

File Management

I/O System Management

Secondary Management

Protection System

Command-Interpreter System

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Process Management

Aprocess is a program in execution. A process needs certain

resources, including CPU time, memory, files, and I/Odevices, to accomplish its task.

The CPU executes one instruction of the process afteranother, until the process completes.

The operating system is responsible for the followingactivities in connection with process management.

Process creation and deletion of both user and system .

process suspension and resumption. Provision of mechanisms for:

process synchronization.

process communication.

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Main Memory Management

Memory is a large array of words or bytes, each with its

own address. It is a repository of quickly accessible datashared by the CPU and I/O devices.

Main memory is a volatile storage device. It loses itscontents in the case of system failure.

The I/O operations implemented via DMA also read andwrite data in memory.

To improve both the utilization of CPU and the speed of thecomputers response to its users, we keep several programsin memory.

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Main Memory Management (Cont.)

The operating system is responsible for the followingactivities in connections with memory management:

Keep track of which parts of memory are currently being

used and by whom.

Decide which processes to load when memory spacebecomes available.

Allocate and deallocate memory space as needed.

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File Management

A file is a collection of related information defined by its

creator. Commonly, files represent programs (both sourceand object forms) and data.

Data such as text files.

Computers can store information on several different typesof physical media.

Magnetic tape, magnetic disk, and optical disk are the mostcommon media.

Each of these media has its own characteristics and physicalorganization.

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File Management (Cont.)

The operating system is responsible for the followingactivities in connections with file management:

File creation and deletion.

Directory creation and deletion. Support of primitives for manipulating files and

directories.

Mapping files onto secondary storage.

File backup on stable (nonvolatile) storage media.

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I/O System Management

The I/O subsystem consists of:

A memory management component including buffering, cachingand spooling.

A general device-driver interface.

Drivers for specific hardware devices.

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Secondary-Storage Management

Since main memory (primary storage) is volatile and toosmall to accommodate all data and programs permanently,the computer system must provide secondary storage to backup main memory.

Most modern computer systems use disks as the principleon-line storage medium, for both programs and data.

The operating system is responsible for the following

activities in connection with disk management: Free space management

Storage allocation

Disk scheduling

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Protection System

Protection refers to a mechanism for controlling access byprograms, processes, or users to both system and userresources.

An unprotected resource cannot defend against use (ormisuse) by an unauthorized or incompetent user.

The protection mechanism must:

distinguish between authorized and unauthorized usage.

specify the controls to be imposed.

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Command-Interpreter System

Many commands are given to the operating system bycontrol statements which deal with:

process creation and management

I/O handling secondary-storage management

main-memory management

file-system access

protection networking

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Command-Interpreter System (Cont.)

The program that reads and interprets control statements iscalled variously:

command-line interpreter

shell (in UNIX)

Its function is to get and execute the next commandstatement.

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Operating System Services

An operating system provides an environment for theexecution of programs.

Program execution

System capability to load a program into memory and to run it. Program must be able to end its execution, either normally orabnormally (indicating error).

I/O operations

Since user programs cannot execute I/O operations directly, theoperating system must provide some means to perform I/O.

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Operating System Services (Cont.)

File-system manipulation Program capability to read, write, create, and delete files.

Communications Exchange of information between processes executing either on the

same computer or on different systems tied together by a network.Implemented via shared memory or message passing.

Error detection Ensure correct computing by detecting errors in the CPU and

memory hardware, in I/O devices, or in user programs.

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Additional Operating System Functions

Additional functions exist not for helping the user, butrather for ensuring efficient system operations.

Resource allocation Allocating resources to multiple users or multiple jobs running at the

same time.

Accounting Keep track of and record which users use how much and what kinds

of computer resources for account billing or for accumulating usagestatistics.

Protection Ensuring that all access to system resources is controlled. Security of the system from outsiders is also important.

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System Calls

System calls provide the interface between a running

program and the operating system.

Generally available as assembly-language instructions.

Languages defined to replace assembly language for systems

programming allow system calls to be made directly (e.g., C, C++)

Three general methods are used to pass parameters betweena running program and the operating system. Pass parameters in registers.

Store the parameters in a table in memory, and the table address ispassed as a parameter in a register.

Push (store) the parameters onto the stack by the program, andpopoff the stack by operating system.

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Passing of Parameters As A Table

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Types of System Calls

Process and job control

File manipulation

Device management

Information maintenance

Communication

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Process and job controlMS-DOS Execution

At System Start-up Running a Program

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Process and job controlUNIX Running Multiple Programs

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Process and job controlCommunication Models

Communication may take place using either message passing orshared memory.

Message Passing Shared Memory

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System Programs

System programs provide a convenient environment forprogram development and execution. They can be dividedinto:

File manipulation

Status information File modification

Programming language support

Program loading and execution

Communications

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System Programs (Contd.)

File manipulation These programs creates, delete copy, rename, print, dump, list and

manipulate files and directories.

Status information Programs simply ask the system for the date, time, amount of

memory or disk space, number of users or similar statusinformation.

The information is printed on terminal or other output device.

File modification Several text editors are available to create and modify the contents of

files and save it on disk or tape.

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System Programs (Contd.)

Programming language support Compilers, assemblers, and interpreters (FORTRAN, COBOL,

Pascal, BASIC and C).

Program loading and execution Once a program is assembled or compiled, it must be loaded into

memory to be executed.

Communications Provide the mechanism for creating virtual connections among

processes, users and different computer systems.

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System Structure

MS-DOS System Structure

MS-DOS written to provide the most functionality in theleast space

Not divided into modules

Although MS-DOS has some structure, its interfaces andlevels of functionality are not well separated

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System Structure

MS-DOS Layer Structure

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System Structure (Contd.)

UNIX System Structure

Limited by hardware functionality, the original UNIXoperating system had limited structuring. The UNIX OSconsists of two separable parts.

Systems programs Consists of everything below the system-call interface and above

the physical hardware

The kernel Provides the file system, CPU scheduling, memory management,

and other operating-system functions; a large number offunctions for one level.

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System Structure

UNIX System Structure

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Layered Approach

The operating system is divided into a number of layers

(levels), each built on top of lower layers. The bottomlayer (layer 0), is the hardware; the highest (layer N) isthe user interface.

With modularity, layers are selected such that each usesfunctions (operations) and services of only lower-levellayers.

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An Operating System Layer

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Layered Approach

Layer 5: User programs

Layer 4: Buffering for input and output devices

Layer 3: Operator-console device driver

Layer 2: Memory management

Layer 1: CPU scheduling

Layer 0: Hardware

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General OS Layers

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