We will focus on operating system concepts

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• We will focus on operating system concepts
• What does it do? How is it implemented?
• Apply to Windows, Linux, Unix, Solaris, Mac OS X.
  
• Will discuss differences between implementations.
  
• Will use programming to enhance our understanding
  of basic concepts.

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Todays Topics

• High level view of Operating System
• Interrupts
• Context Switches
• Multiprogramming
• Timesharing
• Protection of system resources.

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Components of a Computer System
User
User
User
User
System and application Programs
Web Browsing Text Editor
Database Compiler
Operating System
Hardware

• System and Application Programs
• Operating System
• Hardware (CPU, memory, I/O devices).

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Components of a Computer System
App1
App2
App3
App4
System and application Programs
Web Browsing Text Editor
Database iTunes
Operating System
Hardware

• System and Application Programs
• Operating System
• Hardware (CPU, memory, I/O devices)

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• System programs make our lives easier but are not
  part of the Operating System.
• Compilers, editors, linkers, loaders, shells.
• Operating System performs two functions
• Allocates, schedules, manages, and protects
  system resources.
• Provides a convenient interface to system
  resources.
• Dont want to have to write device driver to
  store files on disk.

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Interrupts

• Operating systems are interrupt-driven.
• Perform action, such as giving control of the CPU
  to an application, then does not regain control
  of CPU until an interrupt occurs.

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• CPU and other devices can run concurrently.
• Can start I/O operation and start application
  executing.
• When requested operation completed, device
  generates hardware interrupt.
• User programs generate interrupts when making
  system calls (requesting service from OS)
• TRAP instruction.
• Generally termed software interrupt.

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Interrupts

• When an interrupt is generated, the CPU stops
  what it is doing and execution is transferred to
  the appropriate interrupt handler.
• Addresses of interrupt handlers are found in the
  interrupt vector.
• Table of addresses
• Each device (and TRAP) has a particular offset
  within the table

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Interrupt Vector
100 500 800 1500
0
1
2
3
0 I/O Device 1 1 I/O Device 2 2 NIC 3 TRAP
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Interrupts

• Consider some Joe_Program (nickname is JP or Joe)
  executing and a device issues an interrupt.
• Need to jump to interrupt handler, but

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Interrupts

• Consider some Joe_Program executing and a device
  issues an interrupt.
• Need to jump to interrupt handler, but
• What do we do with Joe?

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Interrupts

• Consider some Joe_Program executing and a device
  issues an interrupt.
• Need to jump to interrupt handler, but
• What do we do with Joe?
• What is the minimal information to save to be
  able to restart his execution?

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Interrupts

• Consider some Joe_Program executing and a device
  issues an interrupt.
• What do we do with Joe?
• What is the minimal information to save to be
  able to restart his execution?
• Address of the next instruction.

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• What if interrupt processing routine needs to use
  all of the CPU registers?

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• What if interrupt processing routine needs to use
  all of the CPU registers?
• Better save a copy of the state of all of the
  registers PJ is using.

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• Once interrupt routine completed, how do we get
  JP restarted?

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• Once interrupt routine completed, how do we get
  JP restarted?
• Restore state of registers
• Set the Instruction Pointer to the next
  instruction JP was going to execute.
• It should appear to JP as if its execution was
  never interrupted.

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• Saving and restoring application state is termed
  a context switch.

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Operating System Structure

• A key concept of operating systems is
  multiprogramming (or multitasking).
• Technique developed because CPU and I/O device
  time were very expensive.
• Basic idea
• Keep multiple jobs in memory.
• When one job blocks on I/O, the CPU immediately
  switches to another job.

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• This keeps CPU and I/O devices fully utilized.
• Without multiprogramming, CPU would be idle
  during I/O operations.
• First developed for batch systems in the 60s.
• No interactions between user and programs.

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Multiprogramming Batch Systems
Problem CPU and I/O very expensive. Solution
Multiplex CPU between multiple jobs.
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Time-Sharing SystemsInteractive Computing

• Logical extension of multiprogramming.
• The CPU is multiplexed among several jobs that
  are kept in memory and on disk (the CPU is
  allocated to a job only if the job is in memory).
• A job swapped in and out of memory to the disk.
• Each user is executing a shell program.
• Takes user command and executes it.
• Waits for the next command.

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Time-Sharing SystemsInteractive Computing

• Goal is to give the illusion that each user has
  own machine.
• Response time is a priority.

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Protection of System Resources

• I/O Devices
• Memory
• CPU
• Files

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Protection of System Resources

• Based on dual-mode execution
• kernel mode and user mode.
• Privileged instructions can be issued only in
  kernel mode.
• Mode bit in PSW, checked on every instruction.

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User process executing
Mode Bit1
Continue Execution
System Call
Trap, mode bit 0
Return. Mode bit 1
Execute System Call
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Protection of I/O Devices

• All I/O instructions are privileged instructions.
• Only accessed through system calls.

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Memory Protection

• Must provide memory protection for the interrupt
  vector, interrupt service routines, and other
  applications address space.
• Two registers that determine the range of legal
  addresses a program may access
• Base register holds the smallest legal physical
  memory address.
• Limit register contains the size of the range
• Memory outside the defined range is protected.

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Use of A Base and Limit Register
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Hardware Address Protection
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CPU (and OS) Protection

• Keep user from monopolizing CPU.
• Ensure OS regains control of CPU.

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CPU Protection

• Timer interrupts computer after specified
  period to ensure operating system maintains
  control.
• Timer is decremented every clock tick.
• When timer reaches the value 0, an interrupt
  occurs.
• Timer commonly used to implement time sharing.

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Privileged Instructions

• Load base and limit registers?

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Privileged Instructions

• Load base and limit registers?
• Set the system timer?

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Privileged Instructions

• Load base and limit registers?
• Set the system timer?
• Read the system clock?

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Privileged Instructions

• Set the system timer?
• Read the system clock?
• Load base and limit registers?
• Open a file?

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Privileged Instructions

• Load base and limit registers?
• Set the system timer?
• Read the system clock?
• Open a file?
• Compile a program and create executable?

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Privileged Instructions

• Load base and limit registers?
• Set the system timer?
• Read the system clock?
• Open a file?
• Compile a program and create executable?
• Enable/disable interrupts?