Chapter 1: Introduction

1. Chapter 1: Introduction

2. Chapter 1: Introduction • 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

3. Objectives • To provide a grand tour of the major operating systems components • To provide coverage of basic computer system organization

4. 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.

5. Operating System • Manages the computer hardware • Varies from machine type to machine type. • Efficiency versus convenience

6. 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

7. Four Components of a Computer System

8. OS from two viewpoints • User View (Ease of use, performance, resource utilization) • PC • Mainframe/Minicomputer terminals • Networks of Workstations and Servers • Handheld computers • System View • OS as a resource allocator • OS as a control program

9. Operating System Definition • OS is a resource allocator • Manages all resources (CPU time, memory space, file space...) • 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. It is especially concerned with the operation and control of I/O devices.

10. Operating System Definition (Cont.) • No universally accepted definition • The fundamental goal of computer systems is to execute user programs and make solving user problems easier. For this, hardware is constructed for which application programs are developed. The common functions of controlling and allocating resources are then brought together into “Operating System”. • “Everything a vendor ships when you order an operating system” is good approximation • But varies wildly • “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

11. Computer Startup • bootstrap program is loaded at power-up or reboot • Typically stored in ROM or EEPROM, generally known as firmware • Initializates all aspects of system, from CPU registers to device controllers to memory contents • Loads operating system kernel and starts execution • The OS then starts executing the first process, such as “init”, and waits for some event to occur. • The occurrence of an event is usually signaled by an interrupt from either hardware or the software. Hardware may trigger an interrupt at any time by sending a signal to the CPU, usually by way of the system bus. Software may trigger an interrupt by executing a special operation called a system call.

12. 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

13. 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. • To ensure orderly access to the shared memory, a memory controller is provided whose function is to synchronize access to memory.

14. 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 (on the stack) • 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 or a user request. • An operating system is interrupt driven.

15. 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 • Interrupts must be handled quickly • A table of pointers (vectors) to interrupts routines can be used • Interrupt vector addresses is then indexed by a unique device number, given with the interrupt request, to provide the address of the interrupt service routine for the interrupting device. • After the interrupt is serviced, the saved return address is loaded into the program counter and the interrupted computation resumes.

16. Interrupt Timeline

17. I/O Structure • Each device controlller is in charge of a specific type of device. Depending on the controller, there may be more than one attached device (eg. SCSI) • Device controller maintains some local buffer storage and a set of special purpose registers. Controller moves the data between the peripheral devices and its local buffer storage. • OSs have a device driver for each device controller. Device driver presents a uniform interface to the device to the rest of OS. • To do I/O • Device driver loads the appropriate registers within the device controller. • Device controller examines the contents of these registers to determine action. • Controller starts the transfer from the device to its local buffer • Once the transfer is complete, the controller informs the device driver via an interrupt. • The device driver then returns control to the operating system, possibly returning the data or a pointer to data if the operation was a read.

18. 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 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.

19. Two I/O Methods Synchronous Asynchronous

20. Device-Status Table

21. Direct Memory Access Structure • Interrupt driven I/O can produce high overhead when used for bulk data movement. To solve this problem, direct memory access (DMA) is used. • After setting up buffers, pointers, and counters for the I/O device, controller transfers an entire block of data directly to/from its own buffer to memory with no intervention by the CPU. • 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 one interrupt per byte. • While the device controller is performing these operations, the CPU is available to accomplish other work. • Some high-end systems use switch rather than bus architecture where the DMA is more effective.

22. Storage Structure • Main memory – only large storage media that the CPU can access directly. Random Access Memory – RAM. • Implemented in semiconductor dynamic random-access memory - DRAM technology. • Interaction is achieved through a sequence of “load” or “store” instructions. • Also, CPU automatically loads instructions from memory • Instruction – execution cycle – von Neumann architecture • An instruction is fetched from memory, stored in Instruction Register. • Instruction is then decoded and may cause operands to be fetched from memory and stored in registers. • Instruction is executed and the result may be stored back in memory.

23. Storage Structure • Secondary storage – 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. • Most programs are stored on a disk until they are loaded into memory.

24. Storage Hierarchy • Storage systems organized in hierarchy. • Speed • Cost • Size • Volatility • Caching – copying information into faster storage system; main memory can be viewed as a last cache for secondary storage.

25. Storage-Device Hierarchy

26. 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

27. Performance of Various Levels of Storage • Movement between levels of storage hierarchy can be explicit or implicit. Data transfer from cache to CPU and registers is a hardware function, whereas transfer from disk to memory is controlled by OS.

28. Migration of Integer A from Disk to Register • Multitasking environments must be careful to use most recent value, not 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 on different computers • Various solutions covered in Chapter 17

29. Computer-System Architecture • Single-Processor Systems • Multiprocessor Systems • Clustered Systems

30. Computer System Architecture • Single Processor Systems • One CPU • May have other special-purpose processors (such as disk controllers) • Run a limited instruction set • Do not run user processes • Sometimes managed by OS • For example a disk controller processor receives a sequence of requests from the main CPU and implements its own disk queue and scheduling algorithm to relieve the main CPU of the overhead of disk scheduling.

31. Multiprocessor systems • Two or more processors in close communication, sharing the computer bus and sometimes the clock, memory, and peripherals. • Advantages: • Increased throughput • The speed-up ratio with N processors is not N however. • Economy of scale • Sharing of peripherals, mass storage, power supplies • Increased reliability • Graceful degredation • Fault tolerance • Failure detection, diagnose and correction • Hardware duplication

32. Multiprocessor Systems • Two types: • Asymmetric multiprocessing in which each processor is assigned a specific task. A master processor controls the system, scheduling and allocating work to slave processors. • Symmetric multiprocessing (SMP) in which each processor performs all tasks within the operating system. All processors are peers.

33. Clustered Systems • Gather together multiple CPUs to accomplish computation • Composed of two or more individual systems coupled together. • High availability service. • Each node can monitor one or more of the others over the LAN. • If the monitored machine fails, the monitoring machine can take ownership of its storage and restart the applications that were running on the failed machine. • Structure • Asymmetric clustering: One machine is in hot-standby mode while the other is running applications. Hot-standby machine only monitors the active server. • Symmetric clustering:Two or more hosts are running applications and are monitoring each other • Parallel Clusters allow multiple hosts to access the same data on the shared storage. May need a distributed lock manager (DLM).

34. 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 (job pool) 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 • 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 • Job scheduling: Which jobs to bring to memory from job pool on disk. • 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 (physical memory versus logical memory)

35. Memory Layout for Multiprogrammed System

36. Operating-System Operations • Interrupt driven by hardware • Software error or request creates exception or trap • Division by zero, invalid memory access, request for operating system service • Other process problems include infinite loop, processes modifying each other or the operating system. An error should cause problems only for the one program that was running. • 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. If tried to be executed in user mode, trapped to OS. • System call changes mode to kernel, return from call resets it to user

37. Operating System Operations • Whenever a trap or interrupt occurs, the hardware switches from user to kernel mode. The system always switches to user mode before passing control to a user program. • When a system call is executed, it is treated by the hardware as a software interrupt. Control passes through the interrupt vector to a service routine in the OS, and the mode bit is set to kernel mode.

38. Transition from User to Kernel Mode • Timer to prevent infinite loop / process hogging resources • Set interrupt after specific period • Operating system decrements counter • When counter zero generate an interrupt • Set up before scheduling process to regain control or terminate program that exceeds allotted time

39. 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

40. 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

41. Memory Management • All data in memory before and after processing • All instructions in memory in order to execute • Memory management determines what is in memory when • Optimizing CPU utilization and computer response to users • To improve utilization, computers must keep several programs in memory, creating a need for memory management • 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

42. Storage 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 • OS implements the abstract concept of a file by managing mass storage media and the devices that control them. • 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 dirs • Mapping files onto secondary storage • Backup files onto stable (non-volatile) storage media

43. 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 • Free-space management • Storage allocation • Disk scheduling • Some storage need not be fast • Tertiary storage includes optical storage, magnetic tape • Still must be managed • Varies between WORM (write-once, read-many-times) and RW (read-write) • OS mount and unmount media in devices, allocating and freeing the devices for exclusive use by processes, and migrating data from secondary to tertiary storage.

44. 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

45. 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

46. Distributed Systems • Collection of physically separate, possibly heterogenous computer systems that are networked to provide the users with access to the various resources • Networks vary by the protocols used (TCP/IP, ATM,...), the distances and the transport media. • Most OSs support TCP/IP. To an OS, a network protocol simply needs a network adapter with a device driver to manage it, as well as software to handle data. • Networks (LAN, WAN, MAN) • Media (copper wire, fiber strands, microwave dishes,...) • Network Operating System is an OS providing features such as file sharing accross the network and that includes a communication scheme that allows different processes on different computers to exchange messages.

47. Special Purpose Systems • Real-Time Embedded Systems • Limited OS features, little or no user interface, preferring to spend their time monitoring and managing hardware devices. • Run real-time operating systems (RTOS). Processing must be done within the defined constraints • Multimedia Systems • Multimedia data consists of audio and video files as well as conventional files. Must be delivered (streamed) according to certain time restrictions (e.g. 30 frames per second). • Handheld Systems • Special purpose embedded OSs. • Small size and memory • Small and slow processor to consume less power • Limited I/O (small size, web clipping) • Wireless technology, such as BlueTooth or 802.11, allowing remote access to e-mail and web browsing.

48. Computing Environments • Traditional computer • Blurring over time • Office environment • PCs connected to a network, terminals attached to mainframe or minicomputers providing batch and timesharing • Now portals allowing networked and remote systems access to same resources • Home networks • Used to be single system, then modems • Now firewalled, networked • Traditional time-sharing systems are uncommon. The same scheduling technique is still in use on workstations and servers, but frequently the processes are all owned by the same user.

49. Computing Environments (Cont.) • Client-Server Computing • Dumb terminals supplanted by smart PCs • Many systems now servers, responding to requests generated by clients • Compute-server provides an interface to client to request services (i.e. database) • File-server provides interface for clients to store and retrieve files

50. Peer-to-Peer Computing • Another model of distributed system • P2P does not distinguish clients and servers • Instead all nodes are considered peers • May each act as client, server or both • Node must join P2P network • Registers its service with central lookup service on network, or • Broadcast request for service and respond to requests for service via discovery protocol • Examples include Napster and Gnutella