Byu cs 345 virtual memory 82 true or false questions

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BYU CS 345 Virtual Memory 82 True or False Questions… The numerical difference between a virtual address and the corresponding physical address is always nonzero. The size of a virtual address space can exceed the total disk capacity in bytes. It is not possible to run an address translation mechanism backward, obtaining a corresponding virtual address from a physical address. Pages should be made as large as possible to minimize the size of page tables. When several processes share a page, their corresponding page table entries must be identical. An optimal replacement policy that always leads to the fewest possible page faults in not implementable.
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BYU CS 345 Virtual Memory 83 Resident Set Management Size – How many pages to bring in Smaller sets allow more processes in memory at one time Larger sets reduce # of page faults for each process Each page has less effect as set grows Fixed Allocation Each process is assigned a fixed number of pages Variable Allocation Allow the number of pages for a given process to vary over time Requires the O.S. to asses the behavior of the processes Scope - what page to remove Local – Only look at that process Global – Look at all processes
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BYU CS 345 Virtual Memory 84 Resident Set Management Fixed Allocation, Local Scope O.S. selects a page from that process to replace Number of pages fixed in advance Too small: leads to thrashing Too big: wastes memory Reduces the available number of processes that can be in memory Variable Allocation, Global Scope Common Page added to a process who experiences a page fault Harder to determine who should lose a page May remove a page from a process that needs it Helped by use of page buffering
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BYU CS 345 Virtual Memory 85 Memory Management Two separate memory management schemes in Unix SVR4 and Solaris Paging System – User page frames Kernel Memory Allocator – Kernel page frames Paging System Data Structures Page Table – One entry for each page of virtual memory for that process Page Frame Number – physical frame # Age – how long in memory w/o reference Copy on Write – 2 processes sharing page: after fork(), waiting for exec() Modify – page modified? Reference – set when page accessed Valid – page is in main memory Protect – are we allowed to write to this page?
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BYU CS 345 Virtual Memory 86 Paging Data Structures Disk Block Descriptor Swap Device Number – Logical device Device Block Number – Block location Type of storage – Swap or executable Page Frame Data Table Page State – Available, in use, in executable, in transfer Reference Count – # processes using page Logical Device – Device holding copy Block Number – Location on device Pfdata pointer – For linked list of pages Swap-use Table Reference Count – # references to page on a storage device Page/storage unit number – Page ID
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BYU CS 345 Virtual Memory 87 SVR4 Page Replacement Clock algorithm variant (Fig 8.23)
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  • Winter '12
  • EricMercer
  • Virtual memory, main memory, page faults, BYU CS

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