COP 3402 meeting -*- Outline -*-

* Operating System (OS) Overview

   Based on chapter 9 of Systems Software by Montagne
   and Operating Systems: Three Easy Pieces
   by Arpaci-Dusseau and Arpaci-Dusseau
     (see https://pages.cs.wisc.edu/~remzi/OSTEP/)

** What is an OS?
------------------------------------------
            OPERATING SYSTEM

Goal: The task of an operating system (OS)
 is to





Other goals




General approach: virtualization, i.e.,
  abstraction of physical resources
  defines APIs for them = system calls

------------------------------------------
        ...
        - make it easy to run programs.
        - provide a standard, convenient interface between programs
          and the computer's hardware.
        - manages sharing of the computer's resources
          (including: the CPU, memory, and devices: storage, printing,
                                                    I/O, etc.)
        ...
        It should have minimal overhead (high performance)
        It should protect users and applications from each other
        It should be reliable (not crash)
        (and: It should be energy efficient, secure, mobile)

        Q: What kinds of I/O devices might you find on a computer?
           keyboard, mouse, screen, printer, touch pad or touch screen,
           microphone, loudspeakers, camera, network (wifi and/or ethernet),
           There can be others (tablet, VR headset)
              including those access through a USB port (Flash drives, ...)

** Some Early History
    Notes from chapter 2 of https://pages.cs.wisc.edu/~remzi/OSTEP/
------------------------------------------
     SOME HISTORY OF OPERATING SYSTEMS
       
Early days: human operator ran jobs
            OS = libraries for I/O

1960s: protection via system calls

       imagine library that can read
         any part of a disk drive:
         Is any information safe then?

   System calls:
      hardware tracks privilege level
         users run in "user mode"
         OS runs in "kernel mode"
      call to system involves:
         a trap (user mode)
          -> trap handler
              starts kernel mode
              checks permissions
         return-from-trap instruction
          => back to user code (user mode)

Multiprogramming (1970s, Unix):
      OS switches between tasks:
      - timesharing and interactivity
      - start of memory protection
      - issues of concurrency
      
------------------------------------------
        Early computers were run in batch mode,
           didn't want humans wasting time "thinking" in front of a
           very expensive (at the time) computer...

        Q: How does user/kernel mode and the trap mechanism give protection?
            The OS can protect some resources (e.g., user identity
            records, permission lists), preventing users from writing
            them and only allowing a user with permission
            to do certain actions.

        Unix eventually found its way into Mac OS
        
        MS-DOS was a great leap backwards compared to Unix
   
** Organization of an OS

*** monolithic

     Example: MS-DOS
     
------------------------------------------
   WAYS TO ORGANIZE AN OS: MONOLITHIC

No distinctions, boundaries between
user and OS programs, all can directly
use the BIOS drivers

    [Application Programs]
        |        |    |     
        v        v    |
    [Systems progs]   |
        |        |    |
        v        |    |
    [OS drivers] |    |
        |        |    |
        v        v    v
    [BIOS device drivers]


Advantages:



Disadvantages:



------------------------------------------

        Q: What is a BIOS?
        The built-in (or basic) I/O system

        ... + simple to implement
            + good performance

        ... - complex code, hard to maintain
            - crash of any part crashes entire OS

*** layered

    Example: Unix/Linux
    
------------------------------------------
             LAYERED OS
 
Each layer can only use 1 layer below it

            [Application Programs]
                    |
                    v
             [Layer N: OS calls]
                    |
                   ...
                    |
                    v
             [Layer 1: OS kernel]
                    |
                    v
             [Layer 0: hardware]

Advantages:



Disadvantages




------------------------------------------
        ... + easier to maintain,
              well-defined interface for each layer
            + easier to debug

        ... - designing layers is hard
            - passing data through layers can be slow

*** micro-kernel

    Example: Mac OS

------------------------------------------
           MICRO-KERNEL OS

Layers,
  but with kernel as small as possible.
Non-essential components
  become (user-space) apps

            [Application Programs]
                    |
                    v
             [Layer L: OS calls]
                    |
                   ...
                    |
                    v
            [Layer 1: micro-kernel]
                    |
                    v
             [Layer 0: hardware]

Advantages:




Disadvantages:




------------------------------------------

        ... + OS becomes more portable (as micro-kernel is smaller)
            + better security and reliabilty (as micro-kernel very small)

        ... - more internal communication in the OS, so worse performance
            - design is harder, requires experience

*** modular

    Example: Solaris OS

------------------------------------------
            MODULAR OS

Kernel has set of basic services,
  other services added as dynamic modules.
Each module has defined API,
  but any module can call any other module


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