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1.     Introduction

• A computer system can be roughly divided into the hardware, the operating system, the application programs, and the users.
• The hardware (i.e., CPU, memory, I/O devices) provides the basic computing resources.
• The application programs (e.g., web browsers, word processors, and compilers) are used to solve user problems.
• The operating system controls and coordinates the use of hardware among various application programs.

• PC Users: an operating system is a program that provides an easy-to-use interface for using the hardware.
• Mainframe/Minicomputer Users: an operating system is a program that helps maximize the system resource utilization.
• Workstation Users: an operating system is a program designed to compromise between individual usability and resource utilization.

• A Resource Manager: the operating system allocates and reclaims the system hardware resources to and from user programs.
• A Control Program: the operating system controls the execution of user programs to prevent errors and improper use of the computer.
• There is no universal definition of what is an operating system. A common definition is that the operating system is the one program running at all times on the computer (i.e., the kernel). All other programs are application programs.

        1.1.3 System Goals
         

• Primary Goal: efficient operation of the computer system.
• In some systems, it is easy of use.
• In general, efficiency is far more important than easy of use.

1.2    Mainframe Systems (Batch, Multiprogrammed, Time-Sharing)

        1.2.1  Batch Systems
                     

• Batch systems allowed automatic job sequencing by a resident operating system.

                     

• Job pool(jobs) -> Job scheduling -> Memory(jobs) -> CPU scheduling -> CPU
• Multiprogramming is the first instance where the operating system must make decisions for the users (So there's a CPU scheduler).
• The objective of multiprogramming is to have some processes running at all times, so as to maximize CPU utilization.

                     

• Time sharing (or multitasking) is a logical extension of multiprogramming. The CPU executes multiple jobs by switching among them, but switches occur so frequently that the user can interact with each program while it is running.
• An interactive (or hands-on) computer system provides direct communication between the user and the system.
• A time-shared operating system uses CPU scheduling and multiprogramming to provide each user with a small portion of a time-shared computer. Each user had at least one separate program in memory.
• A program loaded into memory and executing is commonly referred to as a process.
• Virtual memory is a technique that allows the execution of a job that may not be completely in memory.
  • Programs can be larger than physical memory.
  • Virtual memory abstracts main memory into a large, uniform array of storage, separating logical memory as viewed by the user from physical memory.
• The objective of time-sharing is to switch the CPU among processes so frequently that users can interact with each program while it is running.

1.4    Multiprocessor Systems

          

• Multiprocessor systems (parallel systems, tightly coupled systems) have more than one processor in close communication, sharing the computer bus, the clock, and sometimes memory and peripheral devices.
• The ability to continue providing service proportional to the level surviving hardware is called graceful degradation. Systems designed for graceful degradation are also called fault tolerant.
• Symmetric multiprocessing (SMP):
  • Each processor runs an identical copy of the operating syste.
  • These copies communicate with one another as neede.
  • Each processor performs all tasks within the OS.
  • All processors are peers; no master-slave relationship exists.
• Asymmetric multiprocessing:
  • Each processor is assigned a specific task.
  • A master processor controls the system; the other processors either look to the master for instruction or have predefined tasks.
  • Thus, this defines a master-slave relationship.
• Advantages:
  
  • Increased throughput: get more jobs done.
  • Economy of scale: because of resource sharing, multiprocessor systems are cheaper than multiple single processor systems.
  • Increased reliability: the failure of one processor will not halt the whole system.

• A distributed ystem is a collection of physically separate, possible heterogeneous computer systems that are networked to provide the users with access to the various resources that the system maintains.
• By being able to communicate, distributed systems are able to share computational tasks, and provide a rich set of features to users.
• Distributed systems (loosely coupled systems): each processor has its own local memory. The processors communicate with one another through various communication lines.
• Distributed systems allow sharing of resources on geographically dispersed hosts.
• A network operating system is an OS that
  
  • Provides features such as file sharing across the nework.
  • Includes a communication shceme that allows different processes running on different computers to exchange messages.
• A  computer  in a  network operating  system acts autonomously from all other computers on the network, although it is aware of the network and is able to communicate with other computers.
• A distributed operating system is less autonomous.
• Different operating systems communicate closely enough to provide the illusion that only a single operating system controls the networ.

        1.5.1  Client-Server Systems
                     

• Some systems are assigned as servers to satisfy requests generated by client systems.
• Client-server systems are specialized distributed systems.
• In general, there are two types of server systems
  • Compute-Server Systems: provide an interface to which clients can send requests to perform an action. Then, execute the action and send back the results.
  • File-Server Systems: provide a file system interface where clients can create, update, read and delete files.

        1.5.2  Peer-to Peer Systems

1.6    Clustered Systems

• A clustered system has two or more individual systems connected by a local area networks or interconnection network.
• The key of clustered system is high availability.
• Clustered systems differ from parallel systems in that they are composed of low or more individual systems coupled together. Clustered computers share storage and are closely linked via LAN networking.
• Clustered systems allow multiple machines to perform computations on data contained on shared storage, and let computing continue in the case of failure of some subset of cluster members.
• Asymmetric clustering:
  • One machine is in hot-standby mode while others are running the applications.
  • The host-standby machine (i.e., does nothing but) monitors other machines and becomes active if one server fails.
• Symmetric clustering:
  • Two or more hosts are running applications, and they are monitoring each other.
  • This is more efficient as it uses all available hardware.

1.7    Real-Time Systems

• A real-time operating system has well-defined, fixed time constraints.
• Processing must be done within the defined constraints, or the system will fail.
• A real-time system is used when rigid time requirements have been placed on the operation of a processor or the flow of date.
• Hard real-time system guarantee that critical tasks be completed on time.
• Soft real-time systems prioritize critical tasks. That is, a critical real-time task gets priority over other tasks, and retains that priority until it completes.
• Embedded systems almost always run real-time operating systems.

1.8    Handheld Systems
1.9    Feature Migration
1.10  Computing Environments
        1.10.1          Traditional computing
        1.10.2          Web-Based Computing
        1.10.3          Embedded Computing