decompress kernel

arch/i386/boot/compress/head.S is responsible for decompressed the kernel and move it to 0x100000. when it is executed, the virtual address is 0x1000 for low loaded vmlinux and 0x100000 for high loaded one, which are the same with their physical addresses, respectively.

 

/* check contents of 0x000000 and 0x100000, if not equal, means A20 is enabled. */

  xorl %eax,%eax
1: incl %eax  # check that A20 really IS enabled
  movl %eax,0x000000 # loop forever if it isn't
  cmpl %eax,0x100000
  je 1

 

/* decompress the kernel and jump to it (0x100000) for low loaded kernel */

 subl $16,%esp # place for structure on the stack
 pushl %esp # address of structure as first arg
 call SYMBOL_NAME(decompress_kernel)
 orl  %eax,%eax
 jnz  3f
 xorl %ebx,%ebx
 ljmp $(__KERNEL_CS), $0x10000

 

/* move move_routine_start to 0x1000 and jump to it */

 movl $move_routine_start,%esi
 movl $0x1000,%edi
 movl $move_routine_end,%ecx
 subl %esi,%ecx
 cld
 rep
 movs

 

 popl %esi # discard the address
 popl %esi # low_buffer_start
 popl %ecx # lcount
 popl %edx # high_buffer_start
 popl %eax # hcount
 movl $0x100000,%edi
 cli  # make sure we don't get interrupted
 ljmp $(__KERNEL_CS), $0x1000 # and jump to the move routine

 

/* move hcount from high_buffer_start to 0x100000 and jump to it for high loaded kernel */

move_routine_start:
 rep
 movsb
 movl %edx,%esi
 movl %eax,%ecx # NOTE: rep movsb won't move if %ecx == 0
 rep
 movsb
 xorl %ebx,%ebx
/*
 * Well, the kernel relies on %esp pointing into low mem,
 * with the decompressor loaded high this is no longer true,
 * so we set esp here.
 */
 mov  $0x90000,%esp
 ljmp $(__KERNEL_CS), $0x100000
move_routine_end: