Chapter 2: Operating-System Structures

2.2 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Outline n Operating System Services n User Operating System Interface n System Calls n Types of System Calls n System Programs n Operating System Design and Implementation n Operating System Structure n Virtual Machines n Operating System Generation n System Boot

2.3 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Objectives n To describe the services an OS provides to users, processes, and other systems n To discuss the various ways of structuring an OS n To explain how OS are installed and customized and how they boot

2.4 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Operating System Services n OS functions that are helpful to the user: l User interface (UI)  Command-Line (CLI), Graphics User Interface (GUI), Batch l Program execution - to load a program into memory and to run that program, end execution, either normally or abnormally (indicating error) l I/O operations - A running program may require I/O, which may involve a file or an I/O device l File-system manipulation - to read and write files and directories, create and delete them, search them, list file information, permission management

2.5 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Operating System Services (Cont.) n OS functions that are helpful to the user (Cont): Communications – to exchange information, on the same computer or between computers over a network  via shared memory or through message passing Error detection – to be constantly aware of possible errors  May occur in the CPU and memory hardware, in I/O devices, in user program  Appropriate actions should be taken to ensure correct and consistent computing  Debugging facilities can greatly enhance the user ’ s and programmer ’ s abilities to efficiently use the system

2.6 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Operating System Services (Cont.) n OS functions for ensuring the efficient operation of the system itself via resource sharing l Resource allocation - resources must be allocated to each of concurrent users or jobs  Many types of resources - CPU cycles, main memory, and file storage, and I/O devices l Accounting - To keep track of which users use how much and what kinds of computer resources

2.7 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 l Protection and security - to control use of the information, concurrent processes should not interfere with each other  Protection ensures 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  A chain is only as strong as its weakest link

2.8 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 A View of Operating System Services

2.9 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 User Operating System Interface - CLI n CLI allows direct command entry l Sometimes implemented in kernel, sometimes by systems program Sometimes multiple flavors implemented – shells n To fetch a command from user and execute it l Sometimes commands built-in, sometimes just names of programs  If the latter, adding new features doesn ’ t require shell modification

2.10 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Bourne Shell Command Interpreter

2.11 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 User Operating System Interface - GUI n User-friendly desktop metaphor interface l Usually mouse, keyboard, and monitor l Icons represent files, programs, actions, etc l Various mouse buttons over objects in the interface cause various actions (provide information, options, execute function, open directory or folder ) l Invented at Xerox PARC

2.12 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 The Mac OS X GUI

2.13 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 User Operating System Interface n Many systems now include both CLI and GUI Microsoft Windows: GUI with CLI “ command ” shell Apple Mac OS X: “ Aqua ” GUI interface with UNIX kernel underneath and shells available l Solaris: CLI with optional GUI interfaces (Java Desktop, KDE)

2.14 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 System Calls n Programming interface to the OS services n Typically written in a high-level language (C or C++) n Mostly accessed by programs via a high-level Application Program Interface (API) l Three most common APIs:  Win32 API: Windows  POSIX API: POSIX-based systems (UNIX, Linux, and Mac OS X)  Java API: Java virtual machine (JVM) n Why use APIs rather than system calls? l Portability l Actual system calls might be more detailed and difficult to work with

2.15 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Example of System Calls n System call sequence to copy the contents of one file to another file

2.16 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Example of Standard API Consider the ReadFile() function in the Win32 API — a function for reading from a file n 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

2.17 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 System Call Implementation n Typically, a number associated with each system call l System-call interface maintains a table indexed according to these numbers n The system call interface invokes intended system call in OS kernel and returns status and any return values n The caller needs to know nothing about how the system call is implemented l Just needs to obey API and understand what OS will do as a result l Most details of OS interface hidden by API  Managed by run-time support library (set of functions built into libraries included with compiler)

2.18 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 API – System Call – OS Relationship

2.19 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Standard C Library Example n C program invoking printf() library call, which calls write() system call

2.20 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 System Call Parameter Passing n Often, more information is required than simply identity of desired system call l Exact type and amount of information vary according to OS and call n Three general methods used to pass parameters to the OS l Simplest: pass the parameters in registers l Parameters stored in a block, or table, in memory, and address of block passed as a parameter in a register  E.g. Linux and Solaris l Parameters placed, or pushed, onto the stack by the program and popped off the stack by the OS l Block and stack methods do not limit the number or length of parameters being passed

2.21 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Parameter Passing via Table

2.22 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Types of System Calls n Process control n File management n Device management n Information maintenance n Communications

2.23 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Examples of Windows and Unix System Calls

2.24 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 MS-DOS execution (a) At system startup (b) running a program

2.25 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 FreeBSD Running Multiple Programs

2.26 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 System Programs n System programs provide a convenient environment for program development and execution. They can be divided into: l File manipulation l Status information l File modification l Programming language support l Program loading and execution l Communications Most users ’ view of the OS is defined by system programs, not the actual system calls

2.27 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 System Programs n Provide a convenient environment for program development and execution l Some of them are simply user interfaces to system calls; others are considerably more complex n File management - Create, delete, copy, rename, print, dump, list, and generally manipulate files and directories n Status information l Some ask the system for info - date, time, amount of available memory, disk space, number of users l Others provide detailed performance, logging, and debugging information l Typically, these programs format and print the output to the terminal or other output devices l Some systems implement a registry - used to store and retrieve configuration information

2.28 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 System Programs (cont ’ d) n File modification l Text editors to create and modify files l Special commands to search contents of files or perform transformations of the text n Programming-language support - Compilers, assemblers, debuggers and interpreters n Program loading and execution - Absolute loaders, relocatable loaders, linkage editors, and overlay-loaders, debugging systems for higher-level and machine language n Communications - 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

2.29 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Operating System Design and Implementation Design and Implementation of OS not “ solvable ”, but some approaches have proven successful l Internal structure of different OS can vary widely l Start by defining goals and specifications l Affected by choice of hardware, type of system n Requirements User goals – OS should be convenient to use, easy to learn, reliable, safe, and fast System goals – OS should be easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient

2.30 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Operating System Design and Implementation (Cont.) n Important principle to separate Policy: What will be done? Mechanism: How to do it? n it allows maximum flexibility if policy decisions are to be changed later

2.31 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 OS Structure MS-DOS – written to provide the most functionality in the least space l Not divided into modules l Although MS-DOS has some structure, its interfaces and levels of functionality are not well separated

2.32 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 MS-DOS Layer Structure

2.33 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 UNIX UNIX – limited by hardware functionality, the original UNIX OS had limited structuring, which consists of two separable parts l Systems programs l The kernel  Consists of everything below the system-call interface and above the physical hardware  Provides the file system, CPU scheduling, memory management, and other OS functions; a large number of functions for one level

2.34 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 UNIX System Structure

2.35 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Layered Approach n The OS is divided into a number of layers (levels), each built on top of lower layers l The bottom layer (layer 0) is the hardware l The highest (layer N) is the user interface n With modularity, layers are selected such that each uses functions (operations) and services of only lower-level layers n Major difficulty: appropriately defining the layers n They tend to be less efficient

2.36 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Layered Operating System

2.37 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Microkernel System Structure Moves as much from the kernel into “ user ” space n Communication takes place between user modules using message passing n Benefits: l Easier to extend a microkernel l Easier to port the OS to new architectures l More reliable (less code is running in kernel mode) l More secure n Detriments: l Performance overhead of user space to kernel space communication

2.38 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Mac OS X Structure Hybrid – layered with microkernel

2.39 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Modules n Most modern OS implement kernel modules l Uses object-oriented approach l Each core component is separate l Each talks to the others over known interfaces l Each is loadable as needed within the kernel n Similar to layers, but more flexible l any module can call any other module n Similar to microkernel, but more efficient l No message passing among modules

2.40 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Solaris Loadable Modules

2.41 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Virtual Machines n A virtual machine takes the layered approach to its logical conclusion l It treats hardware and the OS kernel as though they were all hardware l It provides an interface identical to the underlying bare hardware n The OS host creates the illusion of multiple processes, each executing on its own processor with its own (virtual) memory n Each guest provided with a (virtual) copy of underlying computer

2.42 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Virtual Machines (Cont.) n The resources of the physical computer are shared to create the virtual machines l CPU scheduling can create the appearance that users have their own processor l 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

2.43 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Virtual Machines History and Benefits n First appeared commercially in IBM mainframes in 1972 n Fundamentally, multiple execution environments (different OS) can share the same hardware n Protect from each other n Some sharing of file can be permitted, controlled n Commutate with each other, other physical systems via networking n Useful for development, testing n Consolidation of many low-resource use systems onto fewer busier systems n “Open Virtual Machine Format”, standard format of virtual machines, allows a VM to run within many different virtual machine (host) platforms

2.44 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Virtual Machines (Cont) (a) Nonvirtual machine (b) virtual machine Non-virtual Machine Virtual Machine

2.45 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 VMware Architecture

2.46 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 The Java Virtual Machine

2.47 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Operating-System Debugging n Debugging is finding and fixing errors, or bugs n OS generate log files containing error information n Failure of an application can generate core dump file capturing memory of the process n OS failure can generate crash dump file containing kernel memory n Beyond crashes, performance tuning can optimize system performance

2.48 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 n Kernighan’s Law: “Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it.” n DTrace tool in Solaris, FreeBSD, Mac OS X allows live instrumentation on production systems l Probes fire when code is executed, capturing state data and sending it to consumers of those probes

2.49 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Solaris 10 dtrace Following System Call

2.50 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 Operating System Generation n OS are designed to run on any of a class of machines; the system must be configured for each specific computer site n SYSGEN program obtains information concerning the specific configuration of the hardware system Booting – starting a computer by loading the kernel Bootstrap program – code stored in ROM that is able to locate the kernel, load it into memory, and start its execution

2.51 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 14, 2005 System Boot n OS must be made available to hardware so hardware can start it Small piece of code – bootstrap loader, locates the kernel, loads it into memory, and starts it l Sometimes two-step process where boot block at fixed location loads bootstrap loader l When power initialized on system, execution starts at a fixed memory location  Firmware used to hold initial boot code

End of Chapter 2