Operating System (OS) Fundamentals

General Description of the Operating System (OS)

• An OS is a program that controls the execution of application programs
• It serves as an interface between applications and hardware

Objectives of an OS

• Convenience: Makes the computer more comfortable to use
• Efficiency: Allows computer system resources to be used efficiently
• Ability to Evolve: Allows for the effective development, testing, and introduction of new system functions without interfering with service

Layers of a Computer System

• The computer system typically comprises layers like hardware, OS, utilities, application programs, programmers, and end-users

Services Provided by the OS

• Program development is facilitated through editors and debuggers
• Program execution is enabled
• Access to I/O devices is controlled
• File access is managed and controlled
• System access is provided

Error Detection and Response

• The OS handles hardware errors, which can be internal or external, including memory errors and device failures
• Software errors like arithmetic overflows or attempts to access prohibited memory locations are managed
• The OS addresses situations where it is unable to fulfill an application's request

Accounting

• Collecting usage statistics aids in monitoring performance and anticipating future enhancements
• It is also used for billing purposes

Resource Management

• The OS manages resources
• Functions like regular computer software, as a running program
• The OS relinquishes control of the processor

Kernel

• A core part of the OS resides in main memory
• The kernel contains the most frequently used functions
• Also known as the nucleus

OS Evolution

• This involves updating the OS with hardware upgrades, new types of hardware, new services, and corrections

Systems Evolution

• Serial processing lacked an OS
• Machines operated from a console with lights, switches, input devices, and printers, requiring scheduled time
• Setup included loading compilers, source programs, saving compiled programs, and linking

Simple Batch Systems

• These systems use monitors which have software controls and event sequences
• Jobs are batched together, and programs return to the monitor upon completion

Job Control Language (JCL)

• This is a special type of programming language
• It provides instructions to the monitor: specifying which compiler to use and what data to utilize

Hardware Features

• Memory protection prevents modification of the monitor area
• A timer prevents jobs from monopolizing the system
• Privileged instructions can only be executed by the monitor
• Interrupts were not available in early computer models

Memory Protection

• User programs run in user mode, restricting certain instructions
• The monitor operates in system mode, also known as kernel mode
• Privileged instructions are executed and protected memory areas become accessible

Limitations of Early Systems

• I/O devices are slow
• A sample computing usage showed ≅ 3.2% CPU utilization

Uniprogramming

• The processor waits for an I/O instruction to complete before proceeding

Multiprogramming

• When a job needs to wait for I/O, the processor switches to another job

Time Sharing

• Time sharing utilizes multiprogramming to handle multiple interactive jobs
• Processor time is shared among users
• Multiple users can access the system simultaneously via terminals

Compatible Time-Sharing System (CTSS)

• CTSS was an early time-sharing system developed at MIT

Key Achievements in OS

• Processes
• Memory management
• Information protection and security
• Scheduling and resource management
• System structure

Processes

• A process is a program in execution and an instance of a program running on a computer
• It's an entity assignable to and executable on a processor
• It is characterized by a single sequential thread of execution, a current state, & associated system resources

Design Difficulties in System Software

• Incorrect synchronization, failure to ensure signals are received from I/O devices
• Mutual exclusion issues related to resource access
• Non-determinate program operation due to dependencies on external activities
• Deadlocks which are system halts

Process Components

• An executable program
• Associated data required by the program
• The execution context, including all information needed by the OS to manage the process

Memory Management

• Processes isolation, automatic allocation, and modular programming support are key
• Protected access control and long-term storage are enabled

Virtual Memory

• Enables programmers to address memory logically
• Eliminates pauses between the execution of successive processes by managing secondary storage for process data

Virtual Memory and File System

• Implements long-term storage and stores information in named objects called files

Paging

• Allows a process to be made up of fixed-size blocks which are called pages
• A virtual address is a page number and displacement within the page
• Each page can reside anywhere in main memory
• Real or Physical address refers to the location in main memory

Information Protection and Security

• Availability ensures system protection against interruption
• Confidentiality ensures users cannot read unauthorized data
• Integrity of data provides data protection against unauthorized access or modification
• Authenticity verifies the identity and validity of data

Programming and Resource Management Goals

• Fairness ensures equal and just resource access
• Differential responsiveness is the ability to discriminate among different classes of jobs
• Efficiency maximizes throughput and minimizes response time to accommodate as many uses as possible

Key Elements of an OS for Multiprogramming

• The OS requires elements for service calls, as well as interrupt handling, scheduling, and queue management

System Structure

• Viewing the system as a series of levels allows for a subset of related functions being performed at each level
• Each level relies on the level below for primitive functions and decomposes problems into manageable parts

Levels of Process Hardware

• Level 1 includes electronic circuits with registers, memory cells, and logic gates, performing operations like clearing registers or reading memory
• Level 2 involves the processor's instruction set with instructions for operations, such as adding, subtracting, loading and storing memory
• Level 3 adds procedures/subroutines and call/return operations
• Level 4 handles interrupts

Concepts with Multiprogramming

• Level 5 treats processes as programs in execution that can be suspended and resumed
• Level 6 relates to secondary storage devices and the transfer of data blocks
• Level 7 creates logical addressing spaces for processes and organizes virtual address spaces

Dealing with External Objects

• Level 8 involves info and message communication between processes
• Level 9 enables long-term file storage with naming conventions
• Level 10 provides access to external devices through standard interfaces
• Level 11 maintains associations between external and internal identifiers
• Level 12 offers full process support
• Level 13 functions as interface for the actual user

Modern Operating Systems

• Microkernel architecture assigns only essential functions like address spaces, inter process communication (IPC), and basic scheduling to the kernel

Multiprocessing Overview

• Processes are divided into subprocesses that execute simultaneously
• A sub-process is a dispatchable unit of work that can be sequentially executed and interrupted
• The computer process comprises one or more

Symmetric Multiprocessing (SMP)

• In SMP multiple processors share the same main memory and perform identical functions
• All processors are equal

Distributed Operating Systems

• They give the illusion of a single main memory space and single secondary memory space

Object-Oriented Design

• This includes the modular extensions used for small kernels
• Programmers can customize the OS without affecting system integrity

Windows Architecture

• Its modular structure provides flexibility and supports executing on varied hardware platforms
• It also supports applications written for other OS

Windows OS Organization

• A modified microkernel architecture is used
• Many system functions run outside the microkernel in kernel mode
• Modules can be updated or replaced without system-wide rewriting

Kernel Mode Components

• This is a set of systems with basic OS services like memory and process / sub-process management, security, I/O, and between process communication
• It implements core components
• A HAL, or Hardware Abstraction Layer, isolates platform-specific hardware differences
• Device drivers and GUI exist

Windows Executive

• E/S Manager, Cache Manager, Object Manager, Plug and Play Manager, Power Manager, Security Reference Monitor, Memory Manager, Processes/Subprocesses Manager and LPC for configuration are used

User Mode Processes

• These are special system support processes as well as session login and session management processes, environmental management processes, services and applications

Client/Server Model

• Simplifies the executive by enabling construction of varied APIs and improving client reliability
• Each service runs in an independent process, with memory partitioning and restricted client access to hardware
• It has a uniform framework that uses applications to communicate via LPC

Windows Objects

• Encapsulation: Consisting of data and procedures
• Class or instance of an object including generating object instances
• Inheritance is supported and polymorphism and inheritance is included in the executive

Threads and SMP

• OS routines execute on available processors and different routines execute in different, simultaneous, processors
• Multiple subprocesses run simultaneously in various processors with sharing of data and other resources

UNIX

• It has hardware surrounded by software and calls the OS the "system nucleus"
• Comes standard with shell program, standard service package, and C tool component bundle