Re: [PATCH v4 38/63] Documentation: x86: convert boot.txt to reST

Changbin Du
2019-04-25T17:07:37Z

On Wed, Apr 24, 2019 at 02:36:44PM -0300, Mauro Carvalho Chehab wrote:
> Em Wed, 24 Apr 2019 00:29:07 +0800
> Changbin Du <changbin.du@gmail.com> escreveu:
>
> > This converts the plain text documentation to reStructuredText format and
> > add it to Sphinx TOC tree. No essential content change.
> >
> > Signed-off-by: Changbin Du <changbin.du@gmail.com>
> > ---
> > Documentation/x86/boot.rst | 1205 +++++++++++++++++++++++++++++++++++
> > Documentation/x86/boot.txt | 1130 --------------------------------
> > Documentation/x86/index.rst | 2 +
> > 3 files changed, 1207 insertions(+), 1130 deletions(-)
> > create mode 100644 Documentation/x86/boot.rst
> > delete mode 100644 Documentation/x86/boot.txt
> >
> > diff --git a/Documentation/x86/boot.rst b/Documentation/x86/boot.rst
> > new file mode 100644
> > index 000000000000..9f55e832bc47
> > --- /dev/null
> > +++ b/Documentation/x86/boot.rst
> > @@ -0,0 +1,1205 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +===========================
> > +The Linux/x86 Boot Protocol
> > +===========================
> > +
> > +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.
>
> This is a side node, but you should really try to avoid replacing too
> many lines, as it makes a lot harder for reviewers for no good reason.
>
> For example, this is the way I would convert this changelog table:
>
>
> @@ -10,6 +11,7 @@ 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.
>
> @@ -64,33 +66,35 @@ Protocol 2.12: (Kernel 3.8) Added the xloadflags field and extension fields
> 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
> +=============== ===============================================================
>
>
> This is simple enough, preserves the original author's intent and
> makes a lot easier for reviewers to check what you changed.
>
much better. thanks.

> > +
> > +MEMORY LAYOUT
> > +=============
> > +
> > +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 +------------------------+
> > +
> > +
>
> I might be wrong, but it seems that you broke the above ascii
> artwork.
>
You r right and fixed.

> > +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 mark)
> > + X+10000 +------------------------+
> > + | Stack/heap | For use by the kernel real-mode code.
> > + X+08000 +------------------------+
> > + | Kernel setup | The kernel real-mode code.
> > + | Kernel boot sector | The kernel legacy boot sector.
> > + X +------------------------+
> > + | Boot loader | <- Boot sector entry point 0000:7C00
> > + 001000 +------------------------+
> > + | Reserved for MBR/BIOS |
> > + 000800 +------------------------+
> > + | Typically used by MBR |
> > + 000600 +------------------------+
> > + | BIOS use only |
> > + 000000 +------------------------+
>
>
> Same here: it sounds to me that you mistakenly replaced some tabs
> by spaces.
>
> > +
> > +... where the address X is as low as the design of the boot loader
> > +permits.
>
> That seems to be the legend of the artwork. I would indent it, in
> order to be shown inside the artwork.
>
agree.

> > +
> > +
> > +THE REAL-MODE KERNEL HEADER
> > +===========================
> > +
> > +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 Proto Name Meaning
> > + /Size
> > +
> > + 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
>
> This is a table. Please use table markups and fix some wrong indentation
> there, as it makes a lot easier to read it on html, e-pub and pdf formats.
>
> E. g. something like:
>
> ====== ======== ===================== ========================================
> Offset Proto Name Meaning
> /Size
>
> 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
> ====== ======== ===================== ========================================
>
>
done as table.

> > +
> > +(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.
>
> Btw, (1), (2) and (3) here sounds to be footnotes. Perhaps you could use
> ReST footnote markups, if ok for the X86 maintainers.
>
Turned into footnote.

> > +
> > +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.
> > +
> > +
> > +DETAILS OF HEADER FIELDS
> > +========================
> > +
> > +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
>
> Marking this as a literal block sounds plain wrong to me. I suspect that
> you could use this syntax instead:
>
> :Field name: setup_sects
> :Type: read
> :Offset/size: 0x1f1/1
> :Protocol: ALL
>
> Or:
>
> Field name: setup_sects
> -----------------------
>
> Type:
> read
> Offset/size:
> 0x1f1/1
> Protocol:
> ALL
>
> Or (my favorite):
>
> Field name: setup_sects
> -----------------------
>
> :Type: read
> :Offset/size: 0x1f1/1
> :Protocol: ALL
>
> As it is more compact in text, and will provide a much better
> html/pdf output. It will also make (IMHO) a lot easier for
> people to read in text and seek for an specific field.
>
> Of course, whatever we do here should be applied to all similar
> structs inside this file.
>
I convert them to tables. The output looks good. Thanks.

> > +
> > +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 <http://sebastian-plotz.blogspot.de>
> > + 12 OVMF UEFI virtualization stack
>
> Clearly there's something wrong with the last 3 lines, as they aren't
> following the expected indentation.
>
> Anyway, IMO the best would be to use a table, instead:
>
> == =======================================
> 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
> <http://sebastian-plotz.blogspot.de>
> 12 OVMF UEFI virtualization stack
> == =======================================
>
done.

>
>
> > +
> > +Please contact <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.
>
> You need to either add blank lines or add a "- " before the
> two if's above.
>
done.

> > +
> > + 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 (write): KEEP_SEGMENTS
> > + Protocol: 2.07+
> > + - If 0, reload the segment registers in the 32bit entry point.
> > + - If 1, do not reload the segment registers in the 32bit entry point.
> > + Assume that %cs %ds %ss %es are all set to flat segments with
> > + a base of 0 (or the equivalent for their environment).
> > +
> > + 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.
>
> Please indent it the same way as Bit 0.
>
done.

> > +
> > + 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
>
> This is already a table. Just add the markups for it, instead of using '::'
>
> e. g.:
>
> + ========== ==============================
> 0x00000000 The default x86/PC environment
> 0x00000001 lguest
> 0x00000002 Xen
> 0x00000003 Moorestown MID
> 0x00000004 CE4100 TV Platform
> + ========== ==============================
>
done.

>
> > +
> > +::
> > +
> > + 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), and LZ4 (magic number
> > +02 21). 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.
> > +::
> > +
> > + 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.
> > +
> > +
> > +THE IMAGE CHECKSUM
> > +==================
> > +
> > +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.
> > +
> > +
> > +THE KERNEL COMMAND LINE
> > +=======================
> > +
> > +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.
> > +
> > +
> > +MEMORY LAYOUT OF THE REAL-MODE CODE
> > +===================================
> > +
> > +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.
> > +
> > + 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.
> > +
> > +
> > +SAMPLE BOOT CONFIGURATION
> > +=========================
> > +
> > +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
>
> Again, tables. Just do:
>
> 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
> + ============= ===================
>
done.

>
>
> > +
> > +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);
> > + }
> > +
> > +
> > +LOADING THE REST OF THE KERNEL
> > +==============================
> > +
> > +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.
> > +
> > +
> > +SPECIAL COMMAND LINE OPTIONS
> > +============================
> > +
> > +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
> > +Documentation/admin-guide/kernel-parameters.rst 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.
> > +
> > +
> > +RUNNING THE KERNEL
> > +==================
> > +
> > +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!
> > +
> > +
> > +ADVANCED BOOT LOADER HOOKS
> > +==========================
> > +
> > +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.)
> > +
> > +
> > +32-bit BOOT PROTOCOL
> > +====================
> > +
> > +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 that
> > +described in zero-page.txt.
> > +
> > +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.
> > +
> > +64-bit BOOT PROTOCOL
> > +====================
> > +
> > +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 zero-page.txt.
> > +
> > +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.
> > +
> > +EFI HANDOVER PROTOCOL
> > +=====================
> > +
> > +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 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.code32_start
> > + - hdr.cmd_line_ptr
> > + - hdr.ramdisk_image (if applicable)
> > + - hdr.ramdisk_size (if applicable)
> > +
> > +All other fields should be zero.
> > diff --git a/Documentation/x86/boot.txt b/Documentation/x86/boot.txt
> > deleted file mode 100644
> > index f4c2a97bfdbd..000000000000
> > --- a/Documentation/x86/boot.txt
> > +++ /dev/null
> > @@ -1,1130 +0,0 @@
> > - THE LINUX/x86 BOOT PROTOCOL
> > - ---------------------------
> > -
> > -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.
> > -
> > -**** MEMORY LAYOUT
> > -
> > -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 mark)
> > -X+10000 +------------------------+
> > - | Stack/heap | For use by the kernel real-mode code.
> > -X+08000 +------------------------+
> > - | Kernel setup | The kernel real-mode code.
> > - | Kernel boot sector | The kernel legacy boot sector.
> > -X +------------------------+
> > - | Boot loader | <- Boot sector entry point 0000:7C00
> > -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.
> > -
> > -
> > -**** THE REAL-MODE KERNEL HEADER
> > -
> > -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 Proto Name Meaning
> > -/Size
> > -
> > -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
> > -
> > -(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.
> > -
> > -
> > -**** DETAILS OF HEADER FIELDS
> > -
> > -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 <http://sebastian-plotz.blogspot.de>
> > - 12 OVMF UEFI virtualization stack
> > -
> > - Please contact <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 (write): KEEP_SEGMENTS
> > - Protocol: 2.07+
> > - - If 0, reload the segment registers in the 32bit entry point.
> > - - If 1, do not reload the segment registers in the 32bit entry point.
> > - Assume that %cs %ds %ss %es are all set to flat segments with
> > - a base of 0 (or the equivalent for their environment).
> > -
> > - 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), and LZ4 (magic number
> > - 02 21). 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.
> > -
> > -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.
> > -
> > -
> > -**** THE IMAGE CHECKSUM
> > -
> > -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.
> > -
> > -
> > -**** THE KERNEL COMMAND LINE
> > -
> > -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.
> > -
> > -
> > -**** MEMORY LAYOUT OF THE REAL-MODE CODE
> > -
> > -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.
> > -
> > - -> 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.
> > -
> > -
> > -**** SAMPLE BOOT CONFIGURATION
> > -
> > -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);
> > - }
> > -
> > -
> > -**** LOADING THE REST OF THE KERNEL
> > -
> > -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.
> > -
> > -
> > -**** SPECIAL COMMAND LINE OPTIONS
> > -
> > -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
> > -Documentation/admin-guide/kernel-parameters.rst 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.
> > -
> > -
> > -**** RUNNING THE KERNEL
> > -
> > -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!
> > -
> > -
> > -**** ADVANCED BOOT LOADER HOOKS
> > -
> > -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.)
> > -
> > -
> > -**** 32-bit BOOT PROTOCOL
> > -
> > -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 that
> > -described in zero-page.txt.
> > -
> > -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.
> > -
> > -**** 64-bit BOOT PROTOCOL
> > -
> > -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 zero-page.txt.
> > -
> > -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.
> > -
> > -**** EFI HANDOVER PROTOCOL
> > -
> > -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 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,
> > -
> > - o hdr.code32_start
> > - o hdr.cmd_line_ptr
> > - o hdr.ramdisk_image (if applicable)
> > - o hdr.ramdisk_size (if applicable)
> > -
> > -All other fields should be zero.
> > diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst
> > index 7612d3142b2a..8f08caf4fbbb 100644
> > --- a/Documentation/x86/index.rst
> > +++ b/Documentation/x86/index.rst
> > @@ -7,3 +7,5 @@ Linux x86 Support
> > .. toctree::
> > :maxdepth: 2
> > :numbered:
> > +
> > + boot
>
>
>
> Thanks,
> Mauro

--
Cheers,
Changbin Du