Memory Management Chapter 7
Memory Management Subdividing memory to accommodate multiple processes Memory needs to be allocated efficiently to pack as many processes into memory as possible should be able to run a program whose size is larger than the available real memory
Memory Management provides Relocation Programmer (or the compiler) does not know where the program will be placed in memory when it is executed While the program is executing, it may be swapped to disk and returned to main memory at a different location (relocated) Memory references in the code must be translated to actual physical memory addresses
Memory Management provides Protection Processes should not be able to access memory locations that belong to another process without permission Must be done at runtime OS cannot anticipate all of the memory references a program will make Sharing Allow several processes to access the same portion of memory
Fixed Partitioning Memory is divided into equal-sized partitions which are assigned to processes on demand or unequal-sized partitions Memory use is inefficient. Any program, no matter how small, occupies an entire partition. internal fragmentation
Memory Assignment for Fixed Partitioning Equal-size partitions: no strategy is needed; it does not matter Unequal-size partitions assign each process to the smallest partition within which it will fit. This way, we can minimize the wasted memory within a partition Use one queue for each partition or a single queue for all
Dynamic Partitioning Processes are allocated exactly as much memory as required Eventually holes will start appearing in the memory. This is called external fragmentation Must use compaction to shift processes so they are contiguous and all free memory is in one block
Memory Allocation Strategies for Dynamic Partitioning Three popular strategies: First-fit Fastest May have many processes loaded in the front end of memory that must be searched over when trying to find a free block Best-fit Chooses the block that is closest in size to the request Worst performer overall Since smallest block is found for a process external fragments are smallest. Memory compaction must be done more often Next-fit Similar to first fit but starts to search for free blocks beyond the last allocated block Results in an even distribution of the free blocks throughout the memory
Memory Configuration Before and After Allocation of a 16 Mbyte Block
Buddy System Entire space available is treated as a single block of 2U If a request of size s is such that 2U-1 < s ≤ 2U, entire block of 2U is allocated Otherwise block is split into two equal buddies Splitting continues until smallest block greater than or equal to s is generated
Paging Partition memory into small equal-size chunks called frames Divide each process into the same size chunks called pages Operating system maintains a page table for each process contains the frame location for each page in the process Pages of process A
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