Memory Management

1
Memory Management
2

• Why memory management?

• Processes need to be loaded in memory to execute
• Multiprogramming
• The task of subdividing the user area is carried
  out dynamically by the OS -- memory management

3

• Storage organization

4

• What is required of memory management?

• Relocation
• ability to load in relative addresses
• ability to translate relative into absolute
  (real) addresses
• Protection
• processes should not be able to reference memory
  locations of other processes without permission
• it has to be provided at run time
• Sharing
• several processes should be able to access the
  same portion of main memory
• Logical organization
• most programs are organized in modules
• the OS should be able to handle modules or
  segments
• Physical organization
• primary and secondary
• support overlaying

5

• Memory Management Techniques

6

• Virtual memory

DAT
Real
Virtual
7

• Virtual memory requirements

• All memory references within a process are
  logical, translated to physical at run time
• A process may be broken up into a number of
  pieces (pages or segments)
• There is no need for all the pages or all
  segments of a process to be in main memory at the
  time of execution
• Only a resident set of the process needs to be in
  memory

8

• Paging

• Page table for each process
• frame location of each page of the process
• program logical address is a page number plus an
  offset (d) within the page
• page table physical address is a frame number
  plus an offset (d) within the frame
• the processor should use the page table to do the
  translation
• page tables can be big, stored in virtual storage
• Page table mechanisms
• inverted page tables one entry for each real
  page rather than virtual page
• translation lookaside buffer a page table cache
• page fault, roll-in, roll-out, thrashing
• direct and associative mapping

9

• Segmentation

• memory is seen as a set of segments, multiple
  address spaces
• segments can be of variable size
• segment table to each process
• logical address segment number, offset (d)
• physical address memory address, segment length
• data structures (variable length) can be assigned
  to segments
• programs can be developed and maintained by
  segments
• increases code reuse and sharing
• supports protection
• combined paging and segmentation (s,p,d)

10

• Policies for virtual memory

• Fetch policy demand and prepaging
• Placement policy relevant for segmentation
• Replacement policy
• optimal
• first-in, first-out (FIFO)
• least recently used (LRU)
• clock policy and variations (1 used, 0 not)
• selects 0, every time it tests turns 1s in 0s
• second bit for page modification
• page buffering
• Locality temporal and spatial
• Cleaning policy demand, precleaning and buffering

11

• Page fault and page size

Page fault
Page size

• Page size is a hardware design decision (remember
  DAT)
• Small page size decrease internal fragmentation,
  but
• Small page size increase the number of pages in
  table, and
• Small page size increase page fault
• Some computer page sizes 512 bytes - VAX, IBM
  AS/400 4 Kbytes - IBM370, PowerPC 4 Kbytes to 4
  Mbytes - Pentium