1 - Basics of OS

ucla | CS 111 | 2023-01-10T08:35

Table of Contents

Definitions

Big Ideas

What is System Software

System Software

  • an OS is made of a set of system software that provides services/support for higher level applications
  • the system software is usually built with or on the kernel which connects the hardware with the software i.e. firmware

Common System Software

  • memory management
  • persistent storage (drives, paged data)
  • scheduling and synchronization (scheduler, pipeline)
  • interprocess communications (IPC: pipe, etc.)
  • security
  • many OS system software are used as features in higher level applications
  • Other problems handled by OS
    • coordinate separate comms
    • manage shared resources
    • virtualize hardware ans software
    • organize comms
    • protect computing resources

OS Wisdom

  • view services as objects and operations
    • every object is implemented w a data structure

Implementation

  • an implementation is not a specification
  • many spec compliant implementations are possible
  • inappropriate dependencies usually cause problems

Interface Spec

  • an interface spec is a contract which specifies responsibilities of producers/consumers
  • the interface spec is the basis for product/release interoperability

Modularity and Encapsulation

  • complexity of system software hiding the level of abstraction

Policy vs. mechanism

  • policy determines what can and should be done
  • mechanism implements basic operations to follow the policy which don’t dictate/limit policies
  • policies should be malleable without requiring new mechanisms
  • parallelism and asynchronous behavior is powerful and vital but dangerous when unchecked
  • performance and correctness are often at odds and correctness is not always implemented

What is an OS

  • low level software that provides better abstractions of the computer hardware to allow easy/safe/fair use and sharing of hardware/system software resources

What does an OS do

  • manages hardware resources for programs
    • allocates hardware and manage its use (scheduler)
    • enforces privacy and controlled sharing
    • oversees execution of programs and handles problems
  • abstracts the hardware
    • makes it easier to use and improv sw portability
    • optimizes performance
  • provides new abstractions for applications
    • powerful features beyond just hardware

What does an OS look like

  • a set of management and abstraction services run behind the scenes
  • applications see objects and their services

Where does the OS fit in

  • infographic

  • the OS runs above the Hardware through the standard and privileged instruction sets
  • the OS provides a system call interface for system services/libraries to use

What’s special about the OS

  • always in control of the hardware because only the OS has complete access to hardware
    • auto load when machine boots, first software to access hardware, access to privileged instruction set, all memory and IO
  • it is the middle-man for applications’ access to hardware
  • trusted to store/manage data and always act in good faith
  • if it crashes, it takes everything with it (e.g. blue screen of death)

ISAs

  • set of instructions supported by a computer
  • many different ISAs that may be incompatible w/ the OS
    • bus/bandwidth (32 bit → 64 bit)
    • philosophies (RISC vs CISC)
    • competition (x86 vs ARM)

Privileged vs General Instructions

  • modern ISAs divide instruction sets
  • top-level applications can execute general instructions
  • processor must be in special mode to execute privileges instructions
    • can be dangerous in abuse

Platforms

  • ISA don’t completely define a computer
  • PC have functionality beyond user mode instructions
    • different hardware, controllers, memory management, IO bus, BIOS, feature support
  • these variations define a platform
    • the platform on which the OS runs
  • Requires successful OS to run on as many platforms and ISAs
  • implies OS will abstract ISAs
    • and makes minimal assumptions of HW i.e. hw independent (file systems, protocols, processes, etc.)

Binary Distribution Model

  • Binary is aa derivative of the source code which is distributed
    • the OS is written in the source which is compiled to a binary
  • may be multiple binaries per ISA
  • device drivers can be published later or after-market for platform support

Binary Configuration Model

  • the binary will configure to the environment/user based on specific access to hardware/features
  • automatic hardware discovery
    • self identifying busses (device to transfer data)
    • PCI/e, USB, PCMIA, EISA, etc.
  • automatic resource allocation
    • eliminate fixed size resource pools based on config
    • → dynamically re/allocate resources on demand

Functionality of OS

  • as much as necessary with as little as possible
    • OS code is expensive to develop and maintain in price and time
  • functionality that must be in the OOS
    • use of privileged instructions
    • manipulation of OS data structures
    • must maintain security/trust and resource integrity
  • performance code/functionality

Resources

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