Operating System Support

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Chapter 8

• Operating System Support
• Focus on Architecture

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Functions of Operating System

• Managing Resources
• Access to System Utilities
• Access to files
• Access to I/O devices
• Managing Interrupts Bus Control
• Error detection and response
• Accounting
• Scheduling Processes (or tasks)
• Program creation
• Program execution
• I/O Processing
• Managing Memory Utilization
• Partitioning,
• Paging,
• Virtual memory,
• Segmentation

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Types of Operating Systems

• Interactive
• Batch
• Uni-tasking
• Multi-tasking
• Real-Time

4
Batch Operating System Model

• Jobs are batched in queue
• Monitor handles scheduling
• Monitor controls sequence of
• events to process batch
• When one job is finished,
• control returns to Monitor
• which loads next job

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Desirable Hardware Support for OS

• Memory protection
• To protect the Monitor Utilities
• Timer
• To prevent a job monopolizing the system
• Privileged instructions
• Only executed by Monitor
• e.g. I/O, files
• Interrupts
• Allows for relinquishing and regaining control
• DMA
• Allows for optimizing bus usage

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The Case for Multi-programmed Batch Systems

• I/O devices are very slow
• ? Waiting is inefficient use of computer
• When one program is waiting for I/O, another can
  use the CPU

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Multi-Programming with Three Programs
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Utilization Uni-programmed vs Multi-programmed
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Multiprogramming Resource Utilization
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Types of Scheduling
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A Five State Process Model
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Process Control Block Layout
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Scheduling Time Sequence Example
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Key Elements of O/S
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Process Scheduling
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Memory Management

• Uni-programming
• Memory split into two
• One for Operating System (monitor)
• One for currently executing program
• Multi-programming
• User part is sub-divided and shared among
  active processes
• Note Memory size implications
• - 16 bits ? 64K memory addresses
• - 24 bits ? 16M memory addresses
• - 32 bits ? 4G memory addresses

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Swapping

• Problem I/O is so slow compared with CPU that
  even in multi-programming system, CPU can be idle
  most of the time
• Solutions
• Increase amount of main memory
• Expensive
• Swapping

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What is Swapping?

• Long term queue of processes stored on disk
• Processes swapped in as space becomes available
• As a process completes it is moved out of main
  memory
• If none of the processes in memory are ready
  (i.e. all I/O blocked)
• Swap out a blocked process to intermediate queue
• Swap in a ready process or a new process

But swapping is an I/O process Isnt I/O
slow? So why does swapping make sense ?

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Implementation of Swapping
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Partitioning

• Partitioning
• May not be equal size
• Splitting memory into sections to
  allocate to processes
• (including Operating System!)
• Fixed-sized partitions
• Potentially a lot of wasted memory
• Variable-sized partitions
• Process is stored into smallest reasonable hole
• Dynamic partitions
• no room for additional memory allocation
• memory leak need periodic coalescing, or
• need periodic compaction

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Fixed Partitioning
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Effect of Dynamic Partitioning memory leaks
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Relocation Challenges

• Cant expect that process will load into the same
  place in memory as last time
• Instructions contain addresses
• Locations of data
• Addresses for instructions (branching)
• Logical address - relative to beginning of
  program
• Physical address - actual location in memory
• (this time)
• A Solution
• Use Base Address
• Automatic (hardware) Conversion

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Paging

• Split memory into equal sized, small chunks
• - page frames
• Operating System maintains list of free frames
• Then
• Split programs (processes) into equal sized small
  chunks
• pages
• Allocate the required number page frames to a
  process
• - A process does not require contiguous page
  frames
• - Each process has its own page table

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Allocation of Free Frames
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Paging - Logical and Physical Addresses
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Paging Implementations

• Demand paging
• Do not require all pages of a process in memory
• Bring in pages as required
• Page fault
• Required page is not in memory
• Operating System must swap in required page
• May need to swap out a page to make space
• Perhaps select page to throw out based on recent
  history

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Thrashing

• Too many processes in too little memory
• Operating System spends all its time swapping
• Little or no real work is done
• Solutions
• Good page replacement algorithms
• Reduce number of processes running
• Add more memory

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Virtual Memory

• We do not need all of a process in memory for it
  to run - We can swap in pages as required
• So - we can now run processes that are bigger
  than total memory available!
• Differentialtions
• Main memory is called real memory
• User/programmer can see much bigger memory space
  - virtual memory
• Implications
• Tables can become huge that cant fit into memory
  need multiple level tables yech!

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Alternate Inverted Page Table Structure
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Translation Lookaside Buffer

• Every virtual memory reference causes two
  physical memory access
• Fetch page table entry
• Fetch data
• Use special cache for page table(s)

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TLB and Cache Operation (special Cache for
tables)
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Segmentation

• What is it?
• Segmentation is visible to the programmer
• - Paging is not (usually) visible to the
• programmer
• Usually different segments allocated to program
  and data
• May be a number of program and data segments,
  e.g. to support protection levels, priority
  levels, organization, flexibility, etc.

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Advantages of Segmentation

• Simplifies handling of growing data structures
• Allows programs to be altered and recompiled
  independently, without re-linking and re-loading
• Lends itself to sharing among processes
• Lends itself to protection
• Some systems combine segmentation with paging

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OS Review

• Scheduling
• uni-programming
• multi-programming
• time-sharing
• long-term scheduler (queue of all jobs
  potentially schedulable)
• short-term scheduler (queue of processes that
  are ready to execute)
• medium-term scheduling (queue of jobs that can
  reside in memory)
• blocked monitoring (queue of processes blocked
  for resources)
• new ready running blocked exit state
  machine
• Memory management
• partitioning
• paging frames, pages, page fault, page table,
  logical/physical addr
• virtual memory inverted page table,
  Translation Lookaside Buffer
• segmentation

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Pentium and Power PC

• Pentium Intel
• Power PC Motorola IBM
• A 32 bit memory address space is 4 G Bytes
• A 46 bit memory address space is 64 T Bytes
• 1.25 terabytes has been claimed as the capacity
  of a human being's functional memory (according
  to Raymond Kurzweil).
• A Holographic Versatile Disc (HVD) can hold up to
  3.9 terabytes.
• One hour of uncompressed Ultra High Definition
  Video (UHDV) consumes approximately 11.5
  terabytes of data.
• The U.S. Library of Congress has claimed that "as
  of December 31, 2005, the Library has collected
  more than 40 terabytes of data."
• A Protein-coated disc (PCD) can hold 50 terabytes
  of data.
• A 64 bit memory address space is ? (Who cares!)
• The point is that it is not clear to me why we
  care for some at least some years to come.

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Pentium II (Uses hardware for segmentation
paging)

• Unsegmented, unpaged
• virtual address physical address
• Used in Low complexity, High performance systems
• Unsegmented, paged
• Memory viewed as paged linear address space
• Protection and management via paging (Ex
  Berkeley UNIX)
• Segmented, unpaged
• Collection of local address spaces
• Protection to single byte level, Translation
  table needed is on chip when segment is in
  memory, provide predictable access times
• Segmented, paged
• Segmentation used to define logical memory
  partitions subject to access control
• Paging manages allocation of memory within
  partitions (Ex Unix System V)

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Pentium II Address Translation Mechanism

• Segment uses 2 bits to provide 4 levels of
  protection, typically
• 0 OS kernel, 1 OS, 2 apps needing
  special security, 3 general apps

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Pentium II Paging

• Segmentation may be disabled
• In which case linear address space is used
• Two level page table lookup
• First, page directory
• 1024 entries max
• Splits 4G linear memory into 1024 page groups of
  4Mbyte
• Each page table has 1024 entries corresponding to
  4Kbyte pages
• Can use one page directory for all processes, one
  per process or mixture
• Page directory for current process always in
  memory
• Use TLB holding 32 page table entries
• Two page sizes available 4k or 4M

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PowerPC 32-bit Address Translation
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PowerPC Memory Management Hardware

• 32 bit paging with simple segmentation
• or 64 bit paging with more powerful segmentation
• Or, both do block address translation
• Map 4 large blocks of instructions 4 of memory
  to bypass paging
• e.g. OS tables or graphics frame buffers
• 32 bit effective address
• 12 bit byte selector
• 4kbyte pages
• 16 bit page id
• 64k pages per segment
• 4 bits indicate one of 16 segment registers
• Segment registers under OS control

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PowerPC 32-bit Memory Management Formats