21 January 2002
Operating Systems

Read:  Nutt, Chapter 2, 3

User Level View of OS
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What a programmer perceives:
processes  - unit of computation
files      - unit of info storage
resources  - other, specialized components

Resources:  
  managed by the OS - meaning that a user must request access before using it
  e.g., file

  files - often the foundational model for other resources
          open,close,read,write,seek
          similar interface as for pipes

  other resources:  
    processor, memory, tape drive, keyboard, display, pipes
    
    how they are allocated is up to the OS      
    what if allocated poorly?
      synchronization/deadlock - chapters 8-10

Processes:
  components:  code/data/resources/status
  management: 
    fork: create new addres space
          copy text/data/stack to new addr space
          child has access to open files
          what if created in same address space?  
            child could corrupt parent
            separate spaces allow isolation/protection of processes
    quit: terminate process 
    join - merge two or more processes

Threads: lightweight process
  share program
  share resources
  thread scheduler: sometimes part of user program, someteims part of OS
         simple to switch among threads, more simple than process switch

  ex. in Java, gui management done in separate thread which raises events

Will come back to the OS perspective of processes/threads in a couple weeks
(Chapter 6)


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OS Organization

Basic functionality
---

device management
  handling generic devices (disks, tapes, terminals, printers)
  os manages allocation, isolation, sharing
  device drivers need to be inside the OS 
    so if you need to add a device, do you need the OS source code
    no: can add device drivers on the fly to a running OS
  More in ch 5

process and resource management
  process is a special resource
  resources: elements of computing environment needed by a process so that it
             can execute  
  mechanisms for sharing the processor - scheduling
  how to share resources (policies)
  more in ch 6-10
  
memory management
  sharing according to a policy - isolation or sharing
  virtual memory - abstract resource, make system mem look bigger
  memory can also be distributed among multiple machines w/ network
  more in ch 11, 12, 17

file management
  non-volatile storage
  how to store efficiently for retrieval?
    take into consideration the way a disk works
  distributed file systems (e.g., NFS)
  ch 13 and 16