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. [According to comments etc elsewhere on a compressed
  13  * kernel it will end up at 0x1000 + 1Mb I hope so as I assume this. - AC]
  14  *
  15  * Page 0 is deliberately kept safe, since System Management Mode code in
  16  * laptops may need to access the BIOS data stored there.  This is also
  17  * useful for future device drivers that either access the BIOS via VM86
  18  * mode.
  19  */
  20
  21 /*
  22  * High loaded stuff by Hans Lermen & Werner Almesberger, Feb. 1996
  23  */
  24 .text
  25
  26 #include <linux/linkage.h>
  27 #include <asm/segment.h>
  28 #include <asm/page.h>
  29
  30         .globl startup_32
  31
  32 startup_32:
  33         cld
  34         cli
  35         movl $(__BOOT_DS),%eax
  36         movl %eax,%ds
  37         movl %eax,%es
  38         movl %eax,%fs
  39         movl %eax,%gs
  40
  41         lss stack_start,%esp
  42         xorl %eax,%eax
  43 1:      incl %eax               # check that A20 really IS enabled
  44         movl %eax,0x000000      # loop forever if it isn't
  45         cmpl %eax,0x100000
  46         je 1b
  47
  48 /*
  49  * Initialize eflags.  Some BIOS's leave bits like NT set.  This would
  50  * confuse the debugger if this code is traced.
  51  * XXX - best to initialize before switching to protected mode.
  52  */
  53         pushl $0
  54         popfl
  55 /*
  56  * Clear BSS
  57  */
  58         xorl %eax,%eax
  59         movl $_edata,%edi
  60         movl $_end,%ecx
  61         subl %edi,%ecx
  62         cld
  63         rep
  64         stosb
  65 /*
  66  * Do the decompression, and jump to the new kernel..
  67  */
  68         subl $16,%esp   # place for structure on the stack
  69         movl %esp,%eax
  70         pushl %esi      # real mode pointer as second arg
  71         pushl %eax      # address of structure as first arg
  72         call decompress_kernel
  73         orl  %eax,%eax
  74         jnz  3f
  75         popl %esi       # discard address
  76         popl %esi       # real mode pointer
  77         xorl %ebx,%ebx
  78         ljmp $(__BOOT_CS), $__PHYSICAL_START
  79
  80 /*
  81  * We come here, if we were loaded high.
  82  * We need to move the move-in-place routine down to 0x1000
  83  * and then start it with the buffer addresses in registers,
  84  * which we got from the stack.
  85  */
  86 3:
  87         movl $move_routine_start,%esi
  88         movl $0x1000,%edi
  89         movl $move_routine_end,%ecx
  90         subl %esi,%ecx
  91         addl $3,%ecx
  92         shrl $2,%ecx
  93         cld
  94         rep
  95         movsl
  96
  97         popl %esi       # discard the address
  98         popl %ebx       # real mode pointer
  99         popl %esi       # low_buffer_start
 100         popl %ecx       # lcount
 101         popl %edx       # high_buffer_start
 102         popl %eax       # hcount
 103         movl $__PHYSICAL_START,%edi
 104         cli             # make sure we don't get interrupted
 105         ljmp $(__BOOT_CS), $0x1000 # and jump to the move routine
 106
 107 /*
 108  * Routine (template) for moving the decompressed kernel in place,
 109  * if we were high loaded. This _must_ PIC-code !
 110  */
 111 move_routine_start:
 112         movl %ecx,%ebp
 113         shrl $2,%ecx
 114         rep
 115         movsl
 116         movl %ebp,%ecx
 117         andl $3,%ecx
 118         rep
 119         movsb
 120         movl %edx,%esi
 121         movl %eax,%ecx  # NOTE: rep movsb won't move if %ecx == 0
 122         addl $3,%ecx
 123         shrl $2,%ecx
 124         rep
 125         movsl
 126         movl %ebx,%esi  # Restore setup pointer
 127         xorl %ebx,%ebx
 128         ljmp $(__BOOT_CS), $__PHYSICAL_START
 129 move_routine_end: