entry.S

# The xv6 kernel starts executing in this file. This file is linked with
# the kernel C code, so it can refer to kernel symbols such as main().
# The boot block (bootasm.S and bootmain.c) jumps to entry below.
        
# Multiboot header, for multiboot boot loaders like GNU Grub.
# http://www.gnu.org/software/grub/manual/multiboot/multiboot.html
#
# Using GRUB 2, you can boot xv6 from a file stored in a
# Linux file system by copying kernel or kernelmemfs to /boot
# and then adding this menu entry:
#
# menuentry "xv6" {
# 	insmod ext2
# 	set root='(hd0,msdos1)'
# 	set kernel='/boot/kernel'
# 	echo "Loading ${kernel}..."
# 	multiboot ${kernel} ${kernel}
# 	boot
# }

#include "asm.h"
#include "memlayout.h"
#include "mmu.h"
#include "param.h"

# Multiboot header.  Data to direct multiboot loader.
.p2align 2
.text
.globl multiboot_header
multiboot_header:
  #define magic 0x1badb002
  #define flags 0
  .long magic
  .long flags
  .long (-magic-flags)

# By convention, the _start symbol specifies the ELF entry point.
# Since we haven't set up virtual memory yet, our entry point is
# the physical address of 'entry'.
.globl _start
_start = V2P_WO(entry)

# Entering xv6 on boot processor, with paging off.
.globl entry
entry:
  # Turn on page size extension for 4Mbyte pages
  # The kernel tells the paging hardware to allow super pages by setting the CR4_PSE bit
  # 启动PSE后, 虚拟地址的第7位被激活, page directory 直接指向 4MB 大小的 page, 而不是 page table
  movl    %cr4, %eax
  orl     $(CR4_PSE), %eax
  movl    %eax, %cr4
  # Set page directory, V2P_WO将参数减去KERNBASE, 使之成为有效的物理地址
  # 会面会在%cr0中设置CR0_PG位，这两个结合，说明开启了分页并MMU能通过CR3找到页目录
  # 并能进行虚拟地址到物理地址的转换了
  movl    $(V2P_WO(entrypgdir)), %eax
  movl    %eax, %cr3
  # Turn on paging.
  # xv6 sets the flag CR0_PG in %cr0, To enable the paging hardware
  # 开启后, MMU将会通过CR3找到entrypgdir
  movl    %cr0, %eax
  orl     $(CR0_PG|CR0_WP), %eax /*write protect and paging*/
  movl    %eax, %cr0

  # The processor is still executing instructions at low addresses after paging is enabled
  # which works since entrypddir maps low addresses
  # but entry needs to transfer to the kernel's C code, and run it in high memory

  # Set up the stack pointer, %esp point to memory to be used as a stack.
  # All symbols have high addresses, including stack so the stack will still be valid even when entry 0 are removed
  movl $(stack + KSTACKSIZE), %esp

  # Jump to main(main is at high address), and switch to executing at
  # high addresses. The indirect call is needed because
  # the assembler produces a PC-relative instruction
  # for a direct jump.
  mov $main, %eax
  jmp *%eax

.comm stack, KSTACKSIZE    # .comm declares named common area(stack) in bss section for KSTACKSIZE bytes