study material for professional knowledge: 
Operating System Concepts(Basics)

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 systems typically contain: Hardware and Software Hardware - electronic, mechanical, optical devices Software – programs
• Without support software, the computer is of little use.
• An interface between Hardware and User Programs
• An abstraction of the hardware for all the (user) processes
• Hide the complexity of the underlying hardware and give the user a better view of the computer

Computer System Components

• Hardware – provides basic computing resources (CPU, memory, I/O devices).
• Operating system – controls and coordinates the use of the hardware among the various application                           programs for the various users.
• Applications programs – define the ways in which the system resources are used to solve the computing problems of the users (compilers, database systems, video games, business programs).
• Users (people, machines, other computers). 

Abstract View of System Components
         

Operating System Definitions

• Resource allocator – manages and allocates resources.
• Control program – controls the execution of user programs and operations of I/O devices .
• Kernel – the one program running at all times (all else being application programs).

The Goals of an OS

•      Let users run programs:
•      Correctness- Memory boundaries, priorities, steady state
•      Convenience-User should not handle the tiny details (encapsulate/abstract), provide synchronization primitives, system calls, file system, tools
•      Efficiency-Resource Utilization, resource Sharing, Multitasking
•      Fairness (in resource allocation)
•      Among: users, tasks, resources
•      The tradeoff between efficiency and fairness

An OS is a Resource Allocator

• Multiple users  get all computing resources “simultaneously”:
• Cpu time
•  Memory (ram, swap, working set, virtual,..)
•  File system (storage space)
•  I/O devices (display, printers, mouse,..)
•  The OS should give every user the illusion that she is getting all resources to herself (not sharing!) 

Virtual  Continuity

• A process can get “switched in” or “switched out”.
• OS should give the illusion for the process as if it exists in the CPU continuously => Context Switching

Context switching

•       When an event occurs, the operating system saves the state of  the active process and restores the state of the new process. 
•      This mechanism is called a Context Switch.
•      What must get saved? Everything that the next process could or will damage. For example:

              Program counter (PC)
            Program status word (PSW)
              CPU registers (general purpose, floating-point)
            File access pointer(s)
            Memory
Scheduling and Context switch
        Give up the CPU == switch out the current process + switch in another process
Preemptive Scheduling

• There are OS’s where a process is forced to give up the cpu (e.g. when stayed for too long).
• Such systems are implementing a “preemptive scheduling” policy.
• Examples include Windows NT, Unix, - BUT NOT - Windows prior to Win95 ! or Macintosh!

Using Priorities

• Most OS’s provide the priority mechanism
• Priorities are associated with processes
• Priority are used to help the OS to reach fairness

Process
    A process is a program in execution.
    The components of a process are:
            the program to be executed,
            the data on which the program will execute,
            the resources required by the program—such as memory and file(s)—and  the status of the execution.

OS Features Needed for Multiprogramming

•      I/O routine supplied by the system.
•      Memory management – the system must allocate the memory to several jobs.
•     CPU scheduling – the system must choose among several jobs ready to run.
•     Allocation of devices.

Time-Sharing Systems–Interactive Computing
   

• The CPU is multiplexed among several jobs that are kept in memory and on disk (the CPU is allocated to a job only if the job is in memory).
• A job swapped in and out of memory to the disk.
• On-line communication between the user and the system is provided; when the operating system finishes the        execution of one command, it seeks the next “control statement” from the user’s keyboard.
• On-line system must be available for users to access data and code.