Memory Management

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Memory Management
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Memory Manager

• Requirements
• Minimize executable memory access time
• Maximize executable memory size
• Executable memory must be cost-effective
• Todays memory manager
• Allocates primary memory to processes
• Maps process address space to primary memory
• Minimizes access time using cost-effective memory
  configuration
• May use static or dynamic techniques

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Address Space vs Primary Memory
Hardware Primary Memory
Process Address Space
Mapped to object other than memory
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Building the Address Space
Source code
C
Reloc Object code

• Compile time Translate elements

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Primary Secondary Memory
CPU

• CPU can load/store
• Ctl Unit executes code from this memory
• Transient storage

Primary Memory (Executable Memory) e.g. RAM
Secondary Memory e.g. Disk or Tape

• Access using I/O operations
• Persistent storage

Information can be loaded statically or
dynamically
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Static Memory Allocation
Operating System
Unused
In Use
Process 3
Process 0
pi
Process 2
Issue Need a mechanism/policy for loading pis
address space into primary memory
Process 1
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Fixed-Partition Memory Mechanism
Operating System
pi needs ni units
Region 0
N0
pi
ni
Region 1
N1
N2
Region 2
Region 3
N3
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Fixed-Partition Memory -- Best-Fit
Operating System

• Loader must adjust every address in the absolute
  module when placed in memory

Region 0
N0
Region 1
N1
Internal Fragmentation
pi
N2
Region 2
Region 3
N3
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Fixed-Partition Memory -- Worst-Fit
Operating System
pi
Region 0
N0
Region 1
N1
N2
Region 2
Region 3
N3
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Fixed-Partition Memory -- First-Fit
Operating System
pi
Region 0
N0
Region 1
N1
N2
Region 2
Region 3
N3
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Fixed-Partition Memory -- Next-Fit
Operating System
Region 0
N0
pi
Region 1
N1
Pi1
N2
Region 2
Region 3
N3
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Variable Partition Memory Mechanism
Operating System
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Cost of Moving Programs
load R1, 0x02010
3F013010
Program loaded at 0x01000
Consider dynamic techniques
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Dynamic Memory Allocation

• Could use dynamically allocated memory
• Process wants to change the size of its address
  space
• Smaller ? Creates an external fragment
• Larger ? May have to move/relocate the program
• Allocate holes in memory according to
• Best- /Worst- / First- /Next-fit

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Special Case Swapping

• Special case of dynamic memory allocation
• Suppose there is high demand for executable
  memory
• Equitable policy might be to time-multiplex
  processes into the memory (also space-mux)
• Means that process can have its address space
  unloaded when it still needs memory
• Usually only happens when it is blocked

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Dynamic Address Relocation
CPU
Relative Address
0x02010
0x12010
Relocation Register
0x10000
load R1, 0x02010
MAR

• Program loaded at 0x10000 ? Relocation Register
  0x10000
• Program loaded at 0x04000 ? Relocation Register
  0x04000

We never have to change the load module addresses!
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Runtime Bound Checking
CPU
Relative Address
Relocation Register
lt
Limit Register

• Bound checking is inexpensive to add
• Provides excellent memory protection

MAR
Interrupt
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Memory Hierarchies Dynamic Loading
CPU Registers
Primary (Executable)
L1 Cache Memory
L2 Cache Memory
Main Memory
Larger storage
Rotating Magnetic Memory
Faster access
Optical Memory
Secondary
Sequentially Accessed Memory
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Exploiting the Hierarchy

• Upward moves are (usually) copy operations
• Require allocation in upper memory
• Image exists in both higher lower memories
• Updates are first applied to upper memory
• Downward move is (usually) destructive
• Destroy image in upper memory
• Update image in lower memory

• Place frequently-used info high,
  infrequently-used info low in the hierarchy

• Reconfigure as process changes phases

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Memory Mgmt Strategies

• Fixed-Partition used only in batch systems
• Variable-Partition used everywhere (except in
  virtual memory)
• Swapping systems
• Popularized in timesharing
• Relies on dynamic address relocation
• Now dated
• Dynamic Loading (Virtual Memory)
• Exploit the memory hierarchy
• Paging -- mainstream in contemporary systems
• Segmentation -- the future

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NT Memory-mapped Files
Secondary memory
Ordinary file

• Open the file
• Create a section object (that maps file)
• Identify point in address space to place the file

Executable memory
Memory mapped files
Section object