Content

1. Introduction to Memory Management
2. Managing Memory with Bitmaps & Lists
3. Simple Memory Management & Fragmentation

Introduction to Memory Management

• Requirements
  • Isolation
  • Abstraction
  • Sharing
• Goals
  • Low memory overhead
  • Low performance overhead
• Supports
  • Compiler
    • Source file to object file
    • Generates relocatable virtual memeory addresses
  • Linker
    • Links multiple object files to single program on disk
    • Generates absolute virtual memeory addresses
  • OS
    • Loads program into physical memory
    • Sets up virtual memory H/W
  • H/W
    • Translates virtual memory to physical memory
    • S/W too slow for address translation

Managing Memory with Bitmaps & Lists

• After boot loader loads OS code & data in low memory, OS keeps track of allocated & free memory

• Bitmap

  • 1 bit per chunk of memory; in use or not

  • Larger chunk size, lower overhead BUT more internal fragmentation

    |--OS code data--| bit map | chunk1 | chunk2 | ... |
                       01101...
    

• Linked Lists

  • A list of elements for each allocated or free region of memory

  • Allocated bit + starting address + length + pointer

    | A |   f   |  B  | C | f |    D    | E | f | ... |
    
    | a | 0 | 5 | [] | --> | f | 5 | 3 | [] | --> ...
    

  • Searching

    • First fit
    • Best fit
    • Quick fit

• Under which conditions is each approach preferable?

Simple Memory Management & Fragmentation

• Internal fragmentation
  • Program doesn't use entire region -> paddings
• External fragmentation
  • A large region cannot be allocated even enough memory exists
    • Compaction: move processes around, but expensive