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Memory Management & Virtual Memory
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Hierarchy Cache Memory : Provide invisible speedup to main memory
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Hierarchy Virtual Memory : Provide invisible extension to main memory
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Terminology Virtual/Logical Address : Physical Address : Page :
Address program uses Size determined by bits in memory address Physical Address : Determined by available memory Page : Block of memory 4KB+ in Windows/Linux
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Memory Management Memory Management
Turns Virtual addresses used by processor into Physical addresses in memory
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Virtual Memory How Why? Logical space can be independent of physical space Programs don't have to care how many GB ram you have
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Virtual Memory Why? Allow programs to use more main memory than is physically present Hard drive used as extra space
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Virtual Memory How Why? Allows multiple programs to coexist
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Virtual Memory How Why? Allows multiple programs to coexist
Everyone can use same logical addresses
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Virtual Memory How Why? Allows multiple programs to coexist invisibly
Some memory can be shared by processes
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Virtual Memory How Why? Allows multiple programs to coexist invisibly
Memory Management Unit can prevent access to unowned memory
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Terminology Page Table : stores physical address of every page
Paging : copying page from hard drive to RAM Page Fault : accessing a virtual page not in RAM
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Example Assume each page is 4KB (4096 bytes)
32KB Virtual space, 12KB Physical
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Example Assume each page is 4KB (4096 bytes) CPU asks for address 100
100 bytes into page 0
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Example Assume each page is 4KB (4096 bytes) CPU asks for address 100
100 bytes into page 0 Virtual page 0 is in Physical Page 2
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Example Assume each page is 4KB (4096 bytes) CPU asks for address 100
100 bytes into page 0 Virtual page 0 is in Physical Page 2 Physical address is 4K *
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Example 2 Assume each page is 4KB (4096 bytes)
CPU asks for address 4296 200 bytes into page 1
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Example 2 Assume each page is 4KB (4096 bytes)
CPU asks for address 4296 200 bytes into page 1 Not in physical memory Page Fault
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Example 2 Assume each page is 4KB (4096 bytes)
CPU asks for address 4296 200 bytes into page 1 Kick a page out to drive
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Example 2 Assume each page is 4KB (4096 bytes)
CPU asks for address 4296 200 bytes into page 1 Load desired virtual page into physical memory Update Page Table 3
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Example 2 Assume each page is 4KB (4096 bytes)
CPU asks for address 4296 200 bytes into page 1 Virtual page 1 is now in Physical Page 3 Physical address is 4K * 3
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Page Faults Page Fault = 40,000,000 cycles with HDD! 600,000 with SSD
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Page Sizes Small page size More book keeping More physical pages
Less thrashing
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Page Sizes Large page size Less book keeping More temporal locality
If we use it all Fewer physical pages More thrashing
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Page Mapping Memory addresses broken into
Page Offset : Enough bits to specify every address in page Page # : All the rest
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Page Mapping Computer has 32 bit address space, 1GB of physical memory and 4KB pages 1GB = 230 Physical address need 30 bits
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Page Mapping Computer has 32 bit address space, 1GB of physical memory and 4KB pages Pages are 212 bytes 12 bits for offset
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Page Mapping Computer has 32 bit address space, 1GB of physical memory and 4KB pages Pages are 212 bytes 12 bits for offset Virtual page number = 32 – 12 = 20 bits
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Page Mapping Computer has 32 bit address space, 1GB of physical memory and 4KB pages Pages are 212 bytes 12 bits for offset Virtual page number = 32 – 12 = 20 bits Physical page number = 30 – 12 = 18 bits
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Process Need logical address
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Process Find virtual page # and offset in page
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Process Find virtual page # and offset in page Look up in page table…
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Process Find virtual page # and offset in page Look up in page table… Get physical page #
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Process Find virtual page # and offset in page Look up in page table… Get physical page # Combine with offset to make physical address
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Paging & Caching Page table held in main memory by OS
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Paging & Caching Page table held in main memory by OS
Every mapping goes to main memory… Cache is worthless!
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TLB Translation look-aside buffer Cache for page table
Subset of page table Fully associative May be multiple levels
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