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UNIT-I
OVER VIEW OF OPERATING SYSTEM
What is an Operating System? A program that acts as an intermediary between a user of a computer and the computer hardware Operating system goals:
- Execute user programs and make solving user problems easier
- Make the computer system convenient to use
- Use the computer hardware in an efficient manner
Computer System Structure Computer system can be divided into four components
- Hardware – provides basic computing resources CPU, memory, I/O devices
- Operating system Controls and coordinates use of hardware among various applications and users
- Application programs – define the ways in which the system resources are used to solve the computing problems of the users Word processors, compilers, web browsers, database systems, video games
- Users People, machines, other computers
Four Components of a Computer System
Operating System Definition
- OS is a resource allocator
- Manages all resources
- Decides between conflicting requests for efficient and fair resource use
- OS is a control program
- Controls execution of programs to prevent errors and improper use of the computer
- No universally accepted definition
- Everything a vendor ships when you order an operating system” is good approximation But varies wildly
- “The one program running at all times on the computer” is the kernel. Everything else is either a system program (ships with the operating system) or an application program
Computer Startup
- bootstrap program is loaded at power-up or reboot
- Typically stored in ROM or EPROM, generally known as firmware
- Initializes all aspects of system
- Loads operating system kernel and starts execution
Computer System Organization
- Computer-system operation
- One or more CPUs, device controllers connect through common bus providing access to shared memory
- Concurrent execution of CPUs and devices competing for memory cycles
Computer-System Operation
- I/O devices and the CPU can execute concurrently
- Each device controller is in charge of a particular device type
- Each device controller has a local buffer
- CPU moves data from/to main memory to/from local buffers
- I/O is from the device to local buffer of controller
- Device controller informs CPU that it has finished its operation by causing An interrupt
Common Functions of Interrupts
- Interrupt transfers control to the interrupt service routine generally, through the interrupt vector , which contains the addresses of all the service routines
- Device controller transfers blocks of data from buffer storage directly to main memory without CPU intervention
- Only one interrupt is generated per block, rather than the one interrupt per byte
Storage Structure
- Main memory – only large storage media that the CPU can access directly
- Secondary storage – extension of main memory that provides large nonvolatile storage capacity
- Magnetic disks – rigid metal or glass platters covered with magnetic recording material
- Disk surface is logically divided into tracks , which are subdivided into sectors
- The disk controller determines the logical interaction between the device and the computer
Storage Hierarchy
- Storage systems organized in hierarchy
- Speed
- Cost
- Volatility
Caching – copying information into faster storage system; main memory can be viewed as a last cache for secondary storage
Caching
- Important principle, performed at many levels in a computer (in hardware, operating system, software)
- Information in use copied from slower to faster storage temporarily
- Faster storage (cache) checked first to determine if information is there
- If it is, information used directly from the cache (fast)
- If not, data copied to cache and used there
- Cache smaller than storage being cached
- Cache management important design problem
- Cache size and replacement policy
Computer-System Architecture
- Most systems use a single general-purpose processor (PDAs through mainframes)
- Most systems have special-purpose processors as well
- Multiprocessors systems growing in use and importance
- Also known as parallel systems, tightly-coupled systems Advantages include 1.Increased throughput 2.Economy of scale 3.Increased reliability – graceful degradation or fault tolerance Two types 1.Asymmetric Multiprocessing 2.Symmetric Multiprocessing
How a Modern Computer Works Symmetric Multiprocessing Architecture
Operating-System Operations
- Interrupt driven by hardware
- Software error or request creates exception or trap
- Division by zero, request for operating system service
- Other process problems include infinite loop, processes modifying each Other or the operating system
- Dual-mode operation allows OS to protect itself and other system components
- User mode and kernel mode
- Mode bit provided by hardware
Provides ability to distinguish when system is running user code or kernel code Some instructions designated as privileged , only executable in kernel mode System call changes mode to kernel, return from call resets it to user
Transition from User to Kernel Mode
- Timer to prevent infinite loop / process hogging resources
- Set interrupt after specific period
- Operating system decrements counter
- When counter zero generate an interrupt
- Set up before scheduling process to regain control or terminate program that exceeds allotted time
OPERATING SYSTEM FUNCTIONS
Process Management
- A process is a program in execution. It is a unit of work within the system. Program is a passive entity , process is an active entity.
- Process needs resources to accomplish its task
- CPU, memory, I/O, files
- Initialization data
- Process termination requires reclaim of any reusable resources
- Single-threaded process has one program counter specifying location of next instruction to execute
- Process executes instructions sequentially, one at a time, until completion
- Multi-threaded process has one program counter per thread
- Typically system has many processes, some user, some operating system running concurrently on one or more CPUs
- Concurrency by multiplexing the CPUs among the processes / threads
Process Management Activities
- The operating system is responsible for the following activities in connection with process management:
- Creating and deleting both user and system processes
- Suspending and resuming processes
- Providing mechanisms for process synchronization
- Providing mechanisms for process communication
- Providing mechanisms for deadlock handling
Memory Management
- All data in memory before and after processing
- All instructions in memory in order to execute
- Memory management determines what is in memory when
- Optimizing CPU utilization and computer response to users
- Memory management activities
- Keeping track of which parts of memory are currently being used and by whom
- Deciding which processes (or parts thereof) and data to move into and out of memory
- Allocating and deallocating memory space as needed
Storage Management
- OS provides uniform, logical view of information storage
- Abstracts physical properties to logical storage unit - file
- Each medium is controlled by device (i.e., disk drive, tape drive)
Migration of Integer A from Disk to Register
- Multitasking environments must be careful to use most recent value, no matter where it is stored in the storage hierarchy
- Multiprocessor environment must provide cache coherency in hardware such that all CPUs have the most recent value in their cache
- Distributed environment situation even more complex
- Several copies of a datum can exist
I/O Subsystem
- One purpose of OS is to hide peculiarities of hardware devices from the user
- I/O subsystem responsible for
- Memory management of I/O including buffering (storing data temporarily while it is being transferred), caching (storing parts of data in faster storage for performance), spooling (the overlapping of output of one job with input of other jobs)
- General device-driver interface
- Drivers for specific hardware devices
Protection and Security Protection – any mechanism for controlling access of processes or users to resources defined by the OS Security – defense of the system against internal and external attacks
- Huge range, including denial-of-service, worms, viruses, identity theft, theft of service
- Systems generally first distinguish among users, to determine who can do what
- User identities ( user IDs , security IDs) include name and associated number, one per user
- User ID then associated with all files, processes of that user to determine access control
- Group identifier ( group ID ) allows set of users to be defined and controls managed, then also associated with each process, file
- Privilege escalation allows user to change to effective ID with more rights
DISTRIBUTED SYSTEMS Computing Environments Traditional computer
- Blurring over time
- Office environment PCs connected to a network, terminals attached to mainframe or minicomputers providing batch and timesharing Now portals allowing networked and remote systems access to same resources
- Home networks Used to be single system, then modems
Now firewalled, networked
- Client-Server Computing
- Dumb terminals supplanted by smart PCs
- Many systems now servers , responding to requests generated by clients Compute-server provides an interface to client to request services (i.e. database) File-server provides interface for clients to store and retrieve files
Peer-to-Peer Computing
- Another model of distributed system
- P2P does not distinguish clients and servers
- Instead all nodes are considered peers
- May each act as client, server or both
- Node must join P2P network Registers its service with central lookup service on network, or Broadcast request for service and respond to requests for service via discovery protocol
- Examples include Napster and Gnutella
Web-Based Computing
- Web has become ubiquitous
- PCs most prevalent devices
- More devices becoming networked to allow web access
- New category of devices to manage web traffic among similar servers: load balancers
- Use of operating systems like Windows 95, client-side, have evolved into Linux and Windows XP, which can be clients and servers
Open-Source Operating Systems
- Operating systems made available in source-code format rather than just binary closed-source
- Counter to the copy protection and Digital Rights Management (DRM) movement
- Started by Free Software Foundation (FSF), which has “copyleft” GNU Public License (GPL)
- Examples include GNU/Linux, BSD UNIX (including core of Mac OS X), and Sun Solaris
Operating System Services
- One set of operating-system services provides functions that are helpful to the user:
- Protection and security - The owners of information stored in a multiuser or networked computer system may want to control use of that information, concurrent processes should not interfere with each other
- Protection involves ensuring that all access to system resources is controlled
- Security of the system from outsiders requires user authentication, extends to defending external I/O devices from invalid access attempts
- If a system is to be protected and secure, precautions must be instituted throughout it. A chain is only as strong as its weakest link.
User Operating System Interface - CLI
- Command Line Interface (CLI) or command interpreter allows direct command entry Sometimes implemented in kernel, sometimes by systems program Sometimes multiple flavors implemented – shells Primarily fetches a command from user and executes it
- Sometimes commands built-in, sometimes just names of programs
- If the latter, adding new features doesn’t require shell modification
User Operating System Interface - GUI
- User-friendly desktop metaphor interface
- Usually mouse, keyboard, and monitor
- Icons represent files, programs, actions, etc
- Various mouse buttons over objects in the interface cause various actions (provide information, options, execute function, open directory (known as a folder)
- Invented at Xerox PARC
- Many systems now include both CLI and GUI interfaces
- Microsoft Windows is GUI with CLI “command” shell
- Apple Mac OS X as “Aqua” GUI interface with UNIX kernel underneath and shells available
- Solaris is CLI with optional GUI interfaces (Java Desktop, KDE)
Bourne Shell Command Interpreter
The Mac OS X GUI
System Calls
- Programming interface to the services provided by the OS
- Typically written in a high-level language (C or C++)
- Mostly accessed by programs via a high-level Application Program Interface (API) rather than direct system call usenThree 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)
- Why use APIs rather than system calls?(Note that the system-call names used throughout this text are generic)
Example of System Calls
- The system call interface invokes intended system call in OS kernel and returns status of the system call and any return values
- The 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 Relationship
Standard C Library Example
System Call Parameter Passing
- Often, more information is required than simply identity of desired system call
- Exact type and amount of information vary according to OS and call
- Three 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
Parameter Passing via Table
Types of System Calls
- Process control
- File management
- Device management
- Information maintenance
- Communications
- Protection
FreeBSD Running Multiple Programs
System Programs System programs provide a convenient environment for program development and execution. The can be divided into:
- File manipulation
- Status information
- File modification
- Programming language support
- Program loading and execution
- Communications
- Application programs Most users’ view of the operation system is defined by system programs, not the actual system calls
- Provide a convenient environment for program development and execution
- Some of them are simply user interfaces to system calls; others are considerably more complex
- File management - Create, delete, copy, rename, print, dump, list, and generally manipulate files and directories
- Status information
- Some ask the system for info - date, time, amount of available memory, disk space, number of users
- Others provide detailed performance, logging, and debugging information
- Typically, these programs format and print the output to the terminal or other output devices
- Some systems implement a registry - used to store and retrieve configuration information File modification
- Text editors to create and modify files
- Special commands to search contents of files or perform transformations of the text
- Programming-language support - Compilers, assemblers, debuggers and interpreters sometimes provided
- Program loading and execution- Absolute loaders, relocatable loaders, linkage editors, and overlay- loaders, debugging systems for higher-level and machine language
- Communications - Provide the mechanism for creating virtual connections among processes, users, and computer systems
- Allow users to send messages to one another’s screens, browse web pages, send electronic-mail messages, log in remotely, transfer files from one machine to another
Operating System Design and Implementation
- Design and Implementation of OS not “solvable”, but some approaches have proven successful
- Internal structure of different Operating Systems can vary widely
- Start by defining goals and specifications
- Affected by choice of hardware, type of system
- User goals and System goals
- User goals – operating system should be convenient to use, easy to learn, reliable, safe, and fast
- System goals – operating system should be easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient
- Important principle to separate
- Policy: What will be done? Mechanism: How to do it?
- Mechanisms determine how to do something, policies decide what will be done
- The separation of policy from mechanism is a very important principle, it allows maximum flexibility if policy decisions are to be changed later
Simple Structure
- MS-DOS – written to provide the most functionality in the least space
- Not divided into modules
- Although MS-DOS has some structure, its interfaces and levels of Functionality are not well separated
MS-DOS Layer Structure
