linux/Documentation/x86/boot.txt
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   1                     THE LINUX/x86 BOOT PROTOCOL
   2                     ---------------------------
   3
   4On the x86 platform, the Linux kernel uses a rather complicated boot
   5convention.  This has evolved partially due to historical aspects, as
   6well as the desire in the early days to have the kernel itself be a
   7bootable image, the complicated PC memory model and due to changed
   8expectations in the PC industry caused by the effective demise of
   9real-mode DOS as a mainstream operating system.
  10
  11Currently, the following versions of the Linux/x86 boot protocol exist.
  12
  13Old kernels:    zImage/Image support only.  Some very early kernels
  14                may not even support a command line.
  15
  16Protocol 2.00:  (Kernel 1.3.73) Added bzImage and initrd support, as
  17                well as a formalized way to communicate between the
  18                boot loader and the kernel.  setup.S made relocatable,
  19                although the traditional setup area still assumed
  20                writable.
  21
  22Protocol 2.01:  (Kernel 1.3.76) Added a heap overrun warning.
  23
  24Protocol 2.02:  (Kernel 2.4.0-test3-pre3) New command line protocol.
  25                Lower the conventional memory ceiling.  No overwrite
  26                of the traditional setup area, thus making booting
  27                safe for systems which use the EBDA from SMM or 32-bit
  28                BIOS entry points.  zImage deprecated but still
  29                supported.
  30
  31Protocol 2.03:  (Kernel 2.4.18-pre1) Explicitly makes the highest possible
  32                initrd address available to the bootloader.
  33
  34Protocol 2.04:  (Kernel 2.6.14) Extend the syssize field to four bytes.
  35
  36Protocol 2.05:  (Kernel 2.6.20) Make protected mode kernel relocatable.
  37                Introduce relocatable_kernel and kernel_alignment fields.
  38
  39Protocol 2.06:  (Kernel 2.6.22) Added a field that contains the size of
  40                the boot command line.
  41
  42Protocol 2.07:  (Kernel 2.6.24) Added paravirtualised boot protocol.
  43                Introduced hardware_subarch and hardware_subarch_data
  44                and KEEP_SEGMENTS flag in load_flags.
  45
  46Protocol 2.08:  (Kernel 2.6.26) Added crc32 checksum and ELF format
  47                payload. Introduced payload_offset and payload_length
  48                fields to aid in locating the payload.
  49
  50Protocol 2.09:  (Kernel 2.6.26) Added a field of 64-bit physical
  51                pointer to single linked list of struct setup_data.
  52
  53Protocol 2.10:  (Kernel 2.6.31) Added a protocol for relaxed alignment
  54                beyond the kernel_alignment added, new init_size and
  55                pref_address fields.  Added extended boot loader IDs.
  56
  57**** MEMORY LAYOUT
  58
  59The traditional memory map for the kernel loader, used for Image or
  60zImage kernels, typically looks like:
  61
  62        |                        |
  630A0000  +------------------------+
  64        |  Reserved for BIOS     |      Do not use.  Reserved for BIOS EBDA.
  6509A000  +------------------------+
  66        |  Command line          |
  67        |  Stack/heap            |      For use by the kernel real-mode code.
  68098000  +------------------------+      
  69        |  Kernel setup          |      The kernel real-mode code.
  70090200  +------------------------+
  71        |  Kernel boot sector    |      The kernel legacy boot sector.
  72090000  +------------------------+
  73        |  Protected-mode kernel |      The bulk of the kernel image.
  74010000  +------------------------+
  75        |  Boot loader           |      <- Boot sector entry point 0000:7C00
  76001000  +------------------------+
  77        |  Reserved for MBR/BIOS |
  78000800  +------------------------+
  79        |  Typically used by MBR |
  80000600  +------------------------+ 
  81        |  BIOS use only         |
  82000000  +------------------------+
  83
  84
  85When using bzImage, the protected-mode kernel was relocated to
  860x100000 ("high memory"), and the kernel real-mode block (boot sector,
  87setup, and stack/heap) was made relocatable to any address between
  880x10000 and end of low memory. Unfortunately, in protocols 2.00 and
  892.01 the 0x90000+ memory range is still used internally by the kernel;
  90the 2.02 protocol resolves that problem.
  91
  92It is desirable to keep the "memory ceiling" -- the highest point in
  93low memory touched by the boot loader -- as low as possible, since
  94some newer BIOSes have begun to allocate some rather large amounts of
  95memory, called the Extended BIOS Data Area, near the top of low
  96memory.  The boot loader should use the "INT 12h" BIOS call to verify
  97how much low memory is available.
  98
  99Unfortunately, if INT 12h reports that the amount of memory is too
 100low, there is usually nothing the boot loader can do but to report an
 101error to the user.  The boot loader should therefore be designed to
 102take up as little space in low memory as it reasonably can.  For
 103zImage or old bzImage kernels, which need data written into the
 1040x90000 segment, the boot loader should make sure not to use memory
 105above the 0x9A000 point; too many BIOSes will break above that point.
 106
 107For a modern bzImage kernel with boot protocol version >= 2.02, a
 108memory layout like the following is suggested:
 109
 110        ~                        ~
 111        |  Protected-mode kernel |
 112100000  +------------------------+
 113        |  I/O memory hole       |
 1140A0000  +------------------------+
 115        |  Reserved for BIOS     |      Leave as much as possible unused
 116        ~                        ~
 117        |  Command line          |      (Can also be below the X+10000 mark)
 118X+10000 +------------------------+
 119        |  Stack/heap            |      For use by the kernel real-mode code.
 120X+08000 +------------------------+      
 121        |  Kernel setup          |      The kernel real-mode code.
 122        |  Kernel boot sector    |      The kernel legacy boot sector.
 123X       +------------------------+
 124        |  Boot loader           |      <- Boot sector entry point 0000:7C00
 125001000  +------------------------+
 126        |  Reserved for MBR/BIOS |
 127000800  +------------------------+
 128        |  Typically used by MBR |
 129000600  +------------------------+ 
 130        |  BIOS use only         |
 131000000  +------------------------+
 132
 133... where the address X is as low as the design of the boot loader
 134permits.
 135
 136
 137**** THE REAL-MODE KERNEL HEADER
 138
 139In the following text, and anywhere in the kernel boot sequence, "a
 140sector" refers to 512 bytes.  It is independent of the actual sector
 141size of the underlying medium.
 142
 143The first step in loading a Linux kernel should be to load the
 144real-mode code (boot sector and setup code) and then examine the
 145following header at offset 0x01f1.  The real-mode code can total up to
 14632K, although the boot loader may choose to load only the first two
 147sectors (1K) and then examine the bootup sector size.
 148
 149The header looks like:
 150
 151Offset  Proto   Name            Meaning
 152/Size
 153
 15401F1/1  ALL(1   setup_sects     The size of the setup in sectors
 15501F2/2  ALL     root_flags      If set, the root is mounted readonly
 15601F4/4  2.04+(2 syssize         The size of the 32-bit code in 16-byte paras
 15701F8/2  ALL     ram_size        DO NOT USE - for bootsect.S use only
 15801FA/2  ALL     vid_mode        Video mode control
 15901FC/2  ALL     root_dev        Default root device number
 16001FE/2  ALL     boot_flag       0xAA55 magic number
 1610200/2  2.00+   jump            Jump instruction
 1620202/4  2.00+   header          Magic signature "HdrS"
 1630206/2  2.00+   version         Boot protocol version supported
 1640208/4  2.00+   realmode_swtch  Boot loader hook (see below)
 165020C/2  2.00+   start_sys_seg   The load-low segment (0x1000) (obsolete)
 166020E/2  2.00+   kernel_version  Pointer to kernel version string
 1670210/1  2.00+   type_of_loader  Boot loader identifier
 1680211/1  2.00+   loadflags       Boot protocol option flags
 1690212/2  2.00+   setup_move_size Move to high memory size (used with hooks)
 1700214/4  2.00+   code32_start    Boot loader hook (see below)
 1710218/4  2.00+   ramdisk_image   initrd load address (set by boot loader)
 172021C/4  2.00+   ramdisk_size    initrd size (set by boot loader)
 1730220/4  2.00+   bootsect_kludge DO NOT USE - for bootsect.S use only
 1740224/2  2.01+   heap_end_ptr    Free memory after setup end
 1750226/1  2.02+(3 ext_loader_ver  Extended boot loader version
 1760227/1  2.02+(3 ext_loader_type Extended boot loader ID
 1770228/4  2.02+   cmd_line_ptr    32-bit pointer to the kernel command line
 178022C/4  2.03+   ramdisk_max     Highest legal initrd address
 1790230/4  2.05+   kernel_alignment Physical addr alignment required for kernel
 1800234/1  2.05+   relocatable_kernel Whether kernel is relocatable or not
 1810235/1  2.10+   min_alignment   Minimum alignment, as a power of two
 1820236/2  N/A     pad3            Unused
 1830238/4  2.06+   cmdline_size    Maximum size of the kernel command line
 184023C/4  2.07+   hardware_subarch Hardware subarchitecture
 1850240/8  2.07+   hardware_subarch_data Subarchitecture-specific data
 1860248/4  2.08+   payload_offset  Offset of kernel payload
 187024C/4  2.08+   payload_length  Length of kernel payload
 1880250/8  2.09+   setup_data      64-bit physical pointer to linked list
 189                                of struct setup_data
 1900258/8  2.10+   pref_address    Preferred loading address
 1910260/4  2.10+   init_size       Linear memory required during initialization
 192
 193(1) For backwards compatibility, if the setup_sects field contains 0, the
 194    real value is 4.
 195
 196(2) For boot protocol prior to 2.04, the upper two bytes of the syssize
 197    field are unusable, which means the size of a bzImage kernel
 198    cannot be determined.
 199
 200(3) Ignored, but safe to set, for boot protocols 2.02-2.09.
 201
 202If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
 203the boot protocol version is "old".  Loading an old kernel, the
 204following parameters should be assumed:
 205
 206        Image type = zImage
 207        initrd not supported
 208        Real-mode kernel must be located at 0x90000.
 209
 210Otherwise, the "version" field contains the protocol version,
 211e.g. protocol version 2.01 will contain 0x0201 in this field.  When
 212setting fields in the header, you must make sure only to set fields
 213supported by the protocol version in use.
 214
 215
 216**** DETAILS OF HEADER FIELDS
 217
 218For each field, some are information from the kernel to the bootloader
 219("read"), some are expected to be filled out by the bootloader
 220("write"), and some are expected to be read and modified by the
 221bootloader ("modify").
 222
 223All general purpose boot loaders should write the fields marked
 224(obligatory).  Boot loaders who want to load the kernel at a
 225nonstandard address should fill in the fields marked (reloc); other
 226boot loaders can ignore those fields.
 227
 228The byte order of all fields is littleendian (this is x86, after all.)
 229
 230Field name:     setup_sects
 231Type:           read
 232Offset/size:    0x1f1/1
 233Protocol:       ALL
 234
 235  The size of the setup code in 512-byte sectors.  If this field is
 236  0, the real value is 4.  The real-mode code consists of the boot
 237  sector (always one 512-byte sector) plus the setup code.
 238
 239Field name:      root_flags
 240Type:            modify (optional)
 241Offset/size:     0x1f2/2
 242Protocol:        ALL
 243
 244  If this field is nonzero, the root defaults to readonly.  The use of
 245  this field is deprecated; use the "ro" or "rw" options on the
 246  command line instead.
 247
 248Field name:     syssize
 249Type:           read
 250Offset/size:    0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
 251Protocol:       2.04+
 252
 253  The size of the protected-mode code in units of 16-byte paragraphs.
 254  For protocol versions older than 2.04 this field is only two bytes
 255  wide, and therefore cannot be trusted for the size of a kernel if
 256  the LOAD_HIGH flag is set.
 257
 258Field name:     ram_size
 259Type:           kernel internal
 260Offset/size:    0x1f8/2
 261Protocol:       ALL
 262
 263  This field is obsolete.
 264
 265Field name:     vid_mode
 266Type:           modify (obligatory)
 267Offset/size:    0x1fa/2
 268
 269  Please see the section on SPECIAL COMMAND LINE OPTIONS.
 270
 271Field name:     root_dev
 272Type:           modify (optional)
 273Offset/size:    0x1fc/2
 274Protocol:       ALL
 275
 276  The default root device device number.  The use of this field is
 277  deprecated, use the "root=" option on the command line instead.
 278
 279Field name:     boot_flag
 280Type:           read
 281Offset/size:    0x1fe/2
 282Protocol:       ALL
 283
 284  Contains 0xAA55.  This is the closest thing old Linux kernels have
 285  to a magic number.
 286
 287Field name:     jump
 288Type:           read
 289Offset/size:    0x200/2
 290Protocol:       2.00+
 291
 292  Contains an x86 jump instruction, 0xEB followed by a signed offset
 293  relative to byte 0x202.  This can be used to determine the size of
 294  the header.
 295
 296Field name:     header
 297Type:           read
 298Offset/size:    0x202/4
 299Protocol:       2.00+
 300
 301  Contains the magic number "HdrS" (0x53726448).
 302
 303Field name:     version
 304Type:           read
 305Offset/size:    0x206/2
 306Protocol:       2.00+
 307
 308  Contains the boot protocol version, in (major << 8)+minor format,
 309  e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
 310  10.17.
 311
 312Field name:     realmode_swtch
 313Type:           modify (optional)
 314Offset/size:    0x208/4
 315Protocol:       2.00+
 316
 317  Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
 318
 319Field name:     start_sys_seg
 320Type:           read
 321Offset/size:    0x20c/2
 322Protocol:       2.00+
 323
 324  The load low segment (0x1000).  Obsolete.
 325
 326Field name:     kernel_version
 327Type:           read
 328Offset/size:    0x20e/2
 329Protocol:       2.00+
 330
 331  If set to a nonzero value, contains a pointer to a NUL-terminated
 332  human-readable kernel version number string, less 0x200.  This can
 333  be used to display the kernel version to the user.  This value
 334  should be less than (0x200*setup_sects).
 335
 336  For example, if this value is set to 0x1c00, the kernel version
 337  number string can be found at offset 0x1e00 in the kernel file.
 338  This is a valid value if and only if the "setup_sects" field
 339  contains the value 15 or higher, as:
 340
 341        0x1c00  < 15*0x200 (= 0x1e00) but
 342        0x1c00 >= 14*0x200 (= 0x1c00)
 343
 344        0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
 345
 346Field name:     type_of_loader
 347Type:           write (obligatory)
 348Offset/size:    0x210/1
 349Protocol:       2.00+
 350
 351  If your boot loader has an assigned id (see table below), enter
 352  0xTV here, where T is an identifier for the boot loader and V is
 353  a version number.  Otherwise, enter 0xFF here.
 354
 355  For boot loader IDs above T = 0xD, write T = 0xE to this field and
 356  write the extended ID minus 0x10 to the ext_loader_type field.
 357  Similarly, the ext_loader_ver field can be used to provide more than
 358  four bits for the bootloader version.
 359
 360  For example, for T = 0x15, V = 0x234, write:
 361
 362  type_of_loader  <- 0xE4
 363  ext_loader_type <- 0x05
 364  ext_loader_ver  <- 0x23
 365
 366  Assigned boot loader ids:
 367        0  LILO                 (0x00 reserved for pre-2.00 bootloader)
 368        1  Loadlin
 369        2  bootsect-loader      (0x20, all other values reserved)
 370        3  Syslinux
 371        4  Etherboot/gPXE
 372        5  ELILO
 373        7  GRUB
 374        8  U-Boot
 375        9  Xen
 376        A  Gujin
 377        B  Qemu
 378        C  Arcturus Networks uCbootloader
 379        E  Extended             (see ext_loader_type)
 380        F  Special              (0xFF = undefined)
 381
 382  Please contact <hpa@zytor.com> if you need a bootloader ID
 383  value assigned.
 384
 385Field name:     loadflags
 386Type:           modify (obligatory)
 387Offset/size:    0x211/1
 388Protocol:       2.00+
 389
 390  This field is a bitmask.
 391
 392  Bit 0 (read): LOADED_HIGH
 393        - If 0, the protected-mode code is loaded at 0x10000.
 394        - If 1, the protected-mode code is loaded at 0x100000.
 395
 396  Bit 5 (write): QUIET_FLAG
 397        - If 0, print early messages.
 398        - If 1, suppress early messages.
 399                This requests to the kernel (decompressor and early
 400                kernel) to not write early messages that require
 401                accessing the display hardware directly.
 402
 403  Bit 6 (write): KEEP_SEGMENTS
 404        Protocol: 2.07+
 405        - If 0, reload the segment registers in the 32bit entry point.
 406        - If 1, do not reload the segment registers in the 32bit entry point.
 407                Assume that %cs %ds %ss %es are all set to flat segments with
 408                a base of 0 (or the equivalent for their environment).
 409
 410  Bit 7 (write): CAN_USE_HEAP
 411        Set this bit to 1 to indicate that the value entered in the
 412        heap_end_ptr is valid.  If this field is clear, some setup code
 413        functionality will be disabled.
 414
 415Field name:     setup_move_size
 416Type:           modify (obligatory)
 417Offset/size:    0x212/2
 418Protocol:       2.00-2.01
 419
 420  When using protocol 2.00 or 2.01, if the real mode kernel is not
 421  loaded at 0x90000, it gets moved there later in the loading
 422  sequence.  Fill in this field if you want additional data (such as
 423  the kernel command line) moved in addition to the real-mode kernel
 424  itself.
 425
 426  The unit is bytes starting with the beginning of the boot sector.
 427  
 428  This field is can be ignored when the protocol is 2.02 or higher, or
 429  if the real-mode code is loaded at 0x90000.
 430
 431Field name:     code32_start
 432Type:           modify (optional, reloc)
 433Offset/size:    0x214/4
 434Protocol:       2.00+
 435
 436  The address to jump to in protected mode.  This defaults to the load
 437  address of the kernel, and can be used by the boot loader to
 438  determine the proper load address.
 439
 440  This field can be modified for two purposes:
 441
 442  1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
 443
 444  2. if a bootloader which does not install a hook loads a
 445     relocatable kernel at a nonstandard address it will have to modify
 446     this field to point to the load address.
 447
 448Field name:     ramdisk_image
 449Type:           write (obligatory)
 450Offset/size:    0x218/4
 451Protocol:       2.00+
 452
 453  The 32-bit linear address of the initial ramdisk or ramfs.  Leave at
 454  zero if there is no initial ramdisk/ramfs.
 455
 456Field name:     ramdisk_size
 457Type:           write (obligatory)
 458Offset/size:    0x21c/4
 459Protocol:       2.00+
 460
 461  Size of the initial ramdisk or ramfs.  Leave at zero if there is no
 462  initial ramdisk/ramfs.
 463
 464Field name:     bootsect_kludge
 465Type:           kernel internal
 466Offset/size:    0x220/4
 467Protocol:       2.00+
 468
 469  This field is obsolete.
 470
 471Field name:     heap_end_ptr
 472Type:           write (obligatory)
 473Offset/size:    0x224/2
 474Protocol:       2.01+
 475
 476  Set this field to the offset (from the beginning of the real-mode
 477  code) of the end of the setup stack/heap, minus 0x0200.
 478
 479Field name:     ext_loader_ver
 480Type:           write (optional)
 481Offset/size:    0x226/1
 482Protocol:       2.02+
 483
 484  This field is used as an extension of the version number in the
 485  type_of_loader field.  The total version number is considered to be
 486  (type_of_loader & 0x0f) + (ext_loader_ver << 4).
 487
 488  The use of this field is boot loader specific.  If not written, it
 489  is zero.
 490
 491  Kernels prior to 2.6.31 did not recognize this field, but it is safe
 492  to write for protocol version 2.02 or higher.
 493
 494Field name:     ext_loader_type
 495Type:           write (obligatory if (type_of_loader & 0xf0) == 0xe0)
 496Offset/size:    0x227/1
 497Protocol:       2.02+
 498
 499  This field is used as an extension of the type number in
 500  type_of_loader field.  If the type in type_of_loader is 0xE, then
 501  the actual type is (ext_loader_type + 0x10).
 502
 503  This field is ignored if the type in type_of_loader is not 0xE.
 504
 505  Kernels prior to 2.6.31 did not recognize this field, but it is safe
 506  to write for protocol version 2.02 or higher.
 507
 508Field name:     cmd_line_ptr
 509Type:           write (obligatory)
 510Offset/size:    0x228/4
 511Protocol:       2.02+
 512
 513  Set this field to the linear address of the kernel command line.
 514  The kernel command line can be located anywhere between the end of
 515  the setup heap and 0xA0000; it does not have to be located in the
 516  same 64K segment as the real-mode code itself.
 517
 518  Fill in this field even if your boot loader does not support a
 519  command line, in which case you can point this to an empty string
 520  (or better yet, to the string "auto".)  If this field is left at
 521  zero, the kernel will assume that your boot loader does not support
 522  the 2.02+ protocol.
 523
 524Field name:     ramdisk_max
 525Type:           read
 526Offset/size:    0x22c/4
 527Protocol:       2.03+
 528
 529  The maximum address that may be occupied by the initial
 530  ramdisk/ramfs contents.  For boot protocols 2.02 or earlier, this
 531  field is not present, and the maximum address is 0x37FFFFFF.  (This
 532  address is defined as the address of the highest safe byte, so if
 533  your ramdisk is exactly 131072 bytes long and this field is
 534  0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
 535
 536Field name:     kernel_alignment
 537Type:           read/modify (reloc)
 538Offset/size:    0x230/4
 539Protocol:       2.05+ (read), 2.10+ (modify)
 540
 541  Alignment unit required by the kernel (if relocatable_kernel is
 542  true.)  A relocatable kernel that is loaded at an alignment
 543  incompatible with the value in this field will be realigned during
 544  kernel initialization.
 545
 546  Starting with protocol version 2.10, this reflects the kernel
 547  alignment preferred for optimal performance; it is possible for the
 548  loader to modify this field to permit a lesser alignment.  See the
 549  min_alignment and pref_address field below.
 550
 551Field name:     relocatable_kernel
 552Type:           read (reloc)
 553Offset/size:    0x234/1
 554Protocol:       2.05+
 555
 556  If this field is nonzero, the protected-mode part of the kernel can
 557  be loaded at any address that satisfies the kernel_alignment field.
 558  After loading, the boot loader must set the code32_start field to
 559  point to the loaded code, or to a boot loader hook.
 560
 561Field name:     min_alignment
 562Type:           read (reloc)
 563Offset/size:    0x235/1
 564Protocol:       2.10+
 565
 566  This field, if nonzero, indicates as a power of two the minimum
 567  alignment required, as opposed to preferred, by the kernel to boot.
 568  If a boot loader makes use of this field, it should update the
 569  kernel_alignment field with the alignment unit desired; typically:
 570
 571        kernel_alignment = 1 << min_alignment
 572
 573  There may be a considerable performance cost with an excessively
 574  misaligned kernel.  Therefore, a loader should typically try each
 575  power-of-two alignment from kernel_alignment down to this alignment.
 576
 577Field name:     cmdline_size
 578Type:           read
 579Offset/size:    0x238/4
 580Protocol:       2.06+
 581
 582  The maximum size of the command line without the terminating
 583  zero. This means that the command line can contain at most
 584  cmdline_size characters. With protocol version 2.05 and earlier, the
 585  maximum size was 255.
 586
 587Field name:     hardware_subarch
 588Type:           write (optional, defaults to x86/PC)
 589Offset/size:    0x23c/4
 590Protocol:       2.07+
 591
 592  In a paravirtualized environment the hardware low level architectural
 593  pieces such as interrupt handling, page table handling, and
 594  accessing process control registers needs to be done differently.
 595
 596  This field allows the bootloader to inform the kernel we are in one
 597  one of those environments.
 598
 599  0x00000000    The default x86/PC environment
 600  0x00000001    lguest
 601  0x00000002    Xen
 602  0x00000003    Moorestown MID
 603  0x00000004    CE4100 TV Platform
 604
 605Field name:     hardware_subarch_data
 606Type:           write (subarch-dependent)
 607Offset/size:    0x240/8
 608Protocol:       2.07+
 609
 610  A pointer to data that is specific to hardware subarch
 611  This field is currently unused for the default x86/PC environment,
 612  do not modify.
 613
 614Field name:     payload_offset
 615Type:           read
 616Offset/size:    0x248/4
 617Protocol:       2.08+
 618
 619  If non-zero then this field contains the offset from the beginning
 620  of the protected-mode code to the payload.
 621
 622  The payload may be compressed. The format of both the compressed and
 623  uncompressed data should be determined using the standard magic
 624  numbers.  The currently supported compression formats are gzip
 625  (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA
 626  (magic number 5D 00), and XZ (magic number FD 37).  The uncompressed
 627  payload is currently always ELF (magic number 7F 45 4C 46).
 628  
 629Field name:     payload_length
 630Type:           read
 631Offset/size:    0x24c/4
 632Protocol:       2.08+
 633
 634  The length of the payload.
 635
 636Field name:     setup_data
 637Type:           write (special)
 638Offset/size:    0x250/8
 639Protocol:       2.09+
 640
 641  The 64-bit physical pointer to NULL terminated single linked list of
 642  struct setup_data. This is used to define a more extensible boot
 643  parameters passing mechanism. The definition of struct setup_data is
 644  as follow:
 645
 646  struct setup_data {
 647          u64 next;
 648          u32 type;
 649          u32 len;
 650          u8  data[0];
 651  };
 652
 653  Where, the next is a 64-bit physical pointer to the next node of
 654  linked list, the next field of the last node is 0; the type is used
 655  to identify the contents of data; the len is the length of data
 656  field; the data holds the real payload.
 657
 658  This list may be modified at a number of points during the bootup
 659  process.  Therefore, when modifying this list one should always make
 660  sure to consider the case where the linked list already contains
 661  entries.
 662
 663Field name:     pref_address
 664Type:           read (reloc)
 665Offset/size:    0x258/8
 666Protocol:       2.10+
 667
 668  This field, if nonzero, represents a preferred load address for the
 669  kernel.  A relocating bootloader should attempt to load at this
 670  address if possible.
 671
 672  A non-relocatable kernel will unconditionally move itself and to run
 673  at this address.
 674
 675Field name:     init_size
 676Type:           read
 677Offset/size:    0x25c/4
 678
 679  This field indicates the amount of linear contiguous memory starting
 680  at the kernel runtime start address that the kernel needs before it
 681  is capable of examining its memory map.  This is not the same thing
 682  as the total amount of memory the kernel needs to boot, but it can
 683  be used by a relocating boot loader to help select a safe load
 684  address for the kernel.
 685
 686  The kernel runtime start address is determined by the following algorithm:
 687
 688  if (relocatable_kernel)
 689        runtime_start = align_up(load_address, kernel_alignment)
 690  else
 691        runtime_start = pref_address
 692
 693
 694**** THE IMAGE CHECKSUM
 695
 696From boot protocol version 2.08 onwards the CRC-32 is calculated over
 697the entire file using the characteristic polynomial 0x04C11DB7 and an
 698initial remainder of 0xffffffff.  The checksum is appended to the
 699file; therefore the CRC of the file up to the limit specified in the
 700syssize field of the header is always 0.
 701
 702
 703**** THE KERNEL COMMAND LINE
 704
 705The kernel command line has become an important way for the boot
 706loader to communicate with the kernel.  Some of its options are also
 707relevant to the boot loader itself, see "special command line options"
 708below.
 709
 710The kernel command line is a null-terminated string. The maximum
 711length can be retrieved from the field cmdline_size.  Before protocol
 712version 2.06, the maximum was 255 characters.  A string that is too
 713long will be automatically truncated by the kernel.
 714
 715If the boot protocol version is 2.02 or later, the address of the
 716kernel command line is given by the header field cmd_line_ptr (see
 717above.)  This address can be anywhere between the end of the setup
 718heap and 0xA0000.
 719
 720If the protocol version is *not* 2.02 or higher, the kernel
 721command line is entered using the following protocol:
 722
 723        At offset 0x0020 (word), "cmd_line_magic", enter the magic
 724        number 0xA33F.
 725
 726        At offset 0x0022 (word), "cmd_line_offset", enter the offset
 727        of the kernel command line (relative to the start of the
 728        real-mode kernel).
 729        
 730        The kernel command line *must* be within the memory region
 731        covered by setup_move_size, so you may need to adjust this
 732        field.
 733
 734
 735**** MEMORY LAYOUT OF THE REAL-MODE CODE
 736
 737The real-mode code requires a stack/heap to be set up, as well as
 738memory allocated for the kernel command line.  This needs to be done
 739in the real-mode accessible memory in bottom megabyte.
 740
 741It should be noted that modern machines often have a sizable Extended
 742BIOS Data Area (EBDA).  As a result, it is advisable to use as little
 743of the low megabyte as possible.
 744
 745Unfortunately, under the following circumstances the 0x90000 memory
 746segment has to be used:
 747
 748        - When loading a zImage kernel ((loadflags & 0x01) == 0).
 749        - When loading a 2.01 or earlier boot protocol kernel.
 750
 751          -> For the 2.00 and 2.01 boot protocols, the real-mode code
 752             can be loaded at another address, but it is internally
 753             relocated to 0x90000.  For the "old" protocol, the
 754             real-mode code must be loaded at 0x90000.
 755
 756When loading at 0x90000, avoid using memory above 0x9a000.
 757
 758For boot protocol 2.02 or higher, the command line does not have to be
 759located in the same 64K segment as the real-mode setup code; it is
 760thus permitted to give the stack/heap the full 64K segment and locate
 761the command line above it.
 762
 763The kernel command line should not be located below the real-mode
 764code, nor should it be located in high memory.
 765
 766
 767**** SAMPLE BOOT CONFIGURATION
 768
 769As a sample configuration, assume the following layout of the real
 770mode segment:
 771
 772    When loading below 0x90000, use the entire segment:
 773
 774        0x0000-0x7fff   Real mode kernel
 775        0x8000-0xdfff   Stack and heap
 776        0xe000-0xffff   Kernel command line
 777
 778    When loading at 0x90000 OR the protocol version is 2.01 or earlier:
 779
 780        0x0000-0x7fff   Real mode kernel
 781        0x8000-0x97ff   Stack and heap
 782        0x9800-0x9fff   Kernel command line
 783
 784Such a boot loader should enter the following fields in the header:
 785
 786        unsigned long base_ptr; /* base address for real-mode segment */
 787
 788        if ( setup_sects == 0 ) {
 789                setup_sects = 4;
 790        }
 791
 792        if ( protocol >= 0x0200 ) {
 793                type_of_loader = <type code>;
 794                if ( loading_initrd ) {
 795                        ramdisk_image = <initrd_address>;
 796                        ramdisk_size = <initrd_size>;
 797                }
 798
 799                if ( protocol >= 0x0202 && loadflags & 0x01 )
 800                        heap_end = 0xe000;
 801                else
 802                        heap_end = 0x9800;
 803
 804                if ( protocol >= 0x0201 ) {
 805                        heap_end_ptr = heap_end - 0x200;
 806                        loadflags |= 0x80; /* CAN_USE_HEAP */
 807                }
 808
 809                if ( protocol >= 0x0202 ) {
 810                        cmd_line_ptr = base_ptr + heap_end;
 811                        strcpy(cmd_line_ptr, cmdline);
 812                } else {
 813                        cmd_line_magic  = 0xA33F;
 814                        cmd_line_offset = heap_end;
 815                        setup_move_size = heap_end + strlen(cmdline)+1;
 816                        strcpy(base_ptr+cmd_line_offset, cmdline);
 817                }
 818        } else {
 819                /* Very old kernel */
 820
 821                heap_end = 0x9800;
 822
 823                cmd_line_magic  = 0xA33F;
 824                cmd_line_offset = heap_end;
 825
 826                /* A very old kernel MUST have its real-mode code
 827                   loaded at 0x90000 */
 828
 829                if ( base_ptr != 0x90000 ) {
 830                        /* Copy the real-mode kernel */
 831                        memcpy(0x90000, base_ptr, (setup_sects+1)*512);
 832                        base_ptr = 0x90000;              /* Relocated */
 833                }
 834
 835                strcpy(0x90000+cmd_line_offset, cmdline);
 836
 837                /* It is recommended to clear memory up to the 32K mark */
 838                memset(0x90000 + (setup_sects+1)*512, 0,
 839                       (64-(setup_sects+1))*512);
 840        }
 841
 842
 843**** LOADING THE REST OF THE KERNEL
 844
 845The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
 846in the kernel file (again, if setup_sects == 0 the real value is 4.)
 847It should be loaded at address 0x10000 for Image/zImage kernels and
 8480x100000 for bzImage kernels.
 849
 850The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
 851bit (LOAD_HIGH) in the loadflags field is set:
 852
 853        is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
 854        load_address = is_bzImage ? 0x100000 : 0x10000;
 855
 856Note that Image/zImage kernels can be up to 512K in size, and thus use
 857the entire 0x10000-0x90000 range of memory.  This means it is pretty
 858much a requirement for these kernels to load the real-mode part at
 8590x90000.  bzImage kernels allow much more flexibility.
 860
 861
 862**** SPECIAL COMMAND LINE OPTIONS
 863
 864If the command line provided by the boot loader is entered by the
 865user, the user may expect the following command line options to work.
 866They should normally not be deleted from the kernel command line even
 867though not all of them are actually meaningful to the kernel.  Boot
 868loader authors who need additional command line options for the boot
 869loader itself should get them registered in
 870Documentation/kernel-parameters.txt to make sure they will not
 871conflict with actual kernel options now or in the future.
 872
 873  vga=<mode>
 874        <mode> here is either an integer (in C notation, either
 875        decimal, octal, or hexadecimal) or one of the strings
 876        "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
 877        (meaning 0xFFFD).  This value should be entered into the
 878        vid_mode field, as it is used by the kernel before the command
 879        line is parsed.
 880
 881  mem=<size>
 882        <size> is an integer in C notation optionally followed by
 883        (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
 884        << 30, << 40, << 50 or << 60).  This specifies the end of
 885        memory to the kernel. This affects the possible placement of
 886        an initrd, since an initrd should be placed near end of
 887        memory.  Note that this is an option to *both* the kernel and
 888        the bootloader!
 889
 890  initrd=<file>
 891        An initrd should be loaded.  The meaning of <file> is
 892        obviously bootloader-dependent, and some boot loaders
 893        (e.g. LILO) do not have such a command.
 894
 895In addition, some boot loaders add the following options to the
 896user-specified command line:
 897
 898  BOOT_IMAGE=<file>
 899        The boot image which was loaded.  Again, the meaning of <file>
 900        is obviously bootloader-dependent.
 901
 902  auto
 903        The kernel was booted without explicit user intervention.
 904
 905If these options are added by the boot loader, it is highly
 906recommended that they are located *first*, before the user-specified
 907or configuration-specified command line.  Otherwise, "init=/bin/sh"
 908gets confused by the "auto" option.
 909
 910
 911**** RUNNING THE KERNEL
 912
 913The kernel is started by jumping to the kernel entry point, which is
 914located at *segment* offset 0x20 from the start of the real mode
 915kernel.  This means that if you loaded your real-mode kernel code at
 9160x90000, the kernel entry point is 9020:0000.
 917
 918At entry, ds = es = ss should point to the start of the real-mode
 919kernel code (0x9000 if the code is loaded at 0x90000), sp should be
 920set up properly, normally pointing to the top of the heap, and
 921interrupts should be disabled.  Furthermore, to guard against bugs in
 922the kernel, it is recommended that the boot loader sets fs = gs = ds =
 923es = ss.
 924
 925In our example from above, we would do:
 926
 927        /* Note: in the case of the "old" kernel protocol, base_ptr must
 928           be == 0x90000 at this point; see the previous sample code */
 929
 930        seg = base_ptr >> 4;
 931
 932        cli();  /* Enter with interrupts disabled! */
 933
 934        /* Set up the real-mode kernel stack */
 935        _SS = seg;
 936        _SP = heap_end;
 937
 938        _DS = _ES = _FS = _GS = seg;
 939        jmp_far(seg+0x20, 0);   /* Run the kernel */
 940
 941If your boot sector accesses a floppy drive, it is recommended to
 942switch off the floppy motor before running the kernel, since the
 943kernel boot leaves interrupts off and thus the motor will not be
 944switched off, especially if the loaded kernel has the floppy driver as
 945a demand-loaded module!
 946
 947
 948**** ADVANCED BOOT LOADER HOOKS
 949
 950If the boot loader runs in a particularly hostile environment (such as
 951LOADLIN, which runs under DOS) it may be impossible to follow the
 952standard memory location requirements.  Such a boot loader may use the
 953following hooks that, if set, are invoked by the kernel at the
 954appropriate time.  The use of these hooks should probably be
 955considered an absolutely last resort!
 956
 957IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
 958%edi across invocation.
 959
 960  realmode_swtch:
 961        A 16-bit real mode far subroutine invoked immediately before
 962        entering protected mode.  The default routine disables NMI, so
 963        your routine should probably do so, too.
 964
 965  code32_start:
 966        A 32-bit flat-mode routine *jumped* to immediately after the
 967        transition to protected mode, but before the kernel is
 968        uncompressed.  No segments, except CS, are guaranteed to be
 969        set up (current kernels do, but older ones do not); you should
 970        set them up to BOOT_DS (0x18) yourself.
 971
 972        After completing your hook, you should jump to the address
 973        that was in this field before your boot loader overwrote it
 974        (relocated, if appropriate.)
 975
 976
 977**** 32-bit BOOT PROTOCOL
 978
 979For machine with some new BIOS other than legacy BIOS, such as EFI,
 980LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
 981based on legacy BIOS can not be used, so a 32-bit boot protocol needs
 982to be defined.
 983
 984In 32-bit boot protocol, the first step in loading a Linux kernel
 985should be to setup the boot parameters (struct boot_params,
 986traditionally known as "zero page"). The memory for struct boot_params
 987should be allocated and initialized to all zero. Then the setup header
 988from offset 0x01f1 of kernel image on should be loaded into struct
 989boot_params and examined. The end of setup header can be calculated as
 990follow:
 991
 992        0x0202 + byte value at offset 0x0201
 993
 994In addition to read/modify/write the setup header of the struct
 995boot_params as that of 16-bit boot protocol, the boot loader should
 996also fill the additional fields of the struct boot_params as that
 997described in zero-page.txt.
 998
 999After setupping the struct boot_params, the boot loader can load the
100032/64-bit kernel in the same way as that of 16-bit boot protocol.
1001
1002In 32-bit boot protocol, the kernel is started by jumping to the
100332-bit kernel entry point, which is the start address of loaded
100432/64-bit kernel.
1005
1006At entry, the CPU must be in 32-bit protected mode with paging
1007disabled; a GDT must be loaded with the descriptors for selectors
1008__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1009segment; __BOOS_CS must have execute/read permission, and __BOOT_DS
1010must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1011must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
1012address of the struct boot_params; %ebp, %edi and %ebx must be zero.
1013
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