Operating Systems: Introduction and Structure

Chapter 1: Introduction

• An operating system acts as an intermediary between the user of a computer and the computer hardware.

Computer System Structure

• Computer systems consist of four main components: hardware, operating system, application programs, and users.
• Hardware provides the basic computing resources, including CPU, memory, and I/O devices.
• The operating system controls and coordinates the use of hardware among various applications and users.
• Application programs define the ways in which system resources are used to solve computing problems for users, for example, word processors, compilers, web browsers, database systems, and video games.
• Users can be people, machines, or other computers.

Operating Systems (OS)

• An OS is a program that manages a computer's hardware and provides a basis for application programs.
• Goals for OSs include executing user programs, making solving user problems easier, making the computer convenient, and using computer hardware efficiently.

What Operating Systems Do

• From the user's view, the OS aims for convenience, ease of use, and good performance and is not as concerned with resource utilization.
• From the system’s view, the OS is intimately involved with the hardware and acts as a resource allocator of resources like CPU time, memory space, file-storage space, and I/O devices.
• The OS manages all resources and decides between conflicting requests for efficient and fair resource use.
• The OS controls the execution of programs to prevent errors and improper use of the computer.
• The OS is the one program running at all times, known as the kernel, alongside system programs and application programs.

Computer System Organization

• A modern general-purpose computer system is made up of CPUs and device controllers connected via a common bus for shared memory access.
• A bootstrap program, stored in ROM or EPROM (firmware), initializes the system and loads the OS kernel.
• Interrupts, triggered by hardware or software (system calls or monitor calls), signal events.
• Interrupts transfer control to interrupt service routines through the interrupt vector, which contains the addresses of all the service routines.
• A trap or exception is a software-generated interrupt caused by an error or user request.
• The OS is interrupt driven.

Storage Structure Review

• The basic unit of computer storage is the bit, which can contain one of two values: 0 or 1.
• Other storage in a computer is based on collections of bits
• A word is a given computer architecture’s native unit of data
• Data in computer storage is measured and manipulated in bytes and collections of bytes
• The storage hierarchy is:
  • Kilobyte (KB) is 1,024 bytes.
  • Megabyte (MB) is 1,024 to the power of 2 bytes.
  • Gigabyte (GB) is 1,024 to the power of 3 bytes.
  • Terabyte (TB) is 1,024 to the power of 4 bytes.
  • Petabyte (PB) is 1,024 to the power of 5 bytes.

Memory

• Main memory allows direct CPU access via random access and is typically volatile.
• Secondary storage provides large, nonvolatile storage capacity as an extension of main memory.
• Solid-state disks are faster, nonvolatile storage, while hard disks use magnetic recording.
• Disk surfaces are logically divided into tracks subdivided into sectors.
• The disk controller ensures logical interaction between the device and the computer

Multiprocessing Architecture

• Multiprocessor systems, also known as parallel systems, have increased throughput, economy of scale, and reliability.
• Two types of multiprocessing exist: asymmetric, where each processor is assigned a specific task, and symmetric, where each processor performs all tasks.

Operating System Structuring

• Batch systems use multiprogramming for efficiency, organizing jobs so the CPU always has one to execute.
• Job scheduling is used, and the OS switches to another job when one has to wait.
• Timesharing (multitasking) allows users to interact with jobs while running.
• Response time should be less than 1 second.
• Each user has at least one program executing in memory, known as a process.
• Swapping moves processes in and out to run if they don't fit in memory.
• Virtual memory allows for the executions of processes not completely in memory.

Operating System Operations

• The OS is interrupt driven, encompassing both hardware and software interrupts.
• A hardware interrupt is triggered by devices, while a software interrupt (exception or trap) can be caused by errors or requests.
• Dual-mode operation protects the OS and system components via user mode and kernel operating modes.
• A mode bit indicates the current operating mode of a computer: kernel(0) or user (1), it distinguishes between user code and kernel code.
• Certain instructions are privileged and only executable in kernel mode.
• A system call changes the mode to kernel, and returning from the call resets it to user.

Protection and Security

• Protection is any mechanism in the OS that controls access of processes or users to the resources.
• Security is the defense of the system against internal and external attacks, denial-of-service, and viruses.

Computing Environments

• A trend is toward providing more ways to access computing environments through portals that provide Web accessibility to internal servers.
• Network computers (or thin clients) are cost-effective and provide web-based access.
• Mobile computers interconnect via wireless networks.
• Distributed computing involves networked, heterogeneous systems, often using TCP/IP.
• Types of computer networks include Local Area Networks (LAN), Wide Area Networks (WAN), Metropolitan Area Networks (MAN) and Personal Area Networks (PAN).
• Client-Server computing involves clients requesting services from central servers.
• Peer-to-Peer computing allows systems to act as both clients and servers.
• Virtualization enables operating systems to run as applications within other operating systems.
• Cloud Computing delivers computing, storage, and apps as service across a network.
• Cloud types include Public Clouds for public access, Private Clouds run by a company, or Hybrid Clouds include both public.
• Computing as a service
  • Software as a Service provides applications.
  • Platform as a Service provides software stacks for application.
  • Infrastructure as a Service provides servers and storage.

Chapter 2: Operating-System Structures

• An operating system (OS) provides an environment for the execution of programs and services to programs and users

Operating System Services

• An OS provides services to users, such as user interfaces (UI) and program execution.
• UI consists of command-line interface (CLI), graphics user interface (GUI), and batch.
• There is also is I/O operations, such file-system manipulation like creating, deleting, searching, and listing.
• Communications between computers over a network and error detection of possible errors are also provided.
• The OS ensures its efficient operation via resource sharing such as resource allocation, accounting, and protection and security that controls outsider access.
• There is protection where all system resource access is controlled.
• System security requires user authentication and extends the defending of external I/O devices.

User Operating System Interface

• CLI or command interpreter, shell, allows the entry of direct commands and executes commands from users.
• The Graphics User Interface (GUI) is a user-friendly desktop interface.
• Items in GUI are represented as Icons which represent files, programs, actions, etc.
• The actions are carried out by use of a keyboard, mouse or other input devices.
• Many systems now include both CLI and GUI interfaces

System Calls

• They acts as a programming interface to the OS that are typically written in high-level languages like C or C++.
• An application programming interface (API) accesses programs via a high-level use rather than a direct system call.
• Three common APIs exist: Win32 API for Windows, POSIX API for POSIX-based systems, and Java API for the Java virtual machine (JVM).

Common System Calls

• Process control: create, terminate, get attributes, wait, and locks.
• File management: create, delete, open, close, read, write, and reposition.
• Device management: request, release, attach, and detach devices.
• Information maintenance: time, date, system data, file attributes.
• Communications: establish, terminate, send, receive messages, transfer status.
• Protection and security: control access, get/set permissions, allow/deny access.

System Programs

• These provide a convenient environment for program development, execution, and can be divided into specific types.
• File management involves operations such as create, delete, copy, as well as file modification via Text editors that can modify files.
• One can get status on the system, such as date, time, memory, disk space, and detailed performance and logging information
• Programming language support: Compilers, assemblers, debuggers, and interpreters are often supported.
• Program loading and execution
• Communications: Virtual connections for users, browse web pages, messaging, email.
• Background services: Launched at boot, disk, process scheduling, error logging, and printing.
• Application programs: Doesn't pertain to the OS and are run by users.

OS Design

• Implementation of OS design is not "solvable", but approaches have been proven successful.

Design Goals

• User goals should be convenient, easy to learn, reliable, safe, and fast.
• System goals should be easy to design, implement, maintain, flexible, reliable, error-free, and efficient.

Variations

• Varying from early OS programs to early programming languages, such as ALGOL or PL/1 to modern C and C++.
• Usually a combination of languages, lowest levels are in assembly, and Main body programs.

Operating System Structure Types

• Those include, simple structure, complex, layered, and Microkernel.
• MS-DOS is of Simple structure.
• UNIX is more complex.
• Mach uses microkernels.

Chapter 3: Processes

• A program is passive entity and a process becomes an active entity.

Process Control

• This is the fundamental task of any operating system management..

Process Structure

• Text Section program code.
• Data Section that contains global variables.
• Heap contains memory dynamically allocated.
• Stack contains temporary data, function parameters and local variables.

Process Control Block (PCB)

• Operating systems need to consider their dynamic behaviors for better control of processes
• This contains process identification information: numeric identifiers or identifiers of the parent process.
• Processor state information includes running, waiting, and the program counter is the instruction location to next execute.
• CPU registers contains all process-centric registers.
• CPU scheduling information indicates priorities, and scheduling queue pointers.
• Memory-management information shows memory allocated.
• Accounting information indicates CPU use, clock time, and time limits
• I/O status information is for devices allocated, and open files

Process State Types

• new: The process is being created
• running: Instructions are being executed
• waiting: Some event occurs
• ready: The process is to be assigned to a processor
• terminated: The process has finished execution

Schedulers

• Short-term scheduler (or CPU scheduler): Executed next and allocates quickly.
• Long-term scheduler (or job scheduler): Brought into the ready queue and is slow.
• Medium-term scheduler: Multiple programming is needed, process removed from memory and stored to continue.

Interprocess Communication (IPC)

• Communication can be independent or cooperating.
• Cooperating processes cause effects or are affected.
• Two Models of IPC share memory and message pasing.

Chapter 4: Threads

Threads Introduction

• A thread is a unit of CPU utilization that comprises a thread ID, a program counter, a register set, and a stack and shares with the process containing it (code, data, open files, system resources).
• Multithreading enables concurrent drawing and input.
• Most kernels are now multithreaded performing interrupt handling, memory management and managing devices.

Multicore programming

• This provides a mechanism for more efficient management of processors/ multi cores.

Three common models for Multithreading:

• The one-to-one model assigns each user thread to a kernel thread, which enables parallel.
• The main issue for a one-to-one model is one side requires creation of a parallel thread.
• The many-to-one-model can map all user threads to a kenre thread.
• In the Many-to-Many model Many user level threads are mapped to numerous of new threads.

Benefits of Threads

• Benefits include responsiveness if blocked, sharing easier, cheaper execution and scalability.

Thread libraries

• That are available will have an API for use by programmers for easy creation and management control purposes.
• There are three thread libraries in use, POSIX Pthreads Windows and Java.