virtual_memory.theory.txt

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                                  ┃   VIRTUAL_MEMORY   ┃
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Virtual memory:
  - the memory addresses shown to a program are not the hardware one, but an abstraction of it
  - abstraction allows:
     - using several hardware sources, "paging"
     - sharing same hardware addresses to several programs
  - handled by OS, helped by MMU (Memory Management Unit)

Paging:
  - i.e. main memory (e.g. RAM) first, auxiliary (e.g. disk) then
  - [memory|virtual] page: block of virtual memory
  - page frame: block of physical memory

Page table:
  - map between pages and their corresponding page frames
  - page table entry: entry in page table for a given page
  - multi-level paging: paging the page table
  - TLB ("Translation Lookaside Buffer"): cache of the page table

Page fault:
  - "minor": accessing a page stored in main memory, but not present in page table
     - e.g. other programs have loaded page in main memory, but current program did not
  - "major": accessing a page stored in auxiliary memory
     - induces paging|swapping, i.e. move page from auxiliary to main memory
  - "invalid": wrong address error, firing segmentation fault
     - e.g. writing to read-only memory, null pointer dereference, buffer overflow, stack overflow
  - trashing: too frequent paging, due to not enough pages or not enough virtual memory

Fragmentation:
  - pages not fully used, i.e. bigger than their content

Page size:
  - usually 4KB
  - bigger page size:
     - allow page table faster lookup because: smaller page table, and more TLB hits
     - increase fragmentation
     - increase possibility of trashing
     - work best with bigger memory objects, least with small memory objects
  - "huge page size": special mode with very big page size, e.g. when working with huge files
     - "PSE": x86 instruction for 4MB pages