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.h>
  23#include <asm/setup.h>
  24#include "boot.h"
  25
  26SETUPSECTS      = 4                     /* default nr of setup-sectors */
  27BOOTSEG         = 0x07C0                /* original address of boot-sector */
  28SYSSEG          = DEF_SYSSEG            /* system loaded at 0x10000 (65536) */
  29SYSSIZE         = DEF_SYSSIZE           /* system size: # of 16-byte clicks */
  30                                        /* to be loaded */
  31ROOT_DEV        = 0                     /* ROOT_DEV is now written by "build" */
  32SWAP_DEV        = 0                     /* SWAP_DEV is now written by "build" */
  33
  34#ifndef SVGA_MODE
  35#define SVGA_MODE ASK_VGA
  36#endif
  37
  38#ifndef RAMDISK
  39#define RAMDISK 0
  40#endif
  41
  42#ifndef ROOT_RDONLY
  43#define ROOT_RDONLY 1
  44#endif
  45
  46        .code16
  47        .section ".bstext", "ax"
  48
  49        .global bootsect_start
  50bootsect_start:
  51
  52        # Normalize the start address
  53        ljmp    $BOOTSEG, $start2
  54
  55start2:
  56        movw    %cs, %ax
  57        movw    %ax, %ds
  58        movw    %ax, %es
  59        movw    %ax, %ss
  60        xorw    %sp, %sp
  61        sti
  62        cld
  63
  64        movw    $bugger_off_msg, %si
  65
  66msg_loop:
  67        lodsb
  68        andb    %al, %al
  69        jz      bs_die
  70        movb    $0xe, %ah
  71        movw    $7, %bx
  72        int     $0x10
  73        jmp     msg_loop
  74
  75bs_die:
  76        # Allow the user to press a key, then reboot
  77        xorw    %ax, %ax
  78        int     $0x16
  79        int     $0x19
  80
  81        # int 0x19 should never return.  In case it does anyway,
  82        # invoke the BIOS reset code...
  83        ljmp    $0xf000,$0xfff0
  84
  85        .section ".bsdata", "a"
  86bugger_off_msg:
  87        .ascii  "Direct booting from floppy is no longer supported.\r\n"
  88        .ascii  "Please use a boot loader program instead.\r\n"
  89        .ascii  "\n"
  90        .ascii  "Remove disk and press any key to reboot . . .\r\n"
  91        .byte   0
  92
  93
  94        # Kernel attributes; used by setup.  This is part 1 of the
  95        # header, from the old boot sector.
  96
  97        .section ".header", "a"
  98        .globl  hdr
  99hdr:
 100setup_sects:    .byte SETUPSECTS
 101root_flags:     .word ROOT_RDONLY
 102syssize:        .long SYSSIZE
 103ram_size:       .word RAMDISK
 104vid_mode:       .word SVGA_MODE
 105root_dev:       .word ROOT_DEV
 106boot_flag:      .word 0xAA55
 107
 108        # offset 512, entry point
 109
 110        .globl  _start
 111_start:
 112                # Explicitly enter this as bytes, or the assembler
 113                # tries to generate a 3-byte jump here, which causes
 114                # everything else to push off to the wrong offset.
 115                .byte   0xeb            # short (2-byte) jump
 116                .byte   start_of_setup-1f
 1171:
 118
 119        # Part 2 of the header, from the old setup.S
 120
 121                .ascii  "HdrS"          # header signature
 122                .word   0x0207          # header version number (>= 0x0105)
 123                                        # or else old loadlin-1.5 will fail)
 124                .globl realmode_swtch
 125realmode_swtch: .word   0, 0            # default_switch, SETUPSEG
 126start_sys_seg:  .word   SYSSEG
 127                .word   kernel_version-512 # pointing to kernel version string
 128                                        # above section of header is compatible
 129                                        # with loadlin-1.5 (header v1.5). Don't
 130                                        # change it.
 131
 132type_of_loader: .byte   0               # = 0, old one (LILO, Loadlin,
 133                                        #      Bootlin, SYSLX, bootsect...)
 134                                        # See Documentation/i386/boot.txt for
 135                                        # assigned ids
 136
 137# flags, unused bits must be zero (RFU) bit within loadflags
 138loadflags:
 139LOADED_HIGH     = 1                     # If set, the kernel is loaded high
 140CAN_USE_HEAP    = 0x80                  # If set, the loader also has set
 141                                        # heap_end_ptr to tell how much
 142                                        # space behind setup.S can be used for
 143                                        # heap purposes.
 144                                        # Only the loader knows what is free
 145#ifndef __BIG_KERNEL__
 146                .byte   0
 147#else
 148                .byte   LOADED_HIGH
 149#endif
 150
 151setup_move_size: .word  0x8000          # size to move, when setup is not
 152                                        # loaded at 0x90000. We will move setup
 153                                        # to 0x90000 then just before jumping
 154                                        # into the kernel. However, only the
 155                                        # loader knows how much data behind
 156                                        # us also needs to be loaded.
 157
 158code32_start:                           # here loaders can put a different
 159                                        # start address for 32-bit code.
 160#ifndef __BIG_KERNEL__
 161                .long   0x1000          #   0x1000 = default for zImage
 162#else
 163                .long   0x100000        # 0x100000 = default for big kernel
 164#endif
 165
 166ramdisk_image:  .long   0               # address of loaded ramdisk image
 167                                        # Here the loader puts the 32-bit
 168                                        # address where it loaded the image.
 169                                        # This only will be read by the kernel.
 170
 171ramdisk_size:   .long   0               # its size in bytes
 172
 173bootsect_kludge:
 174                .long   0               # obsolete
 175
 176heap_end_ptr:   .word   _end+STACK_SIZE-512
 177                                        # (Header version 0x0201 or later)
 178                                        # space from here (exclusive) down to
 179                                        # end of setup code can be used by setup
 180                                        # for local heap purposes.
 181
 182pad1:           .word   0
 183cmd_line_ptr:   .long   0               # (Header version 0x0202 or later)
 184                                        # If nonzero, a 32-bit pointer
 185                                        # to the kernel command line.
 186                                        # The command line should be
 187                                        # located between the start of
 188                                        # setup and the end of low
 189                                        # memory (0xa0000), or it may
 190                                        # get overwritten before it
 191                                        # gets read.  If this field is
 192                                        # used, there is no longer
 193                                        # anything magical about the
 194                                        # 0x90000 segment; the setup
 195                                        # can be located anywhere in
 196                                        # low memory 0x10000 or higher.
 197
 198ramdisk_max:    .long 0x7fffffff
 199                                        # (Header version 0x0203 or later)
 200                                        # The highest safe address for
 201                                        # the contents of an initrd
 202                                        # The current kernel allows up to 4 GB,
 203                                        # but leave it at 2 GB to avoid
 204                                        # possible bootloader bugs.
 205
 206kernel_alignment:  .long CONFIG_PHYSICAL_ALIGN  #physical addr alignment
 207                                                #required for protected mode
 208                                                #kernel
 209#ifdef CONFIG_RELOCATABLE
 210relocatable_kernel:    .byte 1
 211#else
 212relocatable_kernel:    .byte 0
 213#endif
 214pad2:                   .byte 0
 215pad3:                   .word 0
 216
 217cmdline_size:   .long   COMMAND_LINE_SIZE-1     #length of the command line,
 218                                                #added with boot protocol
 219                                                #version 2.06
 220
 221hardware_subarch:       .long 0                 # subarchitecture, added with 2.07
 222                                                # default to 0 for normal x86 PC
 223
 224hardware_subarch_data:  .quad 0
 225
 226# End of setup header #####################################################
 227
 228        .section ".inittext", "ax"
 229start_of_setup:
 230#ifdef SAFE_RESET_DISK_CONTROLLER
 231# Reset the disk controller.
 232        movw    $0x0000, %ax            # Reset disk controller
 233        movb    $0x80, %dl              # All disks
 234        int     $0x13
 235#endif
 236
 237# Force %es = %ds
 238        movw    %ds, %ax
 239        movw    %ax, %es
 240        cld
 241
 242# Apparently some ancient versions of LILO invoked the kernel with %ss != %ds,
 243# which happened to work by accident for the old code.  Recalculate the stack
 244# pointer if %ss is invalid.  Otherwise leave it alone, LOADLIN sets up the
 245# stack behind its own code, so we can't blindly put it directly past the heap.
 246
 247        movw    %ss, %dx
 248        cmpw    %ax, %dx        # %ds == %ss?
 249        movw    %sp, %dx
 250        je      2f              # -> assume %sp is reasonably set
 251
 252        # Invalid %ss, make up a new stack
 253        movw    $_end, %dx
 254        testb   $CAN_USE_HEAP, loadflags
 255        jz      1f
 256        movw    heap_end_ptr, %dx
 2571:      addw    $STACK_SIZE, %dx
 258        jnc     2f
 259        xorw    %dx, %dx        # Prevent wraparound
 260
 2612:      # Now %dx should point to the end of our stack space
 262        andw    $~3, %dx        # dword align (might as well...)
 263        jnz     3f
 264        movw    $0xfffc, %dx    # Make sure we're not zero
 2653:      movw    %ax, %ss
 266        movzwl  %dx, %esp       # Clear upper half of %esp
 267        sti                     # Now we should have a working stack
 268
 269# We will have entered with %cs = %ds+0x20, normalize %cs so
 270# it is on par with the other segments.
 271        pushw   %ds
 272        pushw   $6f
 273        lretw
 2746:
 275
 276# Check signature at end of setup
 277        cmpl    $0x5a5aaa55, setup_sig
 278        jne     setup_bad
 279
 280# Zero the bss
 281        movw    $__bss_start, %di
 282        movw    $_end+3, %cx
 283        xorl    %eax, %eax
 284        subw    %di, %cx
 285        shrw    $2, %cx
 286        rep; stosl
 287
 288# Jump to C code (should not return)
 289        calll   main
 290
 291# Setup corrupt somehow...
 292setup_bad:
 293        movl    $setup_corrupt, %eax
 294        calll   puts
 295        # Fall through...
 296
 297        .globl  die
 298        .type   die, @function
 299die:
 300        hlt
 301        jmp     die
 302
 303        .size   die, .-die
 304
 305        .section ".initdata", "a"
 306setup_corrupt:
 307        .byte   7
 308        .string "No setup signature found...\n"
 309
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