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
  57Protocol 2.11:  (Kernel 3.6) Added a field for offset of EFI handover
  58                protocol entry point.
  59
  60Protocol 2.12:  (Kernel 3.8) Added the xloadflags field and extension fields
  61                to struct boot_params for loading bzImage and ramdisk
  62                above 4G in 64bit.
  63
  64**** MEMORY LAYOUT
  65
  66The traditional memory map for the kernel loader, used for Image or
  67zImage kernels, typically looks like:
  68
  69        |                        |
  700A0000  +------------------------+
  71        |  Reserved for BIOS     |      Do not use.  Reserved for BIOS EBDA.
  7209A000  +------------------------+
  73        |  Command line          |
  74        |  Stack/heap            |      For use by the kernel real-mode code.
  75098000  +------------------------+      
  76        |  Kernel setup          |      The kernel real-mode code.
  77090200  +------------------------+
  78        |  Kernel boot sector    |      The kernel legacy boot sector.
  79090000  +------------------------+
  80        |  Protected-mode kernel |      The bulk of the kernel image.
  81010000  +------------------------+
  82        |  Boot loader           |      <- Boot sector entry point 0000:7C00
  83001000  +------------------------+
  84        |  Reserved for MBR/BIOS |
  85000800  +------------------------+
  86        |  Typically used by MBR |
  87000600  +------------------------+ 
  88        |  BIOS use only         |
  89000000  +------------------------+
  90
  91
  92When using bzImage, the protected-mode kernel was relocated to
  930x100000 ("high memory"), and the kernel real-mode block (boot sector,
  94setup, and stack/heap) was made relocatable to any address between
  950x10000 and end of low memory. Unfortunately, in protocols 2.00 and
  962.01 the 0x90000+ memory range is still used internally by the kernel;
  97the 2.02 protocol resolves that problem.
  98
  99It is desirable to keep the "memory ceiling" -- the highest point in
 100low memory touched by the boot loader -- as low as possible, since
 101some newer BIOSes have begun to allocate some rather large amounts of
 102memory, called the Extended BIOS Data Area, near the top of low
 103memory.  The boot loader should use the "INT 12h" BIOS call to verify
 104how much low memory is available.
 105
 106Unfortunately, if INT 12h reports that the amount of memory is too
 107low, there is usually nothing the boot loader can do but to report an
 108error to the user.  The boot loader should therefore be designed to
 109take up as little space in low memory as it reasonably can.  For
 110zImage or old bzImage kernels, which need data written into the
 1110x90000 segment, the boot loader should make sure not to use memory
 112above the 0x9A000 point; too many BIOSes will break above that point.
 113
 114For a modern bzImage kernel with boot protocol version >= 2.02, a
 115memory layout like the following is suggested:
 116
 117        ~                        ~
 118        |  Protected-mode kernel |
 119100000  +------------------------+
 120        |  I/O memory hole       |
 1210A0000  +------------------------+
 122        |  Reserved for BIOS     |      Leave as much as possible unused
 123        ~                        ~
 124        |  Command line          |      (Can also be below the X+10000 mark)
 125X+10000 +------------------------+
 126        |  Stack/heap            |      For use by the kernel real-mode code.
 127X+08000 +------------------------+      
 128        |  Kernel setup          |      The kernel real-mode code.
 129        |  Kernel boot sector    |      The kernel legacy boot sector.
 130X       +------------------------+
 131        |  Boot loader           |      <- Boot sector entry point 0000:7C00
 132001000  +------------------------+
 133        |  Reserved for MBR/BIOS |
 134000800  +------------------------+
 135        |  Typically used by MBR |
 136000600  +------------------------+ 
 137        |  BIOS use only         |
 138000000  +------------------------+
 139
 140... where the address X is as low as the design of the boot loader
 141permits.
 142
 143
 144**** THE REAL-MODE KERNEL HEADER
 145
 146In the following text, and anywhere in the kernel boot sequence, "a
 147sector" refers to 512 bytes.  It is independent of the actual sector
 148size of the underlying medium.
 149
 150The first step in loading a Linux kernel should be to load the
 151real-mode code (boot sector and setup code) and then examine the
 152following header at offset 0x01f1.  The real-mode code can total up to
 15332K, although the boot loader may choose to load only the first two
 154sectors (1K) and then examine the bootup sector size.
 155
 156The header looks like:
 157
 158Offset  Proto   Name            Meaning
 159/Size
 160
 16101F1/1  ALL(1   setup_sects     The size of the setup in sectors
 16201F2/2  ALL     root_flags      If set, the root is mounted readonly
 16301F4/4  2.04+(2 syssize         The size of the 32-bit code in 16-byte paras
 16401F8/2  ALL     ram_size        DO NOT USE - for bootsect.S use only
 16501FA/2  ALL     vid_mode        Video mode control
 16601FC/2  ALL     root_dev        Default root device number
 16701FE/2  ALL     boot_flag       0xAA55 magic number
 1680200/2  2.00+   jump            Jump instruction
 1690202/4  2.00+   header          Magic signature "HdrS"
 1700206/2  2.00+   version         Boot protocol version supported
 1710208/4  2.00+   realmode_swtch  Boot loader hook (see below)
 172020C/2  2.00+   start_sys_seg   The load-low segment (0x1000) (obsolete)
 173020E/2  2.00+   kernel_version  Pointer to kernel version string
 1740210/1  2.00+   type_of_loader  Boot loader identifier
 1750211/1  2.00+   loadflags       Boot protocol option flags
 1760212/2  2.00+   setup_move_size Move to high memory size (used with hooks)
 1770214/4  2.00+   code32_start    Boot loader hook (see below)
 1780218/4  2.00+   ramdisk_image   initrd load address (set by boot loader)
 179021C/4  2.00+   ramdisk_size    initrd size (set by boot loader)
 1800220/4  2.00+   bootsect_kludge DO NOT USE - for bootsect.S use only
 1810224/2  2.01+   heap_end_ptr    Free memory after setup end
 1820226/1  2.02+(3 ext_loader_ver  Extended boot loader version
 1830227/1  2.02+(3 ext_loader_type Extended boot loader ID
 1840228/4  2.02+   cmd_line_ptr    32-bit pointer to the kernel command line
 185022C/4  2.03+   initrd_addr_max Highest legal initrd address
 1860230/4  2.05+   kernel_alignment Physical addr alignment required for kernel
 1870234/1  2.05+   relocatable_kernel Whether kernel is relocatable or not
 1880235/1  2.10+   min_alignment   Minimum alignment, as a power of two
 1890236/2  2.12+   xloadflags      Boot protocol option flags
 1900238/4  2.06+   cmdline_size    Maximum size of the kernel command line
 191023C/4  2.07+   hardware_subarch Hardware subarchitecture
 1920240/8  2.07+   hardware_subarch_data Subarchitecture-specific data
 1930248/4  2.08+   payload_offset  Offset of kernel payload
 194024C/4  2.08+   payload_length  Length of kernel payload
 1950250/8  2.09+   setup_data      64-bit physical pointer to linked list
 196                                of struct setup_data
 1970258/8  2.10+   pref_address    Preferred loading address
 1980260/4  2.10+   init_size       Linear memory required during initialization
 1990264/4  2.11+   handover_offset Offset of handover entry point
 200
 201(1) For backwards compatibility, if the setup_sects field contains 0, the
 202    real value is 4.
 203
 204(2) For boot protocol prior to 2.04, the upper two bytes of the syssize
 205    field are unusable, which means the size of a bzImage kernel
 206    cannot be determined.
 207
 208(3) Ignored, but safe to set, for boot protocols 2.02-2.09.
 209
 210If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
 211the boot protocol version is "old".  Loading an old kernel, the
 212following parameters should be assumed:
 213
 214        Image type = zImage
 215        initrd not supported
 216        Real-mode kernel must be located at 0x90000.
 217
 218Otherwise, the "version" field contains the protocol version,
 219e.g. protocol version 2.01 will contain 0x0201 in this field.  When
 220setting fields in the header, you must make sure only to set fields
 221supported by the protocol version in use.
 222
 223
 224**** DETAILS OF HEADER FIELDS
 225
 226For each field, some are information from the kernel to the bootloader
 227("read"), some are expected to be filled out by the bootloader
 228("write"), and some are expected to be read and modified by the
 229bootloader ("modify").
 230
 231All general purpose boot loaders should write the fields marked
 232(obligatory).  Boot loaders who want to load the kernel at a
 233nonstandard address should fill in the fields marked (reloc); other
 234boot loaders can ignore those fields.
 235
 236The byte order of all fields is littleendian (this is x86, after all.)
 237
 238Field name:     setup_sects
 239Type:           read
 240Offset/size:    0x1f1/1
 241Protocol:       ALL
 242
 243  The size of the setup code in 512-byte sectors.  If this field is
 244  0, the real value is 4.  The real-mode code consists of the boot
 245  sector (always one 512-byte sector) plus the setup code.
 246
 247Field name:      root_flags
 248Type:            modify (optional)
 249Offset/size:     0x1f2/2
 250Protocol:        ALL
 251
 252  If this field is nonzero, the root defaults to readonly.  The use of
 253  this field is deprecated; use the "ro" or "rw" options on the
 254  command line instead.
 255
 256Field name:     syssize
 257Type:           read
 258Offset/size:    0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
 259Protocol:       2.04+
 260
 261  The size of the protected-mode code in units of 16-byte paragraphs.
 262  For protocol versions older than 2.04 this field is only two bytes
 263  wide, and therefore cannot be trusted for the size of a kernel if
 264  the LOAD_HIGH flag is set.
 265
 266Field name:     ram_size
 267Type:           kernel internal
 268Offset/size:    0x1f8/2
 269Protocol:       ALL
 270
 271  This field is obsolete.
 272
 273Field name:     vid_mode
 274Type:           modify (obligatory)
 275Offset/size:    0x1fa/2
 276
 277  Please see the section on SPECIAL COMMAND LINE OPTIONS.
 278
 279Field name:     root_dev
 280Type:           modify (optional)
 281Offset/size:    0x1fc/2
 282Protocol:       ALL
 283
 284  The default root device device number.  The use of this field is
 285  deprecated, use the "root=" option on the command line instead.
 286
 287Field name:     boot_flag
 288Type:           read
 289Offset/size:    0x1fe/2
 290Protocol:       ALL
 291
 292  Contains 0xAA55.  This is the closest thing old Linux kernels have
 293  to a magic number.
 294
 295Field name:     jump
 296Type:           read
 297Offset/size:    0x200/2
 298Protocol:       2.00+
 299
 300  Contains an x86 jump instruction, 0xEB followed by a signed offset
 301  relative to byte 0x202.  This can be used to determine the size of
 302  the header.
 303
 304Field name:     header
 305Type:           read
 306Offset/size:    0x202/4
 307Protocol:       2.00+
 308
 309  Contains the magic number "HdrS" (0x53726448).
 310
 311Field name:     version
 312Type:           read
 313Offset/size:    0x206/2
 314Protocol:       2.00+
 315
 316  Contains the boot protocol version, in (major << 8)+minor format,
 317  e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
 318  10.17.
 319
 320Field name:     realmode_swtch
 321Type:           modify (optional)
 322Offset/size:    0x208/4
 323Protocol:       2.00+
 324
 325  Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
 326
 327Field name:     start_sys_seg
 328Type:           read
 329Offset/size:    0x20c/2
 330Protocol:       2.00+
 331
 332  The load low segment (0x1000).  Obsolete.
 333
 334Field name:     kernel_version
 335Type:           read
 336Offset/size:    0x20e/2
 337Protocol:       2.00+
 338
 339  If set to a nonzero value, contains a pointer to a NUL-terminated
 340  human-readable kernel version number string, less 0x200.  This can
 341  be used to display the kernel version to the user.  This value
 342  should be less than (0x200*setup_sects).
 343
 344  For example, if this value is set to 0x1c00, the kernel version
 345  number string can be found at offset 0x1e00 in the kernel file.
 346  This is a valid value if and only if the "setup_sects" field
 347  contains the value 15 or higher, as:
 348
 349        0x1c00  < 15*0x200 (= 0x1e00) but
 350        0x1c00 >= 14*0x200 (= 0x1c00)
 351
 352        0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
 353
 354Field name:     type_of_loader
 355Type:           write (obligatory)
 356Offset/size:    0x210/1
 357Protocol:       2.00+
 358
 359  If your boot loader has an assigned id (see table below), enter
 360  0xTV here, where T is an identifier for the boot loader and V is
 361  a version number.  Otherwise, enter 0xFF here.
 362
 363  For boot loader IDs above T = 0xD, write T = 0xE to this field and
 364  write the extended ID minus 0x10 to the ext_loader_type field.
 365  Similarly, the ext_loader_ver field can be used to provide more than
 366  four bits for the bootloader version.
 367
 368  For example, for T = 0x15, V = 0x234, write:
 369
 370  type_of_loader  <- 0xE4
 371  ext_loader_type <- 0x05
 372  ext_loader_ver  <- 0x23
 373
 374  Assigned boot loader ids (hexadecimal):
 375
 376        0  LILO                 (0x00 reserved for pre-2.00 bootloader)
 377        1  Loadlin
 378        2  bootsect-loader      (0x20, all other values reserved)
 379        3  Syslinux
 380        4  Etherboot/gPXE/iPXE
 381        5  ELILO
 382        7  GRUB
 383        8  U-Boot
 384        9  Xen
 385        A  Gujin
 386        B  Qemu
 387        C  Arcturus Networks uCbootloader
 388        D  kexec-tools
 389        E  Extended             (see ext_loader_type)
 390        F  Special              (0xFF = undefined)
 391       10  Reserved
 392       11  Minimal Linux Bootloader <http://sebastian-plotz.blogspot.de>
 393       12  OVMF UEFI virtualization stack
 394
 395  Please contact <hpa@zytor.com> if you need a bootloader ID
 396  value assigned.
 397
 398Field name:     loadflags
 399Type:           modify (obligatory)
 400Offset/size:    0x211/1
 401Protocol:       2.00+
 402
 403  This field is a bitmask.
 404
 405  Bit 0 (read): LOADED_HIGH
 406        - If 0, the protected-mode code is loaded at 0x10000.
 407        - If 1, the protected-mode code is loaded at 0x100000.
 408
 409  Bit 1 (kernel internal): KASLR_FLAG
 410        - Used internally by the compressed kernel to communicate
 411          KASLR status to kernel proper.
 412          If 1, KASLR enabled.
 413          If 0, KASLR disabled.
 414
 415  Bit 5 (write): QUIET_FLAG
 416        - If 0, print early messages.
 417        - If 1, suppress early messages.
 418                This requests to the kernel (decompressor and early
 419                kernel) to not write early messages that require
 420                accessing the display hardware directly.
 421
 422  Bit 6 (write): KEEP_SEGMENTS
 423        Protocol: 2.07+
 424        - If 0, reload the segment registers in the 32bit entry point.
 425        - If 1, do not reload the segment registers in the 32bit entry point.
 426                Assume that %cs %ds %ss %es are all set to flat segments with
 427                a base of 0 (or the equivalent for their environment).
 428
 429  Bit 7 (write): CAN_USE_HEAP
 430        Set this bit to 1 to indicate that the value entered in the
 431        heap_end_ptr is valid.  If this field is clear, some setup code
 432        functionality will be disabled.
 433
 434Field name:     setup_move_size
 435Type:           modify (obligatory)
 436Offset/size:    0x212/2
 437Protocol:       2.00-2.01
 438
 439  When using protocol 2.00 or 2.01, if the real mode kernel is not
 440  loaded at 0x90000, it gets moved there later in the loading
 441  sequence.  Fill in this field if you want additional data (such as
 442  the kernel command line) moved in addition to the real-mode kernel
 443  itself.
 444
 445  The unit is bytes starting with the beginning of the boot sector.
 446  
 447  This field is can be ignored when the protocol is 2.02 or higher, or
 448  if the real-mode code is loaded at 0x90000.
 449
 450Field name:     code32_start
 451Type:           modify (optional, reloc)
 452Offset/size:    0x214/4
 453Protocol:       2.00+
 454
 455  The address to jump to in protected mode.  This defaults to the load
 456  address of the kernel, and can be used by the boot loader to
 457  determine the proper load address.
 458
 459  This field can be modified for two purposes:
 460
 461  1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
 462
 463  2. if a bootloader which does not install a hook loads a
 464     relocatable kernel at a nonstandard address it will have to modify
 465     this field to point to the load address.
 466
 467Field name:     ramdisk_image
 468Type:           write (obligatory)
 469Offset/size:    0x218/4
 470Protocol:       2.00+
 471
 472  The 32-bit linear address of the initial ramdisk or ramfs.  Leave at
 473  zero if there is no initial ramdisk/ramfs.
 474
 475Field name:     ramdisk_size
 476Type:           write (obligatory)
 477Offset/size:    0x21c/4
 478Protocol:       2.00+
 479
 480  Size of the initial ramdisk or ramfs.  Leave at zero if there is no
 481  initial ramdisk/ramfs.
 482
 483Field name:     bootsect_kludge
 484Type:           kernel internal
 485Offset/size:    0x220/4
 486Protocol:       2.00+
 487
 488  This field is obsolete.
 489
 490Field name:     heap_end_ptr
 491Type:           write (obligatory)
 492Offset/size:    0x224/2
 493Protocol:       2.01+
 494
 495  Set this field to the offset (from the beginning of the real-mode
 496  code) of the end of the setup stack/heap, minus 0x0200.
 497
 498Field name:     ext_loader_ver
 499Type:           write (optional)
 500Offset/size:    0x226/1
 501Protocol:       2.02+
 502
 503  This field is used as an extension of the version number in the
 504  type_of_loader field.  The total version number is considered to be
 505  (type_of_loader & 0x0f) + (ext_loader_ver << 4).
 506
 507  The use of this field is boot loader specific.  If not written, it
 508  is zero.
 509
 510  Kernels prior to 2.6.31 did not recognize this field, but it is safe
 511  to write for protocol version 2.02 or higher.
 512
 513Field name:     ext_loader_type
 514Type:           write (obligatory if (type_of_loader & 0xf0) == 0xe0)
 515Offset/size:    0x227/1
 516Protocol:       2.02+
 517
 518  This field is used as an extension of the type number in
 519  type_of_loader field.  If the type in type_of_loader is 0xE, then
 520  the actual type is (ext_loader_type + 0x10).
 521
 522  This field is ignored if the type in type_of_loader is not 0xE.
 523
 524  Kernels prior to 2.6.31 did not recognize this field, but it is safe
 525  to write for protocol version 2.02 or higher.
 526
 527Field name:     cmd_line_ptr
 528Type:           write (obligatory)
 529Offset/size:    0x228/4
 530Protocol:       2.02+
 531
 532  Set this field to the linear address of the kernel command line.
 533  The kernel command line can be located anywhere between the end of
 534  the setup heap and 0xA0000; it does not have to be located in the
 535  same 64K segment as the real-mode code itself.
 536
 537  Fill in this field even if your boot loader does not support a
 538  command line, in which case you can point this to an empty string
 539  (or better yet, to the string "auto".)  If this field is left at
 540  zero, the kernel will assume that your boot loader does not support
 541  the 2.02+ protocol.
 542
 543Field name:     initrd_addr_max
 544Type:           read
 545Offset/size:    0x22c/4
 546Protocol:       2.03+
 547
 548  The maximum address that may be occupied by the initial
 549  ramdisk/ramfs contents.  For boot protocols 2.02 or earlier, this
 550  field is not present, and the maximum address is 0x37FFFFFF.  (This
 551  address is defined as the address of the highest safe byte, so if
 552  your ramdisk is exactly 131072 bytes long and this field is
 553  0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
 554
 555Field name:     kernel_alignment
 556Type:           read/modify (reloc)
 557Offset/size:    0x230/4
 558Protocol:       2.05+ (read), 2.10+ (modify)
 559
 560  Alignment unit required by the kernel (if relocatable_kernel is
 561  true.)  A relocatable kernel that is loaded at an alignment
 562  incompatible with the value in this field will be realigned during
 563  kernel initialization.
 564
 565  Starting with protocol version 2.10, this reflects the kernel
 566  alignment preferred for optimal performance; it is possible for the
 567  loader to modify this field to permit a lesser alignment.  See the
 568  min_alignment and pref_address field below.
 569
 570Field name:     relocatable_kernel
 571Type:           read (reloc)
 572Offset/size:    0x234/1
 573Protocol:       2.05+
 574
 575  If this field is nonzero, the protected-mode part of the kernel can
 576  be loaded at any address that satisfies the kernel_alignment field.
 577  After loading, the boot loader must set the code32_start field to
 578  point to the loaded code, or to a boot loader hook.
 579
 580Field name:     min_alignment
 581Type:           read (reloc)
 582Offset/size:    0x235/1
 583Protocol:       2.10+
 584
 585  This field, if nonzero, indicates as a power of two the minimum
 586  alignment required, as opposed to preferred, by the kernel to boot.
 587  If a boot loader makes use of this field, it should update the
 588  kernel_alignment field with the alignment unit desired; typically:
 589
 590        kernel_alignment = 1 << min_alignment
 591
 592  There may be a considerable performance cost with an excessively
 593  misaligned kernel.  Therefore, a loader should typically try each
 594  power-of-two alignment from kernel_alignment down to this alignment.
 595
 596Field name:     xloadflags
 597Type:           read
 598Offset/size:    0x236/2
 599Protocol:       2.12+
 600
 601  This field is a bitmask.
 602
 603  Bit 0 (read): XLF_KERNEL_64
 604        - If 1, this kernel has the legacy 64-bit entry point at 0x200.
 605
 606  Bit 1 (read): XLF_CAN_BE_LOADED_ABOVE_4G
 607        - If 1, kernel/boot_params/cmdline/ramdisk can be above 4G.
 608
 609  Bit 2 (read): XLF_EFI_HANDOVER_32
 610        - If 1, the kernel supports the 32-bit EFI handoff entry point
 611          given at handover_offset.
 612
 613  Bit 3 (read): XLF_EFI_HANDOVER_64
 614        - If 1, the kernel supports the 64-bit EFI handoff entry point
 615          given at handover_offset + 0x200.
 616
 617  Bit 4 (read): XLF_EFI_KEXEC
 618        - If 1, the kernel supports kexec EFI boot with EFI runtime support.
 619
 620Field name:     cmdline_size
 621Type:           read
 622Offset/size:    0x238/4
 623Protocol:       2.06+
 624
 625  The maximum size of the command line without the terminating
 626  zero. This means that the command line can contain at most
 627  cmdline_size characters. With protocol version 2.05 and earlier, the
 628  maximum size was 255.
 629
 630Field name:     hardware_subarch
 631Type:           write (optional, defaults to x86/PC)
 632Offset/size:    0x23c/4
 633Protocol:       2.07+
 634
 635  In a paravirtualized environment the hardware low level architectural
 636  pieces such as interrupt handling, page table handling, and
 637  accessing process control registers needs to be done differently.
 638
 639  This field allows the bootloader to inform the kernel we are in one
 640  one of those environments.
 641
 642  0x00000000    The default x86/PC environment
 643  0x00000001    lguest
 644  0x00000002    Xen
 645  0x00000003    Moorestown MID
 646  0x00000004    CE4100 TV Platform
 647
 648Field name:     hardware_subarch_data
 649Type:           write (subarch-dependent)
 650Offset/size:    0x240/8
 651Protocol:       2.07+
 652
 653  A pointer to data that is specific to hardware subarch
 654  This field is currently unused for the default x86/PC environment,
 655  do not modify.
 656
 657Field name:     payload_offset
 658Type:           read
 659Offset/size:    0x248/4
 660Protocol:       2.08+
 661
 662  If non-zero then this field contains the offset from the beginning
 663  of the protected-mode code to the payload.
 664
 665  The payload may be compressed. The format of both the compressed and
 666  uncompressed data should be determined using the standard magic
 667  numbers.  The currently supported compression formats are gzip
 668  (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA
 669  (magic number 5D 00), XZ (magic number FD 37), and LZ4 (magic number
 670  02 21).  The uncompressed payload is currently always ELF (magic
 671  number 7F 45 4C 46).
 672
 673Field name:     payload_length
 674Type:           read
 675Offset/size:    0x24c/4
 676Protocol:       2.08+
 677
 678  The length of the payload.
 679
 680Field name:     setup_data
 681Type:           write (special)
 682Offset/size:    0x250/8
 683Protocol:       2.09+
 684
 685  The 64-bit physical pointer to NULL terminated single linked list of
 686  struct setup_data. This is used to define a more extensible boot
 687  parameters passing mechanism. The definition of struct setup_data is
 688  as follow:
 689
 690  struct setup_data {
 691          u64 next;
 692          u32 type;
 693          u32 len;
 694          u8  data[0];
 695  };
 696
 697  Where, the next is a 64-bit physical pointer to the next node of
 698  linked list, the next field of the last node is 0; the type is used
 699  to identify the contents of data; the len is the length of data
 700  field; the data holds the real payload.
 701
 702  This list may be modified at a number of points during the bootup
 703  process.  Therefore, when modifying this list one should always make
 704  sure to consider the case where the linked list already contains
 705  entries.
 706
 707Field name:     pref_address
 708Type:           read (reloc)
 709Offset/size:    0x258/8
 710Protocol:       2.10+
 711
 712  This field, if nonzero, represents a preferred load address for the
 713  kernel.  A relocating bootloader should attempt to load at this
 714  address if possible.
 715
 716  A non-relocatable kernel will unconditionally move itself and to run
 717  at this address.
 718
 719Field name:     init_size
 720Type:           read
 721Offset/size:    0x260/4
 722
 723  This field indicates the amount of linear contiguous memory starting
 724  at the kernel runtime start address that the kernel needs before it
 725  is capable of examining its memory map.  This is not the same thing
 726  as the total amount of memory the kernel needs to boot, but it can
 727  be used by a relocating boot loader to help select a safe load
 728  address for the kernel.
 729
 730  The kernel runtime start address is determined by the following algorithm:
 731
 732  if (relocatable_kernel)
 733        runtime_start = align_up(load_address, kernel_alignment)
 734  else
 735        runtime_start = pref_address
 736
 737Field name:     handover_offset
 738Type:           read
 739Offset/size:    0x264/4
 740
 741  This field is the offset from the beginning of the kernel image to
 742  the EFI handover protocol entry point. Boot loaders using the EFI
 743  handover protocol to boot the kernel should jump to this offset.
 744
 745  See EFI HANDOVER PROTOCOL below for more details.
 746
 747
 748**** THE IMAGE CHECKSUM
 749
 750From boot protocol version 2.08 onwards the CRC-32 is calculated over
 751the entire file using the characteristic polynomial 0x04C11DB7 and an
 752initial remainder of 0xffffffff.  The checksum is appended to the
 753file; therefore the CRC of the file up to the limit specified in the
 754syssize field of the header is always 0.
 755
 756
 757**** THE KERNEL COMMAND LINE
 758
 759The kernel command line has become an important way for the boot
 760loader to communicate with the kernel.  Some of its options are also
 761relevant to the boot loader itself, see "special command line options"
 762below.
 763
 764The kernel command line is a null-terminated string. The maximum
 765length can be retrieved from the field cmdline_size.  Before protocol
 766version 2.06, the maximum was 255 characters.  A string that is too
 767long will be automatically truncated by the kernel.
 768
 769If the boot protocol version is 2.02 or later, the address of the
 770kernel command line is given by the header field cmd_line_ptr (see
 771above.)  This address can be anywhere between the end of the setup
 772heap and 0xA0000.
 773
 774If the protocol version is *not* 2.02 or higher, the kernel
 775command line is entered using the following protocol:
 776
 777        At offset 0x0020 (word), "cmd_line_magic", enter the magic
 778        number 0xA33F.
 779
 780        At offset 0x0022 (word), "cmd_line_offset", enter the offset
 781        of the kernel command line (relative to the start of the
 782        real-mode kernel).
 783        
 784        The kernel command line *must* be within the memory region
 785        covered by setup_move_size, so you may need to adjust this
 786        field.
 787
 788
 789**** MEMORY LAYOUT OF THE REAL-MODE CODE
 790
 791The real-mode code requires a stack/heap to be set up, as well as
 792memory allocated for the kernel command line.  This needs to be done
 793in the real-mode accessible memory in bottom megabyte.
 794
 795It should be noted that modern machines often have a sizable Extended
 796BIOS Data Area (EBDA).  As a result, it is advisable to use as little
 797of the low megabyte as possible.
 798
 799Unfortunately, under the following circumstances the 0x90000 memory
 800segment has to be used:
 801
 802        - When loading a zImage kernel ((loadflags & 0x01) == 0).
 803        - When loading a 2.01 or earlier boot protocol kernel.
 804
 805          -> For the 2.00 and 2.01 boot protocols, the real-mode code
 806             can be loaded at another address, but it is internally
 807             relocated to 0x90000.  For the "old" protocol, the
 808             real-mode code must be loaded at 0x90000.
 809
 810When loading at 0x90000, avoid using memory above 0x9a000.
 811
 812For boot protocol 2.02 or higher, the command line does not have to be
 813located in the same 64K segment as the real-mode setup code; it is
 814thus permitted to give the stack/heap the full 64K segment and locate
 815the command line above it.
 816
 817The kernel command line should not be located below the real-mode
 818code, nor should it be located in high memory.
 819
 820
 821**** SAMPLE BOOT CONFIGURATION
 822
 823As a sample configuration, assume the following layout of the real
 824mode segment:
 825
 826    When loading below 0x90000, use the entire segment:
 827
 828        0x0000-0x7fff   Real mode kernel
 829        0x8000-0xdfff   Stack and heap
 830        0xe000-0xffff   Kernel command line
 831
 832    When loading at 0x90000 OR the protocol version is 2.01 or earlier:
 833
 834        0x0000-0x7fff   Real mode kernel
 835        0x8000-0x97ff   Stack and heap
 836        0x9800-0x9fff   Kernel command line
 837
 838Such a boot loader should enter the following fields in the header:
 839
 840        unsigned long base_ptr; /* base address for real-mode segment */
 841
 842        if ( setup_sects == 0 ) {
 843                setup_sects = 4;
 844        }
 845
 846        if ( protocol >= 0x0200 ) {
 847                type_of_loader = <type code>;
 848                if ( loading_initrd ) {
 849                        ramdisk_image = <initrd_address>;
 850                        ramdisk_size = <initrd_size>;
 851                }
 852
 853                if ( protocol >= 0x0202 && loadflags & 0x01 )
 854                        heap_end = 0xe000;
 855                else
 856                        heap_end = 0x9800;
 857
 858                if ( protocol >= 0x0201 ) {
 859                        heap_end_ptr = heap_end - 0x200;
 860                        loadflags |= 0x80; /* CAN_USE_HEAP */
 861                }
 862
 863                if ( protocol >= 0x0202 ) {
 864                        cmd_line_ptr = base_ptr + heap_end;
 865                        strcpy(cmd_line_ptr, cmdline);
 866                } else {
 867                        cmd_line_magic  = 0xA33F;
 868                        cmd_line_offset = heap_end;
 869                        setup_move_size = heap_end + strlen(cmdline)+1;
 870                        strcpy(base_ptr+cmd_line_offset, cmdline);
 871                }
 872        } else {
 873                /* Very old kernel */
 874
 875                heap_end = 0x9800;
 876
 877                cmd_line_magic  = 0xA33F;
 878                cmd_line_offset = heap_end;
 879
 880                /* A very old kernel MUST have its real-mode code
 881                   loaded at 0x90000 */
 882
 883                if ( base_ptr != 0x90000 ) {
 884                        /* Copy the real-mode kernel */
 885                        memcpy(0x90000, base_ptr, (setup_sects+1)*512);
 886                        base_ptr = 0x90000;              /* Relocated */
 887                }
 888
 889                strcpy(0x90000+cmd_line_offset, cmdline);
 890
 891                /* It is recommended to clear memory up to the 32K mark */
 892                memset(0x90000 + (setup_sects+1)*512, 0,
 893                       (64-(setup_sects+1))*512);
 894        }
 895
 896
 897**** LOADING THE REST OF THE KERNEL
 898
 899The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
 900in the kernel file (again, if setup_sects == 0 the real value is 4.)
 901It should be loaded at address 0x10000 for Image/zImage kernels and
 9020x100000 for bzImage kernels.
 903
 904The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
 905bit (LOAD_HIGH) in the loadflags field is set:
 906
 907        is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
 908        load_address = is_bzImage ? 0x100000 : 0x10000;
 909
 910Note that Image/zImage kernels can be up to 512K in size, and thus use
 911the entire 0x10000-0x90000 range of memory.  This means it is pretty
 912much a requirement for these kernels to load the real-mode part at
 9130x90000.  bzImage kernels allow much more flexibility.
 914
 915
 916**** SPECIAL COMMAND LINE OPTIONS
 917
 918If the command line provided by the boot loader is entered by the
 919user, the user may expect the following command line options to work.
 920They should normally not be deleted from the kernel command line even
 921though not all of them are actually meaningful to the kernel.  Boot
 922loader authors who need additional command line options for the boot
 923loader itself should get them registered in
 924Documentation/admin-guide/kernel-parameters.rst to make sure they will not
 925conflict with actual kernel options now or in the future.
 926
 927  vga=<mode>
 928        <mode> here is either an integer (in C notation, either
 929        decimal, octal, or hexadecimal) or one of the strings
 930        "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
 931        (meaning 0xFFFD).  This value should be entered into the
 932        vid_mode field, as it is used by the kernel before the command
 933        line is parsed.
 934
 935  mem=<size>
 936        <size> is an integer in C notation optionally followed by
 937        (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
 938        << 30, << 40, << 50 or << 60).  This specifies the end of
 939        memory to the kernel. This affects the possible placement of
 940        an initrd, since an initrd should be placed near end of
 941        memory.  Note that this is an option to *both* the kernel and
 942        the bootloader!
 943
 944  initrd=<file>
 945        An initrd should be loaded.  The meaning of <file> is
 946        obviously bootloader-dependent, and some boot loaders
 947        (e.g. LILO) do not have such a command.
 948
 949In addition, some boot loaders add the following options to the
 950user-specified command line:
 951
 952  BOOT_IMAGE=<file>
 953        The boot image which was loaded.  Again, the meaning of <file>
 954        is obviously bootloader-dependent.
 955
 956  auto
 957        The kernel was booted without explicit user intervention.
 958
 959If these options are added by the boot loader, it is highly
 960recommended that they are located *first*, before the user-specified
 961or configuration-specified command line.  Otherwise, "init=/bin/sh"
 962gets confused by the "auto" option.
 963
 964
 965**** RUNNING THE KERNEL
 966
 967The kernel is started by jumping to the kernel entry point, which is
 968located at *segment* offset 0x20 from the start of the real mode
 969kernel.  This means that if you loaded your real-mode kernel code at
 9700x90000, the kernel entry point is 9020:0000.
 971
 972At entry, ds = es = ss should point to the start of the real-mode
 973kernel code (0x9000 if the code is loaded at 0x90000), sp should be
 974set up properly, normally pointing to the top of the heap, and
 975interrupts should be disabled.  Furthermore, to guard against bugs in
 976the kernel, it is recommended that the boot loader sets fs = gs = ds =
 977es = ss.
 978
 979In our example from above, we would do:
 980
 981        /* Note: in the case of the "old" kernel protocol, base_ptr must
 982           be == 0x90000 at this point; see the previous sample code */
 983
 984        seg = base_ptr >> 4;
 985
 986        cli();  /* Enter with interrupts disabled! */
 987
 988        /* Set up the real-mode kernel stack */
 989        _SS = seg;
 990        _SP = heap_end;
 991
 992        _DS = _ES = _FS = _GS = seg;
 993        jmp_far(seg+0x20, 0);   /* Run the kernel */
 994
 995If your boot sector accesses a floppy drive, it is recommended to
 996switch off the floppy motor before running the kernel, since the
 997kernel boot leaves interrupts off and thus the motor will not be
 998switched off, especially if the loaded kernel has the floppy driver as
 999a demand-loaded module!
1000
1001
1002**** ADVANCED BOOT LOADER HOOKS
1003
1004If the boot loader runs in a particularly hostile environment (such as
1005LOADLIN, which runs under DOS) it may be impossible to follow the
1006standard memory location requirements.  Such a boot loader may use the
1007following hooks that, if set, are invoked by the kernel at the
1008appropriate time.  The use of these hooks should probably be
1009considered an absolutely last resort!
1010
1011IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
1012%edi across invocation.
1013
1014  realmode_swtch:
1015        A 16-bit real mode far subroutine invoked immediately before
1016        entering protected mode.  The default routine disables NMI, so
1017        your routine should probably do so, too.
1018
1019  code32_start:
1020        A 32-bit flat-mode routine *jumped* to immediately after the
1021        transition to protected mode, but before the kernel is
1022        uncompressed.  No segments, except CS, are guaranteed to be
1023        set up (current kernels do, but older ones do not); you should
1024        set them up to BOOT_DS (0x18) yourself.
1025
1026        After completing your hook, you should jump to the address
1027        that was in this field before your boot loader overwrote it
1028        (relocated, if appropriate.)
1029
1030
1031**** 32-bit BOOT PROTOCOL
1032
1033For machine with some new BIOS other than legacy BIOS, such as EFI,
1034LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
1035based on legacy BIOS can not be used, so a 32-bit boot protocol needs
1036to be defined.
1037
1038In 32-bit boot protocol, the first step in loading a Linux kernel
1039should be to setup the boot parameters (struct boot_params,
1040traditionally known as "zero page"). The memory for struct boot_params
1041should be allocated and initialized to all zero. Then the setup header
1042from offset 0x01f1 of kernel image on should be loaded into struct
1043boot_params and examined. The end of setup header can be calculated as
1044follow:
1045
1046        0x0202 + byte value at offset 0x0201
1047
1048In addition to read/modify/write the setup header of the struct
1049boot_params as that of 16-bit boot protocol, the boot loader should
1050also fill the additional fields of the struct boot_params as that
1051described in zero-page.txt.
1052
1053After setting up the struct boot_params, the boot loader can load the
105432/64-bit kernel in the same way as that of 16-bit boot protocol.
1055
1056In 32-bit boot protocol, the kernel is started by jumping to the
105732-bit kernel entry point, which is the start address of loaded
105832/64-bit kernel.
1059
1060At entry, the CPU must be in 32-bit protected mode with paging
1061disabled; a GDT must be loaded with the descriptors for selectors
1062__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1063segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1064must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1065must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
1066address of the struct boot_params; %ebp, %edi and %ebx must be zero.
1067
1068**** 64-bit BOOT PROTOCOL
1069
1070For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader
1071and we need a 64-bit boot protocol.
1072
1073In 64-bit boot protocol, the first step in loading a Linux kernel
1074should be to setup the boot parameters (struct boot_params,
1075traditionally known as "zero page"). The memory for struct boot_params
1076could be allocated anywhere (even above 4G) and initialized to all zero.
1077Then, the setup header at offset 0x01f1 of kernel image on should be
1078loaded into struct boot_params and examined. The end of setup header
1079can be calculated as follows:
1080
1081        0x0202 + byte value at offset 0x0201
1082
1083In addition to read/modify/write the setup header of the struct
1084boot_params as that of 16-bit boot protocol, the boot loader should
1085also fill the additional fields of the struct boot_params as described
1086in zero-page.txt.
1087
1088After setting up the struct boot_params, the boot loader can load
108964-bit kernel in the same way as that of 16-bit boot protocol, but
1090kernel could be loaded above 4G.
1091
1092In 64-bit boot protocol, the kernel is started by jumping to the
109364-bit kernel entry point, which is the start address of loaded
109464-bit kernel plus 0x200.
1095
1096At entry, the CPU must be in 64-bit mode with paging enabled.
1097The range with setup_header.init_size from start address of loaded
1098kernel and zero page and command line buffer get ident mapping;
1099a GDT must be loaded with the descriptors for selectors
1100__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1101segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1102must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1103must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base
1104address of the struct boot_params.
1105
1106**** EFI HANDOVER PROTOCOL
1107
1108This protocol allows boot loaders to defer initialisation to the EFI
1109boot stub. The boot loader is required to load the kernel/initrd(s)
1110from the boot media and jump to the EFI handover protocol entry point
1111which is hdr->handover_offset bytes from the beginning of
1112startup_{32,64}.
1113
1114The function prototype for the handover entry point looks like this,
1115
1116    efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp)
1117
1118'handle' is the EFI image handle passed to the boot loader by the EFI
1119firmware, 'table' is the EFI system table - these are the first two
1120arguments of the "handoff state" as described in section 2.3 of the
1121UEFI specification. 'bp' is the boot loader-allocated boot params.
1122
1123The boot loader *must* fill out the following fields in bp,
1124
1125    o hdr.code32_start
1126    o hdr.cmd_line_ptr
1127    o hdr.ramdisk_image (if applicable)
1128    o hdr.ramdisk_size  (if applicable)
1129
1130All other fields should be zero.
1131
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