CHAP8 - Virtual Memory Chapter 8 Hardware and Control...

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Virtual Memory Chapter 8
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Hardware and Control Structures Memory references are dynamically translated into physical addresses at run time A process may be swapped in and out of main memory such that it occupies different regions A process may be broken up into pieces that do not need to located contiguously in main memory All pieces of a process do not need to be loaded in main memory during execution
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Execution of a Program Operating system brings into main memory a few pieces of the program Resident set - portion of process that is in main memory An interrupt is generated when an address is needed that is not in main memory Operating system places the process in a blocking state
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Execution of a Program Piece of process that contains the logical address is brought into main memory Operating system issues a disk I/O Read request Another process is dispatched to run while the disk I/O takes place An interrupt is issued when disk I/O complete which causes the operating system to place the affected process in the Ready state
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Advantages of Breaking up a Process More processes may be maintained in main memory Only load in some of the pieces of each process With so many processes in main memory, it is very likely a process will be in the Ready state at any particular time A process may be larger than all of main memory
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Types of Memory Real memory Main memory Virtual memory Memory on disk Allows for effective multiprogramming and relieves the user of tight constraints of main memory
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Thrashing Swapping out a piece of a process just before that piece is needed The processor spends most of its time swapping pieces rather than executing user instructions
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Principle of Locality Program and data references within a process tend to cluster Only a few pieces of a process will be needed over a short period of time Possible to make intelligent guesses about which pieces will be needed in the future This suggests that virtual memory may work efficiently
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Support Needed for Virtual Memory Hardware must support paging and segmentation Operating system must be able to management the movement of pages and/or segments between secondary memory and main memory
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Paging Each process has its own page table Each page table entry contains the frame number of the corresponding page in main memory A bit is needed to indicate whether the page is in main memory or not
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Modify Bit in Page Table Another modify bit is needed to indicate if the page has been altered since it was last loaded into main memory If no change has been made, the page does not have to be written to the disk when it needs to be swapped out
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Page Table Entries
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Two-Level Scheme for 32-bit Address
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Page Tables The entire page table may take up too much main memory Page tables are also stored in virtual memory When a process is running, part of its page table is in main memory
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CHAP8 - Virtual Memory Chapter 8 Hardware and Control...

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