Operating Systems Concepts

Module Learning Objectives

  • What is an Operating System?
  • Components of an Operating System
  • Computer System Operation
  • Operating System Functions and Structure
  • Operating System Storage
  • Operating System Cache/Memory
  • Operating System Management
  • Open Source Operating Systems

What is an Operating System?

  • An operating system (OS) is a program that acts as an intermediary between a user of a computer and the computer hardware.
  • OS goals:
    • Execute user programs and make solving user problems easier
    • Make the computer system convenient to use
    • Use the computer hardware in an efficient manner
  • OS tasks:
    • Controls and coordinates use of hardware among various applications and users
  • OS is a resource allocator that manages conflicting requests of resources
  • OS is a control program that controls execution of programs

Bootstrap and Kernel

  • When does an OS start running?
    • Bootstrap program is loaded at power-up or reboot
    • Typically stored in BIOS/UEFI on ROM (Read-Only Memory) or EPROM (erasable programmable read-only memory), generally known as firmware
    • The BIOS is a set of computer instructions in firmware which control input/output operations
    • Loads operating system kernel and starts execution of computer system operation
    • “The one program running at all times on the computer” is the kernel

Computer System

  • Components of a Computer System:

    • Computer Hardware
    • Operating System
    • System/Application Programs
    • Users

  • Operation of a Computer System:

    • One or more CPUs, device controllers connect through a common bus providing access to shared memory
    • Concurrent execution of CPUs and devices competing for memory cycles
    • I/O devices and the CPU can execute concurrently
    • Each device controller is in charge of a particular device type
    • Each device controller has a local buffer
    • CPU moves data from/to main memory to/from local buffers
    • I/O is from the device to the local buffer of the controller
    • Device controller informs CPU that it has finished its operation by causing an interrupt

I/O Structure and Operation

  • After I/O starts, control returns to the user program only upon I/O completion
  • Wait instruction idles the CPU until the next interrupt
  • Wait loop (contention for memory access)
  • At most one I/O request is outstanding at a time, no simultaneous I/O processing
  • Alternatively: system call – request to the OS to allow the user to wait for I/O completion
  • Device-status table contains entry for each I/O device indicating its type, address, and state
  • OS indexes into the I/O device table to determine device status and to modify table entry to include interrupt

Computer Storage

  • The basic unit of computer storage is the bit: a bit can contain one of two values, 0 and 1
  • All other storage in a computer is based on collections of bits
  • Given enough bits, it is possible to represent numbers, letters, images, movies, sounds, documents, processes, programs, etc.
  • A byte is 8 bits, and on most computers it is the smallest convenient chunk of storage
  • A word is the computer architecture’s native unit of data; example: 64-bit registers and 64-bit memory addressing typically have 64-bit (8-byte) words
  • A computer executes many operations in its native word size rather than a byte at a time

Storage Quantities

  • A kilobyte, or KB, is 1,024 bytes
  • A megabyte, or MB, is 1,024^2 bytes
  • A gigabyte, or GB, is 1,024^3 bytes
  • A terabyte, or TB, is 1,024^4 bytes
  • A petabyte, or PB, is 1,024^5 bytes

Computer Storage Structure

  • Main memory – the large storage medium the CPU can access directly
    • Random access
    • Typically volatile
  • Secondary storage – extension of main memory that provides large nonvolatile storage capacity
  • Hard disks – rigid metal or glass platters with magnetic recording material
    • Disk surface is logically divided into tracks, which are subdivided into sectors
    • Disk controller determines the logical interaction between the device and the computer
  • Solid-state disks – faster than hard disks, nonvolatile
    • Various technologies
    • Becoming more popular

Storage Device Hierarchy

  • Registers
  • Cache
  • Main memory
  • Solid-state disk
  • Hard disk
  • Optical disk
  • Magnetic tapes

Storage Performance Levels

  • Level 1: Registers
    • Size: < 1 KB
    • Implementation: custom memory with multiple ports
    • Access time: 0.25-0.5\ \text{ns}
    • Bandwidth: 20{,}000-100{,}000\ \text{MB/sec}
    • Managed by: compiler
  • Level 2: Cache
    • Size: < 16 MB
    • Implementation: CMOS on-chip or off-chip
    • Access time: 0.5-25\ \text{ns}
    • Bandwidth: 5{,}000-10{,}000\ \text{MB/sec}
    • Managed by: hardware
    • Backed by: main memory
  • Level 3: Main memory
    • Size: < 64 GB
    • Implementation: CMOS SRAM
    • Access time: 80-250\ \text{ns}
    • Bandwidth: 1{,}000-5{,}000\ \text{MB/sec}
    • Managed by: operating system
  • Level 4: Solid-state disk
    • Size: < 1 TB
    • Implementation: flash memory
    • Access time: data not explicitly specified in the slide
    • Bandwidth: data not explicitly specified in the slide
    • Managed by: operating system
  • Level 5: Magnetic disk
    • Size: < 10 TB
    • Implementation: magnetic disk
    • Access time: 25{,}000-50{,}000\ \text{ns}
    • Bandwidth: \sim 500\ \text{MB/sec}
    • Managed by: disk
  • Level 6: Magnetic tape (as the next level in the hierarchy beyond disks per the provided sequence)
    • Data not explicitly specified in the excerpt; generally used for archival storage

Caching

  • Important principle: caching is performed at many levels (hardware, OS, software)
  • Idea: information in use is copied from slower storage to faster storage temporarily
  • If data is in the cache, use it directly from the cache (fast)
  • If not in cache, copy data to cache and then use it there
  • Cache is smaller than the storage it caches
  • Cache management is a major design problem: cache size and replacement policy

DMA Structure

  • DMA stands for Direct Memory Access
  • Used for high-speed I/O devices able to transmit information at close to memory speeds
  • Device controller transfers blocks of data from buffer storage directly to main memory without CPU intervention
  • Only one interrupt is generated per block, rather than one interrupt per byte

Operating System Structure

  • Multiprogramming (Batch system) needed for efficiency:
    • A single user cannot keep the CPU and I/O devices busy at all times
    • Multiprogramming organizes jobs (code and data) so CPU always has one to execute
    • A subset of total jobs is kept in memory
    • One job is selected and run via job scheduling
    • When a job has to wait (e.g., for I/O), the OS switches to another job
  • Timesharing (multitasking) is the logical extension in which the CPU switches jobs so frequently that users can interact with each job while it is running
    • Target: response time should be < 1 second
  • Process: a program in execution; each user has at least one program executing in memory
  • CPU Scheduling: when several jobs are ready to run at the same time
  • If processes don’t fit in memory, swapping moves them in and out to run
  • Virtual memory allows execution of processes not completely in memory

Operating System Operations

  • Interrupt-driven (hardware and software)
    • Hardware interrupt by one of the devices
    • Software interrupt (exception or trap): software error (e.g., division by zero) or request for OS service
    • Other problems include infinite loop, processes modifying each other or the OS
  • Dual-mode operation allows the OS to protect itself and other components:
    • User mode and kernel mode
    • Mode bit provided by hardware
    • Ability to distinguish when the system is running user code or kernel code
    • Some instructions are privileged, only executable in kernel mode
    • System call changes mode to kernel, return from call resets it to user

Process Management

  • A process is a program in execution; a unit of work within the system
  • Program vs process:
    • Program is a passive entity; process is an active entity
  • A process needs resources to accomplish its task: CPU, memory, I/O, files; initialization data
  • Process termination requires reclaim of any reusable resources
  • Single-threaded process has one program counter; executes instructions sequentially until completion
  • Multi-threaded process has one program counter per thread
  • In a system there are many processes, some user, some OS; running concurrently on one or more CPUs
  • Concurrency achieved by multiplexing CPUs among the processes/threads

Storage Management

  • OS provides a uniform, logical view of information storage
  • Abstracts physical properties to a logical storage unit – the file
  • Each medium is controlled by a device (disk drive, tape drive)
  • Varying properties include: access speed, capacity, data-transfer rate, access method (sequential or random)
  • OS activities include:
    • Creating and deleting files and directories
    • Primitives to manipulate files and directories
    • Mapping files onto secondary storage
    • Backing up files onto stable (non-volatile) storage media

Memory Management

  • To execute a program, all (or part) of the instructions must be in memory
  • All (or part) of the data needed by the program must be in memory
  • OS performs memory management to determine what should be in memory and when
    • Aims to optimize CPU utilization and user response
  • Memory management activities:
    • Keeping track of which parts of memory are currently used and by whom
    • Deciding which processes (or parts) and data to move into and out of memory
    • Allocating and deallocating memory space as needed

Kernel Data Structures

  • What is the Kernel in the OS?
    • A kernel is a central component of an operating system that manages the operations of computers and hardware
    • It is the core part of an OS
    • It is the bridge between software applications and the hardware of a computer

Kernel Data Structures – Examples

  • Singly Linked List
    • A singly linked list is a data structure consisting of nodes where each node contains data and a pointer to the next node
  • Doubly Linked List
    • A doubly linked list contains three parts: data, a pointer to the previous node, and a pointer to the next node
  • Circular Linked List
    • A circular linked list links the last node back to the first node; there is no distinct start or end node

Open Source Open Source OS

  • Open Source OS basics:
    • Operating systems made available in source-code format rather than just binary (closed-source)
    • Counter to copy protection and Digital Rights Management (DRM) movement
    • Started by Free Software Foundation (FSF), which promotes copyleft GNU Public License (GPL)
    • Examples include GNU/Linux and BSD UNIX (including core of Mac OS X), and many more
  • Open source can be used with virtual machines (VMs) like Hyper-V, VMware, VirtualBox, etc.
  • Open source can be used to run guest operating systems

Practical and Philosophical Implications

  • Open Source OS implications:
    • Transparency and potential for security improvements due to public scrutiny
    • Freedom to modify and customize for specific needs
    • Licensing (e.g., GPL) imposes obligations on modifications and redistribution
  • OS design choices impact performance, reliability, and security:
    • Cache design and replacement policies influence system speed
    • Memory management and virtual memory affect program locality and capacity to run large workloads
    • I/O strategies (interrupts, DMA) affect throughput and CPU utilization
  • Ethical/practical considerations:
    • DRM and copyleft debates influence user rights, software freedom, and vendor control
    • Open source promotes governance models focused on collaboration and transparency

To Dos for this module

  • Quiz: Operating System Basics
  • Essay: Operating System Basics
  • Assignment: Data and OS Architecture Binary Math
  • Discussion: Learning Reflection