21CSC202J Operating Systems Unit I

Introduction to Operating Systems

• Operating System (OS) serves as an intermediary between users and computer hardware.
• Goals of an OS: execute user programs, enhance user convenience, and optimize hardware utilization.

Computer System Components

• Four key components: hardware (CPU, memory, I/O), OS, application programs (e.g., word processors, databases), and users (people or machines).
• OS controls and coordinates hardware among applications and users.

User Perspectives on OS

• User requirements: convenience, ease of use, and performance over resource utilization.
• Different environments: shared systems (mainframes), dedicated workstations, and resource-constrained devices (handhelds).

OS Functions

• OS acts as an allocator of resources, managing demands and ensuring fair usage.
• It controls program execution to prevent errors and improper uses.
• Definitions of OS vary; generally, it comprises the kernel and accompanying system and application programs.

System Startup

• Bootstrapping initializes all system aspects, loading the OS kernel from firmware stored in ROM/EPROM.

Computer Organization and Operation

• Modern systems have multiple CPUs and device controllers connected via a bus, enabling concurrent operations.
• Device controllers manage data flow between I/O devices and main memory.

Interrupts

• Interrupts transfer control to the OS and require saving the CPU's state.
• Types of interrupts: hardware interrupts and software-generated traps (e.g., program errors).
• OS is driven by interrupts, responding to various device and user requests.

I/O Operations and Structure

• Control returns to the user program only upon I/O completion unless otherwise specified.
• Device-status tables track each I/O device status.

Storage Structures

• Main memory is volatile and provides direct access to the CPU, whereas secondary storage (e.g., hard disks) offers larger, non-volatile capacity.
• Storage hierarchy organized by speed, cost, and volatility with caching as a vital mechanism for performance.

Direct Memory Access (DMA)

• Enables high-speed data transfers directly between devices and memory, minimizing CPU intervention and reducing interrupts.

Computer-System Architecture

• Predominantly single processor systems, with increasing adoption of multiprocessor and multi-core systems for better performance and reliability.
• Two multiprocessor types: asymmetric (dedicated tasks) and symmetric (shared tasks).

Clustering Systems

• Multiple interconnected systems provide high availability and fault tolerance.
• Types include asymmetric clustering (hot-standby) and symmetric clustering (active monitoring).

OS Structure and Operations

• Multiprogramming increases CPU usage by organizing jobs, while timesharing enables multiple user interactions.
• Dual-mode operation protects the OS from user processes.
• CPU timer counters prevent resource hogging by interrupting processes after a designated time.

Process Management

• A process is an executing program requiring system resources: CPU, memory, I/O.
• Multi-threading allows concurrent execution within a single process, enhancing efficiency.

Memory Management

• Essential for program execution; it ensures that necessary instructions and data reside in memory during runtime.### Memory Management
• Manages memory allocation and tracks usage to optimize CPU performance and user experience.
• Activities include tracking memory usage, deciding data movement in and out of memory, and allocating/deallocating memory.

Storage Management

• Provides a logical abstraction of physical storage, presenting files instead of raw data.
• Controlled by device drivers with varying access speeds, capacities, and methods (sequential/random).

File-System Management

• Files organized into directories with access control for security.
• OS tasks include creating/deleting files, mapping storage, and backing up data.

Mass-Storage Management

• Disks store data not fitting in main memory or requiring long-term retention.
• Key activities: managing free space, storage allocation, and disk scheduling.
• Tertiary storage includes optical media and magnetic tapes, requiring management despite slower speeds.

Data Migration

• Critical in multitasking environments to ensure the most recent data is accessed.
• In multiprocessor systems, cache coherence is essential to keep data synchronized across CPUs.
• Distributed environments complicate data consistency with multiple copies of information.

I/O Subsystem

• Hides hardware complexities from users; manages data transfer and device operations.
• Responsible for buffering, caching, and spooling to optimize input/output processes.

Protection and Security

• Protection mechanisms control access to resources by processes/users.
• Security focuses on defense against various attacks (e.g., denial-of-service, malware).
• User identification through unique IDs and group IDs that regulate access permissions.

Kernel Data Structures

• Utilizes standard programming structures like linked lists and binary search trees for efficiency.

UNIX Example

• A multitasking OS using fork() to create processes and exec() to load programs.
• Processes exit with a status code indicating success (0) or error (non-zero).

System Programs

• Facilitate program development and execution through several functions: file manipulation, status information, programming support, and communication mechanisms.

Operating System Design and Implementation

• Aimed at balancing user goals (convenience, reliability) and system goals (flexibility, maintenance).
• Separation of policy (what to do) from mechanism (how to do it) enhances adaptability.

Implementation Variations

• Early OSes developed in assembly; modern implementations primarily in C/C++.
• Mix of languages enhances portability but can affect performance.

Operating System Structures

• General-purpose OS may follow various structures: simple (MS-DOS), complex (UNIX), layered, or microkernel (Mach).
• Layered systems allow modular development, while microkernels enhance reliability and security but may introduce overhead.

Modules

• Modern OSes use loadable kernel modules for flexibility and dynamic functionality.
• Interfaces facilitate communication among separate components.

Hybrid Systems

• Combine features from multiple architectures for optimized performance and security.
• Examples include Linux/Solaris (monolithic, modular) and Mac OS X (layered, microkernel hybrid).

Mobile Operating Systems

• iOS, structured on Mac OS X, incorporates touch interfaces and cloud services.
• Android, based on a modified Linux kernel, provides power management and a Java-based development environment.

Operating-System Debugging

• Involves identifying and resolving software bugs with log files and memory dumps.
• Performance tuning executed via trace listings and profiling to analyze and enhance efficiency.### Performance Tuning
• Performance improvement involves removing bottlenecks within an operating system (OS).
• The OS should provide tools for measuring and displaying system behavior.
• Examples of such tools include the “top” program on Unix-based systems and Task Manager on Windows.

DTrace

• DTrace is a powerful tool available on Solaris, FreeBSD, and Mac OS X for live instrumentation on production systems.
• Probes within DTrace activate when specific code is executed, capturing state data for analysis.
• An example use case is tracking the XEventsQueued system call as it transitions from the libc library to the kernel and back.

DTrace Applications (Cont.)

• DTrace can record the duration (in nanoseconds) that each process with UserID 101 spends in a running state on the CPU.

Operating System Generation

• Operating systems are created to function across a variety of machine types, requiring specific configurations for individual hardware setups.
• The SYSGEN program collects hardware configuration data to build a tailored, system-specific kernel that can be more efficient than one-size-fits-all kernels.

System Boot

• System execution begins at a defined memory location upon power initialization, with firmware ROM containing the initial boot code.
• A bootstrap loader, often stored in ROM or EEPROM, is essential for loading the OS into memory and initiating it.
• Booting can be a two-step process where a boot block, loaded by ROM code, finds and loads the bootstrap loader from disk.
• GRUB is a common bootstrap loader that provides options for selecting kernels from various disks and versions, facilitating multi-boot environments.
• Once the kernel is loaded, the operating system becomes operational.