Documentation/i386/boot.txt

   1                      THE LINUX/I386 BOOT PROTOCOL
   2                      ----------------------------
   3
   4                     H. Peter Anvin <hpa@zytor.com>
   5                         Last update 2002-01-01
   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
  38 **** MEMORY LAYOUT
  39
  40 The traditional memory map for the kernel loader, used for Image or
  41 zImage kernels, typically looks like:
  42
  43         |                        |
  44 0A0000  +------------------------+
  45         |  Reserved for BIOS     |      Do not use.  Reserved for BIOS EBDA.
  46 09A000  +------------------------+
  47         |  Stack/heap/cmdline    |      For use by the kernel real-mode code.
  48 098000  +------------------------+
  49         |  Kernel setup          |      The kernel real-mode code.
  50 090200  +------------------------+
  51         |  Kernel boot sector    |      The kernel legacy boot sector.
  52 090000  +------------------------+
  53         |  Protected-mode kernel |      The bulk of the kernel image.
  54 010000  +------------------------+
  55         |  Boot loader           |      <- Boot sector entry point 0000:7C00
  56 001000  +------------------------+
  57         |  Reserved for MBR/BIOS |
  58 000800  +------------------------+
  59         |  Typically used by MBR |
  60 000600  +------------------------+
  61         |  BIOS use only         |
  62 000000  +------------------------+
  63
  64
  65 When using bzImage, the protected-mode kernel was relocated to
  66 0x100000 ("high memory"), and the kernel real-mode block (boot sector,
  67 setup, and stack/heap) was made relocatable to any address between
  68 0x10000 and end of low memory.  Unfortunately, in protocols 2.00 and
  69 2.01 the command line is still required to live in the 0x9XXXX memory
  70 range, and that memory range is still overwritten by the early kernel.
  71 The 2.02 protocol resolves that problem.
  72
  73 It is desirable to keep the "memory ceiling" -- the highest point in
  74 low memory touched by the boot loader -- as low as possible, since
  75 some newer BIOSes have begun to allocate some rather large amounts of
  76 memory, called the Extended BIOS Data Area, near the top of low
  77 memory.  The boot loader should use the "INT 12h" BIOS call to verify
  78 how much low memory is available.
  79
  80 Unfortunately, if INT 12h reports that the amount of memory is too
  81 low, there is usually nothing the boot loader can do but to report an
  82 error to the user.  The boot loader should therefore be designed to
  83 take up as little space in low memory as it reasonably can.  For
  84 zImage or old bzImage kernels, which need data written into the
  85 0x90000 segment, the boot loader should make sure not to use memory
  86 above the 0x9A000 point; too many BIOSes will break above that point.
  87
  88
  89 **** THE REAL-MODE KERNEL HEADER
  90
  91 In the following text, and anywhere in the kernel boot sequence, "a
  92 sector" refers to 512 bytes.  It is independent of the actual sector
  93 size of the underlying medium.
  94
  95 The first step in loading a Linux kernel should be to load the
  96 real-mode code (boot sector and setup code) and then examine the
  97 following header at offset 0x01f1.  The real-mode code can total up to
  98 32K, although the boot loader may choose to load only the first two
  99 sectors (1K) and then examine the bootup sector size.
 100
 101 The header looks like:
 102
 103 Offset  Proto   Name            Meaning
 104 /Size
 105
 106 01F1/1  ALL     setup_sects     The size of the setup in sectors
 107 01F2/2  ALL     root_flags      If set, the root is mounted readonly
 108 01F4/2  ALL     syssize         DO NOT USE - for bootsect.S use only
 109 01F6/2  ALL     swap_dev        DO NOT USE - obsolete
 110 01F8/2  ALL     ram_size        DO NOT USE - for bootsect.S use only
 111 01FA/2  ALL     vid_mode        Video mode control
 112 01FC/2  ALL     root_dev        Default root device number
 113 01FE/2  ALL     boot_flag       0xAA55 magic number
 114 0200/2  2.00+   jump            Jump instruction
 115 0202/4  2.00+   header          Magic signature "HdrS"
 116 0206/2  2.00+   version         Boot protocol version supported
 117 0208/4  2.00+   realmode_swtch  Boot loader hook (see below)
 118 020C/2  2.00+   start_sys       The load-low segment (0x1000) (obsolete)
 119 020E/2  2.00+   kernel_version  Pointer to kernel version string
 120 0210/1  2.00+   type_of_loader  Boot loader identifier
 121 0211/1  2.00+   loadflags       Boot protocol option flags
 122 0212/2  2.00+   setup_move_size Move to high memory size (used with hooks)
 123 0214/4  2.00+   code32_start    Boot loader hook (see below)
 124 0218/4  2.00+   ramdisk_image   initrd load address (set by boot loader)
 125 021C/4  2.00+   ramdisk_size    initrd size (set by boot loader)
 126 0220/4  2.00+   bootsect_kludge DO NOT USE - for bootsect.S use only
 127 0224/2  2.01+   heap_end_ptr    Free memory after setup end
 128 0226/2  N/A     pad1            Unused
 129 0228/4  2.02+   cmd_line_ptr    32-bit pointer to the kernel command line
 130 022C/4  2.03+   initrd_addr_max Highest legal initrd address
 131
 132 For backwards compatibility, if the setup_sects field contains 0, the
 133 real value is 4.
 134
 135 If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
 136 the boot protocol version is "old".  Loading an old kernel, the
 137 following parameters should be assumed:
 138
 139         Image type = zImage
 140         initrd not supported
 141         Real-mode kernel must be located at 0x90000.
 142
 143 Otherwise, the "version" field contains the protocol version,
 144 e.g. protocol version 2.01 will contain 0x0201 in this field.  When
 145 setting fields in the header, you must make sure only to set fields
 146 supported by the protocol version in use.
 147
 148 The "kernel_version" field, if set to a nonzero value, contains a
 149 pointer to a null-terminated human-readable kernel version number
 150 string, less 0x200.  This can be used to display the kernel version to
 151 the user.  This value should be less than (0x200*setup_sects).  For
 152 example, if this value is set to 0x1c00, the kernel version number
 153 string can be found at offset 0x1e00 in the kernel file.  This is a
 154 valid value if and only if the "setup_sects" field contains the value
 155 14 or higher.
 156
 157 Most boot loaders will simply load the kernel at its target address
 158 directly.  Such boot loaders do not need to worry about filling in
 159 most of the fields in the header.  The following fields should be
 160 filled out, however:
 161
 162   vid_mode:
 163         Please see the section on SPECIAL COMMAND LINE OPTIONS.
 164
 165   type_of_loader:
 166         If your boot loader has an assigned id (see table below), enter
 167         0xTV here, where T is an identifier for the boot loader and V is
 168         a version number.  Otherwise, enter 0xFF here.
 169
 170         Assigned boot loader ids:
 171         0  LILO
 172         1  Loadlin
 173         2  bootsect-loader
 174         3  SYSLINUX
 175         4  EtherBoot
 176         5  ELILO
 177         7  GRuB
 178         8  U-BOOT
 179
 180         Please contact <hpa@zytor.com> if you need a bootloader ID
 181         value assigned.
 182
 183   loadflags, heap_end_ptr:
 184         If the protocol version is 2.01 or higher, enter the
 185         offset limit of the setup heap into heap_end_ptr and set the
 186         0x80 bit (CAN_USE_HEAP) of loadflags.  heap_end_ptr appears to
 187         be relative to the start of setup (offset 0x0200).
 188
 189   setup_move_size:
 190         When using protocol 2.00 or 2.01, if the real mode
 191         kernel is not loaded at 0x90000, it gets moved there later in
 192         the loading sequence.  Fill in this field if you want
 193         additional data (such as the kernel command line) moved in
 194         addition to the real-mode kernel itself.
 195
 196   ramdisk_image, ramdisk_size:
 197         If your boot loader has loaded an initial ramdisk (initrd),
 198         set ramdisk_image to the 32-bit pointer to the ramdisk data
 199         and the ramdisk_size to the size of the ramdisk data.
 200
 201         The initrd should typically be located as high in memory as
 202         possible, as it may otherwise get overwritten by the early
 203         kernel initialization sequence.  However, it must never be
 204         located above the address specified in the initrd_addr_max
 205         field.  The initrd should be at least 4K page aligned.
 206
 207   cmd_line_ptr:
 208         If the protocol version is 2.02 or higher, this is a 32-bit
 209         pointer to the kernel command line.  The kernel command line
 210         can be located anywhere between the end of setup and 0xA0000.
 211         Fill in this field even if your boot loader does not support a
 212         command line, in which case you can point this to an empty
 213         string (or better yet, to the string "auto".)  If this field
 214         is left at zero, the kernel will assume that your boot loader
 215         does not support the 2.02+ protocol.
 216
 217   ramdisk_max:
 218         The maximum address that may be occupied by the initrd
 219         contents.  For boot protocols 2.02 or earlier, this field is
 220         not present, and the maximum address is 0x37FFFFFF.  (This
 221         address is defined as the address of the highest safe byte, so
 222         if your ramdisk is exactly 131072 bytes long and this field is
 223         0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
 224
 225
 226 **** THE KERNEL COMMAND LINE
 227
 228 The kernel command line has become an important way for the boot
 229 loader to communicate with the kernel.  Some of its options are also
 230 relevant to the boot loader itself, see "special command line options"
 231 below.
 232
 233 The kernel command line is a null-terminated string up to 255
 234 characters long, plus the final null.
 235
 236 If the boot protocol version is 2.02 or later, the address of the
 237 kernel command line is given by the header field cmd_line_ptr (see
 238 above.)
 239
 240 If the protocol version is *not* 2.02 or higher, the kernel
 241 command line is entered using the following protocol:
 242
 243         At offset 0x0020 (word), "cmd_line_magic", enter the magic
 244         number 0xA33F.
 245
 246         At offset 0x0022 (word), "cmd_line_offset", enter the offset
 247         of the kernel command line (relative to the start of the
 248         real-mode kernel).
 249
 250         The kernel command line *must* be within the memory region
 251         covered by setup_move_size, so you may need to adjust this
 252         field.
 253
 254
 255 **** SAMPLE BOOT CONFIGURATION
 256
 257 As a sample configuration, assume the following layout of the real
 258 mode segment:
 259
 260         0x0000-0x7FFF   Real mode kernel
 261         0x8000-0x8FFF   Stack and heap
 262         0x9000-0x90FF   Kernel command line
 263
 264 Such a boot loader should enter the following fields in the header:
 265
 266         unsigned long base_ptr; /* base address for real-mode segment */
 267
 268         if ( setup_sects == 0 ) {
 269                 setup_sects = 4;
 270         }
 271
 272         if ( protocol >= 0x0200 ) {
 273                 type_of_loader = <type code>;
 274                 if ( loading_initrd ) {
 275                         ramdisk_image = <initrd_address>;
 276                         ramdisk_size = <initrd_size>;
 277                 }
 278                 if ( protocol >= 0x0201 ) {
 279                         heap_end_ptr = 0x9000 - 0x200;
 280                         loadflags |= 0x80; /* CAN_USE_HEAP */
 281                 }
 282                 if ( protocol >= 0x0202 ) {
 283                         cmd_line_ptr = base_ptr + 0x9000;
 284                 } else {
 285                         cmd_line_magic  = 0xA33F;
 286                         cmd_line_offset = 0x9000;
 287                         setup_move_size = 0x9100;
 288                 }
 289         } else {
 290                 /* Very old kernel */
 291
 292                 cmd_line_magic  = 0xA33F;
 293                 cmd_line_offset = 0x9000;
 294
 295                 /* A very old kernel MUST have its real-mode code
 296                    loaded at 0x90000 */
 297
 298                 if ( base_ptr != 0x90000 ) {
 299                         /* Copy the real-mode kernel */
 300                         memcpy(0x90000, base_ptr, (setup_sects+1)*512);
 301                         /* Copy the command line */
 302                         memcpy(0x99000, base_ptr+0x9000, 256);
 303
 304                         base_ptr = 0x90000;              /* Relocated */
 305                 }
 306
 307                 /* It is recommended to clear memory up to the 32K mark */
 308                 memset(0x90000 + (setup_sects+1)*512, 0,
 309                        (64-(setup_sects+1))*512);
 310         }
 311
 312
 313 **** LOADING THE REST OF THE KERNEL
 314
 315 The non-real-mode kernel starts at offset (setup_sects+1)*512 in the
 316 kernel file (again, if setup_sects == 0 the real value is 4.)  It
 317 should be loaded at address 0x10000 for Image/zImage kernels and
 318 0x100000 for bzImage kernels.
 319
 320 The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
 321 bit (LOAD_HIGH) in the loadflags field is set:
 322
 323         is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
 324         load_address = is_bzImage ? 0x100000 : 0x10000;
 325
 326 Note that Image/zImage kernels can be up to 512K in size, and thus use
 327 the entire 0x10000-0x90000 range of memory.  This means it is pretty
 328 much a requirement for these kernels to load the real-mode part at
 329 0x90000.  bzImage kernels allow much more flexibility.
 330
 331
 332 **** SPECIAL COMMAND LINE OPTIONS
 333
 334 If the command line provided by the boot loader is entered by the
 335 user, the user may expect the following command line options to work.
 336 They should normally not be deleted from the kernel command line even
 337 though not all of them are actually meaningful to the kernel.  Boot
 338 loader authors who need additional command line options for the boot
 339 loader itself should get them registered in
 340 Documentation/kernel-parameters.txt to make sure they will not
 341 conflict with actual kernel options now or in the future.
 342
 343   vga=<mode>
 344         <mode> here is either an integer (in C notation, either
 345         decimal, octal, or hexadecimal) or one of the strings
 346         "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
 347         (meaning 0xFFFD).  This value should be entered into the
 348         vid_mode field, as it is used by the kernel before the command
 349         line is parsed.
 350
 351   mem=<size>
 352         <size> is an integer in C notation optionally followed by K, M
 353         or G (meaning << 10, << 20 or << 30).  This specifies the end
 354         of memory to the kernel. This affects the possible placement
 355         of an initrd, since an initrd should be placed near end of
 356         memory.  Note that this is an option to *both* the kernel and
 357         the bootloader!
 358
 359   initrd=<file>
 360         An initrd should be loaded.  The meaning of <file> is
 361         obviously bootloader-dependent, and some boot loaders
 362         (e.g. LILO) do not have such a command.
 363
 364 In addition, some boot loaders add the following options to the
 365 user-specified command line:
 366
 367   BOOT_IMAGE=<file>
 368         The boot image which was loaded.  Again, the meaning of <file>
 369         is obviously bootloader-dependent.
 370
 371   auto
 372         The kernel was booted without explicit user intervention.
 373
 374 If these options are added by the boot loader, it is highly
 375 recommended that they are located *first*, before the user-specified
 376 or configuration-specified command line.  Otherwise, "init=/bin/sh"
 377 gets confused by the "auto" option.
 378
 379
 380 **** RUNNING THE KERNEL
 381
 382 The kernel is started by jumping to the kernel entry point, which is
 383 located at *segment* offset 0x20 from the start of the real mode
 384 kernel.  This means that if you loaded your real-mode kernel code at
 385 0x90000, the kernel entry point is 9020:0000.
 386
 387 At entry, ds = es = ss should point to the start of the real-mode
 388 kernel code (0x9000 if the code is loaded at 0x90000), sp should be
 389 set up properly, normally pointing to the top of the heap, and
 390 interrupts should be disabled.  Furthermore, to guard against bugs in
 391 the kernel, it is recommended that the boot loader sets fs = gs = ds =
 392 es = ss.
 393
 394 In our example from above, we would do:
 395
 396         /* Note: in the case of the "old" kernel protocol, base_ptr must
 397            be == 0x90000 at this point; see the previous sample code */
 398
 399         seg = base_ptr >> 4;
 400
 401         cli();  /* Enter with interrupts disabled! */
 402
 403         /* Set up the real-mode kernel stack */
 404         _SS = seg;
 405         _SP = 0x9000;   /* Load SP immediately after loading SS! */
 406
 407         _DS = _ES = _FS = _GS = seg;
 408         jmp_far(seg+0x20, 0);   /* Run the kernel */
 409
 410 If your boot sector accesses a floppy drive, it is recommended to
 411 switch off the floppy motor before running the kernel, since the
 412 kernel boot leaves interrupts off and thus the motor will not be
 413 switched off, especially if the loaded kernel has the floppy driver as
 414 a demand-loaded module!
 415
 416
 417 **** ADVANCED BOOT TIME HOOKS
 418
 419 If the boot loader runs in a particularly hostile environment (such as
 420 LOADLIN, which runs under DOS) it may be impossible to follow the
 421 standard memory location requirements.  Such a boot loader may use the
 422 following hooks that, if set, are invoked by the kernel at the
 423 appropriate time.  The use of these hooks should probably be
 424 considered an absolutely last resort!
 425
 426 IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
 427 %edi across invocation.
 428
 429   realmode_swtch:
 430         A 16-bit real mode far subroutine invoked immediately before
 431         entering protected mode.  The default routine disables NMI, so
 432         your routine should probably do so, too.
 433
 434   code32_start:
 435         A 32-bit flat-mode routine *jumped* to immediately after the
 436         transition to protected mode, but before the kernel is
 437         uncompressed.  No segments, except CS, are set up; you should
 438         set them up to KERNEL_DS (0x18) yourself.
 439
 440         After completing your hook, you should jump to the address
 441         that was in this field before your boot loader overwrote it.