Operating Systems: Computer System Structure

• Maria Fazio is the professor for Operating Systems at the Università degli Studi di Messina.
• Operating Systems is a course at CdL in Informatica.

Computer System

• Standard components including a monitor, external speaker, keyboard and mouse.
• Internal components include RAM chips, expansion slots, video card, network card, A/V card, internal speaker, computer tower, power supply, cooling fan, floppy drive, CD/DVD-ROM drive, CD-RW drive, hard drive, battery, microprocessor, cooling blades, clock and motherboard.

Von Neumann Architecture

• Memory, Input/Output Interfaces and CPU are the primary blocks.
• The CPU contains the registers and ALU.
• Memory has a mass memory component.
• Interfaces connect to Video/keyboard.
• BUS carries data, addresses and control signals.

Computer System Structure

• Computer systems are composed of hardware, OSs, application programs and users.
• Users include People, machines, other computers who need computing resources.
• Application programs define how the system resources solve user problems.
• Word processors, compilers, web browsers, database systems, and video games are examples of application programs.
• The OS controls and coordinates the use of hardware among applications and users.
• Hardware provides basic computing resources like the CPU, memory, and I/O devices.

Operating System Goals

• OSs act as intermediaries between computer users the computer hardware.
• Execute user programs and make problem-solving easier.
• They aim to make computer systems convenient and use hardware efficiently.

What Operating Systems Do

• Shared computers, like mainframes or minicomputers, must keep all users happy by efficiently managing hardware use and program execution as a resource allocator and control program.
• Dedicated systems, like workstations, use dedicated resources but share from servers.
• Mobile devices are resource-poor and are optimized for usability and battery life with mobile user interfaces such as touch screens and voice recognition.
• Embedded computers in devices and automobiles have little to no user interface, running primarily without user intervention.

Defining Operating Systems

• An OS term covers many roles.
• They exist in various devices such as toasters, ships, spacecraft, game machines, TVs and industrial control systems due to myriad designs and uses.
• OSs emerged when fixed-use military computers became more general and needed resource management and program control.

Different Operating Systems

• Mac OS is a series of graphical user interface-based operating systems developed by Apple Inc. for their Macintosh computers.
• Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution.
• Microsoft Windows is a series of graphical interface operating systems developed, marketed, and sold by Microsoft.
• iOS (previously iPhone OS) is a mobile OS developed and distributed by Apple Inc,.
• Android is a Linux-based operating system designed primarily for touchscreen mobile devices.
• BSD/OS was known for its reliability in server roles.

Evolution of Operating Systems

• Operating Systems have evolved from Serial Processing to Batch Processing, Multiprogramming, Time Sharing, Parallel Systems, Network-based and finally to Distributed Systems.

Operating System Definition

• An OS is the set of basic programs and utilities that enable a computer to run.
• The kernel is the program that runs at all times on the computer
• Everything else is either a system program or an application program.
• System programs ship with the OS but are not part of the kernel.
• Application programs are programs not associated with the OS.
• Today's OS often include middleware with software frameworks for databases, multimedia, and graphics.

Computer System Organization

• Computer system operation comprises of CPUs, device controllers and memory connected by a common bus
• Multiple CPUs and devices concurrently compete for memory cycles

Computer-System Operation

• I/O devices and the CPU can execute concurrently managed by device drivers.
• Each device controller manages a specific device type.
• Each controller has a local buffer.
• The CPU moves data to/from main memory to/from the local buffers.
• The I/O operates from the buffer of controller.
• The device controller informs the CPU of completed operations through an interrupt.

Common Functions of Interrupts

• Interrupts transfer control to an interrupt service routine through the interrupt vector, which contains the addresses of all service routines.
• Interrupt architecture must save the address of the interrupted instruction.
• A trap or exception is a software-generated interrupt caused by an error or a user request.
• An OS is interrupt-driven.

Storage Structure

• Main memory is the only large storage the CPU can access directly using random access and is typically volatile.
• It consists of random-access memory in the form of Dynamic Random-access Memory (DRAM).
• Secondary storage supplements main memory with large, nonvolatile storage capacity.

Storage Structure (Cont.)

• Hard Disk Drives (HDDs) have rigid metal or glass platters covered with magnetic recording material.
• Disk surfaces are logically divided into tracks, which are further subdivided into sectors with interactions is managed by disk controller.
• Non-Volatile Memory (NVM) is faster and nonvolatile.
• Modern Non-Volatile Memory is becoming more popular as capacity and performance increases with price drops.

Storage Definitions and Notation Review

• The bit is the basic unit of computer storage.
• A bit holds either 0 or 1.
• Storage is measured in bytes as 8 bits; this is the smallest convenient chunk of storage on most computers.
• Word is a less common term defined as a given computer architecture's native unit of data made up of one or more bytes.
• A computer executes instructions at its native word size rather than by the byte.
• A computer with 64-bit registers typically has 64-bit (8-byte) words.
• Storage is measured in bytes as the following:
  • A kilobyte (KB) is 1,024 bytes.
  • A megabyte (MB) is 1,024^2 bytes.
  • A gigabyte (GB) is 1,024^3 bytes.
  • A terabyte (TB) is 1,024^4 bytes.
  • A petabyte (PB) is 1,024^5 bytes.

Storage Hierarchy

• Storage systems are organized in a hierarchy based on speed, cost, and volatility.
• Caching copies information into a faster storage system, where main memory acts as a cache for secondary storage.
• Each device controller has a device driver to manage I/O, which provides a uniform interface between the controller and kernel.

Computer-System Architecture

• Most systems use a single general-purpose processor but may have special-purpose processors.
• Parallel or tightly-coupled systems are growing in use.
• Advantages of multiprocessor systems are increased throughput, economy of scale, and increased reliability through graceful degradation or fault tolerance.
• Asymmetric Multiprocessing assigns each processor a specific task.
• Symmetric Multiprocessing assigns each processor performs all tasks.

Clustered Systems

• Clustered systems are like multiprocessor systems but involve multiple systems working together.
• They share storage via a storage-area network (SAN).
• Clustered systems provide high-availability that can survive failures.
• Asymmetric clustering has one machine in hot-standby mode.
• Symmetric clustering has multiple nodes running applications and monitoring each other.
• Some clusters are for high-performance computing (HPC) designed with special applications that use parallelization.
• Some use a distributed lock manager (DLM) to avoid conflicting operations.

PC Motherboard

• Provides all components of a computer

Computer Startup

• A bootstrap program initializes the system and loads the operating system kernel, beginning execution,
• The bootstrap program is loaded at power-up or reboot and is typically stored in ROM or EPROM, as firmware.

How a Modern Computer Works

• Through a thread of execution via instruction execution cycle
• Movement between the CPU, cache memory, device and memory
• Following A von Neumann architecture

Direct Memory Access Structure

• It is essential for high-speed I/O devices able to transmit data close to memory speeds.
• Device controllers transfer blocks of data from buffer storage directly to main memory without CPU intervention.
• Only one interrupt is generated per data block, not per byte.

Multiprogramming (Batch system)

• Single users cannot always keep the CPU and I/O devices busy.
• Multiprogramming organizes code and data so the CPU always has one to execute.
• A subset of total jobs in system is kept in memory.
• One job is selected and run via job scheduling.
• When a job waits (for I/O, for example), the OS switches to another job.

Multitasking (Timesharing)

• Multitasking is a logical extension of batch systems where the CPU switches jobs so frequently that users can interact with each job in real time, enabling interactive computing.
• Response time should be < 1 second.
• Each user has at least one program executing in memory that represents a process.
• If several jobs are ready to run simultaneously CPU scheduling takes place.
• Swapping moves partially executed tasks in or out of memory if processes don't fit in memory.
• Virtual memory allows the execution of processes that are not fully stored in memory.

Process Management

• Is about running a program
• A process is a unit of work within the system.
• Program is a passive entity; process is an active entity.
• Processes need resources like CPU, memory, I/O and files, and initialization data to accomplish its task.
• Process termination requires the reclaim of reusable resources.

Dual-mode Operation

• Dual-mode operation allows the OS to protect itself and other system components by differentiating between user mode and kernel mode.
• Instructions designated as privileged are only executable in kernel mode.

Process Management Activities

• Creating and deleting both user and system processes.
• Suspending and resuming processes.
• Providing mechanisms for process synchronization and communication.
• Providing mechanisms for deadlock handling.

Process Management

• Single-threaded processes have one program counter specify the location of the next instruction.
• It executes instructions sequentially until completion.
• Multi-threaded processes have one program counter per thread.
• A system typically has many user and OS processes running concurrently on one or more CPUs.
• Concurrency uses the multiplexing of processes or threads.

Memory Management

• Executes a program with all or part of the the instructions in memory
• Instructions are executed using the data in memory
• It Optimizes CPU utilization and computer response to users
• Activities include the following:
  • Keeping track of which parts of memory are actively used and by whom.
  • Deciding which processes and data will be moved into or out of memory.
  • The OS allocates and deallocates memory space as needed.

Mass-Storage Management

• Disks are mainly used to store data
• Proper management is of central importance
• The whole speed operations hinges on the disk subsystem and it's algorithms.
• OS activities include the following:
  • Mounting and unmounting
  • Free-space management
  • Storage allocation
  • Disk scheduling
  • Partitioning
  • Protection

Caching

• The most important principle, performed at many levels in the computer.
• Information in use copied from slower to faster storage, temporarily.
• Faster storage (cache) is checked first to determine the presence of information, and used directly if present.
• Cache is smaller than storage being cached
• Includes Cache management, important design problem, Cache size and replacement policy

Characteristics of Various Types of Storage

• Registers are the fastest and smallest storage, followed by cache, main memory, solid-state disk, and magnetic disk, which is slowest and largest.
• Movement between the different levels of storage hierarchy can be explicit or implicit.

I/O Subsystem

• A primary purpose of the OS is to hide the complexities of hardware devices.
• Memory management of I/O includes buffering, caching, and spooling.
• It is responsible for providing the general device-driver interface and drivers for specific hardware.

File-system Management

• The OS provides a uniform and logical view of information storage.
• It abstracts physical properties to logical storage unit as a file
• Each medium is controlled by device (i.e., disk drive, tape drive)
• Varying properties include access speed, capacity, data-transfer rate, access method (sequential or random)
• File-System management includes the following: - Files usually organized into directories - Access control on most systems -OS activities include creating/deleting files/directories; primitives to manipulate files/directories; mapping files onto secondary storage; backing up files onto stable (non-volatile) storage media.

Protection and Security

• Protection includes access to controlling processes, and resources defined by the OS.
• Security is the defense of the system against internal and external attacks like denial-of-service, worms, viruses, and identity theft.
• User identities (user IDs, security IDs) include names and associated numbers, one per user.
• User IDs are associated with all files and processes.
• Group identifiers (group IDs) controlling a set of users are associated with each process and file.
• Privilege escalation is the ability for users to change an effective ID with more rights.

Virtualization

• Operating systems run via applications run within other OSes.
• Emulation is used when the CPU is different (i.e. PowerPC to Intel x86) and is generally the slowest.
• Virtualization allows the OS to be compiled natively for the CPU and running guest OSes.
• In virtualization, VMM (virtual machine Manager) provides virtualization services.

Virtualization (cont.)

• Virtualization is used to explore multiple OSes for compatibility or run Windows as a guest on macOS.
• Used in the software development process.
• Quality assurance is used for testing applications without having multiple systems. VMMs can run natively with not general-purpose hosts.
• VMMs include VMware ESX and Citrix XenServer

Distributed Systems

• A collection of separate, networked systems
• Use a Network as communications path, TCP/IP most common
• Networks include the following:
  • Local Area Network (LAN)
  • Wide Area Network (WAN)
  • Metropolitan Area Network (MAN)
  • Personal Area Network (PAN)
• Network Operating Systems provide features between systems across network
• Communication schemes enable systems to exchange messages

Computing Environments

• Computing environments include basic, mobile, client server, peer-to-peer, cloud computing, and real-time embedded.

Basic Approach

• Utilizes stand-alone general-purpose machines.
• Systems have interconnected with internet Portals
• Provides web access to internal systems.
• Interconnected network, including home systems interconnected wireless networks
• Firewalls are used to protect home computers.

Mobile

• Includes smartphones, tablets, extra OS features like GPS, gyroscope, and augmented reality
• Wireless networks connectivity - IEEE 802.11 wireless or cellular data networks
• Apple iOS and Google Android are among the leaders.

Client-Server Computing

• Dumb terminals supplanted by smart PCs now as client generated servers.
• Compute-server system provides an interface to client to request services (i.e. database)
• File-server system provides interface for clients to store and retrieve files

Peer to Peer

• It does not distinguish clients and servers
• Every node acts as a Client, server, or both
• The node must be able to join
• Includes Napster and Skype

Cloud Computing

• Delivers computing, storage, and apps as a service across a network based on virtualization.
• Amazon EC2 has thousands of servers, VMs, petabytes of storage to share.
• Public cloud is available via the Internet for pay.
• The hybrid cloud includes both public and private cloud components.
• SaaS provides applications like word processing over the Internet
• PaaS supplies a software stack for application via the Internet
• IaaS provides servers or storage over the Internet

Cloud Computing (cont.)

• Cloud computing environments are composed of traditional OSes, VMMs, and other cloud management tools with internet connectivity require safety
• Load balancers spread traffic across multiple applications.

Real-Time Embedded Systems

• Real-time embedded systems are special purpose, with a limited OS, real-time OS.
• May perform tasks without an OS
• They have constraints with processing be done within them and operation only if constraints met.

Kernel Data Structures

• Similar standard programming data structures like singly linked lists, doubly linked lists, and circular linked lists.
• Includes Binary search trees and hash functions