Memory Management

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Memory Management

• Norman White
• Stern School of Business

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ProblemHow does operating system handle memory

• Not a huge problem for non-multiprogramming OS
  like DOS
• All you need to do is separate OS from
  Application program
• Much bigger problem for a multiprogramming system

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Address Space Considerations

• Early computers often had 16 bit or 18 bit
  address spaces (limited by the pointer to a
  location in memory)
• As programs became larger, this was a problem
• As OS starts to support multiprogramming, memory
  needs increase
• Cant fit all programs into a single address
  space.

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Some solutions to address space limitations

• SWAP programs into memory to run
• Have more real memory than the address space
  can hold.
• I.e. 16 bit address implies 216 locations
  (65536)
• Increase real memory to 256K
• Use special addressing registers to point to
  memory to be used by the current program
• Means the operating system now has to move
  programs around to compact memory

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Simple solution Fixed Partitions

• Divide real memory into Fixed Size Chunks
  (partitions)
• Run programs in partitions that are large enough
  for prog to fit.
• Problems-
• Much wasted memory
• Large programs cant run
• Very difficult to schedule

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Better relocation and bounds registers

• Assume MR0 reg points to the REAL memory
  location of location 0 of a an address space
  and MR1 points to the last location (or the
  length)
• OS can now tell hardware to run in relocation
  mode
• Every address reference now has contents of MR0
  added to it
• Hardware will not allow an address beyond MR1
• Memory mgt (OS) responsible for locating programs
  in memory.
• PROBLEM What happens when programs finish?

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Compaction

• As programs finish, memory becomes fragmented
• OS needs to move things around to free up
  contiguous chunks of memory.
• Program size limited by real memory

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More flexible Memory mapping

• Divide memory into fixed size chunks (pages)
• Assign a register for each page (memory map
  register)
• All instructions references use the mapping
  registers to figure out where the pages is in
  real memory
• Address references to locations viewed as
  references to a page and a location with the page
  (in decimal, location 32,357 is viewed as page 32
  displacement 357) (1,000 locations per page)
• Mapping register 32 points to the real memory
  location of page 32.

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Memory mapping example

• As instructions are decoded, each address
  reference is mapped, I.e. the page in the
  reference is used to indicate which memory map
  register to use to find where the page is in
  physical memory. (I.e., 32,567 will be changed to
  78,567 if map register 32 contains 78)
• Advantage - No longer need to move programs
  around to solve the fragmentation problem, just
  update mapping registers
• Problem registers are expensive, as memory
  needs grow, memory mapping needs to many registers

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Solution Virtual Memory

• Memory mapping divided memory into a table
• Page no Real location
• 00 78
• 01 87
• 99 21

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Segmentation and paging

• Divide address space into segments and pages
• Now 32567 becomes
• Segment 3 page 25 displacement 67
• Have a set of segment tables and page tables (1
  page table for each segment) to describe where
  the physical memory locations are for pages in an
  address space
• A single register (Segment Table Origin Register,
  STOR) points to the segment table, entries in the
  segment table point to the page table
• All address references use the tables to
  translate addresses.

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Segment tables and paging (continued)

• Seg number Page table location
• 0 100,000
• 1 80,000
• 2
• 3 60,000
• 9

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Page tables -1 per segment

• Segment 3 example
• Pageno Location
• 00 30,000
• 01 10,000
• 02
• ..
• 25 40,000
• 99 78,000

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So what happens during address translation

• Instruction references location 32567
• Hardware uses STOR to find address of segment
  table
• Relative offset in segment table points to page
  table for segment 3 (60,000)
• Relative offset 25 points to page location for
  segment 3, page 25
• So, 32567 reference is translated to 40067

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Ouch! That is really complex

• Yes, complex and involves lots of overhead
• First, every address reference now generate 2
  extra memory references (to segment table and
  page table)
• Will run verrrry slowly
• Second, lots of table maintenance

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But look what we can do!

• Every address space is completely defined by a
  segment table and page tables
• Changing an address space is accomplished by
  pointing STOR to a new segment table.
• Easy to add on virtual memory!

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Virtual Memory

• Allow programs to run with some of their pages
  missing.
• Hardware generates an interrupt if page table
  entry is missing (add column(s)to page table for
  missing entry)
• External page table used to locate page on disk
• Page paged in, and instruction re-executed

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Ok, thats cool but isnt it very slow?

• Need some tricks to make it go fast
• Add some extra memory logic to help translation
  (translation look-aside buffer, TLB)
• TLB is very high speed associative memory that
  looks up recently referenced segno,pageno
  combinations and replaces them with their real
  locations. If the the info is in TLB, segment
  table and page table not necessary.
• TLB has a limited number of entries. Only recent
  references remembered.

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So what happens?

• Instruction references are translated in the
  processor with minor overhead
• If TLB is fast and large overhead is minimal (lt
  1 typically)
• But OS is much more complex due to interaction
  between memory management and processor management

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Virtual Memory Advantages

• Can now run programs larger than physical memory
  (but still limited by address space)
• Performance is limited by amount of real memory
  (thrashing if REAL lt WORKSET)
• OS can run in separate address space, better for
  security
• File I/O can be combined with virtual memory
  (connect an address space to a segment and page
  table). File can be manipulated as if it were in
  memory
• Programmers (usually) do not need to know size of
  real memory.

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Conclusion

• Virtual Memory is now the standard on almost all
  systems (except cell phones and PDAs)