linux/arch/x86/boot/header.S
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   1/*
   2 *      header.S
   3 *
   4 *      Copyright (C) 1991, 1992 Linus Torvalds
   5 *
   6 *      Based on bootsect.S and setup.S
   7 *      modified by more people than can be counted
   8 *
   9 *      Rewritten as a common file by H. Peter Anvin (Apr 2007)
  10 *
  11 * BIG FAT NOTE: We're in real mode using 64k segments.  Therefore segment
  12 * addresses must be multiplied by 16 to obtain their respective linear
  13 * addresses. To avoid confusion, linear addresses are written using leading
  14 * hex while segment addresses are written as segment:offset.
  15 *
  16 */
  17
  18#include <asm/segment.h>
  19#include <linux/utsrelease.h>
  20#include <asm/boot.h>
  21#include <asm/e820.h>
  22#include <asm/page_types.h>
  23#include <asm/setup.h>
  24#include "boot.h"
  25#include "offsets.h"
  26
  27BOOTSEG         = 0x07C0                /* original address of boot-sector */
  28SYSSEG          = 0x1000                /* historical load address >> 4 */
  29
  30#ifndef SVGA_MODE
  31#define SVGA_MODE ASK_VGA
  32#endif
  33
  34#ifndef RAMDISK
  35#define RAMDISK 0
  36#endif
  37
  38#ifndef ROOT_RDONLY
  39#define ROOT_RDONLY 1
  40#endif
  41
  42        .code16
  43        .section ".bstext", "ax"
  44
  45        .global bootsect_start
  46bootsect_start:
  47
  48        # Normalize the start address
  49        ljmp    $BOOTSEG, $start2
  50
  51start2:
  52        movw    %cs, %ax
  53        movw    %ax, %ds
  54        movw    %ax, %es
  55        movw    %ax, %ss
  56        xorw    %sp, %sp
  57        sti
  58        cld
  59
  60        movw    $bugger_off_msg, %si
  61
  62msg_loop:
  63        lodsb
  64        andb    %al, %al
  65        jz      bs_die
  66        movb    $0xe, %ah
  67        movw    $7, %bx
  68        int     $0x10
  69        jmp     msg_loop
  70
  71bs_die:
  72        # Allow the user to press a key, then reboot
  73        xorw    %ax, %ax
  74        int     $0x16
  75        int     $0x19
  76
  77        # int 0x19 should never return.  In case it does anyway,
  78        # invoke the BIOS reset code...
  79        ljmp    $0xf000,$0xfff0
  80
  81        .section ".bsdata", "a"
  82bugger_off_msg:
  83        .ascii  "Direct booting from floppy is no longer supported.\r\n"
  84        .ascii  "Please use a boot loader program instead.\r\n"
  85        .ascii  "\n"
  86        .ascii  "Remove disk and press any key to reboot . . .\r\n"
  87        .byte   0
  88
  89
  90        # Kernel attributes; used by setup.  This is part 1 of the
  91        # header, from the old boot sector.
  92
  93        .section ".header", "a"
  94        .globl  hdr
  95hdr:
  96setup_sects:    .byte 0                 /* Filled in by build.c */
  97root_flags:     .word ROOT_RDONLY
  98syssize:        .long 0                 /* Filled in by build.c */
  99ram_size:       .word 0                 /* Obsolete */
 100vid_mode:       .word SVGA_MODE
 101root_dev:       .word 0                 /* Filled in by build.c */
 102boot_flag:      .word 0xAA55
 103
 104        # offset 512, entry point
 105
 106        .globl  _start
 107_start:
 108                # Explicitly enter this as bytes, or the assembler
 109                # tries to generate a 3-byte jump here, which causes
 110                # everything else to push off to the wrong offset.
 111                .byte   0xeb            # short (2-byte) jump
 112                .byte   start_of_setup-1f
 1131:
 114
 115        # Part 2 of the header, from the old setup.S
 116
 117                .ascii  "HdrS"          # header signature
 118                .word   0x0209          # header version number (>= 0x0105)
 119                                        # or else old loadlin-1.5 will fail)
 120                .globl realmode_swtch
 121realmode_swtch: .word   0, 0            # default_switch, SETUPSEG
 122start_sys_seg:  .word   SYSSEG          # obsolete and meaningless, but just
 123                                        # in case something decided to "use" it
 124                .word   kernel_version-512 # pointing to kernel version string
 125                                        # above section of header is compatible
 126                                        # with loadlin-1.5 (header v1.5). Don't
 127                                        # change it.
 128
 129type_of_loader: .byte   0               # 0 means ancient bootloader, newer
 130                                        # bootloaders know to change this.
 131                                        # See Documentation/i386/boot.txt for
 132                                        # assigned ids
 133
 134# flags, unused bits must be zero (RFU) bit within loadflags
 135loadflags:
 136LOADED_HIGH     = 1                     # If set, the kernel is loaded high
 137CAN_USE_HEAP    = 0x80                  # If set, the loader also has set
 138                                        # heap_end_ptr to tell how much
 139                                        # space behind setup.S can be used for
 140                                        # heap purposes.
 141                                        # Only the loader knows what is free
 142                .byte   LOADED_HIGH
 143
 144setup_move_size: .word  0x8000          # size to move, when setup is not
 145                                        # loaded at 0x90000. We will move setup
 146                                        # to 0x90000 then just before jumping
 147                                        # into the kernel. However, only the
 148                                        # loader knows how much data behind
 149                                        # us also needs to be loaded.
 150
 151code32_start:                           # here loaders can put a different
 152                                        # start address for 32-bit code.
 153                .long   0x100000        # 0x100000 = default for big kernel
 154
 155ramdisk_image:  .long   0               # address of loaded ramdisk image
 156                                        # Here the loader puts the 32-bit
 157                                        # address where it loaded the image.
 158                                        # This only will be read by the kernel.
 159
 160ramdisk_size:   .long   0               # its size in bytes
 161
 162bootsect_kludge:
 163                .long   0               # obsolete
 164
 165heap_end_ptr:   .word   _end+STACK_SIZE-512
 166                                        # (Header version 0x0201 or later)
 167                                        # space from here (exclusive) down to
 168                                        # end of setup code can be used by setup
 169                                        # for local heap purposes.
 170
 171pad1:           .word   0
 172cmd_line_ptr:   .long   0               # (Header version 0x0202 or later)
 173                                        # If nonzero, a 32-bit pointer
 174                                        # to the kernel command line.
 175                                        # The command line should be
 176                                        # located between the start of
 177                                        # setup and the end of low
 178                                        # memory (0xa0000), or it may
 179                                        # get overwritten before it
 180                                        # gets read.  If this field is
 181                                        # used, there is no longer
 182                                        # anything magical about the
 183                                        # 0x90000 segment; the setup
 184                                        # can be located anywhere in
 185                                        # low memory 0x10000 or higher.
 186
 187ramdisk_max:    .long 0x7fffffff
 188                                        # (Header version 0x0203 or later)
 189                                        # The highest safe address for
 190                                        # the contents of an initrd
 191                                        # The current kernel allows up to 4 GB,
 192                                        # but leave it at 2 GB to avoid
 193                                        # possible bootloader bugs.
 194
 195kernel_alignment:  .long CONFIG_PHYSICAL_ALIGN  #physical addr alignment
 196                                                #required for protected mode
 197                                                #kernel
 198#ifdef CONFIG_RELOCATABLE
 199relocatable_kernel:    .byte 1
 200#else
 201relocatable_kernel:    .byte 0
 202#endif
 203pad2:                   .byte 0
 204pad3:                   .word 0
 205
 206cmdline_size:   .long   COMMAND_LINE_SIZE-1     #length of the command line,
 207                                                #added with boot protocol
 208                                                #version 2.06
 209
 210hardware_subarch:       .long 0                 # subarchitecture, added with 2.07
 211                                                # default to 0 for normal x86 PC
 212
 213hardware_subarch_data:  .quad 0
 214
 215payload_offset:         .long input_data
 216payload_length:         .long input_data_end-input_data
 217
 218setup_data:             .quad 0                 # 64-bit physical pointer to
 219                                                # single linked list of
 220                                                # struct setup_data
 221
 222# End of setup header #####################################################
 223
 224        .section ".inittext", "ax"
 225start_of_setup:
 226#ifdef SAFE_RESET_DISK_CONTROLLER
 227# Reset the disk controller.
 228        movw    $0x0000, %ax            # Reset disk controller
 229        movb    $0x80, %dl              # All disks
 230        int     $0x13
 231#endif
 232
 233# Force %es = %ds
 234        movw    %ds, %ax
 235        movw    %ax, %es
 236        cld
 237
 238# Apparently some ancient versions of LILO invoked the kernel with %ss != %ds,
 239# which happened to work by accident for the old code.  Recalculate the stack
 240# pointer if %ss is invalid.  Otherwise leave it alone, LOADLIN sets up the
 241# stack behind its own code, so we can't blindly put it directly past the heap.
 242
 243        movw    %ss, %dx
 244        cmpw    %ax, %dx        # %ds == %ss?
 245        movw    %sp, %dx
 246        je      2f              # -> assume %sp is reasonably set
 247
 248        # Invalid %ss, make up a new stack
 249        movw    $_end, %dx
 250        testb   $CAN_USE_HEAP, loadflags
 251        jz      1f
 252        movw    heap_end_ptr, %dx
 2531:      addw    $STACK_SIZE, %dx
 254        jnc     2f
 255        xorw    %dx, %dx        # Prevent wraparound
 256
 2572:      # Now %dx should point to the end of our stack space
 258        andw    $~3, %dx        # dword align (might as well...)
 259        jnz     3f
 260        movw    $0xfffc, %dx    # Make sure we're not zero
 2613:      movw    %ax, %ss
 262        movzwl  %dx, %esp       # Clear upper half of %esp
 263        sti                     # Now we should have a working stack
 264
 265# We will have entered with %cs = %ds+0x20, normalize %cs so
 266# it is on par with the other segments.
 267        pushw   %ds
 268        pushw   $6f
 269        lretw
 2706:
 271
 272# Check signature at end of setup
 273        cmpl    $0x5a5aaa55, setup_sig
 274        jne     setup_bad
 275
 276# Zero the bss
 277        movw    $__bss_start, %di
 278        movw    $_end+3, %cx
 279        xorl    %eax, %eax
 280        subw    %di, %cx
 281        shrw    $2, %cx
 282        rep; stosl
 283
 284# Jump to C code (should not return)
 285        calll   main
 286
 287# Setup corrupt somehow...
 288setup_bad:
 289        movl    $setup_corrupt, %eax
 290        calll   puts
 291        # Fall through...
 292
 293        .globl  die
 294        .type   die, @function
 295die:
 296        hlt
 297        jmp     die
 298
 299        .size   die, .-die
 300
 301        .section ".initdata", "a"
 302setup_corrupt:
 303        .byte   7
 304        .string "No setup signature found...\n"
 305
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