18. Paging: Introduction

Question 1

What is the role of page table? Is it a global or per-process data structure?

Answer 1

Store address translation for each of the virtual pages of the address space. It’s per-process data structure.

Question 2

What does virtual address consist of in simple page table implementation?

Answer 2

Virtual Page Number (VPN) and offset

Question 3

What is simplest form of page table? How does it work?

Answer 3

Linear page table. It indexes an array by the VPN and looks up PTE at that index.

Question 4

What kind of bits can be in PTE?

Answer 4

Valid bit - if translation is valid (if the page is allocated).
Protection bits (rwx).
Present bit (if page is in memory or on disk).
Dirty bit (if page was modified)
Reference bit/access bit (if page has been accessed)

Question 5

What is page-table base register and how is it used?

Answer 5

It’s a memory address of the start of page table for the process. This register is used by hardware to find a PTE for each memory reference and lookup a translation there

Question 6

What are upsides and downside of paging in its first iteration?

Answer 6

It’s slow because incurs extra memory reference (to page table) and it’s big in size. Thought, it solves an issue with external fragmentation due to fixed-size pages