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Memory Management and Virtual Memory
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Address Binding – Mapping from one address space to another
 Addresses in a source program are generally symbolic
 A compiler can bind these symbolic source code addresses to relocatable addresses or absolute addresses
 The linker or loader will next bind the re-locatable addresses to
absolute addresses
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Binding at compile time
 It is known at compile time where the process will reside in memory and
absolute address code is generated at this stage
 It is known that a process will reside in memory starting at a fixed location and
therefore the generated compiled code can start from that location
 If starting address of the process changes then we must recompile the code
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Binding at load time
 It is not known at compile time where the process will reside in memory
and re-locatable code is generated at compile time and final binding is
performed at load time
 It is not known that a process will reside in memory at a specific address
location
 If starting address of the process changes then we must only reload the code
Binding at execution time
 The process can be moved during execution from one memory segment
to another, then binding is delayed until run time
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Logical or Virtual Address Space
 Address generated by the CPU is known as the logical address
 Compile-time and load-time address binding create an environment
where logical and physical addresses are the same
 Logical and physical addresses are different when using execution-time
address binding
 The memory mapping performed at execution-time from logical or virtual
address space to physical address space is done by the Memory Management
Unit (MMU)
 The user program never sees the physical addresses and it only deals with
logical addresses
 The MMU hardware converts logical addresses into physical ones when a
memory reference is actually made
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Process Swapping
 In priority-based scheduling schemes, when a higher priority process
arrives and wants service, the memory manager may swap out the lower
priority task and can swap in the higher priority task in it’s place
 In round-robin scheduling systems, when one process’s time slice
expires it can be swapped out and another process is swapped in
 If binding is done at compile or load time then the process cannot be
moved to a different location when it is moved back into memory
 If binding is done at execution time, then it is possible to swap a process
into a different memory space because the physical addresses are
computed during execution time
 To achieve efficient processor utilization execution time for each process
must be long relative to the swap time
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Paging
 Allows the logical address space to be noncontiguous
 Logical memory is broken into blocks called pages
 Physical memory is also broken into blocks of the same size called
frames
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 Page sizes are powers of two
 when addressing a page in memory, the high order address bits are treated as
page numbers when the low order address bits are the page offset to that page
 Therefore every address generated by the CPU is divided into two parts, the
page number and the page offset
 Small pages sizes result in lower page fragmentation
 Larger page sizes require additional overhead involved in each page table entry
and disk I/O is more efficient when the number of data being transferred is
larger
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Frame Table
 The operating system maintains a data structure known as the frame
table that has one entry for each physical page frame indicating
whether the frame is free or allocated and to which page of which
processes it is allocated
Page Table
 Stored in the process control block
 In most operating systems, each process has an associated page table
 Upon start of a process the process page table is reloaded into memory
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 Translation Look-aside Buffers (TLB) are small very fast hardware cache
consisting of a key and value used with page tables to store few of the
page table entries
 When a logical address is generated by the CPU, its page number is
presented to a set of TLB’s that contain page numbers and their
corresponding frame numbers
 If the page number is found in the TLB, its frame number is immediately
available
 Only if it is not a memory reference to the page table must be made to obtain the
frame number
 At this time, the page and frame numbers are added to the TLB to be used for
any subsequent reference to that page
 If the TLB is full of entries, an entry must be selected for replacement
 When new page table is selected (i.e. context switch) the TLB is flushed
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Types of memory
 Virtual memory
 Supported by system hardware and software
 Gives the illusion that a user has a vast linear expanse of storage
 Divided into pages of identical size
 Real memory – Main memory
 Divided into page frames
 Auxiliary memory
 Pages reside in auxiliary memory and may in addition, occupy a page frame
in real memory
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 Page-fault interrupt
 When an executing program refers to a particular item in the virtual memory, the
reference proceeds normally if the item is also in a page frame. Otherwise a
page-fault interrupt occurs to enable the operating system to adjust the
contents of real memory to make a retry of the reference successful.
 Page table
 Hold information about the whereabouts of each page
 Locality of reference
 At any instant, there is a favored subset of all pages that have a high probability
of being referenced in the near future
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 Operating system policies for handling page-fault interrupts
 Fetch policy
 Demand fetching or demand paging
- A page comes into real memory only when a page-fault interrupt results from its absence
- For most computer systems, memory access, ma ranges from 10 to 200 nanosconds as long
as we have no page fault the effective access time is equal to the memory access time.
- effective_access_time = (1 – probability_of_page_fault) * ma + p * page_fault_time
- A page fault interrupt in most situations will cause the reallocation of the CPU to other users
or processes
- It is important to keep the page-fault rate low in a demand-paging system to avoid increase
effective access time and slowing process execution.
 Prepaging
- Pages other than the one demanded by a page fault are brought into page frames
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 Replacement policy
 An increase in the degree of multiprogramming and multiprocessing can lead
to over-allocation of memory in the system.
- While a user process is executing, a page fault occurs.
- The hardware traps to the operating system, which checks its internal tables to see that the
page fault is genuine and not an illegal memory access.
- The operating system determines where the desired page is residing on disk, but there are
no free frames on the free-frame list: all memory is in used.
- The OS can swaps out a process, freeing all its frames, and reducing the level of
multiprogramming or it can use a page replacement policy to replace a frame with the new
desired page.
 Note that system memory is not used only for holding program pages.
Buffers for I/O also consume a significant amount of memory.
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 Demand replacement – replacement occurs only when real memory is full
 Least Recently Used (LRU)
- A true implementation requires hardware support to maintain a stack of pages referenced
- A policy based on examining use bits and dirty bits approximates LRU well.
 First-in, first-out (FIFO)
- Easy to implement with a circular buffer pointer among page frames
- Performance of FIFO is inferior to that of LRU.
 Last-in, first-out (LIFO)
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 Least frequently used (LFU)
- Use counter to determine page usage.
- Requires expensive hardware and other methods generally outperform it
 Most recently used (MRU)
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 Placement policy
 Segmentation
 Cleaning policy
 Opposite of fetch policy
 Determines when a page that has been modified will be written to auxiliary
memory
 Demand cleaning
- A dirty page will be written only when selected for removal by the replacement policy
 Pre cleaning
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 Load-control policy
 Without a mechanism to control the number of active processes, it is very
easy to over commit a virtual memory system
 Thrashing
- Virtual memory management overhead becomes so grave that a precipitous decrease in
system performance is visible upon the moment of memory over commitment.
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 Fixed process allocation policy
 Variable process allocation policy
 Frame allocation algorithms
 Equal allocation
 Proportional allocation
- Allocate available memory to each process according to its size.
- Size of the virtual memory for process pi is si then S =  si .
- If the total number of available frames is m, we allocate ai frames to process pi where:
ai=si/S*m.
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