Download What is an Operating System?

CS149
Operating Systems
Agustin Araya
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What Are the Major Pieces of Functionality of an OS?
...
• Running programs
==> Process management
• Managing main memory
==> Memory management
• Manipulating files (... hard disk)
==> Storage management
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(Cont.)
• Dealing with I/O devices (e.g., printers, keyboard...)
==> Managing I/O systems
• Providing security
• Dealing with distributed systems
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What is an Operating System?
• A program that acts as an intermediary between
a user of a computer and the computer
hardware
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Operating System Goals
1 To execute user programs and make it easy for
the user to solve problems (with the help of a
computer)
...another way of saying it: make the computer
system convenient to use
2 To use the computer hardware in an efficient
manner
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A Look Behind the Scene...
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A Look Behind the Scene...
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A Look Behind the Scene...
OS
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What Do We Mean by a Computer System?
• We just said that a goal of an OS is to make
computer systems easy to use...
• But what is a computer system?
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(Cont.)
• A computer system consists of four major
components...
• Hardware
• provides basic computing resources
• e.g.: CPU, memory, I/O devices
• Operating system
• controls and coordinates use of hardware
among various applications and users
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(Cont.)
• System and Application programs
...define the ways in which the system resources
are used to solve the computing problems of the
users
...e.g.: word processors, compilers, web browsers,
database systems, video games
• Users
• e.g.: people, machines, other computers
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The Four Components of a Computer System
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Important design
issue: what to put
in the OS and
what to leave as
system program
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Unix systems:
small OS...
Microsoft's OSs:
very large OS...
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Operating System Definitions
• OS is a resource allocator
• manages all resources
• decides between conflicting requests for
efficient and fair resource use
• OS is a control program
• controls execution of programs to prevent
errors and improper use of the computer
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E.g.: several
Operating System Definitions
programs need to
run. They
• OS is a resource allocator
compete for the
• manages all resources
CPU, for memory
• decides between conflicting requests for efficient
and fair resource use
• OS is a control program
• controls execution of programs to prevent errors
and improper use of the computer
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(Cont.)
• No universally accepted definition of what an OS
is composed of
• “Everything a vendor ships when you order an
operating system” is a good approximation
• but varies wildly
• Another definition: The OS is “the one program
running at all times on the computer” (the kernel)
• everything else is either a system program
(ships with the operating system) or an
application program
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What should be
part of the kernel?
Should the kernel
be as small as
possible (a microkernel)?
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Different Situations
• A user using a personal computer (PC)...
• A user using a mainframe
• by means of a terminal
• other users are simultaneously connected
• A user using a workstation, more powerful than a PC
• connected to servers
• some resources are shared (e.g., print servers)
• A user using a PDA (personal digital assistant)
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Different Situations
• A user using a personal computer (PC)...
• A user using a mainframe
which are
the OS goals
for each
situation?
• by means of a terminal
• other users are simultaneously connected
• A user using a workstation, more powerful than a PC
• connected to servers
• some resources are shared (e.g., print servers)
• A user using a PDA (personal digital assistant)
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Different Situations
• A user using a personal computer (PC)...
• A user using a mainframe
ease of use
(+resource
utilization)
• by means of a terminal
• other users are simultaneously connected
• A user using a workstation, more powerful than a PC
• connected to servers
• some resources are shared (e.g., print servers)
• A user using a PDA (personal digital assistant)
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Different Situations
• A user using a personal computer (PC)...
• A user using a mainframe
• by means of a terminal
S
maximize
resource
utilization
• other users are simultaneously connected
• A user using a workstation, more powerful than a PC
• connected to servers
• some resources are shared (e.g., print servers)
• A user using a PDA (personal digital assistant)
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Different Situations
S
• A user using a personal computer (PC)...
• A user using a mainframe
• by means of a terminal
• other users are simultaneously connected
ease of use
anda PC
• A user using a workstation, more powerful than
resource
• connected to servers
utilization
• some resources are shared (e.g., print servers)
• A user using a PDA (personal digital assistant)
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Different Situations
S
• A user using a personal computer (PC)...
• A user using a mainframe
• by means of a terminal
• other users are simultaneously connected
• A user using a workstation, more powerful than a PC
• connected to servers
• some resources are shared (e.g., print servers)
ease of use,
battery
• A user using a PDA (personal digital assistant)
performance
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(Cont.)
==>
Different situations lead to different OS goals
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Computer System Organization
• Background knowledge for understanding OSs...
• Review of some basic concepts in computer
organization
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Computer System Organization
(the h/w components)
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The OS is a
program that
manages these
components
Computer System Organization
(the h/w components)
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Computer System Organization
From an OS point
(the h/w components)
of view, we are
interested in an
abstract view of
these components
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Computer System Organization
device
controllers
(hardware)
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The OS interacts
Computer System Organization
with the
controllers, rather
than with the
devices themselves
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Computer System Organization
execute
concurrently
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(Cont.)
• Computers have evolved toward increasing
concurrency
• because it leads to higher resource utilization
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Computer System Organization
Bus
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Computer System Organization
shared
memory
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Interactions of the I/O Devices with
CPU (Processor), Memory, and OS
• Suppose a program requests to read (or write) data
from (to) a file
• How is the information transferred from the file (in
hard disk) to the appropriate addresses of the
memory allocated to the program (or vice versa)?
• The OS will perform key steps of this process...
Note: In this context, interactions with I/O devices refer
to interactions with I/O device controllers
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OS Responsibilities with Respect to I/O Devices
• If the user (who activated the program) can only read
the file, but not write on it
...the OS needs to ensure the program can only use
the appropriate functionality of the I/O device
• The OS does not allow the program to directly access
the device
...rather, it provides appropriate commands
(abstractions)
...and corresponding routines for executing those
commands on the I/O device
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(Cont.)
• The OS must give commands to the I/O devices to
perform the desired operations
• At the same time, the OS needs to deal with the
interrupts triggered by the I/O devices
...it is through interrupts that those devices
communicate with the OS
...e.g., to indicate that an operation terminated or that
an error occurred
• Finally, the transfer of data between the device and
memory must take place...
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How the OS Gives Commands to I/O Devices.
Different Approaches
1 By means of memory-mapped I/O
...certain parts of the address space have been
assigned to I/O devices (rather than to memory)
...so, if such an address is put in the bus, the
corresponding device controller records it and passes
it to the device as a command
In other words, commands to the I/O devices are
given by reading and writing on those addresses...
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(Cont.)
2 Or, by means of specific I/O instructions in the
processor
• (see next slide)
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Communication Between the Processor
and the I/O Device
• Example: a basic printer that prints only one
character at a time
• The status register of the printer has:
• a ‘done’ bit: set to 1 when the printer has printed a
character
• an ‘error’ bit: set to 1 to indicate there is a problem
with the printer
• e.g.: no paper...
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(Cont.)
• The data register of the printer contains the character
to be printed
• Before passing the next character to be printed the
processor must check (poll) the status register...
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(Cont.)
• Polling: “periodically checking status bits to see if it is
time for the next I/O operation”
• But polling has a problem
• what is it?
...
...the processor wastes time checking the status
• What is the alternative approach to polling?
...
...interrupts...
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Interrupt Mechanism
• Varies with different computers, but the basic
functionality is similar
• Events requiring immediate attention from the
processor are indicated by means of an interrupt
• Example: an I/O interrupt...
...the device controller informs the CPU that it has
finished its operation by sending a signal through
the bus
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(Cont.)
• The CPU must pay immediate attention to the
interrupt
• The processors' fetch-execute cycle checks to see if
an "interrupt flag" has been set
• For different interrupts, the CPU must do something
different
• This is achieved by the CPU activating the
appropriate interrupt service routine
(see next slide)
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How the CPU reacts to the interrupt?
• An interrupt vector (a table...) containing the
addresses of each of those service routines is used
• it is stored in memory (e.g., in the first 100
locations)
• When an interrupt is sent, an appropriate index into
the interrupt vector is given with it
• ...in this way the address of the corresponding
service routine is obtained
• and the service routine is activated
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(Cont.)
• After servicing the interrupt the CPU should continue
doing what it was doing earlier...
• Hence, it must save the address of the instruction the
CPU was executing when interrupted
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Coming back to the example of the simple printer
(Now using interrupts)
• ...the printer that prints one character at a time...
• First, the CPU sends the character to be printed to
the printer
• Second, the printer prints it
• Third, the printer sends an I/O interrupt to the CPU
• Fourth, the CPU activates the interrupt service
routine to handle the interrupt...
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A More Detailed Analysis
• Device controllers play a role in transferring data
between I/O devices and memory
• The device controller (a piece of h/w) has:
• special-purpose registers
and
• buffer storage
• ...and it is in charge of transferring data between the
I/O device and its own buffer
• The OS interacts with the device controller by means
of a device driver (a piece of software)
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(Cont.)
• The I/O operation starts with the device driver putting
appropriate info’ in the device controller’s registers
• Depending on the info’ the controller moves the data
from the device (e.g., a hard disk) to its own buffer (or
vice versa)
• By means of an interrupt, the device controller
informs the device driver that the operation ended
• If the operation was a read, the OS would then
proceed to transfer the data from the controller’s
buffer to memory
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(Cont.)
Issue:
• This approach based on interrupts could produce too
much overhead if the amount of data to be
transferred is large
• That is, the CPU becomes too directly involved in the
I/O transference
Alternative approach:
• ‘direct memory access’...
(see next slide)
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Direct Memory Access
• The device controller itself transfers the data between
its own buffer and memory!
• the CPU does not intervene
• The data is transferred in blocks (rather than in bytes)
• when the transfer has concluded, the device
controller sends an interrupt to the device driver
• In this way, far fewer interrupts are generated and the
CPU can do other things
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Simplified Interactions
data
transfers
CPU
I/O request
interrupt
Memory
direct
memory
access
Device
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Hierarchy of Storage Devices
• Fast storage is expensive and volatile
• Slow storage is cheap and non-volatile
• CPU registers
• Cache memory (memory buffer within the CPU)
• Main Memory (Memory/Random access mem.)
• Magnetic disk
• Magnetic tapes
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Hierarchy of Storage Devices
S
• Fast storage is expensive and volatile
• Slow storage is cheap and non-volatile
• CPU registers
• Cache memory (memory buffer within the CPU)
• Main Memory (Memory/Random access mem.)
• Magnetic disk
• Magnetic tapes
less
expensive
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Hierarchy of Storage Devices
S
• Fast storage is expensive and volatile
• Slow storage is cheap and non-volatile
• CPU registers
volatile
• Cache memory (memory buffer within the CPU)
• Main Memory (Memory/Random access mem.)
• Magnetic disk
• Magnetic tapes
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Hierarchy of Storage Devices
S
• Fast storage is expensive and volatile
• Slow storage is cheap and non-volatile
• CPU registers
• Cache memory (memory buffer within the CPU)volatile
• Main Memory (Memory/Random access mem.)
• Magnetic disk
• Magnetic tapes
nonvolatile
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Other Basic Concepts
S
• The instruction-execution cycle...
• etc
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So far...
S
• We have been talking about the basic components of
a computer and their interactions
• Now we will say a word about how those components
can be put together
==> Computer architecture
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Computer Architecture
S
• Single processor
• Multi-processor
• increased throughput
• increased reliability
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(Cont.)
S
• Symmetric multi-processing
• all processors are the same, with the same responsibilities
• Asymmetric multi-processing
• there is a master processor
• the rest of the processors perform specific tasks, and follow
the orders of the master
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Summary.
Important Concepts
•
•
•
•
•
Operating system goals
Components of a Computer System
What is an OS
Different kinds of Computer Systems
(personal computer, mainframe,
handheld...)
• Computer System organization
• Interactions of the I/O Devices with
CPU (Processor), Memory, and OS
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Summary.
Important Concepts
•
•
•
•
Interrupt Mechanism
Direct Memory Access
Hierarchy of Storage Devices
Computer architecture: single- and multiprocessing
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