Operating Systems

 

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 
         • Manage the computer hardware in an efficient manner

Operating-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

Operating-System Structure

Operating System Definition 

• OS as a control program 
        * Controls execution of programs to prevent errors and improper use of the computer 

• OS as a resource allocator 

        * Manages all resources 

        * Decides between conflicting requests for efficient and fair resource use

Operating System Definition – Common Beliefs 

• “Everything a vendor ships when you order an operating system” 

• “The one program running at all times on the computer” - kernel. 

• Everything else is either 

    * a system program (ships with the operating system) , or 

    * an application program.

Computer-Start up – How OS comes to work? 

• BIOS program is loaded at power-up or reboot     

    • Typically stored in EPROM, generally known as firmware 

    • Initializes all aspects of system 

    • Loads operating system kernel and starts execution 

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

Computer System Operation: Interrupts 

• Interrupt transfers control to the interrupt service routine 

• Interrupt architecture must save the address of the interrupted instruction 

• A trap (or an exception) is a software-generated interrupt caused either by an error or a user request 

• An operating system is interrupt driven

Operating-System Operations 

• Multiprogramming (Batch system) needed for efficiency 

• Single user cannot keep CPU and I/O devices busy at all times 

• Multiprogramming organizes jobs (code and data) so CPU always has one to execute 

• A subset of total jobs in system is kept in memory 

• One job selected and run via job scheduling 

• When it has to wait (for I/O for example), OS switches to another job

• Timesharing (multitasking) is logical extension in which CPU switches jobs so frequently that users can interact with each job while it is running, creating interactive computing 

• Response time should be < 1 second 

• Each user has at least one program executing in memory -> process 

• If several jobs ready to run at the same time -> CPU scheduling 

• If processes don’t fit in memory, swapping moves them in and out to run 

• Virtual memory allows execution of processes not completely in memory

• 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 

• Increasingly CPUs support multi-mode operations 

        * i.e. virtual machine manager (VMM) mode for guest VMs 

A system call is the way that a computer program requests a service from the kernel. This may include hardware-related services (for example, accessing a hard disk drive), creation and execution of new processes, and communication with integral kernel services such as process scheduling. 

Operating System - Services 

• Process Management 

• Memory Management 

• Storage Management 

        * File system management 

        * Storage space management 

• Protection and Security

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

• 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 

• To execute a program all (or part) of the instructions must be in memory 

• All (or part) of the data that is needed by the program must be in memory. 

• Memory management determines what is in memory and 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 a device (i.e., disk drive, tape drive) 

• Varying properties include access speed, capacity, datatransfer rate, access method (sequential or random)

• File-System management 

• Files usually organized into directories 

• Access control on most systems to determine who can access what 

• OS activities include 

        * Creating and deleting files and directories 

        * Primitives to manipulate files and directories 

        * Mapping files onto secondary storage 

        * Backup files onto stable (non-volatile) storage media

• Usually disks used to store data that does not fit in main memory or data that must be kept for a “long” period of time 

• Proper management is of central importance 

• Entire speed of computer operation hinges on disk subsystem and its algorithms 

• OS activities 

        * Free-space management 

        * Storage allocation 

        * Disk scheduling

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 

• 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

Computing Environments – Traditional 

• Stand-alone general purpose machines 

• But blurred as most systems interconnect with others (i.e., the Internet) 

• Portals provide web access to internal systems 

• Network computers (thin clients) are like Web terminals 

• Mobile computers interconnect via wireless networks 

• Networking becoming ubiquitous – even home systems use firewalls to protect home computers from Internet attacks

Computing Environments – Mobile 

• Handheld smartphones, tablets, etc 

• What is the functional difference between them and a “traditional” laptop? 

• Extra feature – more OS features (GPS, gyroscope) 

• Allows new types of apps like augmented reality 

• Use wireless, or cellular data networks for connectivity 

• Leaders are Apple iOS and Google Android

Computing Environments – Virtualization 

• Allows operating systems to run applications within other OSes 

• Vast and growing industry 

• Emulation used when source CPU type different from target type (i.e. PowerPC to Intel x86) 

• Generally slowest method 

• When computer language not compiled to native code – Interpretation 

• Virtualization – OS natively compiled for CPU, running guest OSes also natively compiled 

        * Consider VMware running WinXP guests, each running applications, all on native WinXP host OS 

        * VMM (virtual machine Manager) provides virtualization services

• Use cases involve laptops and desktops running multiple OSes for exploration or compatibility 

        *Apple laptop running Mac OS X host, Windows as a guest 

        * Developing apps for multiple OSes without having multiple systems 

        * QA testing applications without having multiple systems 

        * Executing and managing computer environments within data centers

Computing Environments – Client-Server 

• Client-Server Computing 

• Dumb terminals supplanted by smart PCs 

• Many systems now servers, responding to requests generated by clients 

• Compute-server system provides an interface to client to request services (i.e., database) 

• File-server system provides interface for clients to store and retrieve files

Computing Environments – Cloud Computing 

• Delivers computing, storage, even apps as a service across a network 

• Logical extension of virtualization because it uses virtualization as the base for it functionality. 

• Amazon EC2 has thousands of servers, millions of virtual machines, petabytes of storage available across the Internet, pay based on usage 

• Many types 

        • Public cloud – available via Internet to anyone willing to pay 

        • Private cloud – run by a company for the company’s own use 

        • Hybrid cloud – includes both public and private cloud components

• Software as a Service (SaaS) – one or more applications available via the Internet (i.e., word processor) 

• Platform as a Service (PaaS) – software stack ready for application use via the Internet (i.e., a database server) 

• Infrastructure as a Service (IaaS) – servers or storage available over Internet (i.e., storage available for backup use)

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 and BSD UNIX (including core of Mac OS X), and many more 

• Can use VMM like VMware Player (Free on Windows), Virtualbox (open source and free on many platforms - http://www.virtualbox.com) 

• Use to run guest operating systems for exploration

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