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Transcript
Lecture 5 Operating-System Services and Structure These slides are based on slides which are copyright Silberschatz, Galvin and Gagne, 2005 Operating System Definition Intermediary between a user of a computer and the computer hardware. Control program Controls execution of programs to prevent errors and improper use of the computer Resource allocator Manages all resources Decides between conflicting requests for efficient and fair resource use OS 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 Affected by choice of hardware, type of system OS Design and Implementation (Contd.) 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 A View of Operating System Services OS Services Helpful to the User User interface (UI): Command-Line (CLI) Graphics User Interface (GUI) Tangible interfaces User Interface - CLI CLI allows direct command entry Sometimes multiple flavors implemented – shells Primarily fetches a command from user and executes it Sometimes commands built-in, sometimes just names of programs User Interface - GUI User-friendly desktop metaphor interface Usually mouse, keyboard, and monitor Icons represent folders, files, programs, actions, etc. Various mouse buttons over objects in the interface cause various actions. 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) OS Services Helpful to the User (Contd.) Program execution - OS must be able to load a program into memory, run that program, and end execution (either normally or abnormally) I/O operations - A running program may require I/O, which may involve a file or an I/O device. File-system manipulation - The file system is of particular interest. Obviously, programs need to read and write files and directories, create and delete them, search them, list file Information. OS Services Helpful to the User (Contd.) Communications – Processes may exchange information, on the same computer or over a network. Communications may be via shared memory or through message passing. 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 OS Services for Efficient Operation 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 protocol, others (such as I/O devices) may have general request and release protocol. Accounting - To keep track of which users use how much and what kinds of computer resources OS Services for Efficient Operation (Contd.) Protection and security - to control use of shared data (on a multiuser system or a network) 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. Kernel the central component of an OS. It has complete control over everything that occurs in the system. http://en.wikipedia.org/wiki/Kernel_(computer_science) Hierarchical Protection Domains (protection rings) Hardware supported rings were among the more revolutionary concepts introduced by the Multics. However, most generalpurpose UNIX systems use only two rings, even if the hardware it runs on provides more CPU modes than that. X86-processors have four different modes divided into four different rings. http://en.wikipedia.org/wiki/Privilege_ring Monolithic Kernel The monolithic kernel offers everything the OS needs: CPU scheduling, memory management, multiprogramming, interprocess communication (IPC), device access, file systems, network protocols, etc. Newer monolithic kernels have a modular design, which offers run-time adding and removal of services. The whole kernel runs in "kernel mode" in which the software has full control over the machine. The processes running on top of the kernel run in "user mode", in which programs have only access to the kernel services. Modern OS with monolithic kernels: Linux, FreeBSD, NetBSD, Solaris http://tunes.org/~unios/oskernels.html Microkernel Moves as much from the kernel into “user” space 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 Disadvantages: Performance overhead of user space to kernel space communication Example microkernels: Mach, L4, Minix, QNX Hybrid Kernels Similar to microkernels, except that they include additional code in kernel space so that such code can run more swiftly than it would were it in user space. These kernels represent a compromise that was implemented by some developers before it was demonstrated that pure microkernels can provide high performance. Hybrid kernels should not be confused with monolithic kernels that can load modules after booting (such as Linux). Example OS with hybrid kernels: Microsoft Windows, DragonFly BSD, Mac OS X http://www.linfo.org/kernel.html Mac OS X Structure Monolithic vs. Microkernel System Structure Monolithic kernel Microkernel http://en.wikipedia.org/wiki/Kernel_(computer_science)