Operating Systems PDF

OPERATING SYSTEMS General Overview and Terminologies Topic Coverage:  What Operating Systems Do  Computer-System Organization  Computer-System Architecture  Operating-System Structure  Operating-System Operations  Process Management  Memory Management  Storage Management ...

OPERATING SYSTEMS General Overview and Terminologies Topic Coverage:  What Operating Systems Do  Computer-System Organization  Computer-System Architecture  Operating-System Structure  Operating-System Operations  Process Management  Memory Management  Storage Management  Protection and Security  Distributed Systems  Special-Purpose Systems  Computing Environments  Open-Source 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 Use the computer hardware in an efficient manner  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 Four Components of a Computer System Operating System Definition OS is a resource allocator Manages all resources Decides between conflicting requests for efficient and fair resource use OS is a control program Controls execution of programs to prevent errors and improper use of the computer Operating System Definition (Cont…) No universally accepted definition “The one program running at all times on the computer” is the kernel. Everything else is either a system program (ships with the operating system) or an application program. 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 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 Startup bootstrap program is loaded at power-up or reboot Typically stored in ROM or EEPROM, generally known as firmware Initializes all aspects of system including the CPU registers, device controllers, and memory contents Locates and loads operating system kernel and starts execution of the first process (such as “init”) and waits for events to occur. 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  Incoming interrupts are disabled while another interrupt is being processed to prevent a lost interrupt  A trap is a software-generated interrupt caused either by an error such as divide by 0 or a user request (system call).  An operating system is interrupt driven Interrupt Handling The operating system preserves the state of the CPU by storing registers and the program counter Separate segments of code determine what action should be taken for each type of interrupt  I/O Structure  After I/O starts, control returns to user program without waiting for I/O completion ◦ System call – request to the operating system 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 ◦ Operating system indexes into I/O device table to determine device status and to modify table entry to include interrupt  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  Storage Structure  Main memory – only large storage media that the CPU can access directly  Programs and data cannot reside in main memory permanently because: ◦ Main memory is limited (too small) to store all programs and data permanently ◦ Main memory is volatile  So secondary storage is provided – extension of main memory that provides large nonvolatile storage capacity  Magnetic disks – 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 Storage Hierarchy  Storage systems organized in hierarchy by ◦ Speed ◦ Cost ◦ Volatility  The higher levels in the hierarchy are expensive but fast. As we move down the hierarchy, the cost per bit generally decreases, whereas the access time generally increases.  Caching – copying information into faster storage system; main memory can be viewed as a last cache for secondary storage Storage-Device Hierarchy 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 How a Modern Computer Works 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 2. Symmetric Multiprocessing Symmetric Multiprocessing Architecture A Dual-Core Design Distributed Systems  Computation is distributed among several processors. In contrast to tightly-coupled systems, the processors do not share a clock or memory. Each has its own local memory.  Communication is via a network. These systems are termed loosely-coupled or distributed systems. The processors vary in size and function and are called nodes.  Advantages of distributed systems: ◦ Reliability: If one node fails, the remaining nodes can continue operating. So by building enough redundancy, the system will not fail if one or more nodes fail (e.g. redundant web servers). ◦ Computation speedup: Computation can be distributed among various nodes to run concurrently (e.g. load balanced web servers).  Distributed Systems (contd.) Resource Sharing: Software, data, and hardware resources can be shared. E.g. data files in node A can be accessed by a user at node B. Files can be printed at a shared laser printer. Communication: Processes at various nodes can exchange information. Clustered Systems  Are a form of distributed systems. Composed of 2 or more independent machines coupled 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 while other machine/server run applications. The hot-standby machine only monitors the active server. If it fails, the hot-standby machine becomes the active server.  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 Clustered Systems (contd.)  An example is the Beowulf cluster where the cluster master node does the management and provisioning and a set of compute nodes that do computations.  Beowulf Clusters are scalable performance clusters based on commodity hardware, on a private network, with open source software (Linux) infrastructure. Each consists of a cluster of PCs or workstations dedicated to running high-performance computing tasks. Operating System Structure Multiprogramming 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 Operating System Structure (Cont.) 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 Memory Layout for Multiprogrammed System Operating-System Operations  Interrupt driven by hardware  Software error or request creates exception or trap ◦ Division by zero, request for operating system service  Other process problems include infinite loop, processes modifying each other or the operating system  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 -End of Presentation-