Main Memory 2

Main Memory 2

1. Process in Background

   • 1. Addresses in source program are generally symbolic e.g. count
   • 2. A compiler binds symbolic addresses to relocatable addresses
   • 3. Linkage editor/loader editor binds relocatable addresses to absolute addresses

2. Binding

   • Address binding of instructions -> memory addresses can happen at 3 stages
     • Compile Time
       • If memory location is known prior, absolute code can be generated - Recompile if starting location changes
     • Load Time
       • If memory location not known, relocatable code is generated at compile time
     • Execution Time
       • If process can move from one area of memory to another when running, binding is delayed until run time
         • Need hardware support e.g. base and limit registers

3. Swapping

   • Processes can be temporarily removed from memory to a backing store, then brought back into memory to continue execution
     • Backing Store
       • Fast disk to accommodate copies of all memory images - Direct access
     • Roll in, Roll Out
       • Priority-based
         • Lower-priority swapped out for higher priority
     • Usually swapped back into same memory space
     • Ready queue used for read-to-run processes

4. Fragmentation

   • External Fragmentation
     • Total memory space not contiguous
     • Reduce by compaction
       • Shuffle memory contents to place all free memory together in one large block
       • Only possible if relocation's dynamic, and done at run-time
       • Or, by paging/segmentation
   • Internal Fragmentation
     • Allocated memory can be slightly larger than requested memory

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