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   [X]
     * [6]A guide to the Kernel Development Process
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       into the kernel
     * [8]Code of conduct
     * [9]Kernel Maintainer Handbook
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     * [11]Core API Documentation
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     * [15]Linux kernel licensing rules
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     * [18]Kernel Testing Guide
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     * [21]fault-injection
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     * [24]The Linux kernel user's and administrator's guide
     * [25]The kernel build system
     * [26]Reporting issues
     * [27]User-space tools
     * [28]The Linux kernel user-space API guide

     * [29]The Linux kernel firmware guide
     * [30]Open Firmware and Devicetree

     * [31]CPU Architectures
          + [32]ARC architecture
          + [33]ARM Architecture
          + [34]ARM64 Architecture
          + [35]IA-64 Architecture
          + [36]LoongArch Architecture
          + [37]m68k Architecture
          + [38]MIPS-specific Documentation
          + [39]Nios II Specific Documentation
          + [40]OpenRISC Architecture
          + [41]PA-RISC Architecture
          + [42]powerpc
          + [43]RISC-V architecture
          + [44]s390 Architecture
          + [45]SuperH Interfaces Guide
          + [46]Sparc Architecture
          + [47]x86-specific Documentation
               o [48]1. The Linux/x86 Boot Protocol
               o [49]2. DeviceTree Booting
               o [50]3. x86 Feature Flags
               o [51]4. x86 Topology
               o [52]5. Kernel level exception handling
               o [53]6. Kernel Stacks
               o [54]7. Kernel Entries
               o [55]8. Early Printk
               o [56]9. ORC unwinder
               o [57]10. Zero Page
               o [58]11. The TLB
               o [59]12. MTRR (Memory Type Range Register) control
               o [60]13. PAT (Page Attribute Table)
               o [61]14. Hardware-Feedback Interface for scheduling on
                 Intel Hardware
               o [62]15. x86 IOMMU Support
               o [63]16. Intel(R) TXT Overview
               o [64]17. AMD Memory Encryption
               o [65]18. AMD HSMP interface
               o [66]19. Intel Trust Domain Extensions (TDX)
               o [67]20. Page Table Isolation (PTI)
               o [68]21. Microarchitectural Data Sampling (MDS) mitigation
               o [69]22. The Linux Microcode Loader
               o [70]23. User Interface for Resource Control feature
               o [71]24. TSX Async Abort (TAA) mitigation
               o [72]25. Bus lock detection and handling
               o [73]26. USB Legacy support
               o [74]27. i386 Support
               o [75]28. x86_64 Support
               o [76]29. In-Field Scan
               o [77]30. Shared Virtual Addressing (SVA) with ENQCMD
               o [78]31. Software Guard eXtensions (SGX)
               o [79]32. Feature status on x86 architecture
               o [80]33. x86-specific ELF Auxiliary Vectors
               o [81]34. Using XSTATE features in user space applications
          + [82]Xtensa Architecture

     * [83]Unsorted Documentation

     * [84]Translations

This Page

     * [85]Show Source

1. The Linux/x86 Boot Protocol[86]¶

   On the x86 platform, the Linux kernel uses a rather complicated boot
   convention. This has evolved partially due to historical aspects, as
   well as the desire in the early days to have the kernel itself be a
   bootable image, the complicated PC memory model and due to changed
   expectations in the PC industry caused by the effective demise of
   real-mode DOS as a mainstream operating system.

   Currently, the following versions of the Linux/x86 boot protocol exist.

   Old kernels

   zImage/Image support only. Some very early kernels may not even support
   a command line.

   Protocol 2.00

   (Kernel 1.3.73) Added bzImage and initrd support, as well as a
   formalized way to communicate between the boot loader and the kernel.
   setup.S made relocatable, although the traditional setup area still
   assumed writable.

   Protocol 2.01

   (Kernel 1.3.76) Added a heap overrun warning.

   Protocol 2.02

   (Kernel 2.4.0-test3-pre3) New command line protocol. Lower the
   conventional memory ceiling. No overwrite of the traditional setup
   area, thus making booting safe for systems which use the EBDA from SMM
   or 32-bit BIOS entry points. zImage deprecated but still supported.

   Protocol 2.03

   (Kernel 2.4.18-pre1) Explicitly makes the highest possible initrd
   address available to the bootloader.

   Protocol 2.04

   (Kernel 2.6.14) Extend the syssize field to four bytes.

   Protocol 2.05

   (Kernel 2.6.20) Make protected mode kernel relocatable. Introduce
   relocatable_kernel and kernel_alignment fields.

   Protocol 2.06

   (Kernel 2.6.22) Added a field that contains the size of the boot
   command line.

   Protocol 2.07

   (Kernel 2.6.24) Added paravirtualised boot protocol. Introduced
   hardware_subarch and hardware_subarch_data and KEEP_SEGMENTS flag in
   load_flags.

   Protocol 2.08

   (Kernel 2.6.26) Added crc32 checksum and ELF format payload. Introduced
   payload_offset and payload_length fields to aid in locating the
   payload.

   Protocol 2.09

   (Kernel 2.6.26) Added a field of 64-bit physical pointer to single
   linked list of struct setup_data.

   Protocol 2.10

   (Kernel 2.6.31) Added a protocol for relaxed alignment beyond the
   kernel_alignment added, new init_size and pref_address fields. Added
   extended boot loader IDs.

   Protocol 2.11

   (Kernel 3.6) Added a field for offset of EFI handover protocol entry
   point.

   Protocol 2.12

   (Kernel 3.8) Added the xloadflags field and extension fields to struct
   boot_params for loading bzImage and ramdisk above 4G in 64bit.

   Protocol 2.13

   (Kernel 3.14) Support 32- and 64-bit flags being set in xloadflags to
   support booting a 64-bit kernel from 32-bit EFI

   Protocol 2.14

   BURNT BY INCORRECT COMMIT ae7e1238e68f2a472a125673ab506d49158c1889
   (x86/boot: Add ACPI RSDP address to setup_header) DO NOT USE!!! ASSUME
   SAME AS 2.13.

   Protocol 2.15

   (Kernel 5.5) Added the kernel_info and kernel_info.setup_type_max.

   Note

   The protocol version number should be changed only if the setup header
   is changed. There is no need to update the version number if
   boot_params or kernel_info are changed. Additionally, it is recommended
   to use xloadflags (in this case the protocol version number should not
   be updated either) or kernel_info to communicate supported Linux kernel
   features to the boot loader. Due to very limited space available in the
   original setup header every update to it should be considered with
   great care. Starting from the protocol 2.15 the primary way to
   communicate things to the boot loader is the kernel_info.

1.1. Memory Layout[87]¶

   The traditional memory map for the kernel loader, used for Image or
   zImage kernels, typically looks like:
        |                        |
0A0000  +------------------------+
        |  Reserved for BIOS     |      Do not use.  Reserved for BIOS EBDA.
09A000  +------------------------+
        |  Command line          |
        |  Stack/heap            |      For use by the kernel real-mode code.
098000  +------------------------+
        |  Kernel setup          |      The kernel real-mode code.
090200  +------------------------+
        |  Kernel boot sector    |      The kernel legacy boot sector.
090000  +------------------------+
        |  Protected-mode kernel |      The bulk of the kernel image.
010000  +------------------------+
        |  Boot loader           |      <- Boot sector entry point 0000:7C00
001000  +------------------------+
        |  Reserved for MBR/BIOS |
000800  +------------------------+
        |  Typically used by MBR |
000600  +------------------------+
        |  BIOS use only         |
000000  +------------------------+

   When using bzImage, the protected-mode kernel was relocated to 0x100000
   ("high memory"), and the kernel real-mode block (boot sector, setup,
   and stack/heap) was made relocatable to any address between 0x10000 and
   end of low memory. Unfortunately, in protocols 2.00 and 2.01 the
   0x90000+ memory range is still used internally by the kernel; the 2.02
   protocol resolves that problem.

   It is desirable to keep the "memory ceiling" - the highest point in low
   memory touched by the boot loader - as low as possible, since some
   newer BIOSes have begun to allocate some rather large amounts of
   memory, called the Extended BIOS Data Area, near the top of low memory.
   The boot loader should use the "INT 12h" BIOS call to verify how much
   low memory is available.

   Unfortunately, if INT 12h reports that the amount of memory is too low,
   there is usually nothing the boot loader can do but to report an error
   to the user. The boot loader should therefore be designed to take up as
   little space in low memory as it reasonably can. For zImage or old
   bzImage kernels, which need data written into the 0x90000 segment, the
   boot loader should make sure not to use memory above the 0x9A000 point;
   too many BIOSes will break above that point.

   For a modern bzImage kernel with boot protocol version >= 2.02, a
   memory layout like the following is suggested:
              ~                        ~
              |  Protected-mode kernel |
      100000  +------------------------+
              |  I/O memory hole       |
      0A0000  +------------------------+
              |  Reserved for BIOS     |      Leave as much as possible unused
              ~                        ~
              |  Command line          |      (Can also be below the X+10000 mar
k)
      X+10000 +------------------------+
              |  Stack/heap            |      For use by the kernel real-mode co
de.
      X+08000 +------------------------+
              |  Kernel setup          |      The kernel real-mode code.
              |  Kernel boot sector    |      The kernel legacy boot sector.
      X       +------------------------+
              |  Boot loader           |      <- Boot sector entry point 0000:7C
00
      001000  +------------------------+
              |  Reserved for MBR/BIOS |
      000800  +------------------------+
              |  Typically used by MBR |
      000600  +------------------------+
              |  BIOS use only         |
      000000  +------------------------+

... where the address X is as low as the design of the boot loader permits.

1.2. The Real-Mode Kernel Header[88]¶

   In the following text, and anywhere in the kernel boot sequence, "a
   sector" refers to 512 bytes. It is independent of the actual sector
   size of the underlying medium.

   The first step in loading a Linux kernel should be to load the
   real-mode code (boot sector and setup code) and then examine the
   following header at offset 0x01f1. The real-mode code can total up to
   32K, although the boot loader may choose to load only the first two
   sectors (1K) and then examine the bootup sector size.

   The header looks like:

   Offset/Size

   Proto

   Name

   Meaning

   01F1/1

   ALL(1)

   setup_sects

   The size of the setup in sectors

   01F2/2

   ALL

   root_flags

   If set, the root is mounted readonly

   01F4/4

   2.04+(2)

   syssize

   The size of the 32-bit code in 16-byte paras

   01F8/2

   ALL

   ram_size

   DO NOT USE - for bootsect.S use only

   01FA/2

   ALL

   vid_mode

   Video mode control

   01FC/2

   ALL

   root_dev

   Default root device number

   01FE/2

   ALL

   boot_flag

   0xAA55 magic number

   0200/2

   2.00+

   jump

   Jump instruction

   0202/4

   2.00+

   header

   Magic signature "HdrS"

   0206/2

   2.00+

   version

   Boot protocol version supported

   0208/4

   2.00+

   realmode_swtch

   Boot loader hook (see below)

   020C/2

   2.00+

   start_sys_seg

   The load-low segment (0x1000) (obsolete)

   020E/2

   2.00+

   kernel_version

   Pointer to kernel version string

   0210/1

   2.00+

   type_of_loader

   Boot loader identifier

   0211/1

   2.00+

   loadflags

   Boot protocol option flags

   0212/2

   2.00+

   setup_move_size

   Move to high memory size (used with hooks)

   0214/4

   2.00+

   code32_start

   Boot loader hook (see below)

   0218/4

   2.00+

   ramdisk_image

   initrd load address (set by boot loader)

   021C/4

   2.00+

   ramdisk_size

   initrd size (set by boot loader)

   0220/4

   2.00+

   bootsect_kludge

   DO NOT USE - for bootsect.S use only

   0224/2

   2.01+

   heap_end_ptr

   Free memory after setup end

   0226/1

   2.02+(3)

   ext_loader_ver

   Extended boot loader version

   0227/1

   2.02+(3)

   ext_loader_type

   Extended boot loader ID

   0228/4

   2.02+

   cmd_line_ptr

   32-bit pointer to the kernel command line

   022C/4

   2.03+

   initrd_addr_max

   Highest legal initrd address

   0230/4

   2.05+

   kernel_alignment

   Physical addr alignment required for kernel

   0234/1

   2.05+

   relocatable_kernel

   Whether kernel is relocatable or not

   0235/1

   2.10+

   min_alignment

   Minimum alignment, as a power of two

   0236/2

   2.12+

   xloadflags

   Boot protocol option flags

   0238/4

   2.06+

   cmdline_size

   Maximum size of the kernel command line

   023C/4

   2.07+

   hardware_subarch

   Hardware subarchitecture

   0240/8

   2.07+

   hardware_subarch_data

   Subarchitecture-specific data

   0248/4

   2.08+

   payload_offset

   Offset of kernel payload

   024C/4

   2.08+

   payload_length

   Length of kernel payload

   0250/8

   2.09+

   setup_data

   64-bit physical pointer to linked list of struct setup_data

   0258/8

   2.10+

   pref_address

   Preferred loading address

   0260/4

   2.10+

   init_size

   Linear memory required during initialization

   0264/4

   2.11+

   handover_offset

   Offset of handover entry point

   0268/4

   2.15+

   kernel_info_offset

   Offset of the kernel_info

   Note
    1. For backwards compatibility, if the setup_sects field contains 0,
       the real value is 4.
    2. For boot protocol prior to 2.04, the upper two bytes of the syssize
       field are unusable, which means the size of a bzImage kernel cannot
       be determined.
    3. Ignored, but safe to set, for boot protocols 2.02-2.09.

   If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
   the boot protocol version is "old". Loading an old kernel, the
   following parameters should be assumed:
Image type = zImage
initrd not supported
Real-mode kernel must be located at 0x90000.

   Otherwise, the "version" field contains the protocol version, e.g.
   protocol version 2.01 will contain 0x0201 in this field. When setting
   fields in the header, you must make sure only to set fields supported
   by the protocol version in use.

1.3. Details of Header Fields[89]¶

   For each field, some are information from the kernel to the bootloader
   ("read"), some are expected to be filled out by the bootloader
   ("write"), and some are expected to be read and modified by the
   bootloader ("modify").

   All general purpose boot loaders should write the fields marked
   (obligatory). Boot loaders who want to load the kernel at a nonstandard
   address should fill in the fields marked (reloc); other boot loaders
   can ignore those fields.

   The byte order of all fields is littleendian (this is x86, after all.)

   Field name:

   setup_sects

   Type:

   read

   Offset/size:

   0x1f1/1

   Protocol:

   ALL

   The size of the setup code in 512-byte sectors. If this field is 0, the
   real value is 4. The real-mode code consists of the boot sector (always
   one 512-byte sector) plus the setup code.

   Field name:

   root_flags

   Type:

   modify (optional)

   Offset/size:

   0x1f2/2

   Protocol:

   ALL

   If this field is nonzero, the root defaults to readonly. The use of
   this field is deprecated; use the "ro" or "rw" options on the command
   line instead.

   Field name:

   syssize

   Type:

   read

   Offset/size:

   0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)

   Protocol:

   2.04+

   The size of the protected-mode code in units of 16-byte paragraphs. For
   protocol versions older than 2.04 this field is only two bytes wide,
   and therefore cannot be trusted for the size of a kernel if the
   LOAD_HIGH flag is set.

   Field name:

   ram_size

   Type:

   kernel internal

   Offset/size:

   0x1f8/2

   Protocol:

   ALL

   This field is obsolete.

   Field name:

   vid_mode

   Type:

   modify (obligatory)

   Offset/size:

   0x1fa/2

   Please see the section on SPECIAL COMMAND LINE OPTIONS.

   Field name:

   root_dev

   Type:

   modify (optional)

   Offset/size:

   0x1fc/2

   Protocol:

   ALL

   The default root device device number. The use of this field is
   deprecated, use the "root=" option on the command line instead.

   Field name:

   boot_flag

   Type:

   read

   Offset/size:

   0x1fe/2

   Protocol:

   ALL

   Contains 0xAA55. This is the closest thing old Linux kernels have to a
   magic number.

   Field name:

   jump

   Type:

   read

   Offset/size:

   0x200/2

   Protocol:

   2.00+

   Contains an x86 jump instruction, 0xEB followed by a signed offset
   relative to byte 0x202. This can be used to determine the size of the
   header.

   Field name:

   header

   Type:

   read

   Offset/size:

   0x202/4

   Protocol:

   2.00+

   Contains the magic number "HdrS" (0x53726448).

   Field name:

   version

   Type:

   read

   Offset/size:

   0x206/2

   Protocol:

   2.00+

   Contains the boot protocol version, in (major << 8)+minor format, e.g.
   0x0204 for version 2.04, and 0x0a11 for a hypothetical version 10.17.

   Field name:

   realmode_swtch

   Type:

   modify (optional)

   Offset/size:

   0x208/4

   Protocol:

   2.00+

   Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)

   Field name:

   start_sys_seg

   Type:

   read

   Offset/size:

   0x20c/2

   Protocol:

   2.00+

   The load low segment (0x1000). Obsolete.

   Field name:

   kernel_version

   Type:

   read

   Offset/size:

   0x20e/2

   Protocol:

   2.00+

   If set to a nonzero value, contains a pointer to a NUL-terminated
   human-readable kernel version number string, less 0x200. This can be
   used to display the kernel version to the user. This value should be
   less than (0x200*setup_sects).

   For example, if this value is set to 0x1c00, the kernel version number
   string can be found at offset 0x1e00 in the kernel file. This is a
   valid value if and only if the "setup_sects" field contains the value
   15 or higher, as:
0x1c00  < 15*0x200 (= 0x1e00) but
0x1c00 >= 14*0x200 (= 0x1c00)

0x1c00 >> 9 = 14, So the minimum value for setup_secs is 15.

   Field name:

   type_of_loader

   Type:

   write (obligatory)

   Offset/size:

   0x210/1

   Protocol:

   2.00+

   If your boot loader has an assigned id (see table below), enter 0xTV
   here, where T is an identifier for the boot loader and V is a version
   number. Otherwise, enter 0xFF here.

   For boot loader IDs above T = 0xD, write T = 0xE to this field and
   write the extended ID minus 0x10 to the ext_loader_type field.
   Similarly, the ext_loader_ver field can be used to provide more than
   four bits for the bootloader version.

   For example, for T = 0x15, V = 0x234, write:
type_of_loader  <- 0xE4
ext_loader_type <- 0x05
ext_loader_ver  <- 0x23

   Assigned boot loader ids (hexadecimal):

   0

   LILO (0x00 reserved for pre-2.00 bootloader)

   1

   Loadlin

   2

   bootsect-loader (0x20, all other values reserved)

   3

   Syslinux

   4

   Etherboot/gPXE/iPXE

   5

   ELILO

   7

   GRUB

   8

   U-Boot

   9

   Xen

   A

   Gujin

   B

   Qemu

   C

   Arcturus Networks uCbootloader

   D

   kexec-tools

   E

   Extended (see ext_loader_type)

   F

   Special (0xFF = undefined)

   10

   Reserved

   11

   Minimal Linux Bootloader <[90]http://sebastian-plotz.blogspot.de>

   12

   OVMF UEFI virtualization stack

   13

   barebox

   Please contact <[91]hpa@zytor.com> if you need a bootloader ID value
   assigned.

   Field name:

   loadflags

   Type:

   modify (obligatory)

   Offset/size:

   0x211/1

   Protocol:

   2.00+

   This field is a bitmask.

   Bit 0 (read): LOADED_HIGH

     * If 0, the protected-mode code is loaded at 0x10000.
     * If 1, the protected-mode code is loaded at 0x100000.

   Bit 1 (kernel internal): KASLR_FLAG

     * Used internally by the compressed kernel to communicate KASLR
       status to kernel proper.

          + If 1, KASLR enabled.
          + If 0, KASLR disabled.

   Bit 5 (write): QUIET_FLAG

     * If 0, print early messages.
     * If 1, suppress early messages.

     This requests to the kernel (decompressor and early kernel) to not
     write early messages that require accessing the display hardware
     directly.

   Bit 6 (obsolete): KEEP_SEGMENTS

   Protocol: 2.07+
     * This flag is obsolete.

   Bit 7 (write): CAN_USE_HEAP

   Set this bit to 1 to indicate that the value entered in the
   heap_end_ptr is valid. If this field is clear, some setup code
   functionality will be disabled.

   Field name:

   setup_move_size

   Type:

   modify (obligatory)

   Offset/size:

   0x212/2

   Protocol:

   2.00-2.01

   When using protocol 2.00 or 2.01, if the real mode kernel is not loaded
   at 0x90000, it gets moved there later in the loading sequence. Fill in
   this field if you want additional data (such as the kernel command
   line) moved in addition to the real-mode kernel itself.

   The unit is bytes starting with the beginning of the boot sector.

   This field is can be ignored when the protocol is 2.02 or higher, or if
   the real-mode code is loaded at 0x90000.

   Field name:

   code32_start

   Type:

   modify (optional, reloc)

   Offset/size:

   0x214/4

   Protocol:

   2.00+

   The address to jump to in protected mode. This defaults to the load
   address of the kernel, and can be used by the boot loader to determine
   the proper load address.

   This field can be modified for two purposes:

    1. as a boot loader hook (see Advanced Boot Loader Hooks below.)
    2. if a bootloader which does not install a hook loads a relocatable
       kernel at a nonstandard address it will have to modify this field
       to point to the load address.

   Field name:

   ramdisk_image

   Type:

   write (obligatory)

   Offset/size:

   0x218/4

   Protocol:

   2.00+

   The 32-bit linear address of the initial ramdisk or ramfs. Leave at
   zero if there is no initial ramdisk/ramfs.

   Field name:

   ramdisk_size

   Type:

   write (obligatory)

   Offset/size:

   0x21c/4

   Protocol:

   2.00+

   Size of the initial ramdisk or ramfs. Leave at zero if there is no
   initial ramdisk/ramfs.

   Field name:

   bootsect_kludge

   Type:

   kernel internal

   Offset/size:

   0x220/4

   Protocol:

   2.00+

   This field is obsolete.

   Field name:

   heap_end_ptr

   Type:

   write (obligatory)

   Offset/size:

   0x224/2

   Protocol:

   2.01+

   Set this field to the offset (from the beginning of the real-mode code)
   of the end of the setup stack/heap, minus 0x0200.

   Field name:

   ext_loader_ver

   Type:

   write (optional)

   Offset/size:

   0x226/1

   Protocol:

   2.02+

   This field is used as an extension of the version number in the
   type_of_loader field. The total version number is considered to be
   (type_of_loader & 0x0f) + (ext_loader_ver << 4).

   The use of this field is boot loader specific. If not written, it is
   zero.

   Kernels prior to 2.6.31 did not recognize this field, but it is safe to
   write for protocol version 2.02 or higher.

   Field name:

   ext_loader_type

   Type:

   write (obligatory if (type_of_loader & 0xf0) == 0xe0)

   Offset/size:

   0x227/1

   Protocol:

   2.02+

   This field is used as an extension of the type number in type_of_loader
   field. If the type in type_of_loader is 0xE, then the actual type is
   (ext_loader_type + 0x10).

   This field is ignored if the type in type_of_loader is not 0xE.

   Kernels prior to 2.6.31 did not recognize this field, but it is safe to
   write for protocol version 2.02 or higher.

   Field name:

   cmd_line_ptr

   Type:

   write (obligatory)

   Offset/size:

   0x228/4

   Protocol:

   2.02+

   Set this field to the linear address of the kernel command line. The
   kernel command line can be located anywhere between the end of the
   setup heap and 0xA0000; it does not have to be located in the same 64K
   segment as the real-mode code itself.

   Fill in this field even if your boot loader does not support a command
   line, in which case you can point this to an empty string (or better
   yet, to the string "auto".) If this field is left at zero, the kernel
   will assume that your boot loader does not support the 2.02+ protocol.

   Field name:

   initrd_addr_max

   Type:

   read

   Offset/size:

   0x22c/4

   Protocol:

   2.03+

   The maximum address that may be occupied by the initial ramdisk/ramfs
   contents. For boot protocols 2.02 or earlier, this field is not
   present, and the maximum address is 0x37FFFFFF. (This address is
   defined as the address of the highest safe byte, so if your ramdisk is
   exactly 131072 bytes long and this field is 0x37FFFFFF, you can start
   your ramdisk at 0x37FE0000.)

   Field name:

   kernel_alignment

   Type:

   read/modify (reloc)

   Offset/size:

   0x230/4

   Protocol:

   2.05+ (read), 2.10+ (modify)

   Alignment unit required by the kernel (if relocatable_kernel is true.)
   A relocatable kernel that is loaded at an alignment incompatible with
   the value in this field will be realigned during kernel initialization.

   Starting with protocol version 2.10, this reflects the kernel alignment
   preferred for optimal performance; it is possible for the loader to
   modify this field to permit a lesser alignment. See the min_alignment
   and pref_address field below.

   Field name:

   relocatable_kernel

   Type:

   read (reloc)

   Offset/size:

   0x234/1

   Protocol:

   2.05+

   If this field is nonzero, the protected-mode part of the kernel can be
   loaded at any address that satisfies the kernel_alignment field. After
   loading, the boot loader must set the code32_start field to point to
   the loaded code, or to a boot loader hook.

   Field name:

   min_alignment

   Type:

   read (reloc)

   Offset/size:

   0x235/1

   Protocol:

   2.10+

   This field, if nonzero, indicates as a power of two the minimum
   alignment required, as opposed to preferred, by the kernel to boot. If
   a boot loader makes use of this field, it should update the
   kernel_alignment field with the alignment unit desired; typically:
kernel_alignment = 1 << min_alignment

   There may be a considerable performance cost with an excessively
   misaligned kernel. Therefore, a loader should typically try each
   power-of-two alignment from kernel_alignment down to this alignment.

   Field name:

   xloadflags

   Type:

   read

   Offset/size:

   0x236/2

   Protocol:

   2.12+

   This field is a bitmask.

   Bit 0 (read): XLF_KERNEL_64

     * If 1, this kernel has the legacy 64-bit entry point at 0x200.

   Bit 1 (read): XLF_CAN_BE_LOADED_ABOVE_4G

     * If 1, kernel/boot_params/cmdline/ramdisk can be above 4G.

   Bit 2 (read): XLF_EFI_HANDOVER_32

     * If 1, the kernel supports the 32-bit EFI handoff entry point given
       at handover_offset.

   Bit 3 (read): XLF_EFI_HANDOVER_64

     * If 1, the kernel supports the 64-bit EFI handoff entry point given
       at handover_offset + 0x200.

   Bit 4 (read): XLF_EFI_KEXEC

     * If 1, the kernel supports kexec EFI boot with EFI runtime support.

   Field name:

   cmdline_size

   Type:

   read

   Offset/size:

   0x238/4

   Protocol:

   2.06+

   The maximum size of the command line without the terminating zero. This
   means that the command line can contain at most cmdline_size
   characters. With protocol version 2.05 and earlier, the maximum size
   was 255.

   Field name:

   hardware_subarch

   Type:

   write (optional, defaults to x86/PC)

   Offset/size:

   0x23c/4

   Protocol:

   2.07+

   In a paravirtualized environment the hardware low level architectural
   pieces such as interrupt handling, page table handling, and accessing
   process control registers needs to be done differently.

   This field allows the bootloader to inform the kernel we are in one one
   of those environments.

   0x00000000

   The default x86/PC environment

   0x00000001

   lguest

   0x00000002

   Xen

   0x00000003

   Moorestown MID

   0x00000004

   CE4100 TV Platform

   Field name:

   hardware_subarch_data

   Type:

   write (subarch-dependent)

   Offset/size:

   0x240/8

   Protocol:

   2.07+

   A pointer to data that is specific to hardware subarch This field is
   currently unused for the default x86/PC environment, do not modify.

   Field name:

   payload_offset

   Type:

   read

   Offset/size:

   0x248/4

   Protocol:

   2.08+

   If non-zero then this field contains the offset from the beginning of
   the protected-mode code to the payload.

   The payload may be compressed. The format of both the compressed and
   uncompressed data should be determined using the standard magic
   numbers. The currently supported compression formats are gzip (magic
   numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA (magic number
   5D 00), XZ (magic number FD 37), LZ4 (magic number 02 21) and ZSTD
   (magic number 28 B5). The uncompressed payload is currently always ELF
   (magic number 7F 45 4C 46).

   Field name:

   payload_length

   Type:

   read

   Offset/size:

   0x24c/4

   Protocol:

   2.08+

   The length of the payload.

   Field name:

   setup_data

   Type:

   write (special)

   Offset/size:

   0x250/8

   Protocol:

   2.09+

   The 64-bit physical pointer to NULL terminated single linked list of
   struct setup_data. This is used to define a more extensible boot
   parameters passing mechanism. The definition of struct setup_data is as
   follow:
struct setup_data {
        u64 next;
        u32 type;
        u32 len;
        u8  data[0];
};

   Where, the next is a 64-bit physical pointer to the next node of linked
   list, the next field of the last node is 0; the type is used to
   identify the contents of data; the len is the length of data field; the
   data holds the real payload.

   This list may be modified at a number of points during the bootup
   process. Therefore, when modifying this list one should always make
   sure to consider the case where the linked list already contains
   entries.

   The setup_data is a bit awkward to use for extremely large data
   objects, both because the setup_data header has to be adjacent to the
   data object and because it has a 32-bit length field. However, it is
   important that intermediate stages of the boot process have a way to
   identify which chunks of memory are occupied by kernel data.

   Thus setup_indirect struct and SETUP_INDIRECT type were introduced in
   protocol 2.15:
struct setup_indirect {
  __u32 type;
  __u32 reserved;  /* Reserved, must be set to zero. */
  __u64 len;
  __u64 addr;
};

   The type member is a SETUP_INDIRECT | SETUP_* type. However, it cannot
   be SETUP_INDIRECT itself since making the setup_indirect a tree
   structure could require a lot of stack space in something that needs to
   parse it and stack space can be limited in boot contexts.

   Let's give an example how to point to SETUP_E820_EXT data using
   setup_indirect. In this case setup_data and setup_indirect will look
   like this:
struct setup_data {
  __u64 next = 0 or <addr_of_next_setup_data_struct>;
  __u32 type = SETUP_INDIRECT;
  __u32 len = sizeof(setup_indirect);
  __u8 data[sizeof(setup_indirect)] = struct setup_indirect {
    __u32 type = SETUP_INDIRECT | SETUP_E820_EXT;
    __u32 reserved = 0;
    __u64 len = <len_of_SETUP_E820_EXT_data>;
    __u64 addr = <addr_of_SETUP_E820_EXT_data>;
  }
}

   Note

   SETUP_INDIRECT | SETUP_NONE objects cannot be properly distinguished
   from SETUP_INDIRECT itself. So, this kind of objects cannot be provided
   by the bootloaders.

   Field name:

   pref_address

   Type:

   read (reloc)

   Offset/size:

   0x258/8

   Protocol:

   2.10+

   This field, if nonzero, represents a preferred load address for the
   kernel. A relocating bootloader should attempt to load at this address
   if possible.

   A non-relocatable kernel will unconditionally move itself and to run at
   this address.

   Field name:

   init_size

   Type:

   read

   Offset/size:

   0x260/4

   This field indicates the amount of linear contiguous memory starting at
   the kernel runtime start address that the kernel needs before it is
   capable of examining its memory map. This is not the same thing as the
   total amount of memory the kernel needs to boot, but it can be used by
   a relocating boot loader to help select a safe load address for the
   kernel.

   The kernel runtime start address is determined by the following
   algorithm:
if (relocatable_kernel)
runtime_start = align_up(load_address, kernel_alignment)
else
runtime_start = pref_address

   Field name:

   handover_offset

   Type:

   read

   Offset/size:

   0x264/4

   This field is the offset from the beginning of the kernel image to the
   EFI handover protocol entry point. Boot loaders using the EFI handover
   protocol to boot the kernel should jump to this offset.

   See EFI HANDOVER PROTOCOL below for more details.

   Field name:

   kernel_info_offset

   Type:

   read

   Offset/size:

   0x268/4

   Protocol:

   2.15+

   This field is the offset from the beginning of the kernel image to the
   kernel_info. The kernel_info structure is embedded in the Linux image
   in the uncompressed protected mode region.

1.4. The kernel_info[92]¶

   The relationships between the headers are analogous to the various data
   sections:

   setup_header = .data boot_params/setup_data = .bss

   What is missing from the above list? That's right:

   kernel_info = .rodata

   We have been (ab)using .data for things that could go into .rodata or
   .bss for a long time, for lack of alternatives and - especially early
   on - inertia. Also, the BIOS stub is responsible for creating
   boot_params, so it isn't available to a BIOS-based loader (setup_data
   is, though).

   setup_header is permanently limited to 144 bytes due to the reach of
   the 2-byte jump field, which doubles as a length field for the
   structure, combined with the size of the "hole" in struct boot_params
   that a protected-mode loader or the BIOS stub has to copy it into. It
   is currently 119 bytes long, which leaves us with 25 very precious
   bytes. This isn't something that can be fixed without revising the boot
   protocol entirely, breaking backwards compatibility.

   boot_params proper is limited to 4096 bytes, but can be arbitrarily
   extended by adding setup_data entries. It cannot be used to communicate
   properties of the kernel image, because it is .bss and has no
   image-provided content.

   kernel_info solves this by providing an extensible place for
   information about the kernel image. It is readonly, because the kernel
   cannot rely on a bootloader copying its contents anywhere, but that is
   OK; if it becomes necessary it can still contain data items that an
   enabled bootloader would be expected to copy into a setup_data chunk.

   All kernel_info data should be part of this structure. Fixed size data
   have to be put before kernel_info_var_len_data label. Variable size
   data have to be put after kernel_info_var_len_data label. Each chunk of
   variable size data has to be prefixed with header/magic and its size,
   e.g.:
kernel_info:
        .ascii  "LToP"          /* Header, Linux top (structure). */
        .long   kernel_info_var_len_data - kernel_info
        .long   kernel_info_end - kernel_info
        .long   0x01234567      /* Some fixed size data for the bootloaders. */
kernel_info_var_len_data:
example_struct:                 /* Some variable size data for the bootloaders.
*/
        .ascii  "0123"          /* Header/Magic. */
        .long   example_struct_end - example_struct
        .ascii  "Struct"
        .long   0x89012345
example_struct_end:
example_strings:                /* Some variable size data for the bootloaders.
*/
        .ascii  "ABCD"          /* Header/Magic. */
        .long   example_strings_end - example_strings
        .asciz  "String_0"
        .asciz  "String_1"
example_strings_end:
kernel_info_end:

   This way the kernel_info is self-contained blob.

   Note

   Each variable size data header/magic can be any 4-character string,
   without 0 at the end of the string, which does not collide with
   existing variable length data headers/magics.

1.5. Details of the kernel_info Fields[93]¶

   Field name:

   header

   Offset/size:

   0x0000/4

   Contains the magic number "LToP" (0x506f544c).

   Field name:

   size

   Offset/size:

   0x0004/4

   This field contains the size of the kernel_info including
   kernel_info.header. It does not count
   kernel_info.kernel_info_var_len_data size. This field should be used by
   the bootloaders to detect supported fixed size fields in the
   kernel_info and beginning of kernel_info.kernel_info_var_len_data.

   Field name:

   size_total

   Offset/size:

   0x0008/4

   This field contains the size of the kernel_info including
   kernel_info.header and kernel_info.kernel_info_var_len_data.

   Field name:

   setup_type_max

   Offset/size:

   0x000c/4

   This field contains maximal allowed type for setup_data and
   setup_indirect structs.

1.6. The Image Checksum[94]¶

   From boot protocol version 2.08 onwards the CRC-32 is calculated over
   the entire file using the characteristic polynomial 0x04C11DB7 and an
   initial remainder of 0xffffffff. The checksum is appended to the file;
   therefore the CRC of the file up to the limit specified in the syssize
   field of the header is always 0.

1.7. The Kernel Command Line[95]¶

   The kernel command line has become an important way for the boot loader
   to communicate with the kernel. Some of its options are also relevant
   to the boot loader itself, see "special command line options" below.

   The kernel command line is a null-terminated string. The maximum length
   can be retrieved from the field cmdline_size. Before protocol version
   2.06, the maximum was 255 characters. A string that is too long will be
   automatically truncated by the kernel.

   If the boot protocol version is 2.02 or later, the address of the
   kernel command line is given by the header field cmd_line_ptr (see
   above.) This address can be anywhere between the end of the setup heap
   and 0xA0000.

   If the protocol version is not 2.02 or higher, the kernel command line
   is entered using the following protocol:

     * At offset 0x0020 (word), "cmd_line_magic", enter the magic number
       0xA33F.
     * At offset 0x0022 (word), "cmd_line_offset", enter the offset of the
       kernel command line (relative to the start of the real-mode
       kernel).
     * The kernel command line must be within the memory region covered by
       setup_move_size, so you may need to adjust this field.

1.8. Memory Layout of The Real-Mode Code[96]¶

   The real-mode code requires a stack/heap to be set up, as well as
   memory allocated for the kernel command line. This needs to be done in
   the real-mode accessible memory in bottom megabyte.

   It should be noted that modern machines often have a sizable Extended
   BIOS Data Area (EBDA). As a result, it is advisable to use as little of
   the low megabyte as possible.

   Unfortunately, under the following circumstances the 0x90000 memory
   segment has to be used:

     * When loading a zImage kernel ((loadflags & 0x01) == 0).
     * When loading a 2.01 or earlier boot protocol kernel.

   Note

   For the 2.00 and 2.01 boot protocols, the real-mode code can be loaded
   at another address, but it is internally relocated to 0x90000. For the
   "old" protocol, the real-mode code must be loaded at 0x90000.

   When loading at 0x90000, avoid using memory above 0x9a000.

   For boot protocol 2.02 or higher, the command line does not have to be
   located in the same 64K segment as the real-mode setup code; it is thus
   permitted to give the stack/heap the full 64K segment and locate the
   command line above it.

   The kernel command line should not be located below the real-mode code,
   nor should it be located in high memory.

1.9. Sample Boot Configuartion[97]¶

   As a sample configuration, assume the following layout of the real mode
   segment.

   When loading below 0x90000, use the entire segment:

   0x0000-0x7fff

   Real mode kernel

   0x8000-0xdfff

   Stack and heap

   0xe000-0xffff

   Kernel command line

   When loading at 0x90000 OR the protocol version is 2.01 or earlier:

   0x0000-0x7fff

   Real mode kernel

   0x8000-0x97ff

   Stack and heap

   0x9800-0x9fff

   Kernel command line

   Such a boot loader should enter the following fields in the header:
unsigned long base_ptr; /* base address for real-mode segment */

if ( setup_sects == 0 ) {
        setup_sects = 4;
}

if ( protocol >= 0x0200 ) {
        type_of_loader = <type code>;
        if ( loading_initrd ) {
                ramdisk_image = <initrd_address>;
                ramdisk_size = <initrd_size>;
        }

        if ( protocol >= 0x0202 && loadflags & 0x01 )
                heap_end = 0xe000;
        else
                heap_end = 0x9800;

        if ( protocol >= 0x0201 ) {
                heap_end_ptr = heap_end - 0x200;
                loadflags |= 0x80; /* CAN_USE_HEAP */
        }

        if ( protocol >= 0x0202 ) {
                cmd_line_ptr = base_ptr + heap_end;
                strcpy(cmd_line_ptr, cmdline);
        } else {
                cmd_line_magic  = 0xA33F;
                cmd_line_offset = heap_end;
                setup_move_size = heap_end + strlen(cmdline)+1;
                strcpy(base_ptr+cmd_line_offset, cmdline);
        }
} else {
        /* Very old kernel */

        heap_end = 0x9800;

        cmd_line_magic  = 0xA33F;
        cmd_line_offset = heap_end;

        /* A very old kernel MUST have its real-mode code
           loaded at 0x90000 */

        if ( base_ptr != 0x90000 ) {
                /* Copy the real-mode kernel */
                memcpy(0x90000, base_ptr, (setup_sects+1)*512);
                base_ptr = 0x90000;              /* Relocated */
        }

        strcpy(0x90000+cmd_line_offset, cmdline);

        /* It is recommended to clear memory up to the 32K mark */
        memset(0x90000 + (setup_sects+1)*512, 0,
               (64-(setup_sects+1))*512);
}

1.10. Loading The Rest of The Kernel[98]¶

   The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
   in the kernel file (again, if setup_sects == 0 the real value is 4.) It
   should be loaded at address 0x10000 for Image/zImage kernels and
   0x100000 for bzImage kernels.

   The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01 bit
   (LOAD_HIGH) in the loadflags field is set:
is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
load_address = is_bzImage ? 0x100000 : 0x10000;

   Note that Image/zImage kernels can be up to 512K in size, and thus use
   the entire 0x10000-0x90000 range of memory. This means it is pretty
   much a requirement for these kernels to load the real-mode part at
   0x90000. bzImage kernels allow much more flexibility.

1.11. Special Command Line Options[99]¶

   If the command line provided by the boot loader is entered by the user,
   the user may expect the following command line options to work. They
   should normally not be deleted from the kernel command line even though
   not all of them are actually meaningful to the kernel. Boot loader
   authors who need additional command line options for the boot loader
   itself should get them registered in [100]The kernel's command-line
   parameters to make sure they will not conflict with actual kernel
   options now or in the future.

   vga=<mode>
          <mode> here is either an integer (in C notation, either decimal,
          octal, or hexadecimal) or one of the strings "normal" (meaning
          0xFFFF), "ext" (meaning 0xFFFE) or "ask" (meaning 0xFFFD). This
          value should be entered into the vid_mode field, as it is used
          by the kernel before the command line is parsed.

   mem=<size>
          <size> is an integer in C notation optionally followed by (case
          insensitive) K, M, G, T, P or E (meaning << 10, << 20, << 30, <<
          40, << 50 or << 60). This specifies the end of memory to the
          kernel. This affects the possible placement of an initrd, since
          an initrd should be placed near end of memory. Note that this is
          an option to both the kernel and the bootloader!

   initrd=<file>
          An initrd should be loaded. The meaning of <file> is obviously
          bootloader-dependent, and some boot loaders (e.g. LILO) do not
          have such a command.

   In addition, some boot loaders add the following options to the
   user-specified command line:

   BOOT_IMAGE=<file>
          The boot image which was loaded. Again, the meaning of <file> is
          obviously bootloader-dependent.

   auto
          The kernel was booted without explicit user intervention.

   If these options are added by the boot loader, it is highly recommended
   that they are located first, before the user-specified or
   configuration-specified command line. Otherwise, "init=/bin/sh" gets
   confused by the "auto" option.

1.12. Running the Kernel[101]¶

   The kernel is started by jumping to the kernel entry point, which is
   located at segment offset 0x20 from the start of the real mode kernel.
   This means that if you loaded your real-mode kernel code at 0x90000,
   the kernel entry point is 9020:0000.

   At entry, ds = es = ss should point to the start of the real-mode
   kernel code (0x9000 if the code is loaded at 0x90000), sp should be set
   up properly, normally pointing to the top of the heap, and interrupts
   should be disabled. Furthermore, to guard against bugs in the kernel,
   it is recommended that the boot loader sets fs = gs = ds = es = ss.

   In our example from above, we would do:
/* Note: in the case of the "old" kernel protocol, base_ptr must
   be == 0x90000 at this point; see the previous sample code */

seg = base_ptr >> 4;

cli();  /* Enter with interrupts disabled! */

/* Set up the real-mode kernel stack */
_SS = seg;
_SP = heap_end;

_DS = _ES = _FS = _GS = seg;
jmp_far(seg+0x20, 0);   /* Run the kernel */

   If your boot sector accesses a floppy drive, it is recommended to
   switch off the floppy motor before running the kernel, since the kernel
   boot leaves interrupts off and thus the motor will not be switched off,
   especially if the loaded kernel has the floppy driver as a
   demand-loaded module!

1.13. Advanced Boot Loader Hooks[102]¶

   If the boot loader runs in a particularly hostile environment (such as
   LOADLIN, which runs under DOS) it may be impossible to follow the
   standard memory location requirements. Such a boot loader may use the
   following hooks that, if set, are invoked by the kernel at the
   appropriate time. The use of these hooks should probably be considered
   an absolutely last resort!

   IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
   %edi across invocation.

   realmode_swtch:
          A 16-bit real mode far subroutine invoked immediately before
          entering protected mode. The default routine disables NMI, so
          your routine should probably do so, too.

   code32_start:
          A 32-bit flat-mode routine jumped to immediately after the
          transition to protected mode, but before the kernel is
          uncompressed. No segments, except CS, are guaranteed to be set
          up (current kernels do, but older ones do not); you should set
          them up to BOOT_DS (0x18) yourself.

          After completing your hook, you should jump to the address that
          was in this field before your boot loader overwrote it
          (relocated, if appropriate.)

1.14. 32-bit Boot Protocol[103]¶

   For machine with some new BIOS other than legacy BIOS, such as EFI,
   LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
   based on legacy BIOS can not be used, so a 32-bit boot protocol needs
   to be defined.

   In 32-bit boot protocol, the first step in loading a Linux kernel
   should be to setup the boot parameters (struct boot_params,
   traditionally known as "zero page"). The memory for struct boot_params
   should be allocated and initialized to all zero. Then the setup header
   from offset 0x01f1 of kernel image on should be loaded into struct
   boot_params and examined. The end of setup header can be calculated as
   follow:
0x0202 + byte value at offset 0x0201

   In addition to read/modify/write the setup header of the struct
   boot_params as that of 16-bit boot protocol, the boot loader should
   also fill the additional fields of the struct boot_params as described
   in chapter [104]Zero Page.

   After setting up the struct boot_params, the boot loader can load the
   32/64-bit kernel in the same way as that of 16-bit boot protocol.

   In 32-bit boot protocol, the kernel is started by jumping to the 32-bit
   kernel entry point, which is the start address of loaded 32/64-bit
   kernel.

   At entry, the CPU must be in 32-bit protected mode with paging
   disabled; a GDT must be loaded with the descriptors for selectors
   __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
   segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
   must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
   must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
   address of the struct boot_params; %ebp, %edi and %ebx must be zero.

1.15. 64-bit Boot Protocol[105]¶

   For machine with 64bit cpus and 64bit kernel, we could use 64bit
   bootloader and we need a 64-bit boot protocol.

   In 64-bit boot protocol, the first step in loading a Linux kernel
   should be to setup the boot parameters (struct boot_params,
   traditionally known as "zero page"). The memory for struct boot_params
   could be allocated anywhere (even above 4G) and initialized to all
   zero. Then, the setup header at offset 0x01f1 of kernel image on should
   be loaded into struct boot_params and examined. The end of setup header
   can be calculated as follows:
0x0202 + byte value at offset 0x0201

   In addition to read/modify/write the setup header of the struct
   boot_params as that of 16-bit boot protocol, the boot loader should
   also fill the additional fields of the struct boot_params as described
   in chapter [106]Zero Page.

   After setting up the struct boot_params, the boot loader can load
   64-bit kernel in the same way as that of 16-bit boot protocol, but
   kernel could be loaded above 4G.

   In 64-bit boot protocol, the kernel is started by jumping to the 64-bit
   kernel entry point, which is the start address of loaded 64-bit kernel
   plus 0x200.

   At entry, the CPU must be in 64-bit mode with paging enabled. The range
   with setup_header.init_size from start address of loaded kernel and
   zero page and command line buffer get ident mapping; a GDT must be
   loaded with the descriptors for selectors __BOOT_CS(0x10) and
   __BOOT_DS(0x18); both descriptors must be 4G flat segment; __BOOT_CS
   must have execute/read permission, and __BOOT_DS must have read/write
   permission; CS must be __BOOT_CS and DS, ES, SS must be __BOOT_DS;
   interrupt must be disabled; %rsi must hold the base address of the
   struct boot_params.

1.16. EFI Handover Protocol (deprecated)[107]¶

   This protocol allows boot loaders to defer initialisation to the EFI
   boot stub. The boot loader is required to load the kernel/initrd(s)
   from the boot media and jump to the EFI handover protocol entry point
   which is hdr->handover_offset bytes from the beginning of
   startup_{32,64}.

   The boot loader MUST respect the kernel's PE/COFF metadata when it
   comes to section alignment, the memory footprint of the executable
   image beyond the size of the file itself, and any other aspect of the
   PE/COFF header that may affect correct operation of the image as a
   PE/COFF binary in the execution context provided by the EFI firmware.

   The function prototype for the handover entry point looks like this:
efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp)

   `handle' is the EFI image handle passed to the boot loader by the EFI
   firmware, `table' is the EFI system table - these are the first two
   arguments of the "handoff state" as described in section 2.3 of the
   UEFI specification. `bp' is the boot loader-allocated boot params.

   The boot loader must fill out the following fields in bp:
- hdr.cmd_line_ptr
- hdr.ramdisk_image (if applicable)
- hdr.ramdisk_size  (if applicable)

   All other fields should be zero.

   NOTE: The EFI Handover Protocol is deprecated in favour of the ordinary
          PE/COFF
          entry point, combined with the LINUX_EFI_INITRD_MEDIA_GUID based
          initrd loading protocol (refer to [0] for an example of the
          bootloader side of this), which removes the need for any
          knowledge on the part of the EFI bootloader regarding the
          internal representation of boot_params or any
          requirements/limitations regarding the placement of the command
          line and ramdisk in memory, or the placement of the kernel image
          itself.

   [0]
   [108]https://github.com/u-boot/u-boot/commit/ec80b4735a593961fe701cc3a5
   d717d4739b0fd0

   ©The kernel development community. | Powered by [109]Sphinx 5.0.1 &
   [110]Alabaster 0.7.12 | [111]Page source

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