Operating Systems

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Introduction

• Operating Systems
• Fall 2002

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What is Operating System?

• It is a program!
• It is the first piece of software to run after
  boot
• It coordinates the execution of all other
  software
• User programs
• It provides various common services needed by
  users and applications

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Todays plan

• Operating system functionality
• Hardware support for the operating system
• Course overview, bibliography, administrative
  questions

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The Operating System controls the machine
gdb
gcc
User
OS Kernel
grep
diff
Hard ware
Application
date
vi
Operating System
xterm
emacs
Hardware
netscape
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A better picture
Many applications
Application
One Operating System
System calls
Operating System
One Hardware
Privileged instructions
Machine instructions
Hardware
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A typical scenario

• OS executes and schedules an application to run
• Application runs
• CPU executes apps instructions
• OS is not involved
• The system clock interrupts the CPU
• Clock interrupt handler is executed
• The handler is the OS function

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A typical scenario (continued)

• In handler OS chooses another application to run
• Context switch
• The chosen app. runs directly on the hardware
• The app. performs a system call to read from a
  file

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A typical scenario (continued)

• The sys. call causes a trap into the OS
• OS sets up the things for I/O and puts the
  application to sleep
• OS schedules another application to run
• The third application runs
• Note At any given time only one program is
  running
• OS or a user application

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The running application state diagram
Interrupt/ System call
Ready To run
Schedule
I/O completed
Application code runs
OS runs
Wait for I/O completion
Sleep
Resume execution of the app. code
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A question

• The operating system gets an input, performs a
  computation, produces an output, and quits
• Yes or no?

The answer No

• The operating system is a
• reactive program

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The OS is a reactive program

• It is idly waiting for events
• When an event happens, the OS reacts
• It handles the event
• Schedules another application to run
• The event handling must take as little time as
  possible
• Event types
• Interrupts and system calls

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The OS performs Resource Management

• Resources for user programs
• CPU, main memory, disk space
• OS internal resources
• Disk space for paging memory (swap space)
• Entries in system tables
• Process table, open file table
• Statically allocated

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CPU management

• How to share one CPU among many processes
• Time slicing
• Each process is run for a short while and then
  preempted
• Scheduling
• The decision about which application to run next

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Memory management

• Programs need main memory frames to store their
  code, data and stack
• The total amount of memory used by currently
  running programs usually exceed the available
  main memory
• Solution paging
• Temporarily unused pages are stored on disk
  (swapped out)
• When they are needed again, they are brought back
  into the memory (swapped in)

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The OS supports abstractions

• Creates an illusion that each application got the
  whole machine to run on
• In reality an application can be preempted, wait
  for I/O, have its pages being swapped out, etc
• A tree-like file system organization
• Disk controllers can only write/read blocks

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Hardware support for OS

• Support for executing certain instructions in a
  protected mode
• Support for interrupts
• Support for handling interrupts
• Support for system calls
• Support for other services

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CPU execution modes

• CPU has (at least) 2 execution modes
• User mode
• Kernel mode
• Supervisor mode, privileged mode, monitor mode,
  system mode
• The execution mode is indicated by a bit in the
  processor status word (PSW)
• Some CPU instructions are available only when
  executing in the kernel mode

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Kernel Mode

• OS kernel is a collection of functions
  responsible for the most basic services
• OS kernel executes in the kernel mode
• Privileged instructions
• To load/store special CPU registers
• To map memory pages to the address space of a
  specific process
• Instructions to set the interrupt priority level
• Instructions to activate I/O devices

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Protecting Kernel mode

• Is it possible for the user program to cause the
  CPU to enter kernel mode?

• Yes This must be possible (system call)
• The problem how to prevent the user program from
  executing privileged instructions?

• Solution change the program counter (PC) to
  point to the OS code upon switch

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Handling interrupts

• Interrupts cause the CPU to enter kernel mode
• The address of the kernel function to execute is
  loaded from the interrupt vector
• The interrupt vector address and the interrupt
  numbering is a part of the hardware specification
• Handlers are registered during the boot

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Interrupt types (I)

• Asynchronous interrupts are generated by external
  devices at unpredictable times
• Clock interrupt is the basis for time slicing
• Update the system time, preempt/schedule
  processes
• I/O device interrupt
• Informs the OS about completion of a requested I/O

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Interrupt types (II)

• Internal (synchronous) interrupts are generated
  synchronously by CPU as a result of an
  exceptional condition
• An error condition the application is trying to
  perform an illegal operation
• E.g., division by 0, issuing a privileged instr.,
  
• The handler typically kills the application
• A temporary problem
• E.g., the requested page is not in the memory
• Handling bring the page into the memory

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System calls

• Used to request a service from the OS
• A collection of the system calls is the OS API
• Packaged as a library
• Typical system calls
• Open/read/write/close the file
• Get the current time
• Create a new process
• Request more memory

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Handling system calls

• An application executes a special trap (syscall)
  instruction
• Causes the CPU to enter kernel mode and set PC to
  a special system entry point (gate routine)
• The gate routine address is typically stored in a
  predefined interrupt vector entry
• Intel architecture int80
• A single entry serves all system calls (why?)

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An example
open(/tmp/foo) store the system call
number and the parameters in a predefined
kernel memory location trap() retrieve
the response from a predefined kernel memory
location return the response to the calling
application trap() PCint80 // transfer
control to the gate routine Gate
routine switch(sys_call_num) case OPEN
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Other hardware support

• Translating virtual address into a physical
  address
• Assist in supporting the virtual memory
  abstraction
• Support for used bits for memory pages
• Helps to determine which pages can be swapped out
  when needed

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What we are going to study

• Performance evaluation (brief)
• Process handling
• Process concept, scheduling, concurrency control
• Memory management
• Paging, virtual memory
• File system

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Advanced topics

• Distributed systems
• Communication, networking, middleware
• Other possible topics
• Reliable distributed systems
• Real-time systems
• Parallel systems
• Modern storage architectures

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Bibliography

• Notes by Dror Feitelson
• Will be published weekly
• Operating System Concepts, by
• A. Silberschatz, P. Galvin, G. Gagne
• Operating Systems Internals and Design
  Principles, by W. Stallings
• See the notes for more references

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Next

• Performance evaluation