Memory Manager

Addressing

• Physical Addressing
  • Absolute location on memory chip
• Logical Addressing
  • Relative
  • Translated by the hardware and OS to actual physical locations
• Resolved during
  • Development time
  • Compile time (compiler sets special OS addresses)
  • Link time (linker links static libraries)
  • Load time (loading dynamic libraries)
  • Run time

Safety

Physical Addressing (Old)

• Fence Register (very old)
  • For uni-program systems
  • Provide a division between OS and process memory
  • Block a program from accessing any physical memory below the fence register
  • Done by hardware on the MMU
• High/Low Registers
  • For multi-programmed systems
  • Uses high/low registers for each process to define the memory boundaries
  • Still done by hardware but needs for the OS to update the values on each context switch

Logical Addressing

• Base and Limit Registers
  • Logical memory always starts from 0 for every process but is stored in arbitrary physical memory
  • Each process stores the base and limit
    • Base = start of memory in physical memory
    • Limit = end of memory in logical memory
  • Checking
    1. When the logical address is greater than the limit, it traps for illegal access
    2. Add base register to the logical address to get physical address
    3. Use physical address

Approach

Tracking Free Space

• Bitmaps
  • Have one bit represent if each block of memory is free or not
  • Linear growth
• Linked List
  • Option: Track Everything
  • Option: Track Holes

Finding Free Space

• First Fit: First hole big enough
• Next Fit: Next hole big enough
• Best Fit: Smallest hole big enough
• Worst Fit: Find the largest hole available
• Quick Fit: Separate list of commonly requested sizes

Process Partitioning

• Multiple Fixed Partitions

  • Fixed list of fixed size partitions have a fixed size
  • Shared Queue
    • Has a single queue of processes and they get placed in partitions that can fit them
    • Can have a small process take a big partition
  • Separate Queues
    • Looks at how big a process is and then assigns it to a queue for a partition of similar size
    • Decreases internal fragmentation

• Multiple Variable Partitions

  • Partitions can change over time to accommodate process load
  • Prevents internal fragmentation but has external fragmentation
    • Because there is space left over between processes

• Fragmentation

  • When memory is split up into sections that are too small to use
  • Internal
    • Memory is fragmented within a process
    • Examples
      • Unusable amount of space between the heap
      • Small gaps within the heap
    • No solution
  • External
    • Memory is fragmented in the space between processes
    • Solutions
      • Fixed memory partitioning (ugh)
      • Compaction
        • Reorders processes
        • Uses CPU time to reorganize
      • Paging & Segmentation

Loading into Memory

Overlays

• Part of memory stored on disk, swapped out as needed
• User space approach (no OS involvement)
• Ex: cut scene where part of game gets replaced in memory with something that was stored on the disk

Virtual Memory