[OS] Introduction 2

Introduction Part 2

Computer-System Architecture

• Single-processor
• Multi-processor (parallel systems)
  1. Increased throughput
  2. Economy of scale : shared common resources
  3. Increased reliability : if one CPU have problem, then other CPU can do the task
     • Assymmetric Multiprocessing : each processor is assigned a task
     • Symmetric Multiprocessing : each processor performs all tasks
• Multi-core : communicate within one chip, so faster than Multi-processor

Operating System Structure

• Multiprogramming (Batch System) : needed for efficiency
  • Single user cannot keep CPU and I/O devices busy at all times
  • Multiprogramming organizes jobs so CPU always has one to execute
  • A subset of total jobs in system is kept in memory
  • One job selected and run via job scheduling
  • When it has to wati(ex.I/O interrupt), OS switches to another job.
• Timesharing(multitasking) : CPU switches jobs so frequently that users can interact with each job while it is running
  • Several users use 1 CPU
  • Response time should be < 1 second
  • Each user has at least one program executing in memory (process)
  • If several jobs are ready to run at the same time, Os schedules CPU (CPU scheduling)

Operating System Operations

• Interrupt driven
  • Hardware interrupt (by one of the devices) : occurs asynchronously (at any time)
  • Software interrupt (exception or trap)
    • Software error (ex. division by zero)
    • Request for operating system device
    • Other process problems include infinite loop, processes modifying each other in the OS
    • Exception : synchronous interrupts, generated by CPU at the end of the instruction
    • Trap : kind of exception typically with the purpose of debugging
  • Dual mode
    • Dual-mode operation allows OS to protect itsef and other system components
    • User mode (mode bit : 1)
    • Kernel mode (mode bit : 0)
    • Mode bit is provided by hardware
      • provides ability to distinguish when system is running user code or kernel code
      • some instructions designated as privileged, only executable in kernel mode
      • system call changes mode to kernel, return from call resets it to user

Transition from User to Kernel Mode

• Initial phase is controlled by operating system, and instructions are executed on kernel mode
• If user process get the control, then mode change to user mode (mode bit : 1)
• Operating system regain the control by interrupt, trap and system call

• The application runing on user mode use system call to get authorized service by kernel mode

• Timer
  • It is important to guarantee that operating system can continue the control of CPU
  • Timer used to prevent the case if user program is in infinite loop or failure of system call eg.
  • In the kernel mode, OS set a timer to generate an interrupt after some time period (privileged instruction)
  • In the user mode, an application process keeps using the CPU
  • The timer generates a timer interrupt when the timer expires (switched to the kernel mode)
  • The OS can make a decision on the process

Process Management

• Program
  • A collection of instructions that can be executed by a computer to perform a specific task
  • passive entity (static : “saved” state)
• Process
  • A program in execution, a unit of work within the system
  • active entity (dynamic : working in main memory)
• Process needs resources (CPU, memory, I/O, files, etc…) to accomplish its task
• Process termination requires reclaim of any reusuable resources (OS takes all back)
• Single-threaded process has one program counter specifying location of enxt instruction to execute
• Process executes instructions sequentially, one at a time, until completion
• Multi-threaded process has one program counter per thread

Process Management Activities

The operating system is responsible or the following activities in connection with process management

• Creating and deleting both user and system processe
• Suspending and resuming processes
• Providing mechanisms for process synchronization
• Providing mechanisms for process communication
• Providing mechanisms for deadlock handling

Memory Management

• To execute a program, all of the instructions must be in memory
• All of the data that is needed by the program must be in memory
• Memory management determines what is in memory and when optimizing CPU utilization and computer resources to users

Memory Management Activities

• Keeping track of which parts of memory are currently being used and by whom
• Deciding which processes and data to move into and out of the memory
• Allocating and deallocating memory space as needed

Storage Management

• OS provides uniform, logical view of information storage
• File : abstracts physical properties to logical storage unit
• Each medium is controlled by device (ex. disk drive, tape drive)

File-System Management

• Files usually organized into directories
• Access control on most systems to determine who can access what
• OS activities include
  • Creating and deleting files and directories
  • Primitives to manipulate files and directories
  • Mapping files onto secondary storage
  • Backup files onto stable(non-volatile) storage media

Mass-Storage Managment

• Usually disks used to store data that does not fit in main memory or data that must be kept for a long period of time
• OS activities include
  • Free-space managemnet
  • Storage allocation
  • Disk scheduling

Performance of various levels of storage

Migration of data “A” from disk to register

• Multitasking environments must be careful to use most recent value, no matter where it is stored in the storage hierarchy
• Multiprocessor environment must provide cache coherency in hardware such that all CPUs have the most recent value in their cache
  • Assume client A (process A) and client B (process B) share the variable “X”, and the value is 0
  • After Client A write 1 to variable X, Client B read the variable X
  • Variable X that client B read have the value “0”, but Client A have the variable X = 1
  • This is data dismatch problem, so it is important to control cache coherency

I/O subsystem

I/O subsystem responsible for memory managemnt of I/P

• Buffering : storing data temporarily while it is being transferred
• Caching : storing parts of data in faster storage for performance
• Spooling : copying data between different devices, usually fast -> slow device

Protection and Security

• Protection : any mechanism for controlling access of processes or users to resources
• Security : defense of the system against internal and external attacks

Computing Environments - Virtualization

• Allows operating system to run applications within other OS
• VMM (Virtual machine manager) provides virtualization services
• VMM control the access for resource