OPERATING SYSTEMS OVERVIEW

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.

3 An interface between users and hardware - an environment "architecture” Allows convenient usage Allows efficient usage; parallel activity, avoids wasted cycles Provides information protection Gives each user a slice of the resources Acts as a control program. OPERATING SYSTEM OVERVIEW

4 The Layers Of A System Program Interface Humans User Programs O.S. Interface O.S. Hardware Interface/ Privileged Instructions Disk/Tape/Memory

5 A mechanism for scheduling jobs or processes. Scheduling can be as simple as running the next process, or it can use relatively complex rules to pick a running process. A method for simultaneous CPU execution and IO handling. Processing is going on even as IO is occurring in preparation for future CPU work. Off Line Processing; not only are IO and CPU happening concurrently, but some off-board processing is occurring with the IO. OPERATING SYSTEM OVERVIEW Components

6 The CPU is wasted if a job waits for I/O. This leads to: Multiprogramming ( dynamic switching ). While one job waits for a resource, the CPU can find another job to run. It means that several jobs are ready to run and only need the CPU in order to continue. All of this leads to: memory management resource scheduling deadlock protection which are the subject of the rest of this course. OPERATING SYSTEM OVERVIEW Components

7 Other Characteristics include: Time Sharing - multiprogramming environment that's also interactive. Multiprocessing - Tightly coupled systems that communicate via shared memory. Used for scientific applications. Used for speed improvement by putting together a number of off- the-shelf processors. Distributed Systems - Loosely coupled systems that communicate via message passing. Advantages include resource sharing, speed up, reliability, communication. Real Time Systems - Rapid response time is main characteristic. Used in control of applications where rapid response to a stimulus is essential. OPERATING SYSTEM OVERVIEW Characteristics

8 OPERATING SYSTEM OVERVIEW Characteristics 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 or a user request. An operating system is interrupt driven.

9 OPERATING SYSTEM OVERVIEW Hardware Support These are the devices that make up a typical system. Any of these devices can cause an electrical interrupt that grabs the attention of the CPU.

10 OPERATING SYSTEM OVERVIEW Hardware Support Sequence of events for processing an IO request. Comparing Synchronous and Asynchronous IO Operations

11 OPERATING SYSTEM OVERVIEW Hardware Support This is O.S. Bookkeeping. These structures are necessary to keep track of IO in progress.

Computer-System Architecture Most systems use a single general-purpose processor (PDAs through mainframes) 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

Clustered Systems Like multiprocessor systems, but multiple systems working 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 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

16 Very fast storage is very expensive. So the Operating System manages a hierarchy of storage devices in order to make the best use of resources. In fact, considerable effort goes into this support. OPERATING SYSTEM OVERVIEW Storage Hierarchy Fast and Expensive Slow an Cheap

17 Performance: OPERATING SYSTEM OVERVIEW Storage Hierarchy

18 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 OPERATING SYSTEM OVERVIEW Storage Hierarchy

19 The goal is protecting the Operating System and others from malicious or ignorant users. The User/Supervisor Mode and privileged instructions. Concurrent threads might interfere with others. This leads to protection of resources by user/supervisor mode. These resources include: I/O Define I/O instructions as privileged; they can be executed only in Supervisor mode. System calls get us from user to supervisor mode. OPERATING SYSTEM OVERVIEW Protection

20 Memory A user program can only access its own logical memory. For instance, it can't modify supervisor code. Depends on an address translation scheme such as that shown here. OPERATING SYSTEM OVERVIEW Protection

Desktop Systems Personal computers – computer system dedicated to a single user. I/O devices – keyboards, mice, display screens, small printers. User convenience and responsiveness. Can adopt technology developed for larger operating system’ often individuals have sole use of computer and do not need advanced CPU utilization of protection features. May run several different types of operating systems (Windows, MacOS, UNIX, Linux)

Parallel Systems Multiprocessor systems with more than on CPU in close communication. Tightly coupled system – processors share memory and a clock; communication usually takes place through the shared memory. Advantages of parallel system: Increased throughput Economical Increased reliability graceful degradation fail-soft systems

Parallel Systems (Cont.) Symmetric multiprocessing (SMP) Each processor runs and identical copy of the operating system. Many processes can run at once without performance deterioration. Most modern operating systems support SMP Asymmetric multiprocessing Each processor is assigned a specific task; master processor schedules and allocated work to slave processors. More common in extremely large systems

Symmetric Multiprocessing Architecture

Distributed Systems Distribute the computation among several physical processors. Loosely coupled system – each processor has its own local memory; processors communicate with one another through various communications lines, such as high-speed buses or telephone lines. Advantages of distributed systems. Resources Sharing Computation speed up – load sharing Reliability Communications

Distributed Systems (cont) Requires networking infrastructure. Local area networks (LAN) or Wide area networks (WAN) May be either client-server or peer- to-peer systems.

Real-Time Systems Often used as a control device in a dedicated application such as controlling scientific experiments, medical imaging systems, industrial control systems, and some display systems. Well-defined fixed-time constraints. Real-Time systems may be either hard or soft real-time.

Real-Time Systems (Cont.) Hard real-time: Secondary storage limited or absent, data stored in short term memory, or read-only memory (ROM) Conflicts with time-sharing systems, not supported by general-purpose operating systems. Soft real-time Limited utility in industrial control of robotics Useful in applications (multimedia, virtual reality) requiring advanced operating-system features.

Handheld Systems Personal Digital Assistants (PDAs) Cellular telephones Issues: Limited memory Slow processors Small display screens.

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

Open-Source Operating Systems Operating systems made available in source- code format rather than just binary closed- source Counter to the copy protection and Digital Rights Management (DRM) movement Started by Free Software Foundation (FSF), which has “copyleft” GNU Public License (GPL) Examples include GNU/Linux, BSD UNIX (including core of Mac OS X), and Sun Solaris