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Operating Systems
Part I: Introduction
“I think that there is a world
market for five computers”
- Thomas J. Watson (1945)
Why Study Operating Systems?
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We want to have an efficient O/S because it
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consumes more resources than any other program.
is the most complex program.
is necessary for any use of the computer.
is used by many users.
Efficiency is measured through
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Functionality
Performance: Time and Utilization
Convenience and Cost
Goals of This Course
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Understand what an operating system is
Understand the key components of an
operating system
Have a deeper understanding of common
operating systems in the market (e.g.
Windows, Unix, MS-DOS) and the issues
associated with them
To be able to use performance measures
What is an O/S?
•A layer of
abstraction between
the HW and SW
•A resource
coordinator
•Virtual machine
•Reactive system
Operating Systems: A Definition
A collection of programs that integrate the
hardware resources of the computer and make
those resources available to the user in a
productive, timely, and efficient manner
Operating Systems Ease the Pain
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Performs the interface task with the hardware (file
operations, memory paging, etc.) which should have
been done by the user if the OS did not exist
High-level interface (GUI, command line a.k.a. CUI)
The O/S’s capability for multiuser and multitasking
utilize the hardware efficiently
Makes visible the “virtual” component of the system
Allows program interaction
Why are Operating Systems
Difficult to Create and Maintain?
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Size
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Too big for one person; current systems have
millions of lines of code and involve 10-100 man
years to build
Lifetime
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Operating systems remain longer than the
programmers who originally wrote them. Code is
written and rewritten and original intent is forgotten
(Unix designed to be cute, small system - now
several volumes thick!)
Why are Operating Systems
Difficult to Create and Maintain?
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Complexity
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Multitasking
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The system must do difficult things -- deal with ugly
I/O devices, multiplexing/juggling act, handle errors
Must do several things at once.
General purpose
A Brief History: Early 1950’s,
Mainframes Rule!
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Early systems
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No O/S! Programmer is also operator
Large machines run from a console; programs loaded through
switches and card readers
Simple batch systems were the first real OS
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Setup time was a problem -> hire an operator
Operator ran related jobs together
O/S was a simple program stored in one part of memory
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Loads a single job from card reader into memory
Transfers control from one job to the next
Offline Processing
Allowed jobs to be read ahead of time onto tape
Card
Reader
Line
printer
CPU
On-line processing
Card
Reader
Tape
Drive
Tape
Drive
CPU
Off-line processing
Tape
Drive
Line
printer
Tape
Drive
History: Spooling
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Allowed jobs to be read ahead onto disk
Spool (Simultaneous Peripheral Operation OnLine)
disk
Card
Reader
CPU
Line
printer
Multiprogrammed Systems
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Multiprogrammed batch systems provided
increased utilization
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Keeps several jobs in memory simultaneously
I/O processing of one job overlaps with computation
of another
Analogy: Lawyer working on several cases; while
waiting to go to trial on one, can work on another
Needs CPU scheduling
Timesharing/Multitasking Systems
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Timesharing supported
interactive use
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Each user feels as if
he/she has the entire
machine
Tries to optimize
response time
Based on time-slicing;
divide CPU equally
among others
Desktop Systems
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First appeared in the 1970s
More popularly known as personal computers
(PCs)
Breakthroughs in hardware allowed downsizing
from expensive mainframes
Multiprocessor Systems
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Also known as parallel systems or tightly-coupled
systems
Three main advantages
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Increased throughput (more CPUs = more work in less time)
Economy of scale (saves money, CPUs share peripherals)
Increased reliability (provides redundancy and fault tolerance)
Symmetric multiprocessing (SMP): All CPUs do the
same thing
Asymmetric multiprocessing: each CPU has specific
role (usually master-slave)
Distributed (Loosely-Coupled)
Systems
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Facilitates use of geographically distributed computing resources
Supports communications between parts of a job or different jobs
Supports sharing of distributed resources, both hardware and
software
Client-server systems vs. Peer-to-peer systems
Clustered Systems
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Makes several CPUs work together to accomplish
computational task
Most likely share storage and linked through a local
area network (LAN)
Possible clustering schemes:
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Symmetric mode (two or more hosts running applications and
monitoring each other)
Asymmetric clustering (one is in hot standby mode while
another is running applications; switches to backup if active
fails)
Real Time Systems
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Used for specialized applications: subway systems,
flight-control, factories, power plants
Basic idea: O/S guarantees response to physical
events will occur within a fixed amount of time
Problem faced : Schedule activities so as to meet all
time constraints
Hard real time system
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Deadline is critical
Typically used to control a device
Soft real time system
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Deadline is important but not critical
Example : Video applications (Use in PC environment)
Handheld Systems
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Used in PDAs and cellular phones
Common concerns:
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Limited main memory
Processor speed
Small display screens
General Structure of an O/S
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Resident Programs
Programs which are critical to
the operation of the system
KERNEL
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Non-resident Programs
Loaded into memory only when
needed
An Example: MS-DOS Structure
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Memory resident components
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Command interface shell (eg. ver, dir, date, time) :
COMMAND.COM
Set of I/O routines which control each I/O devices
(drivers) -- e.g. BIOS : IO.SYS
File Management System : MSDOS.SYS
Non-resident components
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FORMAT.COM, XCOPY.EXE, LABEL.EXE, etc.
How MS-DOS Programs are Loaded
in Main Memory
At System Start-up
Running a Program
Operating System Components
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Process Management
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Process: a program in execution
Keeps track of each process and it’s state
Create, delete, suspend, resume processes;
synchronize process communications,
handle deadlocks
Possibly support threads (executable parts
of a process)
Operating System Components
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Main Memory Management
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Keep track of which parts of memory are in
use
Allocates and deallocates memory as
needed
Decides which processes must be loaded in
main memory when space becomes
available
Operating System Components
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File Management
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Keeps tracks of available space on the system
Maintains directory structure and hierarchy
Supports file manipulation commands
Keeps track of file information (inode, name,
timestamp)
I/O System Management
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Allows for a standard methodology for access of
each device
Use of device drivers for modularity
Operating System Components
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Secondary Storage Management
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Free space management
Storage allocation
Disk scheduling
Networking
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Transfer protocols
Routing and connection strategies
Operating System Components
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Protection System
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Provide mechanism for controlling access to
programs, processes, or users
Essential in multitasking and multiuser systems
Command Interpreter System
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GUI vs. Command Line Interface
Redirection, Piping, and Parameter Passing
Operating Systems Architectures
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Monolithic
Layered
Virtual Machine
Microkernel
Monolithic
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Easy to implement
“The Big Mess” –
virtually no structure!
Kernel is one large
structure
Each procedure is visible
to every other procedure
Not used anymore
Layered
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Not easy to implement
because some
functionalities are
mutually dependent.
Inefficient because it
requires a high number
of traversals of
interfaces
Virtual Machine
Non-virtual Machine
Virtual Machine
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Each user has a
“virtual machine” and
he can choose which
OS to run on that
machine
Elegant, but does not
deal with questions of
resource management
and responsiveness
Microkernel
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Used in Mac/OSF/NT
Takes out as much
functionality as possible
from kernel -- allows
modularity and portability
across platforms
Interactions between
processes involve the
kernel, thereby requiring
high efficiency in transfer