quartz/content/notes/22-virtual-memory.md

title	aliases	tags
22-virtual-memory		cosc204 lecture

Swapping and virtual memory

swapping

• if there is not enough physical memory we need to sawp processes out of the main ememory to the secondary storage e.g., disk
• 
• when a process is ready, it is swapped into the main memory
• allows more processes to multitask

partially loaded proceses

• dynamic loading
  • load a potion of code when it is called as some code may not need to be executed, e.g., code for handling errors
  • pros
    • process not limited by amount of avilable memory
    • more processes multitasking
    • quicker to swap than entire process

virtual memory

• idea that processes dont need to be fully in memory to run

extends main memory to secondary storage, and allows dynamic loading of processes while they execute

• programmer deals with vmem just like paging scheme
• mem manager in OS kernel controls loading pages of the process into main mem from secondary storage

Demand paging

dont load a page into mem until it is referenced by CPU

implementing

• in paging scheme there are extra bits in the table to provide more information
  • valid/invlid bit
  • protection bits
• in demand paging the mem manager

page replacement algorithms

frame allocation

thrashing