I. Operating System (OS) Overview
 A. 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

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        What kinds of I/O devices might you find on a computer?
 B. Some Early History
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     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
      
------------------------------------------
        How does user/kernel mode and the trap mechanism give protection?
 C. Organization of an OS
  1. monolithic
------------------------------------------
   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:



------------------------------------------
        What is a BIOS?
  2. layered
------------------------------------------
             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




------------------------------------------
  3. micro-kernel
------------------------------------------
           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:




------------------------------------------
  4. modular
------------------------------------------
            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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II. Processes
 A. What is a process?
------------------------------------------
            PROCESS

def: a *process* in an OS is


A process is also called a *task*

OS Goal:



Characteristics:






------------------------------------------
------------------------------------------
         PROGRAM VS. PROCESS

Is a program a process and vice versa?





How does a program become a process?



------------------------------------------
  1. Process Control Block (PCB)
------------------------------------------
      PROCESS CONTROL BLOCK (PCB)

def: a *process control block (PCB)* is





Typical information in a PCB:












------------------------------------------
        Why put the BP, SP, and PC into the PCB?
  2. Process Address Space
------------------------------------------
        PROCESS ADDRESS SPACE

def: a *process address space* is




Layout (in Unix):

     |-------------------|
     |      Stack        |
     |        |          |
     |        v          |
     |                   |
     |                   |
     |-------------------|
     |                   |
     |        ^          |
     |        |          |
     |      Heap         |
     |-------------------|
     |                   |
     |      Data         |
     |                   |
     |-------------------|
     |                   |
     |      Text         |
     |                   |
     |-------------------|

------------------------------------------
  3. Process States
------------------------------------------
         PROCESS STATE

def: a *process state* represents the


Examples:

 Ready:

 Running:

 Waiting:

 Suspended:

 End:

 Abend: 
------------------------------------------
  4. Events
------------------------------------------
     AN OS AS AN EVENT-DRIVEN PROGRAM

                            [Suspended]
                            ^        /
                 dispatch   |stop   resume
  create         /-----\    |      v
    *---> [Ready]      |--> [Running]
            ^   ^         /  |   || EOP
            |   \---------   /   | \->[End]
         I/O|       timer   /SIO |
      interrupt            /     |trap
             \--[Waiting]-/      v
                              [Abend]
        
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        Why would the OS need to stop a running program?