Fundamental Concepts of Operating Systems

Operating System

A layer of abstraction between Hardware and Software

Acts as a "virtual machine" - emulates a machine making it easier for the software to do operations - a resource coordinator, and reactive system

A collection of programs that integrate the hardware resources of the computer and make those resources available to the use in a productive, timely, and efficient manner

Batch Processing

Processes jobs in groups without user interaction.

SPOOLing

Simultaneous Peripheral Operation On-Line.

Reads jobs ahead onto disk for efficiency.

Multiprogrammed Systems

-Provides increased utilization

-Keeps several jobs in memory simultaneously

-I/O processing of one job overlaps with computation of another

-Needs CPU scheduling

-Makes sure the CPU is always busy

Time Sharing Systems

Allows multiple users to share resources interactively.

-Multiple users acting on the same resource

-Supported interactive use

-Each user feels they have the entire machine

-Tries to optimize response time

-Based on time-slicing

"Time slice"

Time Slice

Rapid switching between users for responsiveness.

Desktop Systems

Personal computers that emerged in the 1970s.

Mainframe Systems

Large, powerful computers for bulk data processing.

-Modular, designed with redundancies - so that operations continue even after one part fails

-Adding more storage or peripherals, or maintenance, does not require turning the computer off

-In a year of usage, downtime can only be hours long

I/O Processing

Handles input/output operations for tasks.

Interactive Use

Users engage with the system in real-time.

High-Level Interface

User-friendly interaction through GUI or command line.

Offline Processing

Jobs processed without immediate user interaction.

Multiprocessor Systems

Multiple processors working simultaneously on tasks.

Also known as Parallel Systems/Tightly-Coupled Systems

Advantages of Multiprocessor Systems

Increased Throughput, Economy of Scale, Increased Reliability

Symmetric Multiprocessing (SMP)

All CPUs perform identical tasks.

Asymmetric Multiprocessing

CPUs assigned specific roles for tasks.

Graphics Processing Units (GPUs)

Specialized processors for rendering graphics.

Distributed Systems

Geographically-distributed computing resource utilization.

Also known as Loosely-computed Systems

Clustered Systems

Multiple CPUs working together via a LAN.

Real-Time Systems

Guaranteed response to events within fixed time.

Hard Real-Time Systems - Critical deadlines for device control applications.

Soft Real-Time Systems - Important deadlines, but not critical.

Handheld Systems

Devices like PDAs and mobile phones.

General Structure of an O/S

Resident Programs, Non-Resident Programs

Resident Programs

Critical programs always in memory, like KERNEL.

Non-Resident Programs

Loaded into memory only when needed.

Dual-Mode Operation

User and kernel modes for system security.

Components of an O/S

Management: Process, Main Memory, File, I/O System, Secondary Storage

Networking, Protection System

Process Management

Controls process execution and state tracking.

Main Memory Management

Allocates and deallocates memory for processes.

File Management

Maintains file information and directory structure.

I/O System Management

Standardizes access to input/output devices.

Secondary Storage Management

Manages disk space allocation and scheduling.

Networking

Protocols for data transfer and connection strategies.

Protection System

Controls access to programs and user processes.

Command Interpreter System

Interface for user commands via various input methods.

Architectures

Monolithic, Layered, Virtual Machine, Microkernel

Monolithic Architecture

Single large kernel; minimal structure, no security.

Layered Architecture

Components organized in layers; requires traversals.

Virtual Machine

Multiple systems run on a single machine.

Microkernel

Minimal kernel; controls O/S with modularity.

UNIX Operating System

Developed by Ken Thompson and Dennis Ritchie.

MULTICS

Multiplexed Information and Computer Services

Early time-sharing O/S; faced development issues.

UNICS

Uniplexed Information and Computer Services

Single-task system; precursor to UNIX.

PDP-7

First hardware used to develop UNIX.

PDP-11/20

Second version of UNIX developed here.

Version 3

Used C, Developed in Bell Labs to support Unix

Implements Multiprogramming, Multitasking

Version 6 UNIX

1976, First version distributed outside Bell Labs.

Version 7 UNIX

1978, Considered ancestor of modern UNIX systems.

Unix Design Philosophy

Focus on simplicity, interactivity, and elegance.

UDP Principles

Programs should do one thing well.

Write programs to work together.

Write programs to handle text streams, because that is a universal interface.

3BSD

First implementation of virtual memory in UNIX.

4 BSD

Standard system developed for government use.

4.2BSD

Enabled communication across diverse network facilities.

C Shell

New user interface introduced in 4.2BSD.

Text Editor (vi)

New text editor introduced in 4.2BSD.

Berkeley DARPA Unix

Original project leading to modern Unix systems.

4.4 BSD

Last Berkeley release with significant enhancements.

Includes x.25 networking, new file system organization, enhanced security, improved kernel structure

Berkeley stopped its research after this release

Now

-Currently not limited to Bell, AT&T, Berkeley

-Moved to many different computers

Sun Microsystem - ported BSD to their workstations

DEC - Ultrix, OSF/1

Microsoft - Xenix, Windows/NT heavily influenced by Unix

Santa Cruz Operations - SCO, Linux

-Standardization projects for Unix Environments

IEEE, ISO, ANSI, etc.

1989 - ANSI standardized C (ANSI C)

-Single Unix Specification (SUS)

Latest SUS - two parts:

Base specifications - technically identical to POSIX

X/Open Curses specification

Some parts of the SUS are optional

Unix Design Principles

Guidelines emphasizing simplicity and time-sharing.

Unix Shell

Command interpreter executing user and system commands.

Bourne Shell

First Unix shell with $ prompt by Steve Bourne.

BASH

Bourne Again Shell, free alternative to Bourne shell.

Unix File System

Hierarchical structure managing files and directories. Everything is a file!

Three-tiered Security Structure

Owner, group, universe access levels in Unix.

File Links

Multiple names for the same data clusters.

Ordinary Files

Data stream stored on a device

Directory Files

Files used to access hardware components.

If a file has files below it in the hierarchy, it must be a directory, otherwise, it is an ordinary file or an empty directory file

A directory does NOT contain other files, rather just contain links to them

Different links can be made to the same instance of a file - different directories can link to the same file

System Calls

Interface for user programs to request OS services

Dual-mode Operation

Switching between user mode and kernel mode

Categories of System Calls

Process control, file manipulation, device manipulation

Processes

- Any program in execution

- All CPU activity

Job - unit of work in batch systems.

User Program - A task in time-sharing systems.

Process States

Different stages a process can be in.

Process Control Block (PCB)

Data structure containing process information:

-Process state

-Program Counter

-CPU Registers

-Process Priority

Process Queue

Job Queue - Contains all processes in the system.

Ready Queue - Processes waiting in main memory for execution.

I/O - Processes waiting for I/O device access.

Process Scheduler

Long-term Scheduler, Short-term Scheduler

Long-term Scheduler (Job)

Loads processes from storage to memory.

Short-term Scheduler (CPU)

Allocates CPU to processes in ready queue.

Medium-term Scheduler

Swaps processes in and out of memory.

CPU Scheduling

Selecting processes for execution on CPU.

Process Types

CPU Bound: more time computing

I/O bound: more time doing I/O

Context Switching

Switching from one process to another.

Independent Process

Does not share data with other processes.

Dependent Process

Can affect or be affected by other processes.

Cooperation

Processes working together for shared goals.

Information Sharing, Computation Speedup, Modularity

Information Sharing

Resources shared among processes to avoid duplication.

Computation Speedup

Parallel processes share CPU for faster results.

Modularity

Breaking down large processes into smaller, efficient ones.

Threads

Lightweight processes with shared resources.

Multithreading

Running multiple threads within a single task.

Tasks

Heavyweight processes that can be divided into threads.

Peer Threads

Threads that cooperate to form a task.

Interprocess Communication

Methods for processes to exchange information.

Message Passing

Sending messages between processes via kernel.

Shared Memory

Memory area accessible by multiple processes.

Direct Communication

Explicitly names the process for message passing.

Indirect Communication

Messages sent through a shared mailbox.

Message Queue

Buffer for storing messages in communication.

Bounded Queue

Queue with a limit on message capacity.

Unbounded Queue

Queue limited only by system memory.

Producer-Consumer Problem

Example of dependent processes generating and handling data.

Concurrent Access

Multiple processes accessing shared data simultaneously.