Operating System Concepts

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Operating System Concepts

• Ku-Yaw Chang
• canseco_at_mail.dyu.edu.tw
• Assistant Professor, Department of Computer
  Science and Information Engineering
• Da-Yeh University

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Chapter 9 Memory Management

• Keep several processes in memory to increase CPU
  utilization
• Memory management algorithms
• Require hardware support
• Common strategies
• Paging
• Segmentation

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Chapter 9 Memory Management

1. Background
2. Swapping
3. Contiguous Memory Allocation
4. Paging

1. Segmentation
2. Segmentation with Paging
3. Summary
4. Exercises

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9.1 Background

• Program must be brought (loaded) into memory and
  placed within a process for it to be run.
• Address binding
• A mapping from one address space to another
• A typical instruction-execution cycle
• Fetch an instruction from memory
• Decode the instruction
• May cause operands to be fetched from memory
• Execute the instruction
• Store results back into memory

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9.1.1 Address Binding

• Input queue
• A collection of processes on the disk that are
  waiting to be brought into memory to run the
  program.
• A user program will go through several steps
  before being executed
• Addresses in source program are symbolic
• A compiler binds these symbolic addresses to
  relocatable addresses
• A loader binds these relocatable addresses to
  absolute addresses

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9.1.1 Address Binding

• Compile time
• Absolute code can be generated
• Know at compile time where the process will
  reside in memory
• MS-DOS .COM-format programs are absolute code

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9.1.1 Address Binding

• Load time
• Relocatable code can be generated
• Not known at compile time where the process will
  reside in memory
• Final binding is delayed until load time

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9.1.1 Address Binding

• Execution time
• The process can be moved from one memory segment
  to another
• Binding must be delayed until run time
• Special hardware must be available

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9.1.2 Logical- Versus Physical-Address Space

• Logical address
• An address generated by the CPU
• Compile-time and load-time
• Also called virtual address
• Logical-address space
• The set of all logical addresses
• Physical address
• An address seen by the memory unit
• The one loaded into the memory-address unit
• Execution time
• Logical and physical address spaces differ
• Physical-address space

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9.1.2 Logical- Versus Physical-Address Space

• Memory-Management Unit (MMU)
• A hardware device
• Run-time mapping from virtual to physical
  addresses
• Different methods to accomplish such a mapping
• Logical addresses
• 0 to max
• Physical addresses
• R 0 to R max

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Dynamic RelocationUsing a Relocation Register
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9.1.3 Dynamic Loading

• The entire program and data must be in memory for
  its execution
• The size of a process is limited to the size pf
  physical memory.
• Dynamic Loading
• All routines are kept on disk in a relocatable
  load format
• The main program is loaded into memory and is
  executed
• A routine is not loaded until it is called
• Advantage
• An unused routine is never loaded

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9.1.4 Dynamic Linking andShared Libraries

• Dynamic Linking
• Linking is postponed until execution time
• Small piece of code, called stub, used to locate
  the appropriate memory-resident library routine
• OS checks if routine is in processes memory
  address
• If yes, load the routine into memory
• Stub replaces itself with the address of the
  routine, and executes the routine.
• Dynamic linking is particularly useful for
  libraries.

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9.1.5 Overlays

• Keep in memory only those instructions and data
  that are needed at any given time
• Needed when process is larger than amount of
  memory allocated to it
• Features
• Implemented by user
• No special support needed from operating system
• Programming design is complex

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9.1.5 Overlays
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Chapter 9 Memory Management

1. Background
2. Swapping
3. Contiguous Memory Allocation
4. Paging

1. Segmentation
2. Segmentation with Paging
3. Summary
4. Exercises

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Swapping

• A process can be
• Swapped temporarily out of memory to a backing
  store
• Commonly a fast disk
• Brought back into memory for continued execution
• A process swapped back into
• The same memory space
• Binding is done at assembly or load time
• A different memory space
• Execution-time binding

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Swapping of two processes
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Swapping

• Context-switch time is fairly high
• User process size 1MB
• Transfer rate 5MB per second
• Actual transfer
• 1000KB / 5000 KB per second 200 milliseconds
• An average latency 8 ms
• Total swap time
• 208 208 416 ms
• Time quantum should be substantially larger than
  0.416 seconds.

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Swapping

• Major part of the swap time is transfer time
• Directly proportional to the amount of memory
  swapped
• Factors
• How much memory is actually used
• To reduce swap time
• Be sure the process is completely idle
• Pending I/O

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Chapter 9 Memory Management

1. Background
2. Swapping
3. Contiguous Memory Allocation
4. Paging

1. Segmentation
2. Segmentation with Paging
3. Summary
4. Exercises

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Contiguous Memory Allocation

• Memory
• One for the resident operating system
• In either low or high memory
• Location of the interrupt vector
• One for the user processes
• Contiguous memory allocation
• Each process is contained in a single contiguous
  section of memory

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Memory Protection

• A relocation register with a limit register

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Memory Allocation

• Fixed-sized partitions
• Simplest
• Each partition contain exactly one process
• Degree of multiprogramming is bounded
• Strategies
• First fit
• Best fit
• Worst fit
• Problem
• External fragmentation
• Internal fragmentation
• 50-percent rule
• Given N allocated blocks
• Another 0.5 N blocks will be lost

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Memory Allocation

• Possible solutions to the external fragmentation
• Compaction
• Permit the logical address space of a process to
  be noncontiguous

OS
OS
OS
OS
process 5
process 5
process 5
process 5
process 9
process 9
process 8
process 10
process 2
process 2
process 2
process 2
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Chapter 9 Memory Management

1. Background
2. Swapping
3. Contiguous Memory Allocation
4. Paging

1. Segmentation
2. Segmentation with Paging
3. Summary
4. Exercises

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Paging

• A memory-management scheme that permits the
  physical-address space of a process to be
  noncontiguous
• Frames (physical memory)
• Fixed-sized blocks
• Pages (logical memory)
• Blocks of the same size
• Every address is divided into
• A page number (p)
• An index to a page table
• A page offset (d)

page number
page offset
p
d
m-n
n
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Paging hardware
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Paging Model
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Paging Example
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Free Frames
After Allocation
Before Allocation
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Chapter 9 Memory Management

1. Background
2. Swapping
3. Contiguous Memory Allocation
4. Paging

1. Segmentation
2. Segmentation with Paging
3. Summary
4. Exercises

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Chapter 9 Memory Management

1. Background
2. Swapping
3. Contiguous Memory Allocation
4. Paging

1. Segmentation
2. Segmentation with Paging
3. Summary
4. Exercises

34
Chapter 9 Memory Management

1. Background
2. Swapping
3. Contiguous Memory Allocation
4. Paging

1. Segmentation
2. Segmentation with Paging
3. Summary
4. Exercises

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Chapter 9 Memory Management

1. Background
2. Swapping
3. Contiguous Memory Allocation
4. Paging

1. Segmentation
2. Segmentation with Paging
3. Summary
4. Exercises