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The Operating System Perspective

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● RTE's memory management services●  memory management mechanism,

as viewed from the OS perspective●  opposed to the programming

language perspective

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Virtual Memory (VM)● scheme enables each process to have

its own independent view of memory● OS (with help from the hardware)

maps process memory into real memory

● illusion that more virtual memory exists than physical memory

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Virtual Memory (VM)● process is presented with a virtual

address space, 0-232 ● process thinks is running alone● CPU has no notion of virtual memory ● MMU (memory management unit) –

translates virtual address into physical and sends it to CPU

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Virtual Memory (VM)● there is less RAM installed than 232

● we assume most processes will actually use less RAM than full virtual space.

● when not enough memory in RAM, use disk and swap between RAM and disk

● heavy penalty to go to disk to fetch data

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Virtual Memory (VM)● locality of reference: processes

cluster memory references to small subsets of VM

● virtual memory (caching) is efficient

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Paging - VM implementation● physical memory is broken down into

pieces of small size 4K● physical memory can now be seen as

an array of physical pages●  virtual address space is broken into

pages of same size 4K● virtual page is mapped to

physical page

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Finding the Page of an Address● an address addr resides:

● page number addr >> log(page size) addr/ page size

● offset addr % page size

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● how to break down a virtual address into virtual page number and offset when using pages of 4k on a 32 bit CPU:

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Page Tables● translation table for each process● maps virtual address space into the

physical memory●  OS a translation table for each

process: Page Table

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Page Tables● each row represents a virtual page

and ● column holds the physical page

address in which the virtual page resides

● register stores a pointer to table. ● in context switch, os updates register

with current process' page table address

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Address lifecycle● process executes code involving

memory address, addr● MMU finds the virtual page

number, vpn, in which addr resides● MMU locates vpn entry in page table

- retrieves physical page number ppn ● MMU replaces addr by paddr 

paddr = (addr % page_size) + (ppn << log(page_size)

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example● assume page size = 4096bytes (212

1<<12), ● virtual address 0x80403040 ● vpn =  0x8040 ● by page table vpn->ppn=0x1020● paddr= 0x10203040

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TLB and Context Switches● TLB – translation lookaside buffer● caches recent translation made by

MMU. ● when MMU needs to locate a physical

page for a virtual one - first checks TLB

● in a context switch, TLB is irrelevant

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Demand Paging and Swap● process has memory size of 232

●  what if each process access so much memory?

● a process does not need all of its memory all of the time

● swap unneeded data from memory, and restore it when needed

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Implementation● metadata attribute for each page:1. Access rights: either read, write,

execute or a combination thereof2. Present bit: is the page mapped to a

physical page at all?3. Dirty bit: has the page been written

to since loading it into physical memory?

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Present bit● OS evicts P from main memory -

writes P ● OS sets present bit of P to 0● if in MMU translate page is not

present, MMU throws page fault interrupt - calls OS page fault handler● OS reads saved page from disk into

physical page, and update page table

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Present bit●  lazy page allocation:

●  pretend to allocate the memory: OS updates the page tables of the process

● the present bit is cleared from each of these pages

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Dirty bit● a page needs to be evicted, but the

page is not dirty. ● if the page was already written to

disk earlier, no need to re-copy to disk

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Page Replacement Policy●  The operating system must decide

which page to swap out●  page replacement policy: usually

relies on some kind of LRU (recently used list) 

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