contiguous memory

Concept

• load-store memory model
• assumes:
  • each process occupies a contiguous region
  • physical memory is large enough to containe 1 or more processes with complete memory space

Memory partitioning

Fixed-size partition

• fixed number of partitions
• each process occupies one partition
• easy to manage and allocate
• partition needs to be large enough to contain the largest of the processes
  • smaller processes will lead to wasted memory space -> internal fragmentation

Variable-size partition

• partition based on actual size of process
• OS needs to keep track of occupied and free spaces(holes)
• flexible
• leads to large number of holes
  • smaller holes cannot accommodate larger processes -> external fragmentation

internal fragmentation -> wasted memory inside the partition, external fragmentation -> wasted memory outside the partition

Allocation algorithms

First-fit

• take the first hole that is large enough
  Best-fit
• take the smallest hole that is large enough
• lead to small holes, hard to fit other processes
  Worst-fit
• take the largest hole
• created holes are larger, hopefully other processes can fit

Merge

• join adjacent holes if possible

Compaction

• move the occupied partitions arround to consolidate holes
• less frequently used
  • time consuming -> OS will take up CPU time
  • moved processes have to pause