FINALS

• Memory Management


  Ensuring memory is shared efficiently and free from errors
• Issues in memory management

  • Protecting memory spaces of processes from unauthorized access
  • Ensuring each process has enough memory space to execute
  • Keeping track of memory locations used by each process and free memory
• Absolute address


  Unique, actual address of a memory location
• Relative address


  Address based on a reference point or base address
• Dynamic run-time loading


  1. Relative address generated

  2. Converted to absolute address by CPU

  3. Enabled by memory management unit (MMU)
• Memory management unit (MMU)

  • Converts logical address to physical address
• Main memory allocation strategies

  • Fixed-partition strategies
  • Variable-partition strategies
• Fixed-partition memory allocation


  Main memory divided into fixed number of equal or unequal partitions
• Problems with fixed-partition memory allocation

  • Internal fragmentation
  • External fragmentation
  • Degree of multiprogramming limited
• Variable-partition memory allocation


  Exact memory space needed by a process is allocated, no fixed partitions
• Strategies for variable-partition memory allocation

  • First-fit
  • Best-fit
  • Worst-fit
• External fragmentation can be solved by compaction, where processes are moved towards the beginning of memory to group holes together
• Placement strategies


  1. First-fit

  2. Best-fit

  3. Worst-fit
• If first-fit strategy is used, P4 will be placed in the 4-MB hole
• If best-fit strategy is used, P4 will be placed in the 3-MB hole
• If worst-fit strategy is used, P4 will be placed in the 6-MB hole
• External fragmentation


  The free space is not contiguous
• Compaction


  1. Processes are moved towards the beginning of the main memory

  2. Holes are grouped together, forming one large block of free memory
• After compaction, a 12-MB process can be assigned
• Compaction


  • It is only possible if dynamic run-time loading is used
  • It uses a significant amount of CPU time, especially if it involves moving hundreds of processes
• Paging


  A process is allowed to occupy non-contiguous memory space
• Paging


  • Processes are divided into equal-sized blocks called pages
  • Main memory is divided into equal-sized blocks called frames
  • A page fits a frame
• Logical address to physical address translation


  1. Determine the page number of the logical address

  2. Use the page number to index into the page table

  3. Add the offset field of the logical address to the frame number to form the physical address
• Logical address


  Composed of page number and offset
• Physical address


  Composed of frame number and offset
• The page table maintains the frame numbers of all the pages of the process
• Paging


  • Eliminates external fragmentation
  • Internal fragmentation only occurs on the last page of each process
• Page size


  • If too small, increases the size of the page table
  • If too large, increases internal fragmentation
• Page table implementation


  1. Dedicated registers

  2. Main memory

  3. Cache memory (translation look-aside buffer)
• Virtual memory


  Allows a process to execute even though not all of its pages are inside the main memory
• Locality of reference


  Only some portions of the program are used at any one time
• Demand paging


  1. Initially, all processes are stored in the hard disk

  2. Pages are loaded in main memory only if needed
• Valid-invalid bit


  Identifies if a page is located in main memory or on the hard disk
• If the valid-invalid bit is 1, the page is in main memory. If the valid-invalid bit is 0, the page is on the hard disk.
• Whenever a logical address is generated, the MMU hardware first verifies if the page is in main memory
• If the valid-invalid bit is 0, the page is not in main memory. This is called a page fault.
• Demand paging


  Pages are loaded in main memory only if needed
• Working set


  The group of pages of a process in main memory
• Virtual memory page table

  Uses a valid-invalid bit for each entry to identify if a page is located in main memory or on the hard disk
• If the valid-invalid bit is 1, the page is in main memory. If the valid invalid bit is 0, the page is on the hard disk.