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OS Overview (Services (Program development, Program execution, Access to…

- - - - It includes a processor context (which includes the program counter and stack pointer) and its own data area for a stack (to enable subroutine branching)
      - Executes sequentially and is interruptible so that the processor can turn to another thread.
    - - A collection of one or more threads and associated system resources
  - - - Performance
        
        Greater
      - Availability
        
        Both can do same functions
      - Incremental growth
        
        Additional processors
      - Scaling
        
        Vendors offer a range of products
- - - - Improper synchronization
      - Failed mutual exclusion
      - Nondeterminate program operation
      - Deadlocks
        
        Two or more programs to be hung up waiting for each other
    - - An executable program
      - The associated data needed by the program
      - The execution context of the program:star:
        
        The internal data by which the OS is able to supervise and control the process.
        
        The entire state of the process at any instant is contained in its context
    - - A program in execution
      - An instance of a program running on a computer
      - A unit of activity characterized by a single sequential thread of execution, a current state, and an associated set of system resources
      - The entity that can be assigned to and executed on a processor
  - - - Process isolation
        
        Prevent independent processes from interfering with each other's memory
      - Automatic allocation and management
        
        Programs should be dynamically allocated across the memory hierarchy as required
      - Support of modular programming
        
        Programmers should be able to define program modules, and to create, destroy, and alter the size of modules dynamically
      - Protection and access control
        
        Sharing of memory, at any level of the memory hierarchy, creates the potential for one program to address the memory space of another.
      - Long-term storage
        
        Many application programs require means for storing information for extended periods of time
    - - Implements long-term store
      - Information stored in named objects called files
      - Allows programmers to address memory from a logical point of view
      - When information in virtual memory is not in main memory, program must be suspended in order to transfer that block to main memory
    - - Allows process to be comprised of a number of fixed-size blocks, called pages
      - Virtual addres
        
        Page number and an offset within the page
      - Each page may be located anywhere in main memory
      - Real address or physical address
        
        Main memory address
  - - - Availability
      - Confidentiality
      - Data integrity
      - Authenticity
  - - - Give equal and fair access to resources
    - - Discriminate among different classes of jobs
    - - Maximize throughput, minimize response time, and accommodate as many uses as possible
- - - - No OS
      - Programs in machine code
      - Problems
        
        Scheduling
        
        Reserved time (more or less enough)
        
        Setup time
        
        A single program, called a job, could involve loading the compiler plus the high-level language program (source program) into memory, saving the compiled program (object program), and then loading and linking together the object program and common functions
    - - Use of a piece of software known as the monitor
      - The user no longer has direct access to the processor
      - Monitor
        
        Software that controls the sequence of events
        
        Batch jobs together
        
        Program returns control to monitor when finished
      - Job Control Language (JCL)
        
        Special type of programming language used to provide instructions to the monitor
      - Hardware features
        
        Memory protection
        
        Does not allow the memory area containing the monitor to be altered
        
        Timer
        
        Prevent a single job from monopolizing the system
        
        Privileged instructions
        
        Certain machine level instructions can only be executed by the monitor
        
        Interrupts
        
        Gives the OS more flexibility in relinquishing control to and regaining control from user programs
      - Memory protection
        
        Modes
        
        User mode
        
        System/kernel mode
    - - Multiprogramming :star:
        
        When one job needs to wait for I/O, the processor can switch to the other job
        
        Mode of operation that provides for the interleaved execution of two or more computer programs by a single processor
    - - The processor is shared among multiple users
      - Using multiprogramming to handle multiple interactive jobs
      - Multiple users simultaneously access the system through terminals
      - CTSS
        
        Overwrite processes