Emery Berger

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Operating SystemsCMPSCI 377Lecture 2 OS
Architecture

• Emery Berger
• University of Massachusetts, Amherst

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Last Class Introduction

• Operating system interface between user
  architecture

User-level Applications
virtual machine interface
Operating System
physical machine interface
Hardware
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TodayOS Computer Architecture

• Modern OS Functionality
• Goals of Operating Systems
• Hardware Support for OS Features
• Architecture Basics Details

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Modern OS Functionality

• Provides support for
• Concurrency
• I/O Device Management
• Memory Management
• Files
• Distributed Systems Networks

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Concurrency

• Supports multiple activities, apparently
  occurring simultaneously
• Multiple users
• Several users work as if each has private machine
• Multiple processes/threads
• One on CPU at a time
• Multiple active concurrently

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Modern OS Functionality, Cont.

• I/O
• CPU works while waiting on slow I/O devices
• Memory Management
• Coordinates allocation of RAM
• Moves data between disk main memory
• Files
• Coordinates how disk space is used
• Distributed Systems Networks
• Lets group of PCs to work together on
  distributed h/w

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Operating Systems Governments

• Goals surprisingly like utopian political systems
• Libertarianism
• Socialism
• Communism
• OS researchers benevolent dictators

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Libertarians

• Infinite RAM, CPU
• Protects users from each other

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OS as Utopian Governments

• Socialism
• Safe secure communication
• Protects everyone
• Fair allocation of resources

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Communism

• To each according to his needs
• Centralized control
• Allocates resources efficiently (in theory)

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Theory vs. Practice

• Goals
• Efficient fair resource allocation
• Safe secure communication
• Protect everyone from each other (isolation)
  from govt. (OS)
• Illusion infinite RAM, CPU
• But how can we do this efficiently?
• Hardware support

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Canonical System Hardware

• CPU Processor to perform actual computations
• I/O devices terminal, disks, video, printer
• Memory data programs
• System Bus Communication channel between above

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Services Hardware Support
OS Service Hardware Support
Protection Kernel/User Mode Protected Instructions Base Limit Registers
Interrupts Interrupt Vectors
System Calls Trap Instructions
I/O Interrupts, Memory-Mapping
Synchronization Atomic Instructions
Virtual Memory Translation Lookaside Buffers
Scheduling Timer
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Protection

• OS protects users from each other
• Users cannot read or write other users memory
• Name OSs that do and dont do this!
• Protects self from users
• Safe from errant or malicious users
• Privileged instructions (e.g., halt machine)
• Code data protected

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Kernel ModePrivileged Instructions

• CPU provides kernel mode restricted to OS
• Inaccessible to ordinary users
• Kernel core of operating system
• Privileged instructions registers
• Direct access to I/O
• Modify page table pointers, TLB
• Enable disable interrupts
• Halt the machine, etc.
• Indicated by status bit in protected CPU register

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Protecting MemoryBase and Limit Registers

• Hardware support to protect memory regions
• Loaded by OS before starting program
• CPU checks each reference
• Instruction data addresses
• Ensures reference in range

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Hardware Support
OS Service Hardware Support
Protection Kernel/User Mode Protected Instructions Base Limit Registers
Interrupts Interrupt Vectors
System Calls Trap Instructions
I/O Interrupts, Memory-Mapping
Synchronization Atomic Instructions
Virtual Memory Translation Lookaside Buffers
Scheduling Timer
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Interrupts

• Polling are we there yet? no! (repeat)
• Inefficient use of resources
• Annoys the CPU
• Interrupt silence, then were there
• I/O device has own processor
• When finished, device sends interrupt on bus
• CPU handles interrupt

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CPU Interrupt Handling

• Handling interrupts relatively expensive
• CPU must
• Save hardware state
• Registers, program counter
• Disable interrupts (why?)
• Invoke via in-memory interrupt vector (like trap
  vector, soon)
• Enable interrupts
• Restore hardware state
• Continue execution of interrupted process

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Hardware Support
OS Service Hardware Support
Protection Kernel/User Mode Protected Instructions Base Limit Registers
Interrupts Interrupt Vectors
System Calls Trap Instructions
I/O Interrupts, Memory-Mapping
Synchronization Atomic Instructions
Virtual Memory Translation Lookaside Buffers
Scheduling Timer
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Traps

• Special conditions detected by architecture
• E.g. page fault, write to read-only page,
  overflow, system call
• On detecting trap, hardware must
• Save process state (PC, stack, etc.)
• Transfer control to trap handler (in OS)
• CPU indexes trap vector by trap number
• Jumps to address
• Restore process state and resume

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Memory Traps

• Special case trigger trap on write to protected
  memory area
• Widely used in operating systems
• Debugging
• Distributed virtual memory
• Approximating LRU
• Garbage collection
• Copy-on-write
• Pay performance penalty only when needed

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Hardware Support
OS Service Hardware Support
Protection Kernel/User Mode Protected Instructions Base Limit Registers
Interrupts Interrupt Vectors
System Calls Trap Instructions
I/O Interrupts, Memory-Mapping
Synchronization Atomic Instructions
Virtual Memory Translation Lookaside Buffers
Scheduling Timer
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Memory-Mapped I/O

• Direct access to I/O controller through memory
• Reserve area of memory for communication with
  device (DMA)
• Video RAM
• CPU writes frame buffer
• Video card displays it
• Fast and convenient

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Hardware Support
OS Service Hardware Support
Protection Kernel/User Mode Protected Instructions Base Limit Registers
Interrupts Interrupt Vectors
System Calls Trap Instructions
I/O Interrupts, Memory-Mapping
Synchronization Atomic Instructions
Virtual Memory Translation Lookaside Buffers
Scheduling Timer
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Synchronization

• How can OS synchronize concurrent processes?
• E.g., multiple threads, processes interrupts,
  DMA
• CPU must provide mechanism for atomicity
• Series of instructions that execute as one or not
  at all

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Synchronization How-To

• One approach
• Disable interrupts
• Perform action
• Enable interrupts
• Advantages
• Requires no hardware support
• Conceptually simple
• Disadvantages
• Could cause starvation

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Synchronization How-To, II

• Modern approach atomic instructions
• Small set of instructions that cannot be
  interrupted
• Examples
• Test-and-set (TST)if word contains given
  value, set to new value
• Compare-and-swap (CAS)if word equals value,
  swap old value with new
• Intel LOCK prefix (XCHG, ADD, DEC, etc.)
• Used to implement locks

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Hardware Support
OS Service Hardware Support
Protection Kernel/User Mode Protected Instructions Base Limit Registers
Interrupts Interrupt Vectors
System Calls Trap Instructions
I/O Interrupts, Memory-Mapping
Synchronization Atomic Instructions
Virtual Memory Translation Lookaside Buffers
Scheduling Timer
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Virtual Memory

• Provides illusion of complete access to RAM
• All addresses translated from physical addresses
  into virtual addresses
• OS loads pages from disk as needed
• Keeps track of which pages are in memory (in
  core) and which are on disk
• Many benefits, including
• Allows users to run programs without loading
  entire program into RAM
• May not fit in entirety (think MS Office)

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Translation Lookaside Buffer

• First virtual memory systems performed address
  translation in software
• On every memory access! (s..l..o..w..)
• Modern CPUs contain hardware to do this the TLB
• Hash-based scheme
• Maps virtual addresses to physical addresses
• Fast, fully-associative cache
• Todays workloads are often TLB-miss dominated

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Hardware Support
OS Service Hardware Support
Protection Kernel/User Mode Protected Instructions Base Limit Registers
Interrupts Interrupt Vectors
System Calls Trap Instructions
I/O Interrupts, Memory-Mapping
Synchronization Atomic Instructions
Virtual Memory Translation Lookaside Buffers
Scheduling Timer
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Scheduling Timers

• OS needs timers for
• Time of day
• CPU scheduling
• Fairness limited quantum (e.g., 100ms) for each
  task
• When quantum expires, switch processes
• Uses interrupt vector

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Summary

• OS relies on hardware for many services
• Protection
• Interrupts
• System Calls
• Synchronization
• Virtual memory
• Timers
• Otherwise impossible or impractically slow in
  software