person Tute World Team schedule Updated April 28, 2026

Paging & Page Tables

Paging is the mechanism that makes virtual memory work. It divides both virtual and physical memory into fixed-size chunks (pages), and maintains a translation table that maps virtual pages to physical ones.

Pages — 4KB Chunks

Why divide memory into pages? Managing memory byte-by-byte would be impossibly slow. Pages (4096 bytes on x86-64) are the minimum unit of allocation and protection. Every memory permission (read/write/execute) and every mapping applies to whole pages.

Page Tables — The Translation Map

x86-64 uses four-level page tables. A virtual address is split into five parts, each indexing into the next level of the table:

Virtual address (48 bits used):
  Bits 47-39: PGD index (Page Global Directory)
  Bits 38-30: PUD index (Page Upper Directory)
  Bits 29-21: PMD index (Page Middle Directory)
  Bits 20-12: PTE index (Page Table Entry)
  Bits 11-0:  Offset within the 4KB page

PGD → PUD → PMD → PTE → Physical page address + offset

Each process has its own PGD (top-level page table). CR3 register points to the current process's PGD. On context switch, CR3 is updated, instantly switching to the new process's address space.

The TLB — Translation Lookaside Buffer

Isn't walking 4 levels of tables for every memory access incredibly slow? Yes — without help. The TLB is a hardware cache inside the CPU that stores recent virtual→physical translations. A TLB hit means instant translation (no table walk). A TLB miss triggers the page table walk (handled by hardware on x86). TLB has limited entries — too many different pages = thrashing the TLB.

Context switches flush the TLB (or use Process Context IDs to avoid flushing). This is part of why context switching has overhead.

Page Faults — Three Types

Type	Cause	Action	Cost
Minor	Page in RAM but not mapped in page table yet (e.g., after fork() CoW)	Update page table entry	Very fast — no disk I/O
Major	Page not in RAM — must read from disk (swap or mmap'd file)	Find page on disk, read into RAM, update table	Slow — disk latency (milliseconds)
Segfault	Access to unmapped or protected address	Send SIGSEGV to process	Process may die

# Monitor page faults for a command
/usr/bin/time -v my-program 2>&1 | grep "Page faults"
# Major (I/O required) page faults: 12
# Minor (reclaiming) page faults: 4521

Frequently Asked Questions

What will I learn here?

This page covers the core concepts and techniques you need to understand the topic and progress confidently to the next lesson.

How should I use this page?

Start with the overview, then follow the section links to deepen your understanding. Use the table of contents on the right to jump to specific sections.

What should I read next?

Use the navigation below to continue to the next lesson or explore related topics.