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Operating Systems Design (CS 423)
Elsa L Gunter
2112 SC, UIUC
http://www.cs.illinois.edu/class/cs423/
Based on slides by Sam King and Andrew S Tanenbaum
5/23/2017
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Facilites
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Lab: EWS Machines
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Virtual Machine Emulator
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ssh –Y <netid>@remlnx.ews.illininois.edu
Qemu: http://wiki.qemu.org/Main_Page
Executable available from ~cs423/bin
Will use Linux Kernel 2.6.37
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http://www.kernel.org/
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Course Objectives
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Understanding Major Concepts of Operating
Systems
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Threads and synchronization
Virtualization and Virtual Machine Monitors
I/O and Device Drivers
File Systems
Distributed Systems
Security
Understanding of Interaction Between Hardware
and Kernel
Understanding of the Layers of Abstraction and
Virtualization of Operating Systems
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What is an Operating System?
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Most basically, what is an operating system
are the main things it does?
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What is an Operating System?
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Software layer between hardware and
application software
Application Software
Virtual interface
Operating System
Physical interface
Hardware
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What Does an Operating System Do?
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Abstraction
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Presents application with uniform, (relatively)
simple access to resources
Regulation
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Governs access to resources to guarantee proper
use
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What Does an Operating System Do?
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In any OS area, ask:
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What
What
What
What
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resource(s) is involved?
is physical reality of resource?
abstraction to present to user apps?
protections to guarantee?
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Dual Relationship Between OS and Apps
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User app main program; calls kernel for
services
OS main program; calls user programs as
subroutines
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Main Roles of OS
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Illiusionist
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Make resources seem better than they are
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Examples?
Government
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Parcel out shared resources to multiple apps in a
fair, safe, efficient way
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Tax: Costs CPU, Memory,
Maintains Protections
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Allows good programs to just mind their own business
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Why Study Operating Systems?
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Some day, you may write one (or part of
one)
OS concepts ant techniques common in
other domains
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OS is huge multi-threaded, event drive app
Designing and implementing abstractions and
protections
Caching, indirection, concurrency, atomicity, …
Fun to “open the hood and look inside”
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History of Operating Systems
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Two major forces in OS history
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Equipment was very expensive, steadily
becoming cheaper
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People viewed as increasingly expensive
Systems, including OS growing steadily in
complexity
Original OS Structure
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Simple, library of services, single “thread”
operation
Gave poor utilization of system
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Mainframes and Single Operator
Computers
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Three groups interacted with computers
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Computer designers / developers configure
computers for specific application
Operators collects program, runs it on the
computer, collects output and returns it to the
user
Programmers write programs, submits it to the
operator, waits for output, looks for errors,
repeat
Wastes both computer and human time
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Batch Computers
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Decouple Input, Execution, Output
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One device reads in a collection of programs
Second device (main computer) executes program batch
and outputs results to a “relatively fast” storage
(magnetic tape)
Third device outputs results from tape to line feeder
(basically a dot matrix printer)
Allows for parallel processing of input, execution,
output
Executing multiple programs; protection starts to
become an issue
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Main Time Sharing
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Allow multiple humans to interact at the
same time
Main Idea: Human are very slow I/O
Switch / interleave other activities while
waiting for human
OS much more complicated
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Multiple sources of input
Multiple jobs running in the same time interval
Protection needs much more serious
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Personal Computers
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Computer hardware much much cheaper
Single user
Simple single threaded OS, basically library
routines
No time sharing between jobs
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Wait for printer
No protection
Gradually migrated to full multi-user OS
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Today’s Computer
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Everywhere – microwaves, sewing machines,
washing machines, cars …
Single User computers: Cell phones, gaming
devices, TiVo. Etc
Many start with modern complex OS (eg
Linux)
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Common Modern OS Architecture
Applications
Libraries (e.g. C runtime)
Portable OS
Layer
User Mode
Kernel Mode
Machine Dependant Layer
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Common Modern OS Architecture
Applications
Typical System/User
Interaction
Libraries (e.g. C runtime)
Portable OS
Layer
Machine Dependant Layer
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Common Modern OS Architecture
Applications
Libraries (e.g. C runtime)
Shared Libraries
Portable OS
Layer
Machine Dependant Layer
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Common Modern OS Architecture
Applications
Libraries (e.g. C runtime)
Portable OS
Layer
High-level OS Abstractions
(eg File System, Processes)
Machine Dependant Layer
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Common Modern OS Architecture
Applications
Libraries (e.g. C runtime)
Portable OS
Layer
Machine Dependant Layer
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Low-level (eg process
switching, register
management)
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Common Modern OS Architecture
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OS developer need to be paranoid
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Application software buggy
Hardware unreliable
Users untrustworthy (naïve / malicious)
OS developer need to be engineers
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More is better
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Use as much memory as possible
Faster is better
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Indirections take time, users don’t like to wait
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Monolithic OS Architecture
Applications
Libraries (e.g. C runtime)
Portable OS
Layer
User Mode
Kernel Mode
Machine Dependant Layer
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Alternative: Microkernel Architecture
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Protection: How much, what cost?
Windows NT, Mac OS X microkernel origins
More common on embedded systems
Client
Client
Process Process
Process File
Server
Server
Memmory
… … Server
Microkernel
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Alternative: Virtual Machine Monitors
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VMM like microkernel with simple interface
Performance?
VMM – Microkernel distinction increasing
blurred
Operating System
Virtual Machine Monitor
Identical
interfaces
Hardware
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