Documentation/i386/boot.txt

   1                      THE LINUX/I386 BOOT PROTOCOL
   2                      ----------------------------
   3
   4                     H. Peter Anvin <hpa@zytor.com>
   5                         Last update 2007-03-06
   6
   7 On the i386 platform, the Linux kernel uses a rather complicated boot
   8 convention.  This has evolved partially due to historical aspects, as
   9 well as the desire in the early days to have the kernel itself be a
  10 bootable image, the complicated PC memory model and due to changed
  11 expectations in the PC industry caused by the effective demise of
  12 real-mode DOS as a mainstream operating system.
  13
  14 Currently, four versions of the Linux/i386 boot protocol exist.
  15
  16 Old kernels:    zImage/Image support only.  Some very early kernels
  17                 may not even support a command line.
  18
  19 Protocol 2.00:  (Kernel 1.3.73) Added bzImage and initrd support, as
  20                 well as a formalized way to communicate between the
  21                 boot loader and the kernel.  setup.S made relocatable,
  22                 although the traditional setup area still assumed
  23                 writable.
  24
  25 Protocol 2.01:  (Kernel 1.3.76) Added a heap overrun warning.
  26
  27 Protocol 2.02:  (Kernel 2.4.0-test3-pre3) New command line protocol.
  28                 Lower the conventional memory ceiling.  No overwrite
  29                 of the traditional setup area, thus making booting
  30                 safe for systems which use the EBDA from SMM or 32-bit
  31                 BIOS entry points.  zImage deprecated but still
  32                 supported.
  33
  34 Protocol 2.03:  (Kernel 2.4.18-pre1) Explicitly makes the highest possible
  35                 initrd address available to the bootloader.
  36
  37 Protocol 2.04:  (Kernel 2.6.14) Extend the syssize field to four bytes.
  38
  39 Protocol 2.05:  (Kernel 2.6.20) Make protected mode kernel relocatable.
  40                 Introduce relocatable_kernel and kernel_alignment fields.
  41
  42 Protocol 2.06:  (Kernel 2.6.22) Added a field that contains the size of
  43                 the boot command line
  44
  45
  46 **** MEMORY LAYOUT
  47
  48 The traditional memory map for the kernel loader, used for Image or
  49 zImage kernels, typically looks like:
  50
  51         |                        |
  52 0A0000  +------------------------+
  53         |  Reserved for BIOS     |      Do not use.  Reserved for BIOS EBDA.
  54 09A000  +------------------------+
  55         |  Stack/heap/cmdline    |      For use by the kernel real-mode code.
  56 098000  +------------------------+
  57         |  Kernel setup          |      The kernel real-mode code.
  58 090200  +------------------------+
  59         |  Kernel boot sector    |      The kernel legacy boot sector.
  60 090000  +------------------------+
  61         |  Protected-mode kernel |      The bulk of the kernel image.
  62 010000  +------------------------+
  63         |  Boot loader           |      <- Boot sector entry point 0000:7C00
  64 001000  +------------------------+
  65         |  Reserved for MBR/BIOS |
  66 000800  +------------------------+
  67         |  Typically used by MBR |
  68 000600  +------------------------+
  69         |  BIOS use only         |
  70 000000  +------------------------+
  71
  72
  73 When using bzImage, the protected-mode kernel was relocated to
  74 0x100000 ("high memory"), and the kernel real-mode block (boot sector,
  75 setup, and stack/heap) was made relocatable to any address between
  76 0x10000 and end of low memory.  Unfortunately, in protocols 2.00 and
  77 2.01 the command line is still required to live in the 0x9XXXX memory
  78 range, and that memory range is still overwritten by the early kernel.
  79 The 2.02 protocol resolves that problem.
  80
  81 It is desirable to keep the "memory ceiling" -- the highest point in
  82 low memory touched by the boot loader -- as low as possible, since
  83 some newer BIOSes have begun to allocate some rather large amounts of
  84 memory, called the Extended BIOS Data Area, near the top of low
  85 memory.  The boot loader should use the "INT 12h" BIOS call to verify
  86 how much low memory is available.
  87
  88 Unfortunately, if INT 12h reports that the amount of memory is too
  89 low, there is usually nothing the boot loader can do but to report an
  90 error to the user.  The boot loader should therefore be designed to
  91 take up as little space in low memory as it reasonably can.  For
  92 zImage or old bzImage kernels, which need data written into the
  93 0x90000 segment, the boot loader should make sure not to use memory
  94 above the 0x9A000 point; too many BIOSes will break above that point.
  95
  96
  97 **** THE REAL-MODE KERNEL HEADER
  98
  99 In the following text, and anywhere in the kernel boot sequence, "a
 100 sector" refers to 512 bytes.  It is independent of the actual sector
 101 size of the underlying medium.
 102
 103 The first step in loading a Linux kernel should be to load the
 104 real-mode code (boot sector and setup code) and then examine the
 105 following header at offset 0x01f1.  The real-mode code can total up to
 106 32K, although the boot loader may choose to load only the first two
 107 sectors (1K) and then examine the bootup sector size.
 108
 109 The header looks like:
 110
 111 Offset  Proto   Name            Meaning
 112 /Size
 113
 114 01F1/1  ALL(1   setup_sects     The size of the setup in sectors
 115 01F2/2  ALL     root_flags      If set, the root is mounted readonly
 116 01F4/4  2.04+(2 syssize         The size of the 32-bit code in 16-byte paras
 117 01F8/2  ALL     ram_size        DO NOT USE - for bootsect.S use only
 118 01FA/2  ALL     vid_mode        Video mode control
 119 01FC/2  ALL     root_dev        Default root device number
 120 01FE/2  ALL     boot_flag       0xAA55 magic number
 121 0200/2  2.00+   jump            Jump instruction
 122 0202/4  2.00+   header          Magic signature "HdrS"
 123 0206/2  2.00+   version         Boot protocol version supported
 124 0208/4  2.00+   realmode_swtch  Boot loader hook (see below)
 125 020C/2  2.00+   start_sys       The load-low segment (0x1000) (obsolete)
 126 020E/2  2.00+   kernel_version  Pointer to kernel version string
 127 0210/1  2.00+   type_of_loader  Boot loader identifier
 128 0211/1  2.00+   loadflags       Boot protocol option flags
 129 0212/2  2.00+   setup_move_size Move to high memory size (used with hooks)
 130 0214/4  2.00+   code32_start    Boot loader hook (see below)
 131 0218/4  2.00+   ramdisk_image   initrd load address (set by boot loader)
 132 021C/4  2.00+   ramdisk_size    initrd size (set by boot loader)
 133 0220/4  2.00+   bootsect_kludge DO NOT USE - for bootsect.S use only
 134 0224/2  2.01+   heap_end_ptr    Free memory after setup end
 135 0226/2  N/A     pad1            Unused
 136 0228/4  2.02+   cmd_line_ptr    32-bit pointer to the kernel command line
 137 022C/4  2.03+   initrd_addr_max Highest legal initrd address
 138 0230/4  2.05+   kernel_alignment Physical addr alignment required for kernel
 139 0234/1  2.05+   relocatable_kernel Whether kernel is relocatable or not
 140 0235/3  N/A     pad2            Unused
 141 0238/4  2.06+   cmdline_size    Maximum size of the kernel command line
 142
 143 (1) For backwards compatibility, if the setup_sects field contains 0, the
 144     real value is 4.
 145
 146 (2) For boot protocol prior to 2.04, the upper two bytes of the syssize
 147     field are unusable, which means the size of a bzImage kernel
 148     cannot be determined.
 149
 150 If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
 151 the boot protocol version is "old".  Loading an old kernel, the
 152 following parameters should be assumed:
 153
 154         Image type = zImage
 155         initrd not supported
 156         Real-mode kernel must be located at 0x90000.
 157
 158 Otherwise, the "version" field contains the protocol version,
 159 e.g. protocol version 2.01 will contain 0x0201 in this field.  When
 160 setting fields in the header, you must make sure only to set fields
 161 supported by the protocol version in use.
 162
 163 The "kernel_version" field, if set to a nonzero value, contains a
 164 pointer to a null-terminated human-readable kernel version number
 165 string, less 0x200.  This can be used to display the kernel version to
 166 the user.  This value should be less than (0x200*setup_sects).  For
 167 example, if this value is set to 0x1c00, the kernel version number
 168 string can be found at offset 0x1e00 in the kernel file.  This is a
 169 valid value if and only if the "setup_sects" field contains the value
 170 14 or higher.
 171
 172 Most boot loaders will simply load the kernel at its target address
 173 directly.  Such boot loaders do not need to worry about filling in
 174 most of the fields in the header.  The following fields should be
 175 filled out, however:
 176
 177   vid_mode:
 178         Please see the section on SPECIAL COMMAND LINE OPTIONS.
 179
 180   type_of_loader:
 181         If your boot loader has an assigned id (see table below), enter
 182         0xTV here, where T is an identifier for the boot loader and V is
 183         a version number.  Otherwise, enter 0xFF here.
 184
 185         Assigned boot loader ids:
 186         0  LILO
 187         1  Loadlin
 188         2  bootsect-loader
 189         3  SYSLINUX
 190         4  EtherBoot
 191         5  ELILO
 192         7  GRuB
 193         8  U-BOOT
 194         9  Xen
 195         A  Gujin
 196
 197         Please contact <hpa@zytor.com> if you need a bootloader ID
 198         value assigned.
 199
 200   loadflags, heap_end_ptr:
 201         If the protocol version is 2.01 or higher, enter the
 202         offset limit of the setup heap into heap_end_ptr and set the
 203         0x80 bit (CAN_USE_HEAP) of loadflags.  heap_end_ptr appears to
 204         be relative to the start of setup (offset 0x0200).
 205
 206   setup_move_size:
 207         When using protocol 2.00 or 2.01, if the real mode
 208         kernel is not loaded at 0x90000, it gets moved there later in
 209         the loading sequence.  Fill in this field if you want
 210         additional data (such as the kernel command line) moved in
 211         addition to the real-mode kernel itself.
 212
 213   ramdisk_image, ramdisk_size:
 214         If your boot loader has loaded an initial ramdisk (initrd),
 215         set ramdisk_image to the 32-bit pointer to the ramdisk data
 216         and the ramdisk_size to the size of the ramdisk data.
 217
 218         The initrd should typically be located as high in memory as
 219         possible, as it may otherwise get overwritten by the early
 220         kernel initialization sequence.  However, it must never be
 221         located above the address specified in the initrd_addr_max
 222         field.  The initrd should be at least 4K page aligned.
 223
 224   cmd_line_ptr:
 225         If the protocol version is 2.02 or higher, this is a 32-bit
 226         pointer to the kernel command line.  The kernel command line
 227         can be located anywhere between the end of setup and 0xA0000.
 228         Fill in this field even if your boot loader does not support a
 229         command line, in which case you can point this to an empty
 230         string (or better yet, to the string "auto".)  If this field
 231         is left at zero, the kernel will assume that your boot loader
 232         does not support the 2.02+ protocol.
 233
 234   ramdisk_max:
 235         The maximum address that may be occupied by the initrd
 236         contents.  For boot protocols 2.02 or earlier, this field is
 237         not present, and the maximum address is 0x37FFFFFF.  (This
 238         address is defined as the address of the highest safe byte, so
 239         if your ramdisk is exactly 131072 bytes long and this field is
 240         0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
 241
 242   cmdline_size:
 243         The maximum size of the command line without the terminating
 244         zero. This means that the command line can contain at most
 245         cmdline_size characters. With protocol version 2.05 and
 246         earlier, the maximum size was 255.
 247
 248
 249 **** THE KERNEL COMMAND LINE
 250
 251 The kernel command line has become an important way for the boot
 252 loader to communicate with the kernel.  Some of its options are also
 253 relevant to the boot loader itself, see "special command line options"
 254 below.
 255
 256 The kernel command line is a null-terminated string. The maximum
 257 length can be retrieved from the field cmdline_size.  Before protocol
 258 version 2.06, the maximum was 255 characters.  A string that is too
 259 long will be automatically truncated by the kernel.
 260
 261 If the boot protocol version is 2.02 or later, the address of the
 262 kernel command line is given by the header field cmd_line_ptr (see
 263 above.)  This address can be anywhere between the end of the setup
 264 heap and 0xA0000.
 265
 266 If the protocol version is *not* 2.02 or higher, the kernel
 267 command line is entered using the following protocol:
 268
 269         At offset 0x0020 (word), "cmd_line_magic", enter the magic
 270         number 0xA33F.
 271
 272         At offset 0x0022 (word), "cmd_line_offset", enter the offset
 273         of the kernel command line (relative to the start of the
 274         real-mode kernel).
 275
 276         The kernel command line *must* be within the memory region
 277         covered by setup_move_size, so you may need to adjust this
 278         field.
 279
 280
 281 **** SAMPLE BOOT CONFIGURATION
 282
 283 As a sample configuration, assume the following layout of the real
 284 mode segment (this is a typical, and recommended layout):
 285
 286         0x0000-0x7FFF   Real mode kernel
 287         0x8000-0x8FFF   Stack and heap
 288         0x9000-0x90FF   Kernel command line
 289
 290 Such a boot loader should enter the following fields in the header:
 291
 292         unsigned long base_ptr; /* base address for real-mode segment */
 293
 294         if ( setup_sects == 0 ) {
 295                 setup_sects = 4;
 296         }
 297
 298         if ( protocol >= 0x0200 ) {
 299                 type_of_loader = <type code>;
 300                 if ( loading_initrd ) {
 301                         ramdisk_image = <initrd_address>;
 302                         ramdisk_size = <initrd_size>;
 303                 }
 304                 if ( protocol >= 0x0201 ) {
 305                         heap_end_ptr = 0x9000 - 0x200;
 306                         loadflags |= 0x80; /* CAN_USE_HEAP */
 307                 }
 308                 if ( protocol >= 0x0202 ) {
 309                         cmd_line_ptr = base_ptr + 0x9000;
 310                 } else {
 311                         cmd_line_magic  = 0xA33F;
 312                         cmd_line_offset = 0x9000;
 313                         setup_move_size = 0x9100;
 314                 }
 315         } else {
 316                 /* Very old kernel */
 317
 318                 cmd_line_magic  = 0xA33F;
 319                 cmd_line_offset = 0x9000;
 320
 321                 /* A very old kernel MUST have its real-mode code
 322                    loaded at 0x90000 */
 323
 324                 if ( base_ptr != 0x90000 ) {
 325                         /* Copy the real-mode kernel */
 326                         memcpy(0x90000, base_ptr, (setup_sects+1)*512);
 327                         /* Copy the command line */
 328                         memcpy(0x99000, base_ptr+0x9000, 256);
 329
 330                         base_ptr = 0x90000;              /* Relocated */
 331                 }
 332
 333                 /* It is recommended to clear memory up to the 32K mark */
 334                 memset(0x90000 + (setup_sects+1)*512, 0,
 335                        (64-(setup_sects+1))*512);
 336         }
 337
 338
 339 **** LOADING THE REST OF THE KERNEL
 340
 341 The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
 342 in the kernel file (again, if setup_sects == 0 the real value is 4.)
 343 It should be loaded at address 0x10000 for Image/zImage kernels and
 344 0x100000 for bzImage kernels.
 345
 346 The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
 347 bit (LOAD_HIGH) in the loadflags field is set:
 348
 349         is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
 350         load_address = is_bzImage ? 0x100000 : 0x10000;
 351
 352 Note that Image/zImage kernels can be up to 512K in size, and thus use
 353 the entire 0x10000-0x90000 range of memory.  This means it is pretty
 354 much a requirement for these kernels to load the real-mode part at
 355 0x90000.  bzImage kernels allow much more flexibility.
 356
 357
 358 **** SPECIAL COMMAND LINE OPTIONS
 359
 360 If the command line provided by the boot loader is entered by the
 361 user, the user may expect the following command line options to work.
 362 They should normally not be deleted from the kernel command line even
 363 though not all of them are actually meaningful to the kernel.  Boot
 364 loader authors who need additional command line options for the boot
 365 loader itself should get them registered in
 366 Documentation/kernel-parameters.txt to make sure they will not
 367 conflict with actual kernel options now or in the future.
 368
 369   vga=<mode>
 370         <mode> here is either an integer (in C notation, either
 371         decimal, octal, or hexadecimal) or one of the strings
 372         "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
 373         (meaning 0xFFFD).  This value should be entered into the
 374         vid_mode field, as it is used by the kernel before the command
 375         line is parsed.
 376
 377   mem=<size>
 378         <size> is an integer in C notation optionally followed by K, M
 379         or G (meaning << 10, << 20 or << 30).  This specifies the end
 380         of memory to the kernel. This affects the possible placement
 381         of an initrd, since an initrd should be placed near end of
 382         memory.  Note that this is an option to *both* the kernel and
 383         the bootloader!
 384
 385   initrd=<file>
 386         An initrd should be loaded.  The meaning of <file> is
 387         obviously bootloader-dependent, and some boot loaders
 388         (e.g. LILO) do not have such a command.
 389
 390 In addition, some boot loaders add the following options to the
 391 user-specified command line:
 392
 393   BOOT_IMAGE=<file>
 394         The boot image which was loaded.  Again, the meaning of <file>
 395         is obviously bootloader-dependent.
 396
 397   auto
 398         The kernel was booted without explicit user intervention.
 399
 400 If these options are added by the boot loader, it is highly
 401 recommended that they are located *first*, before the user-specified
 402 or configuration-specified command line.  Otherwise, "init=/bin/sh"
 403 gets confused by the "auto" option.
 404
 405
 406 **** RUNNING THE KERNEL
 407
 408 The kernel is started by jumping to the kernel entry point, which is
 409 located at *segment* offset 0x20 from the start of the real mode
 410 kernel.  This means that if you loaded your real-mode kernel code at
 411 0x90000, the kernel entry point is 9020:0000.
 412
 413 At entry, ds = es = ss should point to the start of the real-mode
 414 kernel code (0x9000 if the code is loaded at 0x90000), sp should be
 415 set up properly, normally pointing to the top of the heap, and
 416 interrupts should be disabled.  Furthermore, to guard against bugs in
 417 the kernel, it is recommended that the boot loader sets fs = gs = ds =
 418 es = ss.
 419
 420 In our example from above, we would do:
 421
 422         /* Note: in the case of the "old" kernel protocol, base_ptr must
 423            be == 0x90000 at this point; see the previous sample code */
 424
 425         seg = base_ptr >> 4;
 426
 427         cli();  /* Enter with interrupts disabled! */
 428
 429         /* Set up the real-mode kernel stack */
 430         _SS = seg;
 431         _SP = 0x9000;   /* Load SP immediately after loading SS! */
 432
 433         _DS = _ES = _FS = _GS = seg;
 434         jmp_far(seg+0x20, 0);   /* Run the kernel */
 435
 436 If your boot sector accesses a floppy drive, it is recommended to
 437 switch off the floppy motor before running the kernel, since the
 438 kernel boot leaves interrupts off and thus the motor will not be
 439 switched off, especially if the loaded kernel has the floppy driver as
 440 a demand-loaded module!
 441
 442
 443 **** ADVANCED BOOT TIME HOOKS
 444
 445 If the boot loader runs in a particularly hostile environment (such as
 446 LOADLIN, which runs under DOS) it may be impossible to follow the
 447 standard memory location requirements.  Such a boot loader may use the
 448 following hooks that, if set, are invoked by the kernel at the
 449 appropriate time.  The use of these hooks should probably be
 450 considered an absolutely last resort!
 451
 452 IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
 453 %edi across invocation.
 454
 455   realmode_swtch:
 456         A 16-bit real mode far subroutine invoked immediately before
 457         entering protected mode.  The default routine disables NMI, so
 458         your routine should probably do so, too.
 459
 460   code32_start:
 461         A 32-bit flat-mode routine *jumped* to immediately after the
 462         transition to protected mode, but before the kernel is
 463         uncompressed.  No segments, except CS, are set up; you should
 464         set them up to KERNEL_DS (0x18) yourself.
 465
 466         After completing your hook, you should jump to the address
 467         that was in this field before your boot loader overwrote it.