Operating Systems: Introduction and Structure

Introduction to Operating Systems

• The operating system acts as an intermediary between a computer user and the hardware
• OS manages hardware resources by exploiting one or more processors
• Ensures the computer hardware is utilized efficiently
• Provides a convenient user experience
• Consists of components designed to provide services to system users

Computer System Structure

• The computer system comprises four main components: hardware, OS, application programs, and users
• Hardware offers fundamental computing resources like the CPU, memory, and I/O devices
• The OS manages and coordinates hardware utilization across various applications and users
• Application programs use system resources to address user computing problems
• Users can be people, machines, or other computers interacting with the system

Basic Elements of a Computer

• The basic elements are the processor, I/O modules, main memory, and system bus

Operating System Definition

• The OS is a resource allocator that efficiently manages all computer resources
• It makes decisions between conflicting resource requests to ensure fair use
• Controls program execution to prevent errors and misuse of the computer
• There isn’t a universally accepted definition of an OS

The Kernel

• The kernel is "the one program running at all times on the computer”
• Everything else is either a system program or an application program

Computer Startup

• A bootstrap program is loaded during system power-up or reboot
• Usually stored in ROM or EPROM, known as firmware
• It initializes the system and loads the OS kernel for execution

Computer System Organization

• CPUs and device controllers are connected via a common bus, enabling access to shared memory
• This setup allows concurrent execution of CPUs and devices, which compete for memory access

Computer-System Operation

• I/O devices and the CPU work simultaneously
• Each device controller manages a specific type of device and has a local buffer
• The CPU transfers data between main memory and device controllers' local buffers
• Device controllers notify the CPU of task completion through interrupts

Interrupts

• Interrupts disrupt the normal sequence of the processor to improve its utilization
• Without interrupt, the processor must pause and wait for slower I/O device
• The interrupt service routine, accessed via the interrupt vector, gains control
• The architecture saves the address of the interrupted instruction
• Incoming interrupts are disabled during interrupt processing to prevent data loss
• A trap, caused by errors or requests, is a software-generated interrupt
• Operating systems are interrupt-driven

Interrupt handling

• Preserves CPU state by storing registers and the program counter
• Determines the type of interrupt by polling or vectored interrupt system
• Executes specific code segments to handle each interrupt type

I/O Structure

• Control returns to the user program only after the I/O operation completes, idling the CPU with wait instructions and potentially leading to a wait loop
• Device status table contains an entry for each I/O device indicating type, address, and state
• The OS uses the device table to find device status and manages interrupts

I/O Completion without Waiting

• The control returns to the user program without waiting by performing a system call or using device-status tables

Direct Memory Access Structure

• Direct Memory Access Structure is used for high-speed I/O devices for direct data transmission to main memory
• It allows device controllers to transfer data blocks to main memory directly
• Only one interrupt is generated per block, improving efficiency

Storage Structures

• Main memory enables direct access by the CPU
• Secondary storage provides large, non-volatile storage capacity that expands main memory
• Magnetic disks have tracks and sectors and are essential components of Secondary Storage
• Disk controllers manage device-computer interactions

Storage Hierarchy

• Storage systems are arranged by speed, cost, and volatility
• Caching involves copying data to faster storage, using main memory as a secondary storage cache

Caching

• Checks faster storage (cache)
• Faster storage used before using slower storage
• Used in Hardware, Operating System and Software
• An element of caching to note is size and replacement policy

Computer System Architecture

• General-purpose Processors are often used
• Special-Purpose Processors are frequently used
• Multiprocessor systems are increasingly common
  • Improves throughout and economy of scale
  • Includes parallel and tightly-coupled systems
  • Fault Tolerance, or graceful degradation, helps reliability
• Two Types of Multiprocessing
  • Asymmetric Multiprocessing
  • Symmetric Multiprocessing

Clustered Systems

• Clustered systems are similar to multiprocessor systems
• Clusters involve multiple systems working together and sharing storage via a SAN
• High Availability is supplied through services that survive failures
• Asymmetric clustering uses a hot-standby machine
• Symmetric clustering runs applications on multiple nodes and supports high-performance computing

Operating System structure

• Multiprogramming maximizes efficiency
• Uses Job Scheduling, so operating system can switch to another job
• Timesharing or multitasking extends this logic, enabling interactive computing with sub-second response times and virtual memory use
• This requires process management and CPU scheduling

Memory Management

• All Data Located in Memory
• Important During Execution
• Optimize Computer Response to User
• Memory Management Activities Include
  • Track memory parts in use
  • Move data in and out of memory
  • Allocate memory as needed

Storage Management

• Provides a logical view of data as files
• File Systems are Often Organized into Directories
  • Access control exists
  • Can included creating, deleting, manipulating primitives

Mass Storage

• Disks store Main Memory Data "Long Term"
  • Managed for the entire Computer Operation
  • Free Space, Allocation, and Disk Scheduling take place

Data Migration

• Multitasking environments need the most recent value
• Provide data coherency across CPUs
• Managing multiple copies on data

I/O Subsystem

• One purpose of the OS is to hide peculiarities
• I/O Subsystems Also Handles the Following
  • Buffering, Caching, and Spooling
  • General drivers for devices

Protection and Security

• Protection controls access to OS defined resources
• Security defends against internal and external attacks
  • May include worms, viruses, theft, or denial-of-service
  • User Identifications and Group identifications are often used
• Privilege escalation elevates user access

###Computing Environments

• Traditional Computing is blurring
• Current Options Include
  • Office, Home Networks -Client-Server, Peer-to-Peer
  • Web-Based and Cloud

Computing Environments

• Client-Server replaces dumb terminals with smart PCs -Compute-server provides an interface (i.e database)
  • File-server provides an interface (i.e file storage)
• Peer-to-Peer is a distributed model that doesn't distinguish
  • Network Nodes, Services, or Discovery protocols

Other environments

• The web is Ubiquitous
• New Devices use Load Balancers for web traffic
• Windows has evolved and can be a client or server
• Cloud computing has delivered across networks

Cloud

• It has thousands of Petabytes of Storage
• Cloud computing is generally available on the internet
• Public, private, and hybrid components

Open-Source Operating Systems

• Available rather than just binary "closed-source"
• A counter to Copy-Protection and DRM movement
• Started by Free Software Foundation
• Examples include GNU/Linux, BSD UNIX and Sun Solaris