Lecture: virtual memory Q&A

Single-level paging

• x86 (32-bit): 4M single-level paging
  • turn on the PSE bit on CR4
  • add the PS bit to page entries
• VA → PA translation
  • linear address
    • bits 31-22: page directory index
    • bits 21-0: offset into 4M page
  • page directory entry
    • bits 31-22: physical 4M page address
    • bits 21-12: reserved
    • bits 11-0: flags
• example
  • xv6: used during booting - see entrypgdir in main.c
  • demo: turn your JOS into using 4M pages
• general concept: superpages (also known as huge pages or large pages)
  • benefits?

Four-level paging

• x86_64: 4K four-level paging
  • pros & cons
  • overhead, performance (what kind of user-space workloads?)
• demo