- THE LINUX/x86 BOOT PROTOCOL
- ---------------------------
+.. 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
Currently, the following versions of the Linux/x86 boot protocol exist.
-Old kernels: zImage/Image support only. Some very early kernels
+============= ============================================================
+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
+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 2.01 (Kernel 1.3.76) Added a heap overrun warning.
-Protocol 2.02: (Kernel 2.4.0-test3-pre3) New command line protocol.
+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
+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 2.04 (Kernel 2.6.14) Extend the syssize field to four bytes.
-Protocol 2.05: (Kernel 2.6.20) Make protected mode kernel relocatable.
+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
+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.
+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
+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
+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
+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 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
+Protocol 2.12 (Kernel 3.8) Added the xloadflags field and extension fields
to struct boot_params for loading bzImage and ramdisk
above 4G in 64bit.
-Protocol 2.13: (Kernel 3.14) Support 32- and 64-bit flags being set in
+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
+============= ============================================================
-**** 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 +------------------------+
+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,
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
+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
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.
+=========== ======== ===================== ============================================
+Offset/Size Proto Name Meaning
+=========== ======== ===================== ============================================
+01F1/1 ALL(1) setup_sects The size of the setup in sectors
+01F2/2 ALL root_flags If set, the root is mounted readonly
+01F4/4 2.04+(2) syssize The size of the 32-bit code in 16-byte paras
+01F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
+01FA/2 ALL vid_mode Video mode control
+01FC/2 ALL root_dev Default root device number
+01FE/2 ALL boot_flag 0xAA55 magic number
+0200/2 2.00+ jump Jump instruction
+0202/4 2.00+ header Magic signature "HdrS"
+0206/2 2.00+ version Boot protocol version supported
+0208/4 2.00+ realmode_swtch Boot loader hook (see below)
+020C/2 2.00+ start_sys_seg The load-low segment (0x1000) (obsolete)
+020E/2 2.00+ kernel_version Pointer to kernel version string
+0210/1 2.00+ type_of_loader Boot loader identifier
+0211/1 2.00+ loadflags Boot protocol option flags
+0212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
+0214/4 2.00+ code32_start Boot loader hook (see below)
+0218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
+021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
+0220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
+0224/2 2.01+ heap_end_ptr Free memory after setup end
+0226/1 2.02+(3) ext_loader_ver Extended boot loader version
+0227/1 2.02+(3) ext_loader_type Extended boot loader ID
+0228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
+022C/4 2.03+ initrd_addr_max Highest legal initrd address
+0230/4 2.05+ kernel_alignment Physical addr alignment required for kernel
+0234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
+0235/1 2.10+ min_alignment Minimum alignment, as a power of two
+0236/2 2.12+ xloadflags Boot protocol option flags
+0238/4 2.06+ cmdline_size Maximum size of the kernel command line
+023C/4 2.07+ hardware_subarch Hardware subarchitecture
+0240/8 2.07+ hardware_subarch_data Subarchitecture-specific data
+0248/4 2.08+ payload_offset Offset of kernel payload
+024C/4 2.08+ payload_length Length of kernel payload
+0250/8 2.09+ setup_data 64-bit physical pointer to linked list
+ of struct setup_data
+0258/8 2.10+ pref_address Preferred loading address
+0260/4 2.10+ init_size Linear memory required during initialization
+0264/4 2.11+ handover_offset Offset of handover entry point
+=========== ======== ===================== ============================================
+
+.. note::
+ (1) For backwards compatibility, if the setup_sects field contains 0, the
+ real value is 4.
+
+ (2) For boot protocol prior to 2.04, the upper two bytes of the syssize
+ field are unusable, which means the size of a bzImage kernel
+ cannot be determined.
+
+ (3) Ignored, but safe to set, for boot protocols 2.02-2.09.
If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
the boot protocol version is "old". Loading an old kernel, the
-following parameters should be assumed:
+following parameters should be assumed::
Image type = zImage
initrd not supported
supported by the protocol version in use.
-**** DETAILS OF HEADER FIELDS
+Details of Harder Fileds
+========================
For each field, some are information from the kernel to the bootloader
("read"), some are expected to be filled out by the bootloader
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
+============ =================
+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
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:
+ 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.
+ 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
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:
+ For example, for T = 0x15, V = 0x234, write::
- type_of_loader <- 0xE4
- ext_loader_type <- 0x05
- ext_loader_ver <- 0x23
+ 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)
+ == =======================================
+ 0 LILO
+ (0x00 reserved for pre-2.00 bootloader)
1 Loadlin
- 2 bootsect-loader (0x20, all other values reserved)
+ 2 bootsect-loader
+ (0x20, all other values reserved)
3 Syslinux
4 Etherboot/gPXE/iPXE
5 ELILO
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
+ 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.
+ 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.
+
+ - 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
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
This field can be modified for two purposes:
- 1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
+ 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.
+ 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
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
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
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
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
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 field with the alignment unit desired; typically::
kernel_alignment = 1 << min_alignment
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+
+============ ==========
+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
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.
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:
+ as follow::
- struct setup_data {
- u64 next;
- u32 type;
- u32 len;
- u8 data[0];
- };
+ 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
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
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
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:
+ The kernel runtime start address is determined by the following algorithm::
- if (relocatable_kernel)
+ if (relocatable_kernel)
runtime_start = align_up(load_address, kernel_alignment)
- else
+ 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
See EFI HANDOVER PROTOCOL below for more details.
-**** THE IMAGE CHECKSUM
+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
syssize field of the header is always 0.
-**** THE KERNEL COMMAND LINE
+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
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 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).
- 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.
+ - 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
+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
- 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.
+.. note::
+ For the 2.00 and 2.01 boot protocols, the real-mode code
+ can be loaded at another address, but it is internally
+ relocated to 0x90000. For the "old" protocol, the
+ real-mode code must be loaded at 0x90000.
When loading at 0x90000, avoid using memory above 0x9a000.
code, nor should it be located in high memory.
-**** SAMPLE BOOT CONFIGURATION
+Sample Boot Configuartion
+=========================
As a sample configuration, assume the following layout of the real
-mode segment:
+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:
+Such a boot loader should enter the following fields in the header::
unsigned long base_ptr; /* base address for real-mode segment */
}
-**** LOADING THE REST OF THE KERNEL
+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.)
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:
+bit (LOAD_HIGH) in the loadflags field is set::
is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
load_address = is_bzImage ? 0x100000 : 0x10000;
much a requirement for these kernels to load the real-mode part at
0x90000. bzImage kernels allow much more flexibility.
-
-**** SPECIAL COMMAND LINE OPTIONS
+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.
gets confused by the "auto" option.
-**** RUNNING THE KERNEL
+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
the kernel, it is recommended that the boot loader sets fs = gs = ds =
es = ss.
-In our example from above, we would do:
+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 */
a demand-loaded module!
-**** ADVANCED BOOT LOADER HOOKS
+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
(relocated, if appropriate.)
-**** 32-bit BOOT PROTOCOL
+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
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:
+follow::
0x0202 + byte value at offset 0x0201
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
+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.
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:
+can be calculated as follows::
0x0202 + byte value at offset 0x0201
must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base
address of the struct boot_params.
-**** EFI HANDOVER PROTOCOL
+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)
which is hdr->handover_offset bytes from the beginning of
startup_{32,64}.
-The function prototype for the handover entry point looks like this,
+The function prototype for the handover entry point looks like this::
efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp)
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,
+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)
+ - hdr.code32_start
+ - hdr.cmd_line_ptr
+ - hdr.ramdisk_image (if applicable)
+ - hdr.ramdisk_size (if applicable)
All other fields should be zero.
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