Operating Systems

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Operating Systems

• CS208

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What is Operating System?

• It is a program.
• It is the first piece of software to run after
  the system boots.
• It coordinates the execution of all other
  software.
• It provides various common services needed by
  users and applications.

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Operating System Definition

• Short definition
• a program that manages a computers resources and
  acts as an intermediary between a user and those
  resources

Application
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Operating System (OS) Functions

• For the User Provides a user interface to let
  the user access hardware and software resources.
• For the System Manages all the system tasks and
  resources to provide security, and fair,
  efficient use.

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Operating System
Applications
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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.

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Parts of an Operating System

• User Interface (Shell)
• Menu
• Command Language
• Graphical User Interface (GUI)
• System Security
• Login control
• Process Management
• CPU Scheduler

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Parts of an Operating System

• Memory Management
• Swapper/Pager
• Temporarily unused pages are stored on disk
  (swapped out)
• When they are needed again, they are brought back
  into the memory (swapped in)
• Garbage Collection
• Resource Management
• File Management
• Device Management

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Operating System Terminology

• Kernel
• Software component that controls the hardware
  directly, and implements the core privileged OS
  functions.
• Process
• An executing program.

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Time Sharing

• Allows multiple processes to run on the same
  computer, seemingly at the same time.
• CPU is multiplexed among several processes that
  are kept in memory (the CPU is allocated to a
  process only if the process is in memory).
• A process is swapped in and out of memory to the
  disk as needed.
• OS must manage process scheduling and provide
  memory protection to keep one program from
  crashing the system or corrupting other programs.

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CPU Time Slicing for Time Sharing

• Only ONE process can be running on the CPU at a
  time.
• Each process is allocated a slice of time in
  the CPU.
• When the time runs out, the process is
  interrupted, and another process is loaded into
  the CPU.
• The act of giving each process a small slice of
  time to run is called time slicing.

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CPU Time Slicing
CPU
Main Memory
Process 1
Current Process
Process 2
Process 3
Repeat until all processes have completed.
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CPU execution modes

• CPUs supports (at least) 2 execution modes
• User mode
• The code of the user programs
• Kernel (supervisor, privileged, monitor, system)
  mode
• The code of OS
• The execution mode is indicated by a bit in the
  processor status word (PSW) (a register in the
  CPU)

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Protecting Kernel mode

• OS code executes in the Kernel mode
• Called via interrupts and system calls
• Only the OS code is allowed to be executed in the
  Kernel mode
• The user code must never be executed in the
  Kernel mode
• The program counter (PC) is only set to point to
  the OS code when the CPU goes to the Kernel mode

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Interrupts

• An interrupt is the way by which the hardware
  informs the OS of special conditions that require
  OS attention
• Interrupts cause the CPU not to execute the next
  instruction
• Instead, the control is passed to OS

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System Calls

• A System Call is used by a process to request a
  service from the OS
• Typical system calls
• Open/read/write/close the file
• Get the current time
• Create a new process
• Request more memory

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A typical scenario

• OS executes and chooses (schedules) an process to
  run
• Process runs
• CPU executes process instructions
• OS is not involved
• The system clock interrupts the CPU
• Clock interrupt handler is executed
• The handler is an OS function

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A typical scenario (continued)

• In the interrupt handler Process time has
  expired. The OS chooses another process to run
• Process runs
• CPU executes process instructions
• OS is not involved
• The process performs a system call to read from a
  file.

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A typical scenario (continued)

• The system call causes a trap into the OS
• OS sets up the I/O
• OS schedules another application to run
• The third application runs.
• NOTE At any given time only ONE program can be
  running
• Either the OS or a user process

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Parallel Systems

• Multi-processor systems with more than one 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
• Economical
• Increased throughput
• Increased reliability

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Parallel Systems

• Symmetric multiprocessing (SMP)
• Each processor runs an 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 allocates work to
  slave processors.
• More common in extremely large systems

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Distributed Systems

• Distribute the computation among several physical
  computers.
• Each has its own CPU, local memory, stable
  storage, I/O paths connecting to the environment
• Interconnections
• 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.
• Systems cooperate to maintain shared state and
  coordinate global information

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Distributed Systems

• Advantages of distributed systems.
• Inherent distribution
• Speedup - improved performance due to load
  sharing
• Fault tolerance/Reliability
• Resource Sharing
• Scalability
• Flexibility

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Distributed Systems

• Network Operating System
• provides file sharing
• provides communication scheme
• runs independently from other computers on the
  network
• Distributed Operating System
• less autonomy between computers
• gives the impression there is a single operating
  system controlling the network.