ch2.pdf

Chapter 2: Operating-System Structures Chapter 2: Operating-System Structures Operating System Services User Operating System Interface System Calls Types of System Calls System Programs Operating System Design and Implementation Operating System Structure Operating Sy...

Chapter 2: Operating-System Structures Chapter 2: Operating-System Structures Operating System Services User Operating System Interface System Calls Types of System Calls System Programs Operating System Design and Implementation Operating System Structure Operating System Debugging Operating System Generation System Boot  Operating System Services Operating systems provide an environment for execution of programs and services to programs and users One set of operating-system services provides functions that are helpful to the user: User interface - Almost all operating systems have a user interface (UI). Varies between Command-Line (CLI), Graphics User Interface (GUI), Batch Program execution - The system must be able to load a program into memory and to run that program, end execution, either normally or abnormally (indicating error) I/O operations - A running program may require I/O, which may involve a file or an I/O device  Operating System Services (Cont.) One set of operating-system services provides functions that are helpful to the user (Cont.): File-system manipulation - Programs need to read and write files and directories, create and delete them, search them, list file Information, permission management. Communications – Processes may exchange information, on the same computer or between computers over a network Communications may be 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 Services (Cont.) Another set of OS functions 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 - CPU cycles, main memory, file storage, I/O devices. 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, extends to defending external I/O devices from invalid access attempts A View of Operating System Services Operating System Interface User Operating System Interface - CLI 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 Bourne Shell Command Interpreter Basic Unix commands Kernel − heart of the OS - interacts with hardware Shell − The shell is the utility that processes your requests. When you type in a command at your terminal, the shell interprets the command and calls the program that you want. Commands and Utilities − There are various commands and utilities such as cp, mv, cat and grep, etc. CLI (Command Line Interface) is a command line program that accepts text input to execute operating system functions. General commands Date – used to display the current system date and time date +%D – displays date only date +%T- displays time only date +%Y – displays the year part of the date date +%H – displays the hour part of time cal – Calendar of the current month cal year - Displays calendar for all months of the specified year cal month year - Displays calendar for the specified month of the year General commands who - Login details of all users such as their IP, Terminal No, User name, who am i - Used to display the login details of the user tty - Used to display the terminal name uname - Displays the Operating System uname –r -Shows version number of the OS (kernel). uname –n - Displays domain name of the server echo "txt" - Displays the given text on the screen echo $HOME - Displays the user's home directory bc - Basic calculator. Press Ctrl+dto quit man cmdname - Manual for the given command. Press q to exit history - To display the commands used by the user since log on. exit - Exit from a process. If shell is the only process then logs out Directory commands pwd - Path of the present working directory mkdir dir - A directory is created in the given name under the current directory mkdir dir1 dir2 - A number of sub-directories can be created cd subdir - Change Directory. If the subdirstarts with / then path starts from root (absolute) otherwise from current working directory. cd - To switch to the home directory. cd / - To switch to the root directory. cd.. - To move back to the parent directory rmdir subdir - Removes an empty sub-directory. File Commands cat >filename - To create a file with some contents. To end typing press Ctrl+d. The >symbol means redirecting output to a file. (>filename - Used to append contents to a file cp src des - Copy files to given location. If already exists, it will be overwritten cp –i src des - Warns the user prior to overwriting the destination file cp –r src des - Copies the entire directory, all its sub-directories and files. mv old new - To rename an existing file or directory. –i option can also be used mv f1 f2 f3 dir - To move a group of files to a directory. sort filename.txt – sort the content in the file sort file.txt > output.txt – redirect the sorted file to another file sort -r inputfile.txt – sort in reverse order File commands mv –v old new - Display name of each file as it is moved. rm file - Used to delete a file or group of files. –i option can also be used rm * - To delete all the files in the directory. rm –r * - Deletes all files and sub-directories rm –f * - To forcibly remove even write-protected files ls – list all files and subdirectories in sorted manner ls name – To check whether a file or directory exists. cmp file1 file2 - Used to compare two files. Displays nothing if files are identical. wc file - It produces a statistics of lines (l), words(w), and characters(c) grep -c "unix" f1.txt - find the number of lines that matches the given string/pattern cat sample2.txt | head -7 | tail -5 File commands chmod 4 = r (read)2 = w (write)1 = x (execute) chmod 600 file.txt (Only the User has read and write permissions.) chmod 644 file.txt three sets of owners of a file or directory: User Group Public chmod ug+x test.txt 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 is “Aqua” GUI interface with UNIX kernel underneath and shells available Unix and Linux have CLI with optional GUI interfaces (CDE, KDE, GNOME) Touchscreen Interfaces Touchscreen devices require new interfaces  Mouse not possible or not desired  Actions and selection based on gestures  Virtual keyboard for text entry  Voice commands. 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 Programming Interface (API) rather than direct system call use 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 Calls System call sequence to copy the contents of one file to another file Example of Standard API System Call Implementation Typically, a number associated with each system call System-call interface maintains a table indexed according to these numbers The system call interface invokes the 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  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 create process, terminate process end, abort load, execute get process attributes, set process attributes wait for time wait event, signal event allocate and free memory Dump memory if error Debugger for determining bugs, single step execution Locks for managing access to shared data between processes Types of System Calls File management create file, delete file open, close file read, write, reposition get and set file attributes Device management request device, release device read, write, reposition get device attributes, set device attributes logically attach or detach devices Types of System Calls (Cont.) Information maintenance get time or date, set time or date get system data, set system data get and set process, file, or device attributes Communications create, delete communication connection send, receive messages if message passing model to host name or process name From client to server Shared-memory model create and gain access to memory regions transfer status information attach and detach remote devices Types of System Calls (Cont.) Protection Control access to resources Get and set permissions Allow and deny user access Examples of Windows and Unix System Calls Standard C Library Example C program invoking printf() library call, which calls write() system call  Example: MS-DOS Single-tasking Shell invoked when system booted Simple method to run program No process created Single memory space Loads program into memory, overwriting all but the kernel Program exit -> shell reloaded At system startup Running a program Example: FreeBSD Unix variant Multitasking User login -> invoke user’s choice of shell Shell executes fork() system call to create process Executes exec() to load program into process Shell waits for process to terminate or continues with user commands Process exits with: code = 0 – no error code > 0 – error code System Call Vs System Programs BASIS FOR SYSTEM CALL SYSTEM PROGRAM COMPARISON Basic Concept Allow user process to request the Creates an environment for program services of operating system. to develop and execute. User View Defines interface to the services Defines a user interface of operating of operating system. system. Request It satisfies the low-level request It satisfies the high-level request of of user program. the user program. Action Invokes the services of operating Initiates a sequence of system calls system. for to satisfy a user request. Language Used Usually written in C and C++. System program are written in high- But where direct hardware access level languages only. is required the call is written using assembly level language. 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 the design 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 Operating System Design and Implementation (Cont.) 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 (example – timer) Specifying and designing an OS is highly creative task of software engineering Implementation Much variation Early OSes in assembly language Then system programming languages like Algol, PL/1 Now C, C++ Actually usually a mix of languages Lowest levels in assembly Main body in C Systems programs in C, C++, scripting languages like PERL, Python, shell scripts More high-level language easier to port to other hardware But slower Emulation can allow an OS to run on non-native hardware Operating System Structure General-purpose OS is very large program Various ways to structure ones Simple structure – MS-DOS More complex -- UNIX Layered – an abstrcation Microkernel -Mach Simple Structure - MS-DOS 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 Non Simple Structure -- UNIX UNIX – limited by hardware functionality, the original UNIX operating system had limited structuring. The UNIX OS consists of two separable parts Systems programs 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 operating-system functions; a large number of functions for one level Traditional UNIX System Structure Layered Approach The 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 Microkernel System Structure Moves as much from the kernel into user space Mach example of microkernel Mac OS X kernel (Darwin) partly based on Mach Communication takes place between user modules using message passing Benefits: Easier to extend a microkernel Easier to port the operating system to new architectures More reliable (less code is running in kernel mode) More secure Detriments: Performance overhead of user space to kernel space communication Microkernel System Structure Application File Device user Program System Driver mode messages messages Interprocess memory CPU kernel Communication managment scheduling mode microkernel hardware Modules Many modern operating systems implement loadable kernel modules Uses object-oriented approach Each core component is separate Each talks to the others over known interfaces Each is loadable as needed within the kernel Overall, similar to layers but with more flexible Linux, Solaris, etc Solaris Modular Approach Hybrid Systems Most modern operating systems are actually not one pure model Hybrid combines multiple approaches to address performance, security, usability needs Linux and Solaris kernels in kernel address space, so monolithic, plus modular for dynamic loading of functionality Windows mostly monolithic, plus microkernel for different subsystem personalities Apple Mac OS X hybrid, layered, Aqua UI plus Cocoa programming environment Below is kernel consisting of Mach microkernel and BSD Unix parts, plus I/O kit and dynamically loadable modules (called kernel extensions) Mac OS X Structure graphical user interface Aqua application environments and services Java Cocoa Quicktime BSD kernel environment BSD Mach I/O kit kernel extensions iOS Apple mobile OS for iPhone, iPad Structured on Mac OS X, added functionality Does not run OS X applications natively Also runs on different CPU architecture (ARM vs. Intel) Cocoa Touch Cocoa Touch Objective-C API for developing apps Media Services Media services layer for graphics, audio, video Core Services Core services provides cloud computing, databases Core OS Core operating system, based on Mac OS X kernel Android Developed by Open Handset Alliance (mostly Google) Open Source Similar stack to IOS Based on Linux kernel but modified Provides process, memory, device-driver management Adds power management Runtime environment includes core set of libraries and Dalvik virtual machine Apps developed in Java plus Android API Java class files compiled to Java bytecode then translated to executable than runs in Dalvik VM Libraries include frameworks for web browser (webkit), database (SQLite), multimedia, smaller libc Android Architecture Applications Application Framework Libraries Android runtime SQLite openGL Core Libraries surface media Dalvik manager framework virtual machine webkit libc Linux kernel System Boot When power initialized on system, execution starts at a fixed memory location Firmware ROM used to hold initial boot code Operating system must be made available to hardware so hardware can start it Small piece of code – bootstrap loader, stored in ROM or EEPROM locates the kernel, loads it into memory, and starts it Sometimes two-step process where boot block at fixed location loaded by ROM code, which loads bootstrap loader from disk Common bootstrap loader, GRUB, allows selection of kernel from multiple disks, versions, kernel options Kernel loads and system is then running End of Chapter 2