before start_kernel

arch/i386/kernel/head.S is responsible for turning on paging mechnisam, and prepare execution envrionment for the first process. The virtual address is from PAGE_OFFSET + 0x100000, and physical address is from 0x100000.

 

 /* enable the paging mechnism, so later address is all virtual address which is physical address + PAGE_OFFSET*/

 movl $0x101000,%eax
 movl %eax,%cr3  /* set the page table pointer.. */
 movl %cr0,%eax
 orl $0x80000000,%eax
 movl %eax,%cr0  /* ..and set paging (PG) bit */
 jmp 1f   /* flush the prefetch-queue */
1:
 movl $1f,%eax
 jmp *%eax  /* make sure eip is relocated */
1:
 /* Set up the stack pointer */
 lss stack_start,%esp

 

/* set gdtr and idtr to the final one. */

 lgdt gdt_descr
 lidt idt_descr
 ljmp $(__KERNEL_CS),$1f
1: movl $(__KERNEL_DS),%eax # reload all the segment registers
 movl %ax,%ds  # after changing gdt.
 movl %ax,%es
 movl %ax,%fs
 movl %ax,%gs
#ifdef __SMP__
 movl $(__KERNEL_DS), %eax
 mov  %ax,%ss  # Reload the stack pointer (segment only)
#else
 lss stack_start,%esp # Load processor stack

 

/* move to start_kernel - the modern age. */

 xorl %eax,%eax
 lldt %ax
 cld   # gcc2 wants the direction flag cleared at all times
 call SYMBOL_NAME(start_kernel)