structure of operating system

Operating System Concepts / Silberschatz / Ch 3 Oper Sys Strcutures Slide 1

Chapter 3: Operating-System StructuresChapter 3: Operating-System Structures

System ComponentsOperating System ServicesSystem CallsSystem ProgramsSystem Structure Virtual MachinesSystem Design and Implementation


Common System ComponentsCommon System ComponentsA system as large and complex as an OS can be

created only by partitioning it into smaller pieces

Process Management Main Memory ManagementFile ManagementI/O System ManagementSecondary ManagementNetworkingProtection SystemCommand-Interpreter System


Process ManagementProcess ManagementA process is a program in execution. A process needs certain resources, including CPU time, memory, files, and I/O devices, to accomplish its task. A process is the unit of work in a system

The operating system is responsible for the following activities in connection with process management. Process creation and deletion. Process suspension and resumption. Provision of mechanisms for:

process synchronization process communication


Main-Memory ManagementMain-Memory ManagementMemory is a large array of words or bytes, each with its own address. It is a repository of quickly accessible data shared by the CPU and I/O devices. The CPU reads instructions from main memory (MM) during

the instruction-fetch cycle and both reads and writes data from MM during the data-fetch cycle

Main memory is a volatile storage device. It loses its contents in the case of system failure.To improve both CPU utilization and response speed, several programs are kept in memoryThe following are some of the activities in connections with memory management that are handled by the operating system : Keep track of which parts of memory are currently being

used and by whom. Decide which processes to load when memory space becomes

available. Allocate and deallocate memory space as needed.


File ManagementFile Management

A file is a collection of related information defined by its creator. Commonly, files represent programs (both source and object forms) and data.The operating system is responsible for the following activities 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.


I/O System ManagementI/O System ManagementHide the peculiarities of specific hardware devices from the userThe I/O system consists of: A memory management component that includes

buffering, caching and spooling A general device-driver interface Drivers for specific hardware devices


Secondary-Storage ManagementSecondary-Storage ManagementSince main memory (primary storage) is volatile and too small to accommodate all data and programs permanently, the computer system must provide secondary storage to back up main memory.Most modern computer systems use disks as the principle on-line storage medium, for both programs and data.Because secondary storage is used frequently, it must be used efficiently or else it will become a processing bottleneckThe operating system is responsible for the following activities in connection with disk management: Free space management Storage allocation Disk scheduling


Networking (Distributed Systems)Networking (Distributed Systems)A distributed system is a collection processors that do not share memory or a clock. Each processor has its own local memory.The processors in the system are connected through a communication network.Communication takes place using a protocol.A distributed system provides user access to various system resources.Access to a shared resource allows: Computation speed-up Increased data availability Enhanced reliability


Protection SystemProtection System

Protection refers to a mechanism for controlling access by programs, processes, or users to both system and user resources.The protection mechanism must: distinguish between authorized and unauthorized

usage. specify the controls to be imposed. provide a means of enforcement.


Command-Interpreter SystemCommand-Interpreter SystemServes as the interface between the user and the OS User friendly, mouse based windows environment in the

Macintosh and in Microsoft Windows In MS-DOS and UNIX, commands are typed on a keyboard

and displayed on a screen or printing terminal with the Enter or Return key indicating that a command is complete and ready to be executed

Many commands are given to the operating system by control statements which deal with: process creation and management I/O handling secondary-storage management main-memory management file-system access protection networking


Command-Interpreter System (Cont.)Command-Interpreter System (Cont.)

The program that reads and interprets control statements is called variously:

command-line interpreter shell (in UNIX)

Its function is to get and execute the next command

statement.


System CallsSystem CallsSystem calls provide the interface between a running program and the operating system. For example – open input file, create output file, print

message to console, terminate with error or normally Generally available as routines written in C and C++ Certain low-level tasks (direct hardware access) may be

written in assembly-languageMostly accessed by programs via a high-level Application Program Interface (API) rather than direct system call use Provides portability (underlying hardware handled by OS) Hides the detail from the programmer

Three most common APIs are Win32 API for Windows, POSIX API for POSIX-based systems (including virtually all versions of UNIX, Linux, and Mac OS X), and Java API for the Java virtual machine (JVM)


Example of System CallsExample of System CallsSystem call sequence to copy the contents of one file to another file


Example of Standard APIExample of Standard APIConsider the ReadFile() function in theWin32 API—a function for reading from a file

A description of the parameters passed to ReadFile() HANDLE file—the file to be read LPVOID buffer—a buffer where the data will be read into and written

from DWORD bytesToRead—the number of bytes to be read into the buffer LPDWORD bytesRead—the number of bytes read during the last read LPOVERLAPPED ovl—indicates if overlapped I/O is being used


System Call ImplementationSystem Call ImplementationTypically, a number associated with each system call System-call interface maintains a table indexed according

to these numbers

The system call interface invokes intended system call in OS kernel and returns status of the system call and any return valuesThe caller need know nothing about how the system call is implemented Just needs to obey API and understand what OS will do as

a result call Most details of OS interface hidden from programmer by

API Managed by run-time support library (set of functions built

into libraries included with compiler)


API – System Call – OS RelationshipAPI – System Call – OS Relationship


Standard C Library ExampleStandard C Library ExampleC program invoking printf() library call, which calls write() system call


System Call Parameter PassingSystem Call Parameter PassingOften, more information is required than simply identity of desired system call Exact type and amount of information vary according to

OS and callThree general methods used to pass parameters to the OS Simplest: pass the parameters in registers

In some cases, may be more parameters than registers Parameters stored in a block, or table, in memory, and

address of block passed as a parameter in a register This approach taken by Linux and Solaris

Parameters placed, or pushed, onto the stack by the program and popped off the stack by the operating system

Block and stack methods do not limit the number or length of parameters being passed


Passing Parameters as a TablePassing Parameters as a Table


Types of System CallsTypes of System Calls

Process control end, abort, load, execute, allocate/free memory

File management Create/delete file, open, close, read, write

Device management Request/release device, read, write

Information maintenance Get/set date or time, get process, get/set system

data

Communications Send/receive message, create/delete comm link


UNIX System CallsUNIX System Calls


MS-DOS ExecutionMS-DOS ExecutionMS-DOS is an example of a single tasking system Has command interpreter (shell) which is invoked at

startup time Loads the program into memory, writing over most of

the shell to give the program as much memory as possible

The instruction pointer is set to the first instruction of the program and it begins running

Programs either terminates normally or abends providing a return code that is saved in system memory

The resident portion of the shell resumes execution, reloads remainder of the shell and makes the previous error code available to the user or next program


MS-DOS ExecutionMS-DOS Execution

At System Start-up Running a Program


UNIX Running Multiple ProgramsUNIX Running Multiple ProgramsFreeBSD is a multitasking system Shell accepts a command to run a program but, unlike

MS-DOS, continues to run while the other program is executing

To start a new process, the shell executes a fork system call which enables another program to be loaded into memory and executed

A process can run in the “background” but then it can not accept input from the keyboard (as it is dedicated to the shell) but can read and writes files

The shell again waits for further commands – run another program, monitor the progress of the running processes, change priorities, etc.

When a process terminates it issues an exit system call returning a status code


UNIX Running Multiple ProgramsUNIX Running Multiple Programs


System ProgramsSystem ProgramsSystem programs provide a convenient environment for program development and execution. The can be divided into: File manipulation Status information (date,time, # of users, free memory etc.) File modification (text editors) Programming language support (compilers, assemblers, etc) Program loading and execution (loaders, linkage editors) Communications (create virtual connections among processes,

users and different computer systems) System utilities or application programs (web browsers, word

processing, spreadsheets etc)

Most users’ view of the operating system is defined by system programs, not the actual system calls.


OS Design ImplementationOS Design ImplementationMechanisms determine how to do something, policies decide what will be done.The separation of policy from mechanism is a very important principle in designing OSs It allows maximum flexibility if policy decisions are to be

changed later. The mechanism that implements the policy to give priority to

I/O intensive processes over CPU intensive processes should be written in a general way so that if the policy is changed no or minimal change to the mechanism would be required

The timer provides is a mechanism providing CPU protection. How long it runs for a particular user is a policy decision.


System ImplementationSystem ImplementationTraditionally written in assembly language, operating systems can now be written in higher-level languages.Code written in a high-level language: can be written faster. is more compact. is easier to understand and debug.

Opponents to OSs written high-level language claim slower performance and increased storageProponents argue: Modern compilers can produce highly optimized code Todays OSs run on highly complex hardware which can

overwhelm the programmer with details Better data structures and algorithms will truly improve

the performance of OSs Only a small amount of code is critical to high

performance – bottlenecks can be replaced with assembler code later on

An operating system is far easier to port (move to some other hardware) if it is written in a high-level language.


Operating System Structure Operating System Structure

MS-DOS – written to provide the most functionality in the least space Not well divided into modules Designed without realizing it

was going to become so popular

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

Vulnerable to system crashes when a user program fails

Written for Intel 8088 which had no dual mode or hardware protection

MS-DOS layer structureMS-DOS layer structure


Operating System StructureOperating System Structure

UNIX – the original UNIX operating system had limited structuring due to limited hardware functionality. The UNIX OS consists of two separable parts. Systems programs The kernel – series of interfaces and device drivers

Consists of everything below the system-call interface and above the physical hardware

Provides the file system, CPU scheduling, memory management, and other operating-system functions; a large number of functions for one level.

All this functionality in one level makes UNIX difficult to enhance – difficult to determine impact of change in one part of the kernel on other parts


UNIX System StructureUNIX System Structure

kernel


Layered ApproachLayered ApproachThe operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface.With modularity, layers are selected such that each uses functions (operations) and services of only lower-level layers. Simplifies debugging and system

verification – each layer only uses those below it, so by testing from bottom up, you isolate errors to the layer being tested

Requires careful definition of layers

Less efficient as each layer adds overhead to the system increasing overall time for a system call to execute

An operating system layerAn operating system layer


Layered ApproachLayered ApproachThese limitations have led to the design of OSs with fewer layers each with more functionalityProvides most of the advantages of modularized code while avoiding the difficulties of layer definition and interactionOS/2, a descendent of MS-DOS, adds multitasking and dual-mode operation Designed in a more layered approach than MS-DOS Direct access to low-level facilities is not allowed; this gives the

OS more control over the hardware and more knowledge of which resources each user program is using

The first release of Windows NT had a highly layered approach but it delivered low performance compared to Windows 95 NT 4.0 addressed that problem in part by moving layers from

user space to kernel space and more closely integrating them


OS/2 Layer StructureOS/2 Layer Structure


Microkernel System Structure Microkernel System Structure In the mid 1980s, researchers at Carnegie Mellon developed the Mach microkernel OSMoves nonessential components from the kernel into “user” space resulting in a smaller kernelMain function of the microkernel is to provide a communication facility between the client program and various services that are also running in user space Communication takes place between user modules using message

passing.Benefits: easier to extend a microkernel

all new services are added to user space easier to port the operating system to new architectures more reliable and secure

less code is running in kernel modeApple MacOS X Server OS is based on the Mach kernel maps system calls into messages to the appropriate user-level services


Modules Modules Best, current technology for OS design involves using object oriented programming techniques to create a modular kernelKernel dynamically links in additional services either during boot time or run time Uses dynamically loadable modules (Solaris, Linux, Mac

OS X) Kernel provides core services and others implemented

dynamically Similar but more efficient than microkernel design

because modules do not need to invoke message passing in order to communicate


Solaris loadable modulesSolaris loadable modules


Virtual MachinesVirtual MachinesA virtual machine takes the layered approach to its logical conclusion. It treats hardware and the operating system kernel as though they were all hardware.A virtual machine not provide any additional functionality but provides an interface identical to the underlying bare hardware. Each process is provided with a (virtual) copy of the underlying computerThe operating system creates the illusion that a process has its own processor with its own (virtual) memory.The resources of the physical computer are shared to create the virtual machines. CPU scheduling can create the appearance that users

have their own processor. Spooling and a file system can provide virtual card

readers and virtual line printers. A normal user time-sharing terminal serves as the

virtual machine operator’s console.


System ModelsSystem Models

Non-virtual Machine Virtual Machine


Virtual Machines Virtual Machines The virtual-machine concept provides complete protection of system resources since each virtual machine is isolated from all other virtual machines. This isolation, however, permits no direct sharing of resources.A virtual-machine system is a perfect vehicle for operating-systems research and development. System development is done on the virtual machine, instead of on a physical machine and so does not disrupt normal system operation.The virtual machine concept is difficult to implement due to the effort required to provide an exact duplicate to the underlying machineVirtual machines are a means to solve system compatibility problems Windows applications to run on Linux-based computers

Intel instructions are translated into the native instruction set


Java Virtual MachineJava Virtual Machine

A platform is the hardware or software environment in which a program runs. Some of the most popular platforms are Windows 2000, Linux, Solaris, and MacOS.Most platforms can be described as a combination of the operating system and hardware. The Java platform differs from most other platforms in that it's a software-only platform that runs on top of other hardware-based platforms. Compiled Java programs are platform-neutral bytecodes executed by a Java Virtual Machine (JVM).The JVM is specific for each system and it abstracts the system in a standard way to the Java program eliminating code changes when ported from one platform to another


Communication ModelsCommunication ModelsCommunication between processes may take place using either message passing or shared memory.Message passing Information is exchanged through an interprocess

communication facility provided by the OS Computers have host names, processes have process names

for identification purposes Useful when smaller number of data need to be exchanged Easier to implement than shared memory

Shared memory Processes use map memory system calls to gain access to

regions of memory owned by other processes Allows maximum speed and convenience of communication Problems arise in the area of protection and synchronization


Communication ModelsCommunication Models

Message Passing Shared Memory

structure of operating system

Education

system structureschapter

memory management component

oper sys strcuturesslide

various system resources

concepts silberschatz

memory space

case of system failure

disk management