Virtual Memory: Address Translation

  • Virtual address bits are decided by ISA (Instruction Set Architecture), and P address bits are based on RAM available. In the cases considered here, there is 32-bit ISA and 256MB RAM.
  • Page offsets depend upon frame/page size. Here each page has a size of 4KB.

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  • This is how an address in RAM is translated.

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  • When T. address (Page) is on Disk.
  • In this case, CPU generates an exception: Page Fault
  • Hardware gives control to OS’s Page Fault Exception handler.
  • OS decides which page to move out from RAM to replace with this recently accessed one.
  • If the page is dirty, it has to be written on the disk too.[ The bit indicates that its associated block of memory has been modified and has not yet been saved to storage. ], then update page table. Now OS jumps to the same instruction what caused fault.
  • Page Faults are very expensive.

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