AOS Lecture Notes - Lesson 1 - Introduction to AOS

Principle of Abstraction

Introduction

• Most computers will have multiple applications open. How do they all work together?
• OS’s have evolved over time. We will see some of how the current state of the art has come about.
• Abstractions!
  • hardware resources
  • OS functionality
  • OS abstractions
• By end of the lesson, we will be ready to discuss the primary OS abstractions

• Lots of review of basics OS stuff. Going to omit many of these slides from notes

• Abstraction is an interface that allows us to hide all the details

The power of abstractions

• Google Earth, at some level, is electrons and holes.
• An ascending list of all the abstractions might be:
  • Transistors
  • Logic Gates
  • Sequential and combination logic elements
  • machine organization (data path and control)
  • Instruction set architecture
    • abstraction defined by a processor (Intel Inside, yo)
  • System software
    • OS, compilers, etc
  • Applications
    • high level language that leverages all lower layers
• Application is written at the top of the hierarchy, and traverses the stack below it. The stack below it provides all it needs for successful and safe execution
  • 

Hardware Resources

• OS role is to control access to the physical hardware
• Hardware ranges from smartphone to cloud servers, but really they’re all pretty similar in terms of internal organization
  • same for all manifestations
    • CPU is connected by a conduit we call the Bus
    • Memory has instructions and data needed for CPU to execute. CPU uses Bus to access memory
    • Storage is needed for persistence of files, and other data. Also accessed by CPU via a Controller, attached to the Bus
    • Peripherals such as mic/keyboard/etc all accessed via Controllers attached the Bus
    • Network is interfaced through Controller/Bus, same as everything else
    • Bus is a conduit for communication, serves the purpose of connecting CPU to other components
    • Depending on sophistication/speed of device may have various other capabilities
      • e.g. network or storage may access memory directly (DMA)
  • Second example given, with a system bus in addition to the IO bus
    • System bus is for synchronous communication between CPU and Memory
    • IO bus is for the devices to communicate with the CPU
    • The intent is that the individual needs of these devices is less than the cumulative bandwidth for the CPU to communicate with the memory. System bus > IO bus
    • System bus connects via a bridge to the IO bus
    • Bridge is a specialized IO processor for scheduling the devices to communicate with the memory (DMA) or the CPU
    • Other high end devices, such as frame buffer for a graphics display, that may hang off the system bus to take advantage of its high-speed nature
    • 

OS Functionality

• Section begins with a ‘quiz’ that lists some of the functionalities provided by an OS
  • OS is a resource manager
  • OS provides a consistent interface to the hardware resources (CPU, memory, etc)
  • OS schedules applications on the CPU
• What is an OS?
  • Protected access to hardware resources
  • Arbitrates among competing requests
  • 
  • OS provides well defined APIs for accessing hardware resources managed by the OS
    • resources are provided as services
    • applications make requests for resources to the OS
    • services are provided as responses to those API calls
• What happens when you click the mouse on your computer?
  • It generates a CPU interrupt
    • using an interrupt line, one of the lines on the bus
    • 
  • CPU is running whatever else. An interrupt is a hardware mechanism for alerting the processor that something external requires the attention of the CPU. OS fields the interrupt.
  • OS schedules itself to run on the CPU to handle the interrupt, figures out what its intended for, and passes it to the intended recipient program for action.

Managing the CPU and Memory

• Recall the initial imagery from beginning of this lecture, where a computer is running multiple apps seemingly simultaneously, and all of them must play nicely with each other.
• OS must protect these applications from one another and themselves, while staying out of the way as much as possible.

Catering to Resource Requirements

• Resource needs of applications
  • cpu
  • memory
  • peripheral devices
• App launch time
  • OS doesn’t know much about the app, but at least has enough information to create memory footprint
    • this is provided by the OS loader
    • 
  • app asks for additional resources at runtime, OS will serve as broker for running applications

Modern Operating Systems

• Program
  • static image loaded into memory
• Process
  • a program in execution
  • program + state of the running program (which evolves as the program executes)
  • in the simplest examples, there is a single thread of control coursing through the program
  • However, picture two (or many) threads of control running parallel through the program
  • The OS must arbitrate between competing requests from these threads
  • Therefore a process = program + state of all threads executing in the program
• Memory related OS abstraction
  • address space for each process distinct from one another
  • implement the abstraction using hardware capabilities