Chapter 1 and 2 Introduction (1) 2.pdf

Chapter 1: Introduction Operating System Concepts – 10h Edition Silberschatz, Galvin and Gagne ©2018  Chapter 1: Introduction What Operating Systems Do Computer-System Organization Computer-System Ar...

Chapter 1: Introduction Operating System Concepts – 10h Edition Silberschatz, Galvin and Gagne ©2018  Chapter 1: Introduction What Operating Systems Do Computer-System Organization Computer-System Architecture Operating-System Operations Resource Management Security and Protection Virtualization Distributed Systems Kernel Data Structures Computing Environments Free/Libre and Open-Source Operating Systems Operating System Concepts – 10th Edition 1.2 Silberschatz, Galvin and Gagne ©2018  Objectives Describe the general organization of a computer system and the role of interrupts Describe the components in a modern, multiprocessor computer system Illustrate the transition from user mode to kernel mode Discuss how operating systems are used in various computing environments Provide examples of free and open-source operating systems Operating System Concepts – 10th Edition 1.3 Silberschatz, Galvin and Gagne ©2018  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 Operating System Concepts – 10th Edition 1.4 Silberschatz, Galvin and Gagne ©2018  Abstract View of Components of Computer Operating System Concepts – 10th Edition 1.5 Silberschatz, Galvin and Gagne ©2018  What Operating Systems Do Depends on the point of view Users want convenience, ease of use and good performance Don’t care about resource utilization But shared computer such as mainframe or minicomputer must keep all users happy Operating system is a resource allocator and control program making efficient use of HW and managing execution of user programs Users of dedicate systems such as workstations have dedicated resources but frequently use shared resources from servers Mobile devices like smartphones and tables are resource poor, optimized for usability and battery life Mobile user interfaces such as touch screens, voice recognition Some computers have little or no user interface, such as embedded computers in devices and automobiles Run primarily without user intervention Operating System Concepts – 10th Edition 1.6 Silberschatz, Galvin and Gagne ©2018  Defining Operating Systems Term OS covers many roles Because of myriad designs and uses of OSes Present in toasters through ships, spacecraft, game machines, TVs and industrial control systems Born when fixed use computers for military became more general purpose and needed resource management and program control Operating System Concepts – 10th Edition 1.7 Silberschatz, Galvin and Gagne ©2018  Operating System Definition (Cont.) No universally accepted definition “Everything a vendor ships when you order an operating system” is a good approximation But varies wildly “The one program running at all times on the computer” is the kernel, part of the operating system Everything else is either a system program (ships with the operating system, but not part of the kernel) , or an application program, all programs not associated with the operating system Today’s OSes for general purpose and mobile computing also include middleware – a set of software frameworks that provide addition services to application developers such as databases, multimedia, graphics Operating System Concepts – 10th Edition 1.8 Silberschatz, Galvin and Gagne ©2018  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 Operating System Concepts – 10th Edition 1.9 Silberschatz, Galvin and Gagne ©2018  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 Each device controller type has an operating system device driver to manage it 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 Operating System Concepts – 10th Edition 1.10 Silberschatz, Galvin and Gagne ©2018  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 Interrupt architecture must save the address of the interrupted instruction A trap or exception is a software-generated interrupt caused either by an error or a user request An operating system is interrupt driven Operating System Concepts – 10th Edition 1.11 Silberschatz, Galvin and Gagne ©2018  Interrupt Timeline Operating System Concepts – 10th Edition 1.12 Silberschatz, Galvin and Gagne ©2018  Interrupt Handling The operating system preserves the state of the CPU by storing registers and the program counter Determines which type of interrupt has occurred: polling vectored interrupt system Separate segments of code determine what action should be taken for each type of interrupt Operating System Concepts – 10th Edition 1.14 Silberschatz, Galvin and Gagne ©2018  Interrupt-drive I/O Cycle Operating System Concepts – 10th Edition 1.15 Silberschatz, Galvin and Gagne ©2018  I/O Structure After I/O starts, control returns to user program only upon I/O completion Wait instruction idles the CPU until the next interrupt Wait loop (contention for memory access) At most one I/O request is outstanding at a time, no simultaneous I/O processing After I/O starts, control returns to user program without waiting for I/O completion System call – request to the OS to allow user to wait for I/O completion Device-status table contains entry for each I/O device indicating its type, address, and state OS indexes into I/O device table to determine device status and to modify table entry to include interrupt Operating System Concepts – 10th Edition 1.16 Silberschatz, Galvin and Gagne ©2018  Storage Structure Main memory – only large storage media that the CPU can access directly Random access Typically volatile Typically random-access memory in the form of Dynamic Random- access Memory (DRAM) Secondary storage – extension of main memory that provides large nonvolatile storage capacity Hard Disk Drives (HDD) – 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 Non-volatile memory (NVM) devices– faster than hard disks, nonvolatile Various technologies Becoming more popular as capacity and performance increases, price drops Operating System Concepts – 10th Edition 1.17 Silberschatz, Galvin and Gagne ©2018  Storage Definitions and Notation Review The basic unit of computer storage is the bit. A bit can contain one of two values, 0 and 1. All other storage in a computer is based on collections of bits. Given enough bits, it is amazing how many things a computer can represent: numbers, letters, images, movies, sounds, documents, and programs, to name a few. A byte is 8 bits, and on most computers it is the smallest convenient chunk of storage. For example, most computers don’t have an instruction to move a bit but do have one to move a byte. A less common term is word, which is a given computer architecture’s native unit of data. A word is made up of one or more bytes. For example, a computer that has 64-bit registers and 64-bit memory addressing typically has 64-bit (8-byte) words. A computer executes many operations in its native word size rather than a byte at a time. Computer storage, along with most computer throughput, is generally measured and manipulated in bytes and collections of bytes. A kilobyte , or KB , is 1,024 bytes; a megabyte , or MB , is 1,0242 bytes; a gigabyte , or GB , is 1,0243 bytes; a terabyte , or TB , is 1,0244 bytes; and a petabyte , or PB , is 1,0245 bytes. Computer manufacturers often round off these numbers and say that a megabyte is 1 million bytes and a gigabyte is 1 billion bytes. Networking measurements are an exception to this general rule; they are given in bits (because networks move data a bit at a time). Operating System Concepts – 10th Edition 1.18 Silberschatz, Galvin and Gagne ©2018  Storage Hierarchy Storage systems organized in hierarchy Speed Cost Volatility Caching – copying information into faster storage system; main memory can be viewed as a cache for secondary storage Device Driver for each device controller to manage I/O Provides uniform interface between controller and kernel Operating System Concepts – 10th Edition 1.19 Silberschatz, Galvin and Gagne ©2018  Storage-Device Hierarchy Operating System Concepts – 10th Edition 1.20 Silberschatz, Galvin and Gagne ©2018  How a Modern Computer Works A von Neumann architecture Operating System Concepts – 10th Edition 1.21 Silberschatz, Galvin and Gagne ©2018  Direct Memory Access Structure Used for high-speed I/O devices able to transmit information at close to memory speeds 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 Operating System Concepts – 10th Edition 1.22 Silberschatz, Galvin and Gagne ©2018  Computer-System Architecture Most systems use a single general-purpose processor 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 – each processor is assigned a specie task. 2. Symmetric Multiprocessing – each processor performs all tasks Operating System Concepts – 10th Edition 1.23 Silberschatz, Galvin and Gagne ©2018  Symmetric Multiprocessing Architecture Operating System Concepts – 10th Edition 1.24 Silberschatz, Galvin and Gagne ©2018  A Dual-Core Design Multi-chip and multicore Systems containing all chips Chassis containing multiple separate systems Operating System Concepts – 10th Edition 1.25 Silberschatz, Galvin and Gagne ©2018  Non-Uniform Memory Access System Operating System Concepts – 10th Edition 1.26 Silberschatz, Galvin and Gagne ©2018  Clustered Systems Like multiprocessor systems, but multiple systems working together Usually sharing storage via a storage-area network (SAN) Provides a high-availability service which survives failures  Asymmetric clustering has one machine in hot-standby mode  Symmetric clustering has multiple nodes running applications, monitoring each other Some clusters are for high-performance computing (HPC)  Applications must be written to use parallelization Some have distributed lock manager (DLM) to avoid conflicting operations Operating System Concepts – 10th Edition 1.27 Silberschatz, Galvin and Gagne ©2018  Clustered Systems Operating System Concepts – 10th Edition 1.28 Silberschatz, Galvin and Gagne ©2018  PC Motherboard Operating System Concepts – 10th Edition 1.29 Silberschatz, Galvin and Gagne ©2018  Operating-System Operations Bootstrap program – simple code to initialize the system, load the kernel Kernel loads Starts system daemons (services provided outside of the kernel) Kernel interrupt driven (hardware and software) Hardware interrupt by one of the devices Software interrupt (exception or trap):  Software error (e.g., division by zero)  Request for operating system service – system call  Other process problems include infinite loop, processes modifying each other or the operating system Operating System Concepts – 10th Edition 1.30 Silberschatz, Galvin and Gagne ©2018  Operating System Structure : Uni-programming Main features are: Poor CPU utilization Poor Memory Utilization Poor I/O utilization No overlapping of CPU and I/O Operating System Concepts – 10th Edition 1.31 Silberschatz, Galvin and Gagne ©2018  Operating System Structure: Multiprogramming Multiprogramming is needed for efficiency Single user cannot keep CPU and I/O devices busy at all times Multiprogramming organizes jobs (code and data) so that 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 a job has to wait (for I/O for example), OS switches the CPU to another job OS Features Needed for Multiprogramming I/O routine supplied by the system. Memory management – the system must allocate the memory to several jobs. CPU scheduling – the system must choose among several jobs ready to run. Allocation of devices. Unlike sitting idle in a non- multiprogrammed system Operating System Concepts – 10th Edition 1.32 Silberschatz, Galvin and Gagne ©2018  Memory Layout for Multiprogrammed System ▪ Several jobs are kept in main memory at the same time, and the CPU is multiplexed among them. One job is executed at a time. The system operates as follows: OS picks and begins to execute one of the jobs in memory. The job may have to wait for some IO task (Typing a command, reading from disk,…): In non-multiprogramming systems, the CPU would sit idle. In multiprogramming systems, OS switches the CPU to another job in memory. Operating System Concepts – 10th Edition 1.33 Silberschatz, Galvin and Gagne ©2018  Dual-mode and Multimode Operation 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 Operating System Concepts – 10th Edition 1.34 Silberschatz, Galvin and Gagne ©2018  Transition from User to Kernel Mode Timer to prevent infinite loop / process hogging resources Timer is set to interrupt the computer after some time period Keep a counter that is decremented by the physical clock Operating system set the counter (privileged instruction) When counter zero generate an interrupt Set up before scheduling process to regain control or terminate program that exceeds allotted time Operating System Concepts – 10th Edition 1.35 Silberschatz, Galvin and Gagne ©2018  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 Operating System Concepts – 10th Edition 1.36 Silberschatz, Galvin and Gagne ©2018  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 Operating System Concepts – 10th Edition 1.37 Silberschatz, Galvin and Gagne ©2018  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 Operating System Concepts – 10th Edition 1.38 Silberschatz, Galvin and Gagne ©2018  File-system 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)  Varying properties include access speed, capacity, data- transfer 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 Operating System Concepts – 10th Edition 1.39 Silberschatz, Galvin and Gagne ©2018  Mass-Storage Management 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 Mounting and unmounting Free-space management Storage allocation Disk scheduling Partitioning Protection Some storage need not be fast Tertiary storage includes optical storage, magnetic tape Still must be managed – by OS or applications Operating System Concepts – 10th Edition 1.40 Silberschatz, Galvin and Gagne ©2018  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 Operating System Concepts – 10th Edition 1.41 Silberschatz, Galvin and Gagne ©2018  Characteristics of Various Types of Storage Movement between levels of storage hierarchy can be explicit or implicit Operating System Concepts – 10th Edition 1.42 Silberschatz, Galvin and Gagne ©2018  Migration of data “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 Various solutions covered in Chapter 19 Operating System Concepts – 10th Edition 1.43 Silberschatz, Galvin and Gagne ©2018  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 Operating System Concepts – 10th Edition 1.44 Silberschatz, Galvin and Gagne ©2018  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 Operating System Concepts – 10th Edition 1.45 Silberschatz, Galvin and Gagne ©2018  A View of Operating System Services Questions: -What part here is the OS? -What are system calls? -When are user mode and kernel mode being used here? Operating System Concepts – 10th Edition 1.46 Silberschatz, Galvin and Gagne ©2018  Services provided by Operating Systems One set of operating-system services provides functions that are helpful to the user (programmer): User interface - Almost all operating systems have a user interface (UI)  User interface types: Command-Line (CLI), Graphics User Interface (GUI), Batch interface, Touch Screen interface. Program execution - load a program into memory and run it, end execution, either normally or abnormally (indicating error) I/O operations – since user programs cannot execute I/O operations directly, the operating system must provide some means to perform I/O. File-system manipulation (for users and their programs) - read and write files and directories, create and delete them, search them, list file Information, permission management Silberschatz, Galvin and Gagne ©2018 Operating System Concepts – 10th Edition 1.47  Services provided by Operating Systems Communications – Processes may exchange information, on the same computer or between computers over a network  Communications may be implemented via shared memory or through message passing (packets moved by the OS) Error detection – OS needs to be constantly aware of possible errors  May occur in the CPU and memory hardware, in I/O devices, in user program  For each type of error, OS should take the appropriate action to ensure correct and consistent computing  Debugging facilities can greatly enhance the user’s and programmer’s abilities to efficiently use the system Operating System Concepts – 10th Edition 1.48 Silberschatz, Galvin and Gagne ©2018  Services provided by Operating Systems Another set of OS services exists for ensuring the efficient operation of the system itself via resource sharing Resource allocation - When multiple users or multiple jobs running concurrently, resources must be allocated to each of them  Many types of resources - Some (such as CPU cycles, main memory, and file storage) may have special allocation code, others (such as I/O devices) may have general request and release code Accounting - To keep track of which users use how much and what kinds of computer resources 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 by means of a password, extends to defending external I/O devices from invalid access attempts Operating System Concepts – 10th Edition 1.49 Silberschatz, Galvin and Gagne ©2018  User Operating System Interface - CLI Command Line Interface (CLI) or command interpreter allows direct command entry using the keyboard. Not user-friendly The main function of the command interpreter is to get and execute the next user-specified command: create, delete, print, copy, execute, …etc Interpreter (interpreting a user command to system calls) can be implemented in the kernel, or by system programs Some systems have multiple interpreters to choose from – shells (e.g., in UNIX: Bourne shell, Bourne-again shell, C shell, Korn shell) Interpreter commands are implemented in two general ways: 1) Commands are implemented as code section in the interpreter (command interpreter jumps to the required section of its code). o Commands can be issued at any path (internal commands) o The interpreter size depends on the number of commands. o Adding new commands is difficult and requires kernel change Operating System Concepts – 10th Edition 1.50 Silberschatz, Galvin and Gagne ©2018  User Operating System Interface - CLI 2) Commands are implemented as standalone system programs stored on the secondary storage, loaded and executed when needed. o Commands can be issued at specific path (external commands) o The command interpreter has fixed code of fixed size. o Programmers can add new commands to the system easily by creating new files with the proper names. This it doesn’t require kernel change. o This way is implemented in UNIX Unix command: rm file.txt Search the secondary storage for a file named file.txt Remove (rm) file.txt Operating System Concepts – 10th Edition 1.51 Silberschatz, Galvin and Gagne ©2018  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) Selection from menus Invented at Xerox PARC research facility Because a mouse is impractical for most mobile systems, smartphones typically use a touchscreen interface. 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 Operating System Concepts – 10th Edition 1.52 Silberschatz, Galvin and Gagne ©2018  Using CLI or GUI? When is it better to use CLI and when GUI? System administrators who manage computers and power users who have deep knowledge of a system frequently use the command-line interface. For them, it is more efficient, giving them faster access to the activities they need to perform. On some systems, only a subset of system functions is available via the GUI, leaving the less common tasks to those who are command-line knowledgeable. CLIs usually make repetitive tasks easier, in part because they have their own programmability. For example, if a frequent task requires a set of command-line steps, those steps can be written into a file, and that file can be run just like a program. File commands are interpreted by the CLI. These shell scripts are very common on systems that are command-line oriented, such as UNIX and Linux. Operating System Concepts – 10th Edition 1.53 Silberschatz, Galvin and Gagne ©2018  Bourne Shell Command Interpreter (Unix) Operating System Concepts – 10th Edition 1.54 Silberschatz, Galvin and Gagne ©2018  The Mac OS X GUI Operating System Concepts – 10th Edition 1.55 Silberschatz, Galvin and Gagne ©2018  The iPad touchscreen Operating System Concepts – 10th Edition 1.56 Silberschatz, Galvin and Gagne ©2018  System Calls System calls provide the interface between a running program (a process) and the operating system. Typically written in a high-level language (C or C++) Certain low-level tasks (where hardware must be accessed directly) may require Assembly-language instructions System calls, provide interface to the OS available services, is the mechanism by which a program requests services from an operating system. On Unix, Unix-like and other POSIX-compatible OSs, popular system calls are open, read, write, close, wait, exec, fork, exit, and kill. Many of today's operating systems have hundreds of system calls. Linux has 319 different system calls. FreeBSD (free Unix-like OS) has almost 330. Operating System Concepts – 10th Edition 1.57 Silberschatz, Galvin and Gagne ©2018  Examples of Windows and Unix System Calls Operating System Concepts – 10th Edition 1.58 Silberschatz, Galvin and Gagne ©2018  Example of using System Calls How many system calls do we need to copy the contents of a file to another? Get the names of the two files. o Prompt the user for the file names (write to screen, receive input) o Let the user point to the files and select from menu using GUI (many I/O system calls) Open the input file and create the output file Validate the entered file names and handle error conditions (input file does not exist, file is protected against access). Possible reactions: o Abort (system call), delete existing file (system call), ask the user (system calls sequence). Loop: read from input file and write to output file (system calls) Error handling – hardware malfunction, EOF, no more disk space, … Close both files Report success to the user Terminate normally (final system call) ▪ Even simple programs make heavy use of the operating system. Frequently, systems execute thousands Operating System Concepts – 10 Edition th 1.59 of system calls per second. Silberschatz, Galvin and Gagne ©2018  Example of using System Calls How many system calls do we need to copy the contents of a file to another? Get the names of the two files. o Prompt the user for the file names (write to screen, receive input) o Let the user point to the files and select from menu using GUI (many I/O system calls) Open the input file and create the output file Validate the entered file names and handle error conditions (input file does not exist, file is protected against access). Possible reactions: o Abort (system call), delete existing file (system call), ask the user (system calls sequence). Loop: read from input file and write to output file (system calls) Error handling – hardware malfunction, EOF, no more disk space, … Close both files Report success to the user Terminate normally (final system call) ▪ Even simple programs make heavy use of the operating system. Frequently, systems execute thousands Operating System Concepts – 10 Edition th 1.60 of system calls per second. Silberschatz, Galvin and Gagne ©2018  System Calls (cont.) - API Most of us do not work directly with system calls, but rather use Application Programming Interface (APIs or system programs) Typically, application developers design programs according to an application programming interface (API). The API specifies the set of functions that are available to the application programmer, including parameters that are passed to each function and expected return values. Three most common APIs are: Windows API for Windows, POSIX API for POSIX-based systems (including virtually all versions of UNIX, Linux, and Mac OS X), Java API for the Java virtual machine (JVM) A programmer can access an API via a library of code provided by OS. In the case of C language programs under UNIX /Linux, the library is called libc. Why use APIs rather than system calls? Program portability (compiling on different machines) Actual system calls are more detailed and harder to work with than Operating System Concepts – 10th Edition 1.61 Silberschatz, Galvin and Gagne ©2018  Steps in requesting services from OS Requesting services from OS is accomplished by using: 1) System Calls Process traps to OS Interrupt Handler OS switched to kernel (Supervisor) mode Desired ISR function executed Returns to requesting process 2) Message Passing A requesting process constructs a message indicating needed service (function) A process invokes send system call to pass message to OS The sending process blocks …… OS receives message OS initiates function execution Upon function completion, OS returns “OK” Process unblock… Silberschatz, Galvin and Gagne ©2018 Operating System Concepts – 10th Edition 1.62  Standard C Library Example C program invoking printf() a library call, which calls write() system call Used in many cases as a portion of the system call interface Operating System Concepts – 10th Edition 1.63 Silberschatz, Galvin and Gagne ©2018  Types of System Calls System calls are grouped into 5 categories: Process and job control: end, abort, create, terminate, load, execute,… File manipulation: create, delete, open, close, read, write, … Device manipulation: request, release, read, write, reposition, … Information maintenance: get time & date, set time & date, … Communications: open close connection, send receive message, transfer status info, … Operating System Concepts – 10th Edition 1.64 Silberschatz, Galvin and Gagne ©2018  Examples of Windows and Unix System Calls parent process to wait for the child process termination Sets the input mode of a console's input buffer or the output mode of a console screen buffer allocates a shared memory segment map files or devices into memory four-digit octal number that UNIX uses to determine the file permission for newly created files changes the user and group ownership of a Operating System Concepts – 10th Edition 1.65 file. Silberschatz, Galvin and Gagne ©2018  System Programs System programs provide a convenient environment for program development and execution. Some system programs are simply user interfaces to system calls, while other are more complex. System programs can be divided into: File manipulation : Create, delete, copy, rename, print, dump, list, and generally manipulate files and directories. Status information: date, time, free memory, free disk space,… File modification: text editor create and modify the file contents. Programming language support: compilers, assemblers, interpreters, …. Program loading and execution: absolute (relocatable) loaders, linkage-editors, debuggers,… Communications: provide mechanisms for creating virtual connections among processes, users, computers to exchange messages. Application programs: programs for solving common problems, or perform common operations: editors, spreadsheets, games,… Most users’ view of the OS is defined by the system programs, not the actual system calls. Operating System Concepts – 10th Edition 1.66 Silberschatz, Galvin and Gagne ©2018  Communication Models There are two common models of communication: In a message-passing model, information is exchanged through an IPC facility provided by an OS. A connection must be opened before the communication takes place. The name of both parties must be known. Each computer has a host name, each process has a process name. The source of communication (client) and the receiving daemon (server) exchange messages by “read-write” message system calls. In a shared-memory model, processes use “map memory” system call to gain access to areas of memory owned by other users. Processes exchange info by reading- writing these shared areas. Operating System Concepts – 10th Edition 1.67 Silberschatz, Galvin and Gagne ©2018  End of Chapter 1 Operating System Concepts – 10h Edition Silberschatz, Galvin and Gagne ©2018