Lecture: address spaces

Preparation

• read OSPP §8, Address Translation
• read kern/entrypgdir.c again in your JOS (and compare it with xv6’s entrypgdir in main.c)

Overview

• last lecture
  • isolation and sharing
  • kernel, user space, system calls
  • kernel organization: monolithic kernel, microkernel, exokernel
• virtual addresses
  • popular in modern OSes: not just for isolation; more examples next week
  • both kernel and user space are virtual addresses
  • again, even pointers in kernel are not using physical addresses
• MMU (memory management unit)
  • hardware support: VA → PA translation [ draw the workflow ]
  • cache: TLB (translation lookaside buffer)
  • segmentation vs. paging
• paging
  • page: fixed-size memory chunk (e.g., 4KB/2MB/4MB on x86)
  • page table: address translation map
  • entry: physical address, plus flags
    • P/W/U: present/writable/user
    • A/D: accessed/dirty
    • see xv6’s mmu.h
  • isolation
    • per-process page table: switch with process
    • kernel (un)sets W, etc.
• x86 control registers
  • CR0: turn on paging
  • CR3: pointer to page table
  • CR2: will use it later for page faults

Two-level paging

• x86
  • let’s start with the 4K two-level paging
  • read through JOS’s kern/entrypgdir.c
  • there are many other plans
• VA → PA translation
  • linear address
    • bits 31-22: page directory index
    • bits 21-12: page table index
    • bits 11-0: offset into 4K page
  • page directory entry (PDE)
    • bits 31-12: page table address
    • bits 11-0: flags
  • page table entry (PTE)
    • bits 31-12: physical 4K page address
    • bits 11-0: flags