1.. SPDX-License-Identifier: GPL-2.0
   4The Linux/x86 Boot Protocol
   7On the x86 platform, the Linux kernel uses a rather complicated boot
   8convention.  This has evolved partially due to historical aspects, as
   9well as the desire in the early days to have the kernel itself be a
  10bootable image, the complicated PC memory model and due to changed
  11expectations in the PC industry caused by the effective demise of
  12real-mode DOS as a mainstream operating system.
  14Currently, the following versions of the Linux/x86 boot protocol exist.
  16=============   ============================================================
  17Old kernels     zImage/Image support only.  Some very early kernels
  18                may not even support a command line.
  20Protocol 2.00   (Kernel 1.3.73) Added bzImage and initrd support, as
  21                well as a formalized way to communicate between the
  22                boot loader and the kernel.  setup.S made relocatable,
  23                although the traditional setup area still assumed
  24                writable.
  26Protocol 2.01   (Kernel 1.3.76) Added a heap overrun warning.
  28Protocol 2.02   (Kernel 2.4.0-test3-pre3) New command line protocol.
  29                Lower the conventional memory ceiling.  No overwrite
  30                of the traditional setup area, thus making booting
  31                safe for systems which use the EBDA from SMM or 32-bit
  32                BIOS entry points.  zImage deprecated but still
  33                supported.
  35Protocol 2.03   (Kernel 2.4.18-pre1) Explicitly makes the highest possible
  36                initrd address available to the bootloader.
  38Protocol 2.04   (Kernel 2.6.14) Extend the syssize field to four bytes.
  40Protocol 2.05   (Kernel 2.6.20) Make protected mode kernel relocatable.
  41                Introduce relocatable_kernel and kernel_alignment fields.
  43Protocol 2.06   (Kernel 2.6.22) Added a field that contains the size of
  44                the boot command line.
  46Protocol 2.07   (Kernel 2.6.24) Added paravirtualised boot protocol.
  47                Introduced hardware_subarch and hardware_subarch_data
  48                and KEEP_SEGMENTS flag in load_flags.
  50Protocol 2.08   (Kernel 2.6.26) Added crc32 checksum and ELF format
  51                payload. Introduced payload_offset and payload_length
  52                fields to aid in locating the payload.
  54Protocol 2.09   (Kernel 2.6.26) Added a field of 64-bit physical
  55                pointer to single linked list of struct setup_data.
  57Protocol 2.10   (Kernel 2.6.31) Added a protocol for relaxed alignment
  58                beyond the kernel_alignment added, new init_size and
  59                pref_address fields.  Added extended boot loader IDs.
  61Protocol 2.11   (Kernel 3.6) Added a field for offset of EFI handover
  62                protocol entry point.
  64Protocol 2.12   (Kernel 3.8) Added the xloadflags field and extension fields
  65                to struct boot_params for loading bzImage and ramdisk
  66                above 4G in 64bit.
  68Protocol 2.13   (Kernel 3.14) Support 32- and 64-bit flags being set in
  69                xloadflags to support booting a 64-bit kernel from 32-bit
  70                EFI
  73                ae7e1238e68f2a472a125673ab506d49158c1889
  74                (x86/boot: Add ACPI RSDP address to setup_header)
  75                DO NOT USE!!! ASSUME SAME AS 2.13.
  77Protocol 2.15   (Kernel 5.5) Added the kernel_info and kernel_info.setup_type_max.
  78=============   ============================================================
  80.. note::
  81     The protocol version number should be changed only if the setup header
  82     is changed. There is no need to update the version number if boot_params
  83     or kernel_info are changed. Additionally, it is recommended to use
  84     xloadflags (in this case the protocol version number should not be
  85     updated either) or kernel_info to communicate supported Linux kernel
  86     features to the boot loader. Due to very limited space available in
  87     the original setup header every update to it should be considered
  88     with great care. Starting from the protocol 2.15 the primary way to
  89     communicate things to the boot loader is the kernel_info.
  92Memory Layout
  95The traditional memory map for the kernel loader, used for Image or
  96zImage kernels, typically looks like::
  98                |                        |
  99        0A0000  +------------------------+
 100                |  Reserved for BIOS     |      Do not use.  Reserved for BIOS EBDA.
 101        09A000  +------------------------+
 102                |  Command line          |
 103                |  Stack/heap            |      For use by the kernel real-mode code.
 104        098000  +------------------------+
 105                |  Kernel setup          |      The kernel real-mode code.
 106        090200  +------------------------+
 107                |  Kernel boot sector    |      The kernel legacy boot sector.
 108        090000  +------------------------+
 109                |  Protected-mode kernel |      The bulk of the kernel image.
 110        010000  +------------------------+
 111                |  Boot loader           |      <- Boot sector entry point 0000:7C00
 112        001000  +------------------------+
 113                |  Reserved for MBR/BIOS |
 114        000800  +------------------------+
 115                |  Typically used by MBR |
 116        000600  +------------------------+
 117                |  BIOS use only         |
 118        000000  +------------------------+
 120When using bzImage, the protected-mode kernel was relocated to
 1210x100000 ("high memory"), and the kernel real-mode block (boot sector,
 122setup, and stack/heap) was made relocatable to any address between
 1230x10000 and end of low memory. Unfortunately, in protocols 2.00 and
 1242.01 the 0x90000+ memory range is still used internally by the kernel;
 125the 2.02 protocol resolves that problem.
 127It is desirable to keep the "memory ceiling" -- the highest point in
 128low memory touched by the boot loader -- as low as possible, since
 129some newer BIOSes have begun to allocate some rather large amounts of
 130memory, called the Extended BIOS Data Area, near the top of low
 131memory.  The boot loader should use the "INT 12h" BIOS call to verify
 132how much low memory is available.
 134Unfortunately, if INT 12h reports that the amount of memory is too
 135low, there is usually nothing the boot loader can do but to report an
 136error to the user.  The boot loader should therefore be designed to
 137take up as little space in low memory as it reasonably can.  For
 138zImage or old bzImage kernels, which need data written into the
 1390x90000 segment, the boot loader should make sure not to use memory
 140above the 0x9A000 point; too many BIOSes will break above that point.
 142For a modern bzImage kernel with boot protocol version >= 2.02, a
 143memory layout like the following is suggested::
 145                ~                        ~
 146                |  Protected-mode kernel |
 147        100000  +------------------------+
 148                |  I/O memory hole       |
 149        0A0000  +------------------------+
 150                |  Reserved for BIOS     |      Leave as much as possible unused
 151                ~                        ~
 152                |  Command line          |      (Can also be below the X+10000 mark)
 153        X+10000 +------------------------+
 154                |  Stack/heap            |      For use by the kernel real-mode code.
 155        X+08000 +------------------------+
 156                |  Kernel setup          |      The kernel real-mode code.
 157                |  Kernel boot sector    |      The kernel legacy boot sector.
 158        X       +------------------------+
 159                |  Boot loader           |      <- Boot sector entry point 0000:7C00
 160        001000  +------------------------+
 161                |  Reserved for MBR/BIOS |
 162        000800  +------------------------+
 163                |  Typically used by MBR |
 164        000600  +------------------------+
 165                |  BIOS use only         |
 166        000000  +------------------------+
 168  ... where the address X is as low as the design of the boot loader permits.
 171The Real-Mode Kernel Header
 174In the following text, and anywhere in the kernel boot sequence, "a
 175sector" refers to 512 bytes.  It is independent of the actual sector
 176size of the underlying medium.
 178The first step in loading a Linux kernel should be to load the
 179real-mode code (boot sector and setup code) and then examine the
 180following header at offset 0x01f1.  The real-mode code can total up to
 18132K, although the boot loader may choose to load only the first two
 182sectors (1K) and then examine the bootup sector size.
 184The header looks like:
 186===========     ========        =====================   ============================================
 187Offset/Size     Proto           Name                    Meaning
 188===========     ========        =====================   ============================================
 18901F1/1          ALL(1)          setup_sects             The size of the setup in sectors
 19001F2/2          ALL             root_flags              If set, the root is mounted readonly
 19101F4/4          2.04+(2)        syssize                 The size of the 32-bit code in 16-byte paras
 19201F8/2          ALL             ram_size                DO NOT USE - for bootsect.S use only
 19301FA/2          ALL             vid_mode                Video mode control
 19401FC/2          ALL             root_dev                Default root device number
 19501FE/2          ALL             boot_flag               0xAA55 magic number
 1960200/2          2.00+           jump                    Jump instruction
 1970202/4          2.00+           header                  Magic signature "HdrS"
 1980206/2          2.00+           version                 Boot protocol version supported
 1990208/4          2.00+           realmode_swtch          Boot loader hook (see below)
 200020C/2          2.00+           start_sys_seg           The load-low segment (0x1000) (obsolete)
 201020E/2          2.00+           kernel_version          Pointer to kernel version string
 2020210/1          2.00+           type_of_loader          Boot loader identifier
 2030211/1          2.00+           loadflags               Boot protocol option flags
 2040212/2          2.00+           setup_move_size         Move to high memory size (used with hooks)
 2050214/4          2.00+           code32_start            Boot loader hook (see below)
 2060218/4          2.00+           ramdisk_image           initrd load address (set by boot loader)
 207021C/4          2.00+           ramdisk_size            initrd size (set by boot loader)
 2080220/4          2.00+           bootsect_kludge         DO NOT USE - for bootsect.S use only
 2090224/2          2.01+           heap_end_ptr            Free memory after setup end
 2100226/1          2.02+(3)        ext_loader_ver          Extended boot loader version
 2110227/1          2.02+(3)        ext_loader_type         Extended boot loader ID
 2120228/4          2.02+           cmd_line_ptr            32-bit pointer to the kernel command line
 213022C/4          2.03+           initrd_addr_max         Highest legal initrd address
 2140230/4          2.05+           kernel_alignment        Physical addr alignment required for kernel
 2150234/1          2.05+           relocatable_kernel      Whether kernel is relocatable or not
 2160235/1          2.10+           min_alignment           Minimum alignment, as a power of two
 2170236/2          2.12+           xloadflags              Boot protocol option flags
 2180238/4          2.06+           cmdline_size            Maximum size of the kernel command line
 219023C/4          2.07+           hardware_subarch        Hardware subarchitecture
 2200240/8          2.07+           hardware_subarch_data   Subarchitecture-specific data
 2210248/4          2.08+           payload_offset          Offset of kernel payload
 222024C/4          2.08+           payload_length          Length of kernel payload
 2230250/8          2.09+           setup_data              64-bit physical pointer to linked list
 224                                                        of struct setup_data
 2250258/8          2.10+           pref_address            Preferred loading address
 2260260/4          2.10+           init_size               Linear memory required during initialization
 2270264/4          2.11+           handover_offset         Offset of handover entry point
 2280268/4          2.15+           kernel_info_offset      Offset of the kernel_info
 229===========     ========        =====================   ============================================
 231.. note::
 232  (1) For backwards compatibility, if the setup_sects field contains 0, the
 233      real value is 4.
 235  (2) For boot protocol prior to 2.04, the upper two bytes of the syssize
 236      field are unusable, which means the size of a bzImage kernel
 237      cannot be determined.
 239  (3) Ignored, but safe to set, for boot protocols 2.02-2.09.
 241If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
 242the boot protocol version is "old".  Loading an old kernel, the
 243following parameters should be assumed::
 245        Image type = zImage
 246        initrd not supported
 247        Real-mode kernel must be located at 0x90000.
 249Otherwise, the "version" field contains the protocol version,
 250e.g. protocol version 2.01 will contain 0x0201 in this field.  When
 251setting fields in the header, you must make sure only to set fields
 252supported by the protocol version in use.
 255Details of Header Fields
 258For each field, some are information from the kernel to the bootloader
 259("read"), some are expected to be filled out by the bootloader
 260("write"), and some are expected to be read and modified by the
 261bootloader ("modify").
 263All general purpose boot loaders should write the fields marked
 264(obligatory).  Boot loaders who want to load the kernel at a
 265nonstandard address should fill in the fields marked (reloc); other
 266boot loaders can ignore those fields.
 268The byte order of all fields is littleendian (this is x86, after all.)
 270============    ===========
 271Field name:     setup_sects
 272Type:           read
 273Offset/size:    0x1f1/1
 274Protocol:       ALL
 275============    ===========
 277  The size of the setup code in 512-byte sectors.  If this field is
 278  0, the real value is 4.  The real-mode code consists of the boot
 279  sector (always one 512-byte sector) plus the setup code.
 281============    =================
 282Field name:     root_flags
 283Type:           modify (optional)
 284Offset/size:    0x1f2/2
 285Protocol:       ALL
 286============    =================
 288  If this field is nonzero, the root defaults to readonly.  The use of
 289  this field is deprecated; use the "ro" or "rw" options on the
 290  command line instead.
 292============    ===============================================
 293Field name:     syssize
 294Type:           read
 295Offset/size:    0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
 296Protocol:       2.04+
 297============    ===============================================
 299  The size of the protected-mode code in units of 16-byte paragraphs.
 300  For protocol versions older than 2.04 this field is only two bytes
 301  wide, and therefore cannot be trusted for the size of a kernel if
 302  the LOAD_HIGH flag is set.
 304============    ===============
 305Field name:     ram_size
 306Type:           kernel internal
 307Offset/size:    0x1f8/2
 308Protocol:       ALL
 309============    ===============
 311  This field is obsolete.
 313============    ===================
 314Field name:     vid_mode
 315Type:           modify (obligatory)
 316Offset/size:    0x1fa/2
 317============    ===================
 319  Please see the section on SPECIAL COMMAND LINE OPTIONS.
 321============    =================
 322Field name:     root_dev
 323Type:           modify (optional)
 324Offset/size:    0x1fc/2
 325Protocol:       ALL
 326============    =================
 328  The default root device device number.  The use of this field is
 329  deprecated, use the "root=" option on the command line instead.
 331============    =========
 332Field name:     boot_flag
 333Type:           read
 334Offset/size:    0x1fe/2
 335Protocol:       ALL
 336============    =========
 338  Contains 0xAA55.  This is the closest thing old Linux kernels have
 339  to a magic number.
 341============    =======
 342Field name:     jump
 343Type:           read
 344Offset/size:    0x200/2
 345Protocol:       2.00+
 346============    =======
 348  Contains an x86 jump instruction, 0xEB followed by a signed offset
 349  relative to byte 0x202.  This can be used to determine the size of
 350  the header.
 352============    =======
 353Field name:     header
 354Type:           read
 355Offset/size:    0x202/4
 356Protocol:       2.00+
 357============    =======
 359  Contains the magic number "HdrS" (0x53726448).
 361============    =======
 362Field name:     version
 363Type:           read
 364Offset/size:    0x206/2
 365Protocol:       2.00+
 366============    =======
 368  Contains the boot protocol version, in (major << 8)+minor format,
 369  e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
 370  10.17.
 372============    =================
 373Field name:     realmode_swtch
 374Type:           modify (optional)
 375Offset/size:    0x208/4
 376Protocol:       2.00+
 377============    =================
 379  Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
 381============    =============
 382Field name:     start_sys_seg
 383Type:           read
 384Offset/size:    0x20c/2
 385Protocol:       2.00+
 386============    =============
 388  The load low segment (0x1000).  Obsolete.
 390============    ==============
 391Field name:     kernel_version
 392Type:           read
 393Offset/size:    0x20e/2
 394Protocol:       2.00+
 395============    ==============
 397  If set to a nonzero value, contains a pointer to a NUL-terminated
 398  human-readable kernel version number string, less 0x200.  This can
 399  be used to display the kernel version to the user.  This value
 400  should be less than (0x200*setup_sects).
 402  For example, if this value is set to 0x1c00, the kernel version
 403  number string can be found at offset 0x1e00 in the kernel file.
 404  This is a valid value if and only if the "setup_sects" field
 405  contains the value 15 or higher, as::
 407        0x1c00  < 15*0x200 (= 0x1e00) but
 408        0x1c00 >= 14*0x200 (= 0x1c00)
 410        0x1c00 >> 9 = 14, So the minimum value for setup_secs is 15.
 412============    ==================
 413Field name:     type_of_loader
 414Type:           write (obligatory)
 415Offset/size:    0x210/1
 416Protocol:       2.00+
 417============    ==================
 419  If your boot loader has an assigned id (see table below), enter
 420  0xTV here, where T is an identifier for the boot loader and V is
 421  a version number.  Otherwise, enter 0xFF here.
 423  For boot loader IDs above T = 0xD, write T = 0xE to this field and
 424  write the extended ID minus 0x10 to the ext_loader_type field.
 425  Similarly, the ext_loader_ver field can be used to provide more than
 426  four bits for the bootloader version.
 428  For example, for T = 0x15, V = 0x234, write::
 430        type_of_loader  <- 0xE4
 431        ext_loader_type <- 0x05
 432        ext_loader_ver  <- 0x23
 434  Assigned boot loader ids (hexadecimal):
 436        == =======================================
 437        0  LILO
 438           (0x00 reserved for pre-2.00 bootloader)
 439        1  Loadlin
 440        2  bootsect-loader
 441           (0x20, all other values reserved)
 442        3  Syslinux
 443        4  Etherboot/gPXE/iPXE
 444        5  ELILO
 445        7  GRUB
 446        8  U-Boot
 447        9  Xen
 448        A  Gujin
 449        B  Qemu
 450        C  Arcturus Networks uCbootloader
 451        D  kexec-tools
 452        E  Extended (see ext_loader_type)
 453        F  Special (0xFF = undefined)
 454        10 Reserved
 455        11 Minimal Linux Bootloader
 456           <>
 457        12 OVMF UEFI virtualization stack
 458        == =======================================
 460  Please contact <> if you need a bootloader ID value assigned.
 462============    ===================
 463Field name:     loadflags
 464Type:           modify (obligatory)
 465Offset/size:    0x211/1
 466Protocol:       2.00+
 467============    ===================
 469  This field is a bitmask.
 471  Bit 0 (read): LOADED_HIGH
 473        - If 0, the protected-mode code is loaded at 0x10000.
 474        - If 1, the protected-mode code is loaded at 0x100000.
 476  Bit 1 (kernel internal): KASLR_FLAG
 478        - Used internally by the compressed kernel to communicate
 479          KASLR status to kernel proper.
 481            - If 1, KASLR enabled.
 482            - If 0, KASLR disabled.
 484  Bit 5 (write): QUIET_FLAG
 486        - If 0, print early messages.
 487        - If 1, suppress early messages.
 489                This requests to the kernel (decompressor and early
 490                kernel) to not write early messages that require
 491                accessing the display hardware directly.
 493  Bit 6 (obsolete): KEEP_SEGMENTS
 495        Protocol: 2.07+
 497        - This flag is obsolete.
 499  Bit 7 (write): CAN_USE_HEAP
 501        Set this bit to 1 to indicate that the value entered in the
 502        heap_end_ptr is valid.  If this field is clear, some setup code
 503        functionality will be disabled.
 506============    ===================
 507Field name:     setup_move_size
 508Type:           modify (obligatory)
 509Offset/size:    0x212/2
 510Protocol:       2.00-2.01
 511============    ===================
 513  When using protocol 2.00 or 2.01, if the real mode kernel is not
 514  loaded at 0x90000, it gets moved there later in the loading
 515  sequence.  Fill in this field if you want additional data (such as
 516  the kernel command line) moved in addition to the real-mode kernel
 517  itself.
 519  The unit is bytes starting with the beginning of the boot sector.
 521  This field is can be ignored when the protocol is 2.02 or higher, or
 522  if the real-mode code is loaded at 0x90000.
 524============    ========================
 525Field name:     code32_start
 526Type:           modify (optional, reloc)
 527Offset/size:    0x214/4
 528Protocol:       2.00+
 529============    ========================
 531  The address to jump to in protected mode.  This defaults to the load
 532  address of the kernel, and can be used by the boot loader to
 533  determine the proper load address.
 535  This field can be modified for two purposes:
 537    1. as a boot loader hook (see Advanced Boot Loader Hooks below.)
 539    2. if a bootloader which does not install a hook loads a
 540       relocatable kernel at a nonstandard address it will have to modify
 541       this field to point to the load address.
 543============    ==================
 544Field name:     ramdisk_image
 545Type:           write (obligatory)
 546Offset/size:    0x218/4
 547Protocol:       2.00+
 548============    ==================
 550  The 32-bit linear address of the initial ramdisk or ramfs.  Leave at
 551  zero if there is no initial ramdisk/ramfs.
 553============    ==================
 554Field name:     ramdisk_size
 555Type:           write (obligatory)
 556Offset/size:    0x21c/4
 557Protocol:       2.00+
 558============    ==================
 560  Size of the initial ramdisk or ramfs.  Leave at zero if there is no
 561  initial ramdisk/ramfs.
 563============    ===============
 564Field name:     bootsect_kludge
 565Type:           kernel internal
 566Offset/size:    0x220/4
 567Protocol:       2.00+
 568============    ===============
 570  This field is obsolete.
 572============    ==================
 573Field name:     heap_end_ptr
 574Type:           write (obligatory)
 575Offset/size:    0x224/2
 576Protocol:       2.01+
 577============    ==================
 579  Set this field to the offset (from the beginning of the real-mode
 580  code) of the end of the setup stack/heap, minus 0x0200.
 582============    ================
 583Field name:     ext_loader_ver
 584Type:           write (optional)
 585Offset/size:    0x226/1
 586Protocol:       2.02+
 587============    ================
 589  This field is used as an extension of the version number in the
 590  type_of_loader field.  The total version number is considered to be
 591  (type_of_loader & 0x0f) + (ext_loader_ver << 4).
 593  The use of this field is boot loader specific.  If not written, it
 594  is zero.
 596  Kernels prior to 2.6.31 did not recognize this field, but it is safe
 597  to write for protocol version 2.02 or higher.
 599============    =====================================================
 600Field name:     ext_loader_type
 601Type:           write (obligatory if (type_of_loader & 0xf0) == 0xe0)
 602Offset/size:    0x227/1
 603Protocol:       2.02+
 604============    =====================================================
 606  This field is used as an extension of the type number in
 607  type_of_loader field.  If the type in type_of_loader is 0xE, then
 608  the actual type is (ext_loader_type + 0x10).
 610  This field is ignored if the type in type_of_loader is not 0xE.
 612  Kernels prior to 2.6.31 did not recognize this field, but it is safe
 613  to write for protocol version 2.02 or higher.
 615============    ==================
 616Field name:     cmd_line_ptr
 617Type:           write (obligatory)
 618Offset/size:    0x228/4
 619Protocol:       2.02+
 620============    ==================
 622  Set this field to the linear address of the kernel command line.
 623  The kernel command line can be located anywhere between the end of
 624  the setup heap and 0xA0000; it does not have to be located in the
 625  same 64K segment as the real-mode code itself.
 627  Fill in this field even if your boot loader does not support a
 628  command line, in which case you can point this to an empty string
 629  (or better yet, to the string "auto".)  If this field is left at
 630  zero, the kernel will assume that your boot loader does not support
 631  the 2.02+ protocol.
 633============    ===============
 634Field name:     initrd_addr_max
 635Type:           read
 636Offset/size:    0x22c/4
 637Protocol:       2.03+
 638============    ===============
 640  The maximum address that may be occupied by the initial
 641  ramdisk/ramfs contents.  For boot protocols 2.02 or earlier, this
 642  field is not present, and the maximum address is 0x37FFFFFF.  (This
 643  address is defined as the address of the highest safe byte, so if
 644  your ramdisk is exactly 131072 bytes long and this field is
 645  0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
 647============    ============================
 648Field name:     kernel_alignment
 649Type:           read/modify (reloc)
 650Offset/size:    0x230/4
 651Protocol:       2.05+ (read), 2.10+ (modify)
 652============    ============================
 654  Alignment unit required by the kernel (if relocatable_kernel is
 655  true.)  A relocatable kernel that is loaded at an alignment
 656  incompatible with the value in this field will be realigned during
 657  kernel initialization.
 659  Starting with protocol version 2.10, this reflects the kernel
 660  alignment preferred for optimal performance; it is possible for the
 661  loader to modify this field to permit a lesser alignment.  See the
 662  min_alignment and pref_address field below.
 664============    ==================
 665Field name:     relocatable_kernel
 666Type:           read (reloc)
 667Offset/size:    0x234/1
 668Protocol:       2.05+
 669============    ==================
 671  If this field is nonzero, the protected-mode part of the kernel can
 672  be loaded at any address that satisfies the kernel_alignment field.
 673  After loading, the boot loader must set the code32_start field to
 674  point to the loaded code, or to a boot loader hook.
 676============    =============
 677Field name:     min_alignment
 678Type:           read (reloc)
 679Offset/size:    0x235/1
 680Protocol:       2.10+
 681============    =============
 683  This field, if nonzero, indicates as a power of two the minimum
 684  alignment required, as opposed to preferred, by the kernel to boot.
 685  If a boot loader makes use of this field, it should update the
 686  kernel_alignment field with the alignment unit desired; typically::
 688        kernel_alignment = 1 << min_alignment
 690  There may be a considerable performance cost with an excessively
 691  misaligned kernel.  Therefore, a loader should typically try each
 692  power-of-two alignment from kernel_alignment down to this alignment.
 694============    ==========
 695Field name:     xloadflags
 696Type:           read
 697Offset/size:    0x236/2
 698Protocol:       2.12+
 699============    ==========
 701  This field is a bitmask.
 703  Bit 0 (read): XLF_KERNEL_64
 705        - If 1, this kernel has the legacy 64-bit entry point at 0x200.
 707  Bit 1 (read): XLF_CAN_BE_LOADED_ABOVE_4G
 709        - If 1, kernel/boot_params/cmdline/ramdisk can be above 4G.
 711  Bit 2 (read): XLF_EFI_HANDOVER_32
 713        - If 1, the kernel supports the 32-bit EFI handoff entry point
 714          given at handover_offset.
 716  Bit 3 (read): XLF_EFI_HANDOVER_64
 718        - If 1, the kernel supports the 64-bit EFI handoff entry point
 719          given at handover_offset + 0x200.
 721  Bit 4 (read): XLF_EFI_KEXEC
 723        - If 1, the kernel supports kexec EFI boot with EFI runtime support.
 726============    ============
 727Field name:     cmdline_size
 728Type:           read
 729Offset/size:    0x238/4
 730Protocol:       2.06+
 731============    ============
 733  The maximum size of the command line without the terminating
 734  zero. This means that the command line can contain at most
 735  cmdline_size characters. With protocol version 2.05 and earlier, the
 736  maximum size was 255.
 738============    ====================================
 739Field name:     hardware_subarch
 740Type:           write (optional, defaults to x86/PC)
 741Offset/size:    0x23c/4
 742Protocol:       2.07+
 743============    ====================================
 745  In a paravirtualized environment the hardware low level architectural
 746  pieces such as interrupt handling, page table handling, and
 747  accessing process control registers needs to be done differently.
 749  This field allows the bootloader to inform the kernel we are in one
 750  one of those environments.
 752  ==========    ==============================
 753  0x00000000    The default x86/PC environment
 754  0x00000001    lguest
 755  0x00000002    Xen
 756  0x00000003    Moorestown MID
 757  0x00000004    CE4100 TV Platform
 758  ==========    ==============================
 760============    =========================
 761Field name:     hardware_subarch_data
 762Type:           write (subarch-dependent)
 763Offset/size:    0x240/8
 764Protocol:       2.07+
 765============    =========================
 767  A pointer to data that is specific to hardware subarch
 768  This field is currently unused for the default x86/PC environment,
 769  do not modify.
 771============    ==============
 772Field name:     payload_offset
 773Type:           read
 774Offset/size:    0x248/4
 775Protocol:       2.08+
 776============    ==============
 778  If non-zero then this field contains the offset from the beginning
 779  of the protected-mode code to the payload.
 781  The payload may be compressed. The format of both the compressed and
 782  uncompressed data should be determined using the standard magic
 783  numbers.  The currently supported compression formats are gzip
 784  (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA
 785  (magic number 5D 00), XZ (magic number FD 37), LZ4 (magic number
 786  02 21) and ZSTD (magic number 28 B5). The uncompressed payload is
 787  currently always ELF (magic number 7F 45 4C 46).
 789============    ==============
 790Field name:     payload_length
 791Type:           read
 792Offset/size:    0x24c/4
 793Protocol:       2.08+
 794============    ==============
 796  The length of the payload.
 798============    ===============
 799Field name:     setup_data
 800Type:           write (special)
 801Offset/size:    0x250/8
 802Protocol:       2.09+
 803============    ===============
 805  The 64-bit physical pointer to NULL terminated single linked list of
 806  struct setup_data. This is used to define a more extensible boot
 807  parameters passing mechanism. The definition of struct setup_data is
 808  as follow::
 810        struct setup_data {
 811                u64 next;
 812                u32 type;
 813                u32 len;
 814                u8  data[0];
 815        };
 817  Where, the next is a 64-bit physical pointer to the next node of
 818  linked list, the next field of the last node is 0; the type is used
 819  to identify the contents of data; the len is the length of data
 820  field; the data holds the real payload.
 822  This list may be modified at a number of points during the bootup
 823  process.  Therefore, when modifying this list one should always make
 824  sure to consider the case where the linked list already contains
 825  entries.
 827  The setup_data is a bit awkward to use for extremely large data objects,
 828  both because the setup_data header has to be adjacent to the data object
 829  and because it has a 32-bit length field. However, it is important that
 830  intermediate stages of the boot process have a way to identify which
 831  chunks of memory are occupied by kernel data.
 833  Thus setup_indirect struct and SETUP_INDIRECT type were introduced in
 834  protocol 2.15::
 836    struct setup_indirect {
 837      __u32 type;
 838      __u32 reserved;  /* Reserved, must be set to zero. */
 839      __u64 len;
 840      __u64 addr;
 841    };
 843  The type member is a SETUP_INDIRECT | SETUP_* type. However, it cannot be
 844  SETUP_INDIRECT itself since making the setup_indirect a tree structure
 845  could require a lot of stack space in something that needs to parse it
 846  and stack space can be limited in boot contexts.
 848  Let's give an example how to point to SETUP_E820_EXT data using setup_indirect.
 849  In this case setup_data and setup_indirect will look like this::
 851    struct setup_data {
 852      __u64 next = 0 or <addr_of_next_setup_data_struct>;
 853      __u32 type = SETUP_INDIRECT;
 854      __u32 len = sizeof(setup_indirect);
 855      __u8 data[sizeof(setup_indirect)] = struct setup_indirect {
 856        __u32 type = SETUP_INDIRECT | SETUP_E820_EXT;
 857        __u32 reserved = 0;
 858        __u64 len = <len_of_SETUP_E820_EXT_data>;
 859        __u64 addr = <addr_of_SETUP_E820_EXT_data>;
 860      }
 861    }
 863.. note::
 864     SETUP_INDIRECT | SETUP_NONE objects cannot be properly distinguished
 865     from SETUP_INDIRECT itself. So, this kind of objects cannot be provided
 866     by the bootloaders.
 868============    ============
 869Field name:     pref_address
 870Type:           read (reloc)
 871Offset/size:    0x258/8
 872Protocol:       2.10+
 873============    ============
 875  This field, if nonzero, represents a preferred load address for the
 876  kernel.  A relocating bootloader should attempt to load at this
 877  address if possible.
 879  A non-relocatable kernel will unconditionally move itself and to run
 880  at this address.
 882============    =======
 883Field name:     init_size
 884Type:           read
 885Offset/size:    0x260/4
 886============    =======
 888  This field indicates the amount of linear contiguous memory starting
 889  at the kernel runtime start address that the kernel needs before it
 890  is capable of examining its memory map.  This is not the same thing
 891  as the total amount of memory the kernel needs to boot, but it can
 892  be used by a relocating boot loader to help select a safe load
 893  address for the kernel.
 895  The kernel runtime start address is determined by the following algorithm::
 897        if (relocatable_kernel)
 898        runtime_start = align_up(load_address, kernel_alignment)
 899        else
 900        runtime_start = pref_address
 902============    ===============
 903Field name:     handover_offset
 904Type:           read
 905Offset/size:    0x264/4
 906============    ===============
 908  This field is the offset from the beginning of the kernel image to
 909  the EFI handover protocol entry point. Boot loaders using the EFI
 910  handover protocol to boot the kernel should jump to this offset.
 912  See EFI HANDOVER PROTOCOL below for more details.
 914============    ==================
 915Field name:     kernel_info_offset
 916Type:           read
 917Offset/size:    0x268/4
 918Protocol:       2.15+
 919============    ==================
 921  This field is the offset from the beginning of the kernel image to the
 922  kernel_info. The kernel_info structure is embedded in the Linux image
 923  in the uncompressed protected mode region.
 926The kernel_info
 929The relationships between the headers are analogous to the various data
 932  setup_header = .data
 933  boot_params/setup_data = .bss
 935What is missing from the above list? That's right:
 937  kernel_info = .rodata
 939We have been (ab)using .data for things that could go into .rodata or .bss for
 940a long time, for lack of alternatives and -- especially early on -- inertia.
 941Also, the BIOS stub is responsible for creating boot_params, so it isn't
 942available to a BIOS-based loader (setup_data is, though).
 944setup_header is permanently limited to 144 bytes due to the reach of the
 9452-byte jump field, which doubles as a length field for the structure, combined
 946with the size of the "hole" in struct boot_params that a protected-mode loader
 947or the BIOS stub has to copy it into. It is currently 119 bytes long, which
 948leaves us with 25 very precious bytes. This isn't something that can be fixed
 949without revising the boot protocol entirely, breaking backwards compatibility.
 951boot_params proper is limited to 4096 bytes, but can be arbitrarily extended
 952by adding setup_data entries. It cannot be used to communicate properties of
 953the kernel image, because it is .bss and has no image-provided content.
 955kernel_info solves this by providing an extensible place for information about
 956the kernel image. It is readonly, because the kernel cannot rely on a
 957bootloader copying its contents anywhere, but that is OK; if it becomes
 958necessary it can still contain data items that an enabled bootloader would be
 959expected to copy into a setup_data chunk.
 961All kernel_info data should be part of this structure. Fixed size data have to
 962be put before kernel_info_var_len_data label. Variable size data have to be put
 963after kernel_info_var_len_data label. Each chunk of variable size data has to
 964be prefixed with header/magic and its size, e.g.::
 966  kernel_info:
 967          .ascii  "LToP"          /* Header, Linux top (structure). */
 968          .long   kernel_info_var_len_data - kernel_info
 969          .long   kernel_info_end - kernel_info
 970          .long   0x01234567      /* Some fixed size data for the bootloaders. */
 971  kernel_info_var_len_data:
 972  example_struct:                 /* Some variable size data for the bootloaders. */
 973          .ascii  "0123"          /* Header/Magic. */
 974          .long   example_struct_end - example_struct
 975          .ascii  "Struct"
 976          .long   0x89012345
 977  example_struct_end:
 978  example_strings:                /* Some variable size data for the bootloaders. */
 979          .ascii  "ABCD"          /* Header/Magic. */
 980          .long   example_strings_end - example_strings
 981          .asciz  "String_0"
 982          .asciz  "String_1"
 983  example_strings_end:
 984  kernel_info_end:
 986This way the kernel_info is self-contained blob.
 988.. note::
 989     Each variable size data header/magic can be any 4-character string,
 990     without \0 at the end of the string, which does not collide with
 991     existing variable length data headers/magics.
 994Details of the kernel_info Fields
 997============    ========
 998Field name:     header
 999Offset/size:    0x0000/4
1000============    ========
1002  Contains the magic number "LToP" (0x506f544c).
1004============    ========
1005Field name:     size
1006Offset/size:    0x0004/4
1007============    ========
1009  This field contains the size of the kernel_info including kernel_info.header.
1010  It does not count kernel_info.kernel_info_var_len_data size. This field should be
1011  used by the bootloaders to detect supported fixed size fields in the kernel_info
1012  and beginning of kernel_info.kernel_info_var_len_data.
1014============    ========
1015Field name:     size_total
1016Offset/size:    0x0008/4
1017============    ========
1019  This field contains the size of the kernel_info including kernel_info.header
1020  and kernel_info.kernel_info_var_len_data.
1022============    ==============
1023Field name:     setup_type_max
1024Offset/size:    0x000c/4
1025============    ==============
1027  This field contains maximal allowed type for setup_data and setup_indirect structs.
1030The Image Checksum
1033From boot protocol version 2.08 onwards the CRC-32 is calculated over
1034the entire file using the characteristic polynomial 0x04C11DB7 and an
1035initial remainder of 0xffffffff.  The checksum is appended to the
1036file; therefore the CRC of the file up to the limit specified in the
1037syssize field of the header is always 0.
1040The Kernel Command Line
1043The kernel command line has become an important way for the boot
1044loader to communicate with the kernel.  Some of its options are also
1045relevant to the boot loader itself, see "special command line options"
1048The kernel command line is a null-terminated string. The maximum
1049length can be retrieved from the field cmdline_size.  Before protocol
1050version 2.06, the maximum was 255 characters.  A string that is too
1051long will be automatically truncated by the kernel.
1053If the boot protocol version is 2.02 or later, the address of the
1054kernel command line is given by the header field cmd_line_ptr (see
1055above.)  This address can be anywhere between the end of the setup
1056heap and 0xA0000.
1058If the protocol version is *not* 2.02 or higher, the kernel
1059command line is entered using the following protocol:
1061  - At offset 0x0020 (word), "cmd_line_magic", enter the magic
1062    number 0xA33F.
1064  - At offset 0x0022 (word), "cmd_line_offset", enter the offset
1065    of the kernel command line (relative to the start of the
1066    real-mode kernel).
1068  - The kernel command line *must* be within the memory region
1069    covered by setup_move_size, so you may need to adjust this
1070    field.
1073Memory Layout of The Real-Mode Code
1076The real-mode code requires a stack/heap to be set up, as well as
1077memory allocated for the kernel command line.  This needs to be done
1078in the real-mode accessible memory in bottom megabyte.
1080It should be noted that modern machines often have a sizable Extended
1081BIOS Data Area (EBDA).  As a result, it is advisable to use as little
1082of the low megabyte as possible.
1084Unfortunately, under the following circumstances the 0x90000 memory
1085segment has to be used:
1087        - When loading a zImage kernel ((loadflags & 0x01) == 0).
1088        - When loading a 2.01 or earlier boot protocol kernel.
1090.. note::
1091     For the 2.00 and 2.01 boot protocols, the real-mode code
1092     can be loaded at another address, but it is internally
1093     relocated to 0x90000.  For the "old" protocol, the
1094     real-mode code must be loaded at 0x90000.
1096When loading at 0x90000, avoid using memory above 0x9a000.
1098For boot protocol 2.02 or higher, the command line does not have to be
1099located in the same 64K segment as the real-mode setup code; it is
1100thus permitted to give the stack/heap the full 64K segment and locate
1101the command line above it.
1103The kernel command line should not be located below the real-mode
1104code, nor should it be located in high memory.
1107Sample Boot Configuartion
1110As a sample configuration, assume the following layout of the real
1111mode segment.
1113    When loading below 0x90000, use the entire segment:
1115        =============   ===================
1116        0x0000-0x7fff   Real mode kernel
1117        0x8000-0xdfff   Stack and heap
1118        0xe000-0xffff   Kernel command line
1119        =============   ===================
1121    When loading at 0x90000 OR the protocol version is 2.01 or earlier:
1123        =============   ===================
1124        0x0000-0x7fff   Real mode kernel
1125        0x8000-0x97ff   Stack and heap
1126        0x9800-0x9fff   Kernel command line
1127        =============   ===================
1129Such a boot loader should enter the following fields in the header::
1131        unsigned long base_ptr; /* base address for real-mode segment */
1133        if ( setup_sects == 0 ) {
1134                setup_sects = 4;
1135        }
1137        if ( protocol >= 0x0200 ) {
1138                type_of_loader = <type code>;
1139                if ( loading_initrd ) {
1140                        ramdisk_image = <initrd_address>;
1141                        ramdisk_size = <initrd_size>;
1142                }
1144                if ( protocol >= 0x0202 && loadflags & 0x01 )
1145                        heap_end = 0xe000;
1146                else
1147                        heap_end = 0x9800;
1149                if ( protocol >= 0x0201 ) {
1150                        heap_end_ptr = heap_end - 0x200;
1151                        loadflags |= 0x80; /* CAN_USE_HEAP */
1152                }
1154                if ( protocol >= 0x0202 ) {
1155                        cmd_line_ptr = base_ptr + heap_end;
1156                        strcpy(cmd_line_ptr, cmdline);
1157                } else {
1158                        cmd_line_magic  = 0xA33F;
1159                        cmd_line_offset = heap_end;
1160                        setup_move_size = heap_end + strlen(cmdline)+1;
1161                        strcpy(base_ptr+cmd_line_offset, cmdline);
1162                }
1163        } else {
1164                /* Very old kernel */
1166                heap_end = 0x9800;
1168                cmd_line_magic  = 0xA33F;
1169                cmd_line_offset = heap_end;
1171                /* A very old kernel MUST have its real-mode code
1172                   loaded at 0x90000 */
1174                if ( base_ptr != 0x90000 ) {
1175                        /* Copy the real-mode kernel */
1176                        memcpy(0x90000, base_ptr, (setup_sects+1)*512);
1177                        base_ptr = 0x90000;              /* Relocated */
1178                }
1180                strcpy(0x90000+cmd_line_offset, cmdline);
1182                /* It is recommended to clear memory up to the 32K mark */
1183                memset(0x90000 + (setup_sects+1)*512, 0,
1184                       (64-(setup_sects+1))*512);
1185        }
1188Loading The Rest of The Kernel
1191The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
1192in the kernel file (again, if setup_sects == 0 the real value is 4.)
1193It should be loaded at address 0x10000 for Image/zImage kernels and
11940x100000 for bzImage kernels.
1196The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
1197bit (LOAD_HIGH) in the loadflags field is set::
1199        is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
1200        load_address = is_bzImage ? 0x100000 : 0x10000;
1202Note that Image/zImage kernels can be up to 512K in size, and thus use
1203the entire 0x10000-0x90000 range of memory.  This means it is pretty
1204much a requirement for these kernels to load the real-mode part at
12050x90000.  bzImage kernels allow much more flexibility.
1207Special Command Line Options
1210If the command line provided by the boot loader is entered by the
1211user, the user may expect the following command line options to work.
1212They should normally not be deleted from the kernel command line even
1213though not all of them are actually meaningful to the kernel.  Boot
1214loader authors who need additional command line options for the boot
1215loader itself should get them registered in
1216Documentation/admin-guide/kernel-parameters.rst to make sure they will not
1217conflict with actual kernel options now or in the future.
1219  vga=<mode>
1220        <mode> here is either an integer (in C notation, either
1221        decimal, octal, or hexadecimal) or one of the strings
1222        "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
1223        (meaning 0xFFFD).  This value should be entered into the
1224        vid_mode field, as it is used by the kernel before the command
1225        line is parsed.
1227  mem=<size>
1228        <size> is an integer in C notation optionally followed by
1229        (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
1230        << 30, << 40, << 50 or << 60).  This specifies the end of
1231        memory to the kernel. This affects the possible placement of
1232        an initrd, since an initrd should be placed near end of
1233        memory.  Note that this is an option to *both* the kernel and
1234        the bootloader!
1236  initrd=<file>
1237        An initrd should be loaded.  The meaning of <file> is
1238        obviously bootloader-dependent, and some boot loaders
1239        (e.g. LILO) do not have such a command.
1241In addition, some boot loaders add the following options to the
1242user-specified command line:
1244  BOOT_IMAGE=<file>
1245        The boot image which was loaded.  Again, the meaning of <file>
1246        is obviously bootloader-dependent.
1248  auto
1249        The kernel was booted without explicit user intervention.
1251If these options are added by the boot loader, it is highly
1252recommended that they are located *first*, before the user-specified
1253or configuration-specified command line.  Otherwise, "init=/bin/sh"
1254gets confused by the "auto" option.
1257Running the Kernel
1260The kernel is started by jumping to the kernel entry point, which is
1261located at *segment* offset 0x20 from the start of the real mode
1262kernel.  This means that if you loaded your real-mode kernel code at
12630x90000, the kernel entry point is 9020:0000.
1265At entry, ds = es = ss should point to the start of the real-mode
1266kernel code (0x9000 if the code is loaded at 0x90000), sp should be
1267set up properly, normally pointing to the top of the heap, and
1268interrupts should be disabled.  Furthermore, to guard against bugs in
1269the kernel, it is recommended that the boot loader sets fs = gs = ds =
1270es = ss.
1272In our example from above, we would do::
1274        /* Note: in the case of the "old" kernel protocol, base_ptr must
1275           be == 0x90000 at this point; see the previous sample code */
1277        seg = base_ptr >> 4;
1279        cli();  /* Enter with interrupts disabled! */
1281        /* Set up the real-mode kernel stack */
1282        _SS = seg;
1283        _SP = heap_end;
1285        _DS = _ES = _FS = _GS = seg;
1286        jmp_far(seg+0x20, 0);   /* Run the kernel */
1288If your boot sector accesses a floppy drive, it is recommended to
1289switch off the floppy motor before running the kernel, since the
1290kernel boot leaves interrupts off and thus the motor will not be
1291switched off, especially if the loaded kernel has the floppy driver as
1292a demand-loaded module!
1295Advanced Boot Loader Hooks
1298If the boot loader runs in a particularly hostile environment (such as
1299LOADLIN, which runs under DOS) it may be impossible to follow the
1300standard memory location requirements.  Such a boot loader may use the
1301following hooks that, if set, are invoked by the kernel at the
1302appropriate time.  The use of these hooks should probably be
1303considered an absolutely last resort!
1305IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
1306%edi across invocation.
1308  realmode_swtch:
1309        A 16-bit real mode far subroutine invoked immediately before
1310        entering protected mode.  The default routine disables NMI, so
1311        your routine should probably do so, too.
1313  code32_start:
1314        A 32-bit flat-mode routine *jumped* to immediately after the
1315        transition to protected mode, but before the kernel is
1316        uncompressed.  No segments, except CS, are guaranteed to be
1317        set up (current kernels do, but older ones do not); you should
1318        set them up to BOOT_DS (0x18) yourself.
1320        After completing your hook, you should jump to the address
1321        that was in this field before your boot loader overwrote it
1322        (relocated, if appropriate.)
132532-bit Boot Protocol
1328For machine with some new BIOS other than legacy BIOS, such as EFI,
1329LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
1330based on legacy BIOS can not be used, so a 32-bit boot protocol needs
1331to be defined.
1333In 32-bit boot protocol, the first step in loading a Linux kernel
1334should be to setup the boot parameters (struct boot_params,
1335traditionally known as "zero page"). The memory for struct boot_params
1336should be allocated and initialized to all zero. Then the setup header
1337from offset 0x01f1 of kernel image on should be loaded into struct
1338boot_params and examined. The end of setup header can be calculated as
1341        0x0202 + byte value at offset 0x0201
1343In addition to read/modify/write the setup header of the struct
1344boot_params as that of 16-bit boot protocol, the boot loader should
1345also fill the additional fields of the struct boot_params as
1346described in chapter Documentation/x86/zero-page.rst.
1348After setting up the struct boot_params, the boot loader can load the
134932/64-bit kernel in the same way as that of 16-bit boot protocol.
1351In 32-bit boot protocol, the kernel is started by jumping to the
135232-bit kernel entry point, which is the start address of loaded
135332/64-bit kernel.
1355At entry, the CPU must be in 32-bit protected mode with paging
1356disabled; a GDT must be loaded with the descriptors for selectors
1357__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1358segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1359must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1360must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
1361address of the struct boot_params; %ebp, %edi and %ebx must be zero.
136364-bit Boot Protocol
1366For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader
1367and we need a 64-bit boot protocol.
1369In 64-bit boot protocol, the first step in loading a Linux kernel
1370should be to setup the boot parameters (struct boot_params,
1371traditionally known as "zero page"). The memory for struct boot_params
1372could be allocated anywhere (even above 4G) and initialized to all zero.
1373Then, the setup header at offset 0x01f1 of kernel image on should be
1374loaded into struct boot_params and examined. The end of setup header
1375can be calculated as follows::
1377        0x0202 + byte value at offset 0x0201
1379In addition to read/modify/write the setup header of the struct
1380boot_params as that of 16-bit boot protocol, the boot loader should
1381also fill the additional fields of the struct boot_params as described
1382in chapter Documentation/x86/zero-page.rst.
1384After setting up the struct boot_params, the boot loader can load
138564-bit kernel in the same way as that of 16-bit boot protocol, but
1386kernel could be loaded above 4G.
1388In 64-bit boot protocol, the kernel is started by jumping to the
138964-bit kernel entry point, which is the start address of loaded
139064-bit kernel plus 0x200.
1392At entry, the CPU must be in 64-bit mode with paging enabled.
1393The range with setup_header.init_size from start address of loaded
1394kernel and zero page and command line buffer get ident mapping;
1395a GDT must be loaded with the descriptors for selectors
1396__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1397segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1398must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1399must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base
1400address of the struct boot_params.
1402EFI Handover Protocol (deprecated)
1405This protocol allows boot loaders to defer initialisation to the EFI
1406boot stub. The boot loader is required to load the kernel/initrd(s)
1407from the boot media and jump to the EFI handover protocol entry point
1408which is hdr->handover_offset bytes from the beginning of
1411The boot loader MUST respect the kernel's PE/COFF metadata when it comes
1412to section alignment, the memory footprint of the executable image beyond
1413the size of the file itself, and any other aspect of the PE/COFF header
1414that may affect correct operation of the image as a PE/COFF binary in the
1415execution context provided by the EFI firmware.
1417The function prototype for the handover entry point looks like this::
1419    efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp)
1421'handle' is the EFI image handle passed to the boot loader by the EFI
1422firmware, 'table' is the EFI system table - these are the first two
1423arguments of the "handoff state" as described in section 2.3 of the
1424UEFI specification. 'bp' is the boot loader-allocated boot params.
1426The boot loader *must* fill out the following fields in bp::
1428  - hdr.cmd_line_ptr
1429  - hdr.ramdisk_image (if applicable)
1430  - hdr.ramdisk_size  (if applicable)
1432All other fields should be zero.
1434NOTE: The EFI Handover Protocol is deprecated in favour of the ordinary PE/COFF
1435      entry point, combined with the LINUX_EFI_INITRD_MEDIA_GUID based initrd
1436      loading protocol (refer to [0] for an example of the bootloader side of
1437      this), which removes the need for any knowledge on the part of the EFI
1438      bootloader regarding the internal representation of boot_params or any
1439      requirements/limitations regarding the placement of the command line
1440      and ramdisk in memory, or the placement of the kernel image itself.