1. Memory Management
Chapter 7

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. Memory Configuration
Before and After Allocation of a
16 Mbyte Block

10. 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

13. 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

14. Page Tables 4 5 6