root/arch/i386/boot/compressed/head.S

/* */

   1 /*
   2  *  linux/boot/head.S
   3  *
   4  *  Copyright (C) 1991, 1992, 1993  Linus Torvalds
   5  */
   6 
   7 /*
   8  *  head.S contains the 32-bit startup code.
   9  *
  10  * NOTE!!! Startup happens at absolute address 0x00001000, which is also where
  11  * the page directory will exist. The startup code will be overwritten by
  12  * the page directory.
  13  *
  14  * Page 0 is deliberately kept safe, since System Management Mode code in 
  15  * laptops may need to access the BIOS data stored there.  This is also
  16  * useful for future device drivers that either access the BIOS via VM86 
  17  * mode.
  18  */
  19 .text
  20 
  21 #define __ASSEMBLY__
  22 #include <linux/linkage.h>
  23 #include <asm/segment.h>
  24 
  25         .globl startup_32
  26 startup_32:
  27         cld
  28         cli
  29         movl $(KERNEL_DS),%eax
  30         mov %ax,%ds
  31         mov %ax,%es
  32         mov %ax,%fs
  33         mov %ax,%gs
  34         lss SYMBOL_NAME(stack_start),%esp
  35         xorl %eax,%eax
  36 1:      incl %eax               # check that A20 really IS enabled
  37         movl %eax,0x000000      # loop forever if it isn't
  38         cmpl %eax,0x100000
  39         je 1b
  40 /*
  41  * Initialize eflags.  Some BIOS's leave bits like NT set.  This would
  42  * confuse the debugger if this code is traced.
  43  * XXX - best to initialize before switching to protected mode.
  44  */
  45         pushl $0
  46         popfl
  47 /*
  48  * Clear BSS
  49  */
  50         xorl %eax,%eax
  51         movl $ SYMBOL_NAME(_edata),%edi
  52         movl $ SYMBOL_NAME(_end),%ecx
  53         subl %edi,%ecx
  54         cld
  55         rep
  56         stosb
  57 /*
  58  * Do the decompression, and jump to the new kernel..
  59  */
  60         call SYMBOL_NAME(decompress_kernel)
  61         ljmp $(KERNEL_CS), $0x100000

/* */