Virtual Memory III

What is demand paging?

Bring a page into memory only when it is needed

How does the OS keep track of whether a page is in RAM or not?

Use a valid bit.

How do we choose a valid bit?

legal and in memory

What happens when a bit is invalid.

A trap is generated.

What happens during a page fault?

• find a free frame (from free frame list)
• schedule disk access to load the page into frame
• kick process of CPU and put in blocked/waiting state
• Update process page table w/ new logical/physical memory paging
• Update the valid bit
• Set process state to Ready
• Process will rerun the instruction that caused the trap

Is the page table always rerun after a page fault is resolved?

yes

What would we change in order to increase the page fault rate?

change page fault rate.

What should we do to decrease the page fault rate?

• Increase memory size
• Limit size of the process address space
• Tell programmers to develop programs w/ small address spaces

What is copy-on-write?

don't copy address pace and initially share all pages.
only copy a page when parent or child modifies a page.

What is a dirty bit?

A dirty bit marks a page whenever a process is written to a page. If a page is evicted then it is only written to disk if it is dirty. This saves useless disk writes.