Documentation/i386/boot.txt

   1                      THE LINUX/I386 BOOT PROTOCOL
   2                      ----------------------------
   3
   4                     H. Peter Anvin <hpa@zytor.com>
   5                         Last update 2007-05-23
   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, the following 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         |  Command line          |
  56         |  Stack/heap            |      For use by the kernel real-mode code.
  57 098000  +------------------------+
  58         |  Kernel setup          |      The kernel real-mode code.
  59 090200  +------------------------+
  60         |  Kernel boot sector    |      The kernel legacy boot sector.
  61 090000  +------------------------+
  62         |  Protected-mode kernel |      The bulk of the kernel image.
  63 010000  +------------------------+
  64         |  Boot loader           |      <- Boot sector entry point 0000:7C00
  65 001000  +------------------------+
  66         |  Reserved for MBR/BIOS |
  67 000800  +------------------------+
  68         |  Typically used by MBR |
  69 000600  +------------------------+
  70         |  BIOS use only         |
  71 000000  +------------------------+
  72
  73
  74 When using bzImage, the protected-mode kernel was relocated to
  75 0x100000 ("high memory"), and the kernel real-mode block (boot sector,
  76 setup, and stack/heap) was made relocatable to any address between
  77 0x10000 and end of low memory. Unfortunately, in protocols 2.00 and
  78 2.01 the 0x90000+ memory range is still used internally by the 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 For a modern bzImage kernel with boot protocol version >= 2.02, a
  97 memory layout like the following is suggested:
  98
  99         ~                        ~
 100         |  Protected-mode kernel |
 101 100000  +------------------------+
 102         |  I/O memory hole       |
 103 0A0000  +------------------------+
 104         |  Reserved for BIOS     |      Leave as much as possible unused
 105         ~                        ~
 106         |  Command line          |      (Can also be below the X+10000 mark)
 107 X+10000 +------------------------+
 108         |  Stack/heap            |      For use by the kernel real-mode code.
 109 X+08000 +------------------------+
 110         |  Kernel setup          |      The kernel real-mode code.
 111         |  Kernel boot sector    |      The kernel legacy boot sector.
 112 X       +------------------------+
 113         |  Boot loader           |      <- Boot sector entry point 0000:7C00
 114 001000  +------------------------+
 115         |  Reserved for MBR/BIOS |
 116 000800  +------------------------+
 117         |  Typically used by MBR |
 118 000600  +------------------------+
 119         |  BIOS use only         |
 120 000000  +------------------------+
 121
 122 ... where the address X is as low as the design of the boot loader
 123 permits.
 124
 125
 126 **** THE REAL-MODE KERNEL HEADER
 127
 128 In the following text, and anywhere in the kernel boot sequence, "a
 129 sector" refers to 512 bytes.  It is independent of the actual sector
 130 size of the underlying medium.
 131
 132 The first step in loading a Linux kernel should be to load the
 133 real-mode code (boot sector and setup code) and then examine the
 134 following header at offset 0x01f1.  The real-mode code can total up to
 135 32K, although the boot loader may choose to load only the first two
 136 sectors (1K) and then examine the bootup sector size.
 137
 138 The header looks like:
 139
 140 Offset  Proto   Name            Meaning
 141 /Size
 142
 143 01F1/1  ALL(1   setup_sects     The size of the setup in sectors
 144 01F2/2  ALL     root_flags      If set, the root is mounted readonly
 145 01F4/4  2.04+(2 syssize         The size of the 32-bit code in 16-byte paras
 146 01F8/2  ALL     ram_size        DO NOT USE - for bootsect.S use only
 147 01FA/2  ALL     vid_mode        Video mode control
 148 01FC/2  ALL     root_dev        Default root device number
 149 01FE/2  ALL     boot_flag       0xAA55 magic number
 150 0200/2  2.00+   jump            Jump instruction
 151 0202/4  2.00+   header          Magic signature "HdrS"
 152 0206/2  2.00+   version         Boot protocol version supported
 153 0208/4  2.00+   realmode_swtch  Boot loader hook (see below)
 154 020C/2  2.00+   start_sys       The load-low segment (0x1000) (obsolete)
 155 020E/2  2.00+   kernel_version  Pointer to kernel version string
 156 0210/1  2.00+   type_of_loader  Boot loader identifier
 157 0211/1  2.00+   loadflags       Boot protocol option flags
 158 0212/2  2.00+   setup_move_size Move to high memory size (used with hooks)
 159 0214/4  2.00+   code32_start    Boot loader hook (see below)
 160 0218/4  2.00+   ramdisk_image   initrd load address (set by boot loader)
 161 021C/4  2.00+   ramdisk_size    initrd size (set by boot loader)
 162 0220/4  2.00+   bootsect_kludge DO NOT USE - for bootsect.S use only
 163 0224/2  2.01+   heap_end_ptr    Free memory after setup end
 164 0226/2  N/A     pad1            Unused
 165 0228/4  2.02+   cmd_line_ptr    32-bit pointer to the kernel command line
 166 022C/4  2.03+   initrd_addr_max Highest legal initrd address
 167 0230/4  2.05+   kernel_alignment Physical addr alignment required for kernel
 168 0234/1  2.05+   relocatable_kernel Whether kernel is relocatable or not
 169 0235/3  N/A     pad2            Unused
 170 0238/4  2.06+   cmdline_size    Maximum size of the kernel command line
 171
 172 (1) For backwards compatibility, if the setup_sects field contains 0, the
 173     real value is 4.
 174
 175 (2) For boot protocol prior to 2.04, the upper two bytes of the syssize
 176     field are unusable, which means the size of a bzImage kernel
 177     cannot be determined.
 178
 179 If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
 180 the boot protocol version is "old".  Loading an old kernel, the
 181 following parameters should be assumed:
 182
 183         Image type = zImage
 184         initrd not supported
 185         Real-mode kernel must be located at 0x90000.
 186
 187 Otherwise, the "version" field contains the protocol version,
 188 e.g. protocol version 2.01 will contain 0x0201 in this field.  When
 189 setting fields in the header, you must make sure only to set fields
 190 supported by the protocol version in use.
 191
 192
 193 **** DETAILS OF HEADER FIELDS
 194
 195 For each field, some are information from the kernel to the bootloader
 196 ("read"), some are expected to be filled out by the bootloader
 197 ("write"), and some are expected to be read and modified by the
 198 bootloader ("modify").
 199
 200 All general purpose boot loaders should write the fields marked
 201 (obligatory).  Boot loaders who want to load the kernel at a
 202 nonstandard address should fill in the fields marked (reloc); other
 203 boot loaders can ignore those fields.
 204
 205 The byte order of all fields is littleendian (this is x86, after all.)
 206
 207 Field name:     setup_secs
 208 Type:           read
 209 Offset/size:    0x1f1/1
 210 Protocol:       ALL
 211
 212   The size of the setup code in 512-byte sectors.  If this field is
 213   0, the real value is 4.  The real-mode code consists of the boot
 214   sector (always one 512-byte sector) plus the setup code.
 215
 216 Field name:      root_flags
 217 Type:            modify (optional)
 218 Offset/size:     0x1f2/2
 219 Protocol:        ALL
 220
 221   If this field is nonzero, the root defaults to readonly.  The use of
 222   this field is deprecated; use the "ro" or "rw" options on the
 223   command line instead.
 224
 225 Field name:     syssize
 226 Type:           read
 227 Offset/size:    0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
 228 Protocol:       2.04+
 229
 230   The size of the protected-mode code in units of 16-byte paragraphs.
 231   For protocol versions older than 2.04 this field is only two bytes
 232   wide, and therefore cannot be trusted for the size of a kernel if
 233   the LOAD_HIGH flag is set.
 234
 235 Field name:     ram_size
 236 Type:           kernel internal
 237 Offset/size:    0x1f8/2
 238 Protocol:       ALL
 239
 240   This field is obsolete.
 241
 242 Field name:     vid_mode
 243 Type:           modify (obligatory)
 244 Offset/size:    0x1fa/2
 245
 246   Please see the section on SPECIAL COMMAND LINE OPTIONS.
 247
 248 Field name:     root_dev
 249 Type:           modify (optional)
 250 Offset/size:    0x1fc/2
 251 Protocol:       ALL
 252
 253   The default root device device number.  The use of this field is
 254   deprecated, use the "root=" option on the command line instead.
 255
 256 Field name:     boot_flag
 257 Type:           read
 258 Offset/size:    0x1fe/2
 259 Protocol:       ALL
 260
 261   Contains 0xAA55.  This is the closest thing old Linux kernels have
 262   to a magic number.
 263
 264 Field name:     jump
 265 Type:           read
 266 Offset/size:    0x200/2
 267 Protocol:       2.00+
 268
 269   Contains an x86 jump instruction, 0xEB followed by a signed offset
 270   relative to byte 0x202.  This can be used to determine the size of
 271   the header.
 272
 273 Field name:     header
 274 Type:           read
 275 Offset/size:    0x202/4
 276 Protocol:       2.00+
 277
 278   Contains the magic number "HdrS" (0x53726448).
 279
 280 Field name:     version
 281 Type:           read
 282 Offset/size:    0x206/2
 283 Protocol:       2.00+
 284
 285   Contains the boot protocol version, in (major << 8)+minor format,
 286   e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
 287   10.17.
 288
 289 Field name:     readmode_swtch
 290 Type:           modify (optional)
 291 Offset/size:    0x208/4
 292 Protocol:       2.00+
 293
 294   Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
 295
 296 Field name:     start_sys
 297 Type:           read
 298 Offset/size:    0x20c/4
 299 Protocol:       2.00+
 300
 301   The load low segment (0x1000).  Obsolete.
 302
 303 Field name:     kernel_version
 304 Type:           read
 305 Offset/size:    0x20e/2
 306 Protocol:       2.00+
 307
 308   If set to a nonzero value, contains a pointer to a NUL-terminated
 309   human-readable kernel version number string, less 0x200.  This can
 310   be used to display the kernel version to the user.  This value
 311   should be less than (0x200*setup_sects).
 312
 313   For example, if this value is set to 0x1c00, the kernel version
 314   number string can be found at offset 0x1e00 in the kernel file.
 315   This is a valid value if and only if the "setup_sects" field
 316   contains the value 15 or higher, as:
 317
 318         0x1c00  < 15*0x200 (= 0x1e00) but
 319         0x1c00 >= 14*0x200 (= 0x1c00)
 320
 321         0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
 322
 323 Field name:     type_of_loader
 324 Type:           write (obligatory)
 325 Offset/size:    0x210/1
 326 Protocol:       2.00+
 327
 328   If your boot loader has an assigned id (see table below), enter
 329   0xTV here, where T is an identifier for the boot loader and V is
 330   a version number.  Otherwise, enter 0xFF here.
 331
 332   Assigned boot loader ids:
 333         0  LILO                 (0x00 reserved for pre-2.00 bootloader)
 334         1  Loadlin
 335         2  bootsect-loader      (0x20, all other values reserved)
 336         3  SYSLINUX
 337         4  EtherBoot
 338         5  ELILO
 339         7  GRuB
 340         8  U-BOOT
 341         9  Xen
 342         A  Gujin
 343         B  Qemu
 344
 345   Please contact <hpa@zytor.com> if you need a bootloader ID
 346   value assigned.
 347
 348 Field name:     loadflags
 349 Type:           modify (obligatory)
 350 Offset/size:    0x211/1
 351 Protocol:       2.00+
 352
 353   This field is a bitmask.
 354
 355   Bit 0 (read): LOADED_HIGH
 356         - If 0, the protected-mode code is loaded at 0x10000.
 357         - If 1, the protected-mode code is loaded at 0x100000.
 358
 359   Bit 7 (write): CAN_USE_HEAP
 360         Set this bit to 1 to indicate that the value entered in the
 361         heap_end_ptr is valid.  If this field is clear, some setup code
 362         functionality will be disabled.
 363
 364 Field name:     setup_move_size
 365 Type:           modify (obligatory)
 366 Offset/size:    0x212/2
 367 Protocol:       2.00-2.01
 368
 369   When using protocol 2.00 or 2.01, if the real mode kernel is not
 370   loaded at 0x90000, it gets moved there later in the loading
 371   sequence.  Fill in this field if you want additional data (such as
 372   the kernel command line) moved in addition to the real-mode kernel
 373   itself.
 374
 375   The unit is bytes starting with the beginning of the boot sector.
 376
 377   This field is can be ignored when the protocol is 2.02 or higher, or
 378   if the real-mode code is loaded at 0x90000.
 379
 380 Field name:     code32_start
 381 Type:           modify (optional, reloc)
 382 Offset/size:    0x214/4
 383 Protocol:       2.00+
 384
 385   The address to jump to in protected mode.  This defaults to the load
 386   address of the kernel, and can be used by the boot loader to
 387   determine the proper load address.
 388
 389   This field can be modified for two purposes:
 390
 391   1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
 392
 393   2. if a bootloader which does not install a hook loads a
 394      relocatable kernel at a nonstandard address it will have to modify
 395      this field to point to the load address.
 396
 397 Field name:     ramdisk_image
 398 Type:           write (obligatory)
 399 Offset/size:    0x218/4
 400 Protocol:       2.00+
 401
 402   The 32-bit linear address of the initial ramdisk or ramfs.  Leave at
 403   zero if there is no initial ramdisk/ramfs.
 404
 405 Field name:     ramdisk_size
 406 Type:           write (obligatory)
 407 Offset/size:    0x21c/4
 408 Protocol:       2.00+
 409
 410   Size of the initial ramdisk or ramfs.  Leave at zero if there is no
 411   initial ramdisk/ramfs.
 412
 413 Field name:     bootsect_kludge
 414 Type:           kernel internal
 415 Offset/size:    0x220/4
 416 Protocol:       2.00+
 417
 418   This field is obsolete.
 419
 420 Field name:     heap_end_ptr
 421 Type:           write (obligatory)
 422 Offset/size:    0x224/2
 423 Protocol:       2.01+
 424
 425   Set this field to the offset (from the beginning of the real-mode
 426   code) of the end of the setup stack/heap, minus 0x0200.
 427
 428 Field name:     cmd_line_ptr
 429 Type:           write (obligatory)
 430 Offset/size:    0x228/4
 431 Protocol:       2.02+
 432
 433   Set this field to the linear address of the kernel command line.
 434   The kernel command line can be located anywhere between the end of
 435   the setup heap and 0xA0000; it does not have to be located in the
 436   same 64K segment as the real-mode code itself.
 437
 438   Fill in this field even if your boot loader does not support a
 439   command line, in which case you can point this to an empty string
 440   (or better yet, to the string "auto".)  If this field is left at
 441   zero, the kernel will assume that your boot loader does not support
 442   the 2.02+ protocol.
 443
 444 Field name:     initrd_addr_max
 445 Type:           read
 446 Offset/size:    0x22c/4
 447 Protocol:       2.03+
 448
 449   The maximum address that may be occupied by the initial
 450   ramdisk/ramfs contents.  For boot protocols 2.02 or earlier, this
 451   field is not present, and the maximum address is 0x37FFFFFF.  (This
 452   address is defined as the address of the highest safe byte, so if
 453   your ramdisk is exactly 131072 bytes long and this field is
 454   0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
 455
 456 Field name:     kernel_alignment
 457 Type:           read (reloc)
 458 Offset/size:    0x230/4
 459 Protocol:       2.05+
 460
 461   Alignment unit required by the kernel (if relocatable_kernel is true.)
 462
 463 Field name:     relocatable_kernel
 464 Type:           read (reloc)
 465 Offset/size:    0x234/1
 466 Protocol:       2.05+
 467
 468   If this field is nonzero, the protected-mode part of the kernel can
 469   be loaded at any address that satisfies the kernel_alignment field.
 470   After loading, the boot loader must set the code32_start field to
 471   point to the loaded code, or to a boot loader hook.
 472
 473 Field name:     cmdline_size
 474 Type:           read
 475 Offset/size:    0x238/4
 476 Protocol:       2.06+
 477
 478   The maximum size of the command line without the terminating
 479   zero. This means that the command line can contain at most
 480   cmdline_size characters. With protocol version 2.05 and earlier, the
 481   maximum size was 255.
 482
 483
 484 **** THE KERNEL COMMAND LINE
 485
 486 The kernel command line has become an important way for the boot
 487 loader to communicate with the kernel.  Some of its options are also
 488 relevant to the boot loader itself, see "special command line options"
 489 below.
 490
 491 The kernel command line is a null-terminated string. The maximum
 492 length can be retrieved from the field cmdline_size.  Before protocol
 493 version 2.06, the maximum was 255 characters.  A string that is too
 494 long will be automatically truncated by the kernel.
 495
 496 If the boot protocol version is 2.02 or later, the address of the
 497 kernel command line is given by the header field cmd_line_ptr (see
 498 above.)  This address can be anywhere between the end of the setup
 499 heap and 0xA0000.
 500
 501 If the protocol version is *not* 2.02 or higher, the kernel
 502 command line is entered using the following protocol:
 503
 504         At offset 0x0020 (word), "cmd_line_magic", enter the magic
 505         number 0xA33F.
 506
 507         At offset 0x0022 (word), "cmd_line_offset", enter the offset
 508         of the kernel command line (relative to the start of the
 509         real-mode kernel).
 510
 511         The kernel command line *must* be within the memory region
 512         covered by setup_move_size, so you may need to adjust this
 513         field.
 514
 515
 516 **** MEMORY LAYOUT OF THE REAL-MODE CODE
 517
 518 The real-mode code requires a stack/heap to be set up, as well as
 519 memory allocated for the kernel command line.  This needs to be done
 520 in the real-mode accessible memory in bottom megabyte.
 521
 522 It should be noted that modern machines often have a sizable Extended
 523 BIOS Data Area (EBDA).  As a result, it is advisable to use as little
 524 of the low megabyte as possible.
 525
 526 Unfortunately, under the following circumstances the 0x90000 memory
 527 segment has to be used:
 528
 529         - When loading a zImage kernel ((loadflags & 0x01) == 0).
 530         - When loading a 2.01 or earlier boot protocol kernel.
 531
 532           -> For the 2.00 and 2.01 boot protocols, the real-mode code
 533              can be loaded at another address, but it is internally
 534              relocated to 0x90000.  For the "old" protocol, the
 535              real-mode code must be loaded at 0x90000.
 536
 537 When loading at 0x90000, avoid using memory above 0x9a000.
 538
 539 For boot protocol 2.02 or higher, the command line does not have to be
 540 located in the same 64K segment as the real-mode setup code; it is
 541 thus permitted to give the stack/heap the full 64K segment and locate
 542 the command line above it.
 543
 544 The kernel command line should not be located below the real-mode
 545 code, nor should it be located in high memory.
 546
 547
 548 **** SAMPLE BOOT CONFIGURATION
 549
 550 As a sample configuration, assume the following layout of the real
 551 mode segment:
 552
 553     When loading below 0x90000, use the entire segment:
 554
 555         0x0000-0x7fff   Real mode kernel
 556         0x8000-0xdfff   Stack and heap
 557         0xe000-0xffff   Kernel command line
 558
 559     When loading at 0x90000 OR the protocol version is 2.01 or earlier:
 560
 561         0x0000-0x7fff   Real mode kernel
 562         0x8000-0x97ff   Stack and heap
 563         0x9800-0x9fff   Kernel command line
 564
 565 Such a boot loader should enter the following fields in the header:
 566
 567         unsigned long base_ptr; /* base address for real-mode segment */
 568
 569         if ( setup_sects == 0 ) {
 570                 setup_sects = 4;
 571         }
 572
 573         if ( protocol >= 0x0200 ) {
 574                 type_of_loader = <type code>;
 575                 if ( loading_initrd ) {
 576                         ramdisk_image = <initrd_address>;
 577                         ramdisk_size = <initrd_size>;
 578                 }
 579
 580                 if ( protocol >= 0x0202 && loadflags & 0x01 )
 581                         heap_end = 0xe000;
 582                 else
 583                         heap_end = 0x9800;
 584
 585                 if ( protocol >= 0x0201 ) {
 586                         heap_end_ptr = heap_end - 0x200;
 587                         loadflags |= 0x80; /* CAN_USE_HEAP */
 588                 }
 589
 590                 if ( protocol >= 0x0202 ) {
 591                         cmd_line_ptr = base_ptr + heap_end;
 592                         strcpy(cmd_line_ptr, cmdline);
 593                 } else {
 594                         cmd_line_magic  = 0xA33F;
 595                         cmd_line_offset = heap_end;
 596                         setup_move_size = heap_end + strlen(cmdline)+1;
 597                         strcpy(base_ptr+cmd_line_offset, cmdline);
 598                 }
 599         } else {
 600                 /* Very old kernel */
 601
 602                 heap_end = 0x9800;
 603
 604                 cmd_line_magic  = 0xA33F;
 605                 cmd_line_offset = heap_end;
 606
 607                 /* A very old kernel MUST have its real-mode code
 608                    loaded at 0x90000 */
 609
 610                 if ( base_ptr != 0x90000 ) {
 611                         /* Copy the real-mode kernel */
 612                         memcpy(0x90000, base_ptr, (setup_sects+1)*512);
 613                         base_ptr = 0x90000;              /* Relocated */
 614                 }
 615
 616                 strcpy(0x90000+cmd_line_offset, cmdline);
 617
 618                 /* It is recommended to clear memory up to the 32K mark */
 619                 memset(0x90000 + (setup_sects+1)*512, 0,
 620                        (64-(setup_sects+1))*512);
 621         }
 622
 623
 624 **** LOADING THE REST OF THE KERNEL
 625
 626 The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
 627 in the kernel file (again, if setup_sects == 0 the real value is 4.)
 628 It should be loaded at address 0x10000 for Image/zImage kernels and
 629 0x100000 for bzImage kernels.
 630
 631 The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
 632 bit (LOAD_HIGH) in the loadflags field is set:
 633
 634         is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
 635         load_address = is_bzImage ? 0x100000 : 0x10000;
 636
 637 Note that Image/zImage kernels can be up to 512K in size, and thus use
 638 the entire 0x10000-0x90000 range of memory.  This means it is pretty
 639 much a requirement for these kernels to load the real-mode part at
 640 0x90000.  bzImage kernels allow much more flexibility.
 641
 642
 643 **** SPECIAL COMMAND LINE OPTIONS
 644
 645 If the command line provided by the boot loader is entered by the
 646 user, the user may expect the following command line options to work.
 647 They should normally not be deleted from the kernel command line even
 648 though not all of them are actually meaningful to the kernel.  Boot
 649 loader authors who need additional command line options for the boot
 650 loader itself should get them registered in
 651 Documentation/kernel-parameters.txt to make sure they will not
 652 conflict with actual kernel options now or in the future.
 653
 654   vga=<mode>
 655         <mode> here is either an integer (in C notation, either
 656         decimal, octal, or hexadecimal) or one of the strings
 657         "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
 658         (meaning 0xFFFD).  This value should be entered into the
 659         vid_mode field, as it is used by the kernel before the command
 660         line is parsed.
 661
 662   mem=<size>
 663         <size> is an integer in C notation optionally followed by
 664         (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
 665         << 30, << 40, << 50 or << 60).  This specifies the end of
 666         memory to the kernel. This affects the possible placement of
 667         an initrd, since an initrd should be placed near end of
 668         memory.  Note that this is an option to *both* the kernel and
 669         the bootloader!
 670
 671   initrd=<file>
 672         An initrd should be loaded.  The meaning of <file> is
 673         obviously bootloader-dependent, and some boot loaders
 674         (e.g. LILO) do not have such a command.
 675
 676 In addition, some boot loaders add the following options to the
 677 user-specified command line:
 678
 679   BOOT_IMAGE=<file>
 680         The boot image which was loaded.  Again, the meaning of <file>
 681         is obviously bootloader-dependent.
 682
 683   auto
 684         The kernel was booted without explicit user intervention.
 685
 686 If these options are added by the boot loader, it is highly
 687 recommended that they are located *first*, before the user-specified
 688 or configuration-specified command line.  Otherwise, "init=/bin/sh"
 689 gets confused by the "auto" option.
 690
 691
 692 **** RUNNING THE KERNEL
 693
 694 The kernel is started by jumping to the kernel entry point, which is
 695 located at *segment* offset 0x20 from the start of the real mode
 696 kernel.  This means that if you loaded your real-mode kernel code at
 697 0x90000, the kernel entry point is 9020:0000.
 698
 699 At entry, ds = es = ss should point to the start of the real-mode
 700 kernel code (0x9000 if the code is loaded at 0x90000), sp should be
 701 set up properly, normally pointing to the top of the heap, and
 702 interrupts should be disabled.  Furthermore, to guard against bugs in
 703 the kernel, it is recommended that the boot loader sets fs = gs = ds =
 704 es = ss.
 705
 706 In our example from above, we would do:
 707
 708         /* Note: in the case of the "old" kernel protocol, base_ptr must
 709            be == 0x90000 at this point; see the previous sample code */
 710
 711         seg = base_ptr >> 4;
 712
 713         cli();  /* Enter with interrupts disabled! */
 714
 715         /* Set up the real-mode kernel stack */
 716         _SS = seg;
 717         _SP = heap_end;
 718
 719         _DS = _ES = _FS = _GS = seg;
 720         jmp_far(seg+0x20, 0);   /* Run the kernel */
 721
 722 If your boot sector accesses a floppy drive, it is recommended to
 723 switch off the floppy motor before running the kernel, since the
 724 kernel boot leaves interrupts off and thus the motor will not be
 725 switched off, especially if the loaded kernel has the floppy driver as
 726 a demand-loaded module!
 727
 728
 729 **** ADVANCED BOOT LOADER HOOKS
 730
 731 If the boot loader runs in a particularly hostile environment (such as
 732 LOADLIN, which runs under DOS) it may be impossible to follow the
 733 standard memory location requirements.  Such a boot loader may use the
 734 following hooks that, if set, are invoked by the kernel at the
 735 appropriate time.  The use of these hooks should probably be
 736 considered an absolutely last resort!
 737
 738 IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
 739 %edi across invocation.
 740
 741   realmode_swtch:
 742         A 16-bit real mode far subroutine invoked immediately before
 743         entering protected mode.  The default routine disables NMI, so
 744         your routine should probably do so, too.
 745
 746   code32_start:
 747         A 32-bit flat-mode routine *jumped* to immediately after the
 748         transition to protected mode, but before the kernel is
 749         uncompressed.  No segments, except CS, are guaranteed to be
 750         set up (current kernels do, but older ones do not); you should
 751         set them up to BOOT_DS (0x18) yourself.
 752
 753         After completing your hook, you should jump to the address
 754         that was in this field before your boot loader overwrote it
 755         (relocated, if appropriate.)