Memory management

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Memory management

• Ref Stallings
• G.Anuradha

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What is memory management?

• The task of subdivision of user portion of memory
  to accommodate multiple processes is carried out
  dynamically by the operating system and is known
  as memory management
• Memory management terms

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Memory management requirements
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Memory management requirements Contd

• Relocation
• Users generally dont know where they will be
  placed in main memory
• May want to swap in at a different place
• Must deal with user pointers
• Generally handled by hardware
• Protection
• Prevent processes from interfering with the O.S.
  or other processes
• Often integrated with relocation

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Memory management requirements Contd

• Sharing
• Allow processes to share data/programs
• Logical Organization
• Main memory and secondary memory are organized
  into linear/1D address space of segments and
  words.
• Secondary memory is also similarly organized
• Most programs are modularized
• Advantages of modular approach
• Written and compiled independently
• Can have different degrees of protection
• Module level of sharing
• Physical Organization
• Transferring data in and out of main memory to
  secondary memory cant be assigned to programmers
• Manage memory disk transfers (System
  responsibility)

Segmentation
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Memory Partitioning

• Fixed partitioning
• Dynamic partitioning
• Simple Paging
• Simple segmentation
• Virtual memory paging
• Virtual memory segmentation

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Fixed partitioning
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Difficulties in equal size fixed partitions

• A program may be too big to fit into a partition.
  In such cases overlays can be used
• Main memory utilization is extremely inefficient.
  
• Leads to internal fragmentation
• BLOCK OF DATA LOADED IS SMALLER THAN THE PARTITION

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Placement algorithm

• With equal size partition a process can be loaded
  into a partition as long as there is an available
  partition
• With unequal size partitions there are two
  possible ways to assign processes to partitions
• One process queue per partition
• Single partition

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Memory assignment for fixed partitioning
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Advantages and disadvantages of each of the
approaches

• One process queue per partition
• Advantages
• Internal fragmentation is reduced
• Disadvantages
• Not optimal from the system point of view
• Single queue
• Advantages
• Degree of flexibility, simple, requires minimal
  OS S/w and processing overhead
• Disadvantages
• Limits the number of active processes in the
  system
• Small jobs will not utilize partition space
  efficiently

IBM Mainframe OS. OS/MFT
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Dynamic partitioning
Create partitions as programs loaded Avoids
internal fragmentation, but must deal with
external fragmentation
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Effect of dynamic partitioning
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Dynamic partitioning

• External fragmentation-As time goes on more and
  more fragments are added and the effective
  utilization declines
• External fragmentation can be overcome using
  compaction
• Compaction
• Time consuming

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Placement algorithm

• Best-fit- Chooses the block that is closest in
  size to the request
• First-fit- scans memory from the beginning and
  chooses the first available block that is large
  enough
• Next-fit-Scan memory from the location of the
  last placement and chooses the next available
  block that is large enough

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Which amongst them is the best?

• First fit-
• Simple, best and fastest
• Next fit-
• Next to first fit. Requires compaction in this
  case
• Best fit-
• Worst performer. Memory compaction should be done
  as frequently as possible

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Buddy system

• Overcomes the drawbacks of both fixed and dynamic
  partitioning schemes

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Algorithm of buddy system

• The entire space available for allocation is
  treated as a single block of size 2U
• If a request of size s ST is
  made then the entire block is allocated
• Otherwise the block is split into two of size
  2U-1
• This process continues until the smallest block
  greater than or equal to s is generated and
  allocated to the request

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Example of a buddy system
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Free representation of buddy system
Modified version used in UNIX kernel memory
allocation
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Relocation
Not a major problem with fixed-sized
partitions Easy to load process back into the
same partition Otherwise need to deal with a
process loaded into a new location Memory
addresses may change When loaded into a new
partition If compaction is used To solve this
problem a distinction is made among several types
of addresses.
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Relocation

• Different types of address
• Logical address- reference to a memory location
  independent of the current assignment of data to
  memory
• Relative address- example of logical address in
  which the address is expressed as a location
  relative to some known point
• Physical address- actual location in main memory
• A hardware mechanism is needed for translating
  the relative addresses to physical main memory
  addresses at the time of execution of the
  instruction that contain the reference

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Hardware support for relocation
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Base/Bounds Relocation

• Base Register
• Holds beginning physical address
• Add to all program addresses
• Bounds Register
• Used to detect accesses beyond the end of the
  allocated memory
• Provides protection to system
• Easy to move programs in memory
• Change base/bounds registers
• Largely replaced by paging

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Paging

• Problems with unequal fixed-size and
  variable-size partitions are external and
  internal fragmentations respectively.
• If the process is also divided into chunks of
  same size - pages
• Memory is divided into chunks called frames
• Then a page can be framed into a page frame
• Then there will be only internal fragmentation
  especially in the last page of the process

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Assignment of Process pages to free frames
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Page tables

• When a new process D is brought in, it can still
  be loaded even though there is no contiguous
  memory location to store the process.
• For this the OS maintains a page table for each
  process
• The page table shows the frame location for each
  page of the process
• Within the program, each logical address consists
  of a page number and an offset within the page

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Page table contd

• In a simple partition, a logical address is the
  location of a word relative to the beginning of
  the program
• The processor translates it into physical
  address.
• With paging the logical-physical address
  translation is done by hardware

Logical address page number, offset
Physical address Frame number, offset
processor
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Data structures when process D is stored in main
memory
Paging similar to fixed size partitioning Differen
ce- 1. partitions are small 2. Program may
occupy more than one partition 3.
Partitions need not be contiguous
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Computation of logical and physical addresses

• Page size typically a power of 2 to simplify the
  paging hardware
• Example (16-bit address, 1K pages)
• Relative address of 1502 is
• Top 6 bits (000001) page Page number is 1
• Bottom 10 bits (0111011110) offset within
  page, in this case 478
• Thus a program can consist of a maximum of 26
  64 pages of 1K bytes each.

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Why page size is in multiples of 2?

• The logical addressing scheme is transparent to
  programmer, assembler, linker.
• Easy to implement a function in hardware to
  perform dynamic address translation at runtime.
• Steps in address translation

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Logical to physical address translation using
paging
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Hardware support

• OS has its own method for storing page tables.
  Pointer to page table is stored with the other
  register values in the PCB, which is reloaded
  whenever the process is loaded.
• Page table may be stored in special registers if
  the number of pages is small.
• Page table may be stored in physical memory, and
  a special register, page-table base register,
  points to the page table.(Problem is time taken
  for accessing)

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Implementation of Page Table

• Page table is kept in main memory
• Page-table base register (PTBR) points to the
  page table
• Page-table length register (PRLR) indicates size
  of the page table
• In this scheme every data/instruction access
  requires two memory accesses. One for the page
  table and one for the data/instruction.
• The two memory access problem can be solved by
  the use of a special fast-lookup hardware cache
  called associative memory or translation
  look-aside buffers (TLBs)
• Some TLBs store address-space identifiers (ASIDs)
  in each TLB entry uniquely identifies each
  process to provide address-space protection for
  that process

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Hardware support contd

• Use translation look-aside buffer (TLB). TLB
  stores recently used pairs (page , frame ).
• It compares the input page against the stored
  ones. If a match is found, the corresponding
  frame is the output. Thus, no physical memory
  access is required.
• The comparison is carried out in parallel and is
  fast.
• TLB normally has 64 to 1,024 entries.

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Paging Hardware With TLB
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Memory Protection

• Memory protection implemented by associating
  protection bit with each frame
• Valid-invalid bit attached to each entry in the
  page table
• valid indicates that the associated page is in
  the process logical address space, and is thus a
  legal page
• invalid indicates that the page is not in the
  process logical address space

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Valid (v) or Invalid (i) Bit In A Page Table
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Shared pages

• Possibility of sharing common code
• This happens in the case if the code is
  re-entrant code(pure code).
• Re-entrant code is non-self modifying code it
  never changes during execution. Two or more
  processes can simultaneously utilize the same
  code.

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Shared Pages Example
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Structure of page table

• Hierarchical paging
• Hashed page tables
• Inverted page tables

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Hierarchical paging
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Summary

1. Main memory is divided into equal sized frames
2. Each process is divided into frame-sized pages
3. When a process is brought in all of its pages are
   loaded into available frames and a page table is
   set up
4. This approach solves the problems in partitioning

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Segmentation

• Segmentation is a memory management scheme that
  supports user view of memory
• Logical address space is a collection of segments
• Each segment has a name and length
• Each address has a segment and a offset within a
  segment

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Users View of a Program
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Logical View of Segmentation
1
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user space
physical memory space
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Segmentation Hardware
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Example of Segmentation
Segment 2 , ref to byte 534300534353 Segment
3, ref to byte 85232008524052 Segment 0, ref
to byte 1222 63001222
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Implementation of Segment Tables

• Just like page tables the segment tables can be
  kept in registers and accessed
• When the program contains a large number of
  segments a segment table base register and
  segment table limit register is kept in the
  memory and checks are performed

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Protection and sharing

• Segments are always protected becos its
  semantically defined
• Code and data can be shared in segmentation.
  Segments are shared when entries in the segment
  tables of two different processes point to the
  same physical location

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