Operating Systems: An Overview

Introduction to Operating Systems

• An operating system (OS) manages hardware and coordinates application programs for users.
• A computer system consists of hardware, software, and data.
• The OS ensures resources are used correctly.
• An OS is like a government; it provides an environment for other programs to function within.
• Operating systems can be viewed from the perspective of the user and the system.

User View

• The user's view depends on the interface being used.
• Single-user computers are designed to maximize the user's work or play. The OS is optimized for ease of use and performance.
• Multi-user computers share resources to maximize resource utilization. Available CPU time, memory, and I/O are used efficiently.
• Handheld smartphones and tablets have touch screens and are optimized for usability and battery life.
• Embedded computers in home devices and automobiles feature numeric keypads, indicator lights and run without intervention.

System View

• From the computer's viewpoint, the OS is deeply involved with hardware.
• The OS acts as a resource allocator and manages resources and decides between conflicting requests efficiently.
• The OS as a control program controls the execution of programs to prevent errors and improper use.

Defining the Operating System

• An OS is an intermediary between computer users and hardware.
• OS goals are:
  • To execute user programs and solve user problems easily.
  • To make the system user-friendly.
  • To use the hardware efficiently.

Computer System Structure

• A computer system comprises four components:
  • Hardware: Provides computing resources like CPU, memory, and I/O devices.
  • Operating System: Controls hardware use across applications and users.
  • Application Programs: Use resources to solve users' computing problems, for example, word processors, compilers, web browsers, databases, and video games.
  • Users: People, machines, or other computers.

Operating System Definition

• There is no universal definition.
• A good approximation is: everything a vendor ships when ordering an operating system".
• "The one program running at all times on the computer" is the kernel. Other components are either system programs (shipped with the OS) or application programs.

Computer System Organization

• During computer startup, the bootstrap program loads at power-up or reboot.
  • It's stored in ROM or EPROM and known as firmware.
  • It initializes the system and loads the OS kernel, then starts execution.

Computer System Operation

• After kernel loading and execution, it provides services to the system and users.
• Some services are provided by system programs loaded into memory at boot as system processes or daemons.
• On UNIX, the first process is init which starts other daemons with interrupts signaling events.

Computer System Architecture

• Single general-purpose processor systems have special-purpose processors too.
• Multiprocessor systems have increased in use and importance. They're known as parallel systems or tightly-coupled systems.
  • Throughput increases, as there's less time to complete tasks with more processors.
  • There's also economy of scale - less cost when sharing peripherals, storage, and power supplies
  • Reliability is increased through functions distributed among processors, allowing graceful degradation and fault tolerance.
• Symmetric Multiprocessing: Each processor performs all tasks.
• Asymmetric Multiprocessing: Each processor is assigned a specific task.

Multicore Systems

• Modern CPUs include multiple computing cores on a single chip, called multicore systems.
• Multicore systems are more efficient due to:
  • Faster on-chip communication.
  • Lower power consumption than multiple single-core chips.
• Multicore systems are multiprocessor systems, but not all multiprocessor systems are multicore.
• A multiprocessor system has multiple CPUs working in parallel.
• A multicore CPU duplicates CPU components.
• Simultaneous multithreading duplicates a smaller subset, making the core look like two processors.

Clustered Systems

• Clustered systems work together, like multiprocessor systems
• They share storage via a storage-area network (SAN)
• They provide high availability, surviving failures
  • Asymmetric clustering has a hot-standby (master) server which monitors active ones (slaves).
  • Symmetric clustering runs applications across multiple nodes/machines.
• Some clusters are for high-performance computing (HPC) and require parallelization
• Some have a distributed lock manager (DLM) to avoid conflicting operations.

Operating System Structure

• A single user cannot keep the CPU and I/O devices busy all the time.
• Multiprogramming organizes jobs (code and data) for continuous CPU execution.
• A subset of total jobs is kept in memory.
• Batch systems run one job at a time through job scheduling. The OS switches jobs when a job has to wait.
• Timesharing systems allow users to interact with each job while it is running, termed interactive computing.

Timesharing Systems

• Timesharing can be called multitasking and the response time is < 1 second.
• Each user has at least one program, called a process, executing in memory
• CPU scheduling occurs if processes are ready to run.
• Swapping moves processes in and out of memory if processes do not fit in memory.
• Virtual memory allows processes to execute, even when not completely in memory.

Operating System Operations

• A mechanism protects the OS and other system components.
• There are two modes of operations:
  • User mode.
  • Kernel mode.
• A mode bit (0 or 1) provided by hardware distinguishes if user code or kernel code is running.
• Privileged instructions execute in kernel mode only.
• System calls from users change the mode from user to kernel.
• Returning from a system call resets the mode to user.

Hardware Protection

• Privileged instructions are issued only in kernel mode.
• Examples:
  • I/O instruction.
  • Halt.
  • Changing execution mode.
  • Modifying memory-management registers and timer.
  • Switching interrupt services on/off.
• Other hardware protections beside dual mode: I/O, memory, and CPU protection, and kernel mode.

Timer

• A timer, as a hardware device, prevents processes from infinite loops (process hogging of resources).
• A timer is decremented by the physical clock
• The timer is set to interrupt after a time period
• The OS sets the counter (privileged instruction)
• An interrupt generates when it reaches zero
• The OS sets the counter's value to regain control or terminate programs exceeding allotted time

Protection and Security

• Protection controls access to resources.
• Security defends the system
  • There is a range of defenses, including denial-of-service, worms, viruses, identity theft, and theft of service
• Systems distinguish among users:
  • User identities include a name and associated number, one per user
  • User IDs associate with files and processes
  • A group ID allows users to be defined and controls managed
  • Privilege escalation enables a user to change to an effective ID with higher rights.

Process Management

• A process is a program in execution and unit of work; the program is a passive entity, the process is an active entity.
• Processes require resources: CPU, memory, I/O, files, and initialization data
• Process termination reclaims resources
• Single-threaded processes have one program counter for the next instruction's location. The instructions execute sequentially, one at a time, until completion
• Multi-threaded processes have one program counter per thread
• Systems have many processes, running concurrently on one or more CPUs; concurrency is done by multiplexing.

Process Management Activities

• The OS handles
  • Creating and deleting user and system processes.
  • Suspending and resuming processes.
  • Providing mechanisms for process synchronization and communication.
  • Providing mechanisms for deadlock handling.

Memory Management

• Memory management deals with all data and instructions in the course of processing.
• Memory management determines what resides in memory by optimizing CPU utilization and computer response
• Memory management :
  • Keeps track of parts of memory
  • Decides which processes and data to move in and out of memory
  • Allocates and deallocates memory space.

Storage Management

• The OS provides a logical view of information storage and abstracts physical properties to a logical storage unit file.
• Each medium is controlled by a device (e.g., disk drive, tape drive). Their properties include the following - Access speed, capacity, data-transfer rate, and access method
• Access control determines who accesses directories.
• The OS activities are: creating/deleting files/directories and manipulating files and directories, as well as mapping files onto storage, etc.

Mass Storage Management

• Disks store programs for long periods of time and that does not fit into memory
• The programs use the disk as the source and destination of their processing
• OS activities in connection with disk management are:
  • Free-space management.
  • Storage allocation and disk scheduling.
• Tertiary storage is managed by the OS or applications. Tertiary storage includes:
  • Optical storage
  • Magnetic tape
• Storage varies between WORM (write-once, read-many-times) and RW (read-write).

Migration of Integer A from Disk to Register

• In hierarchical storage the same data (“A”) appears in different storage system levels.
• The value of "A" matches only after writing the new value from the magnetic disk.
• In multiprocessors, each CPU can execute a local value of “A” in parallel and must ensure the value updates quickly. This is called cache coherency
• In distributed environments this becomes complex
  • Several copies of the same file is kept on different computers

I/O Subsystem

• The purpose of an OS is to hide the functions of the hardware.
• The I/O subsystem is responsible for:
  • Memory management of I/O, the buffering, caching and spooling
  • The general device driver interface
  • Drivers for specific hardware devices

Types of Operating Systems

• Early systems did not have operating systems; they ran from a console with switches, lights, and input/output devices.
• The programmer acted as an operator.
• Setup included loading the compiler, source program, saving the compiled program and loading and linking.
• They involved one user and serial processing.
• A weakness was CPU idleness and wasted time.

Categories of Operating Systems

• Five categories:
• Batch.
• Interactive.
• Real-Time.
• Hybrid.
• Embedded.
• Distinguishing features: Response time and how data enters the system.

Batch Systems

• Input relied on punched cards or tape.
• Jobs collected and executed as one batch
  • Which reduced setup time.
• A resident monitor or batch monitor was then introduced
  • This was a basic operating system with initial control.
  • Control transfers to jobs and back to the monitor at the end of the job.
  • Use automatic job sequencing and transfers control from one job to another.

Interactive Systems

• Interactive systems:
  • Are faster than batch systems.
  • Are slower than real-time systems.
• Were introduced to provide fast turnaround when debugging.
• Time-sharing software developed for operating systems.

Real-Time Systems (RTOS)

• Essential components are reliability, speed and time limit sensitivity
• Must work in time-critical environments (Space flights, air traffic control)
• Commonly used for: Industrial processes, medical equipment, electricity and telephone switching.
• Must be 100% responsive at all times.

Hybrid Systems

• Hybrid systems are a combination of batch and interactive
• Appear to be interactive. Users access the system with fast responses.
• Accept and run batch programs in the background under conditions of light loads
• Take advantage from free time between high-low demand usage
• Many large computer systems are hybrids

Embedded Systems

• Embedded systems are computers placed inside other products.
• These add features and capabilities with special Operating System requirements
• Specific set of programs need to be performed
• They are not interchangeable among systems
• They use a small kernel and flexible functions

Computing Environments

• Traditional computing
• Mobile computing
• Distributed systems
• Virtualization

Computing Environments - Traditional

• Traditional environments feature, stand-alone general purpose machines.
• Most systems interconnect with others (i.e., the Internet)
• Portals provide access to internal systems.
• Mobile computers use wireless networks while network computers use thin clients or Web terminals.
• Networking is becoming ubiquitous; home systems use firewalls to protect computers.

Computing Environments - Mobile

• Smartphones, tablets, and handheld devices.
• The features include: more OS features(GPS -- Waze),augmented reality, wireless such as IEEE 802.11, cellular data etc

Computing Environments - Distributed

• There are heterogenous systems networked together
• They use a network path, often times TCP/IP
  • Local Area Networks (LAN), Wide Are Networks (WAN) and personal area networks
• They use network operating systems which offer the sharing data between systems
• An illusion of a single system across networks due to communications scheme.

Virtualization

• Virtualization makes the OS able to run applications in other OSes
• In Emulation the source CPU type differs. This is generally the slowest process where languages are not compiled to native
• Virtualization has OS natively compile for the CPU.
• For example, WMware runs WinXP as a guest with apps, running within the native windows system
• Virtual Machine manager (VMM) manages this