Documentation / x86 / boot.txt

Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

			     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.

• [ x86 ]
• 00-INDEX
• amd-memory-encryption.txt
• boot.txt
• earlyprintk.txt
• entry_64.txt
• exception-tables.txt
• [ i386 ]
• intel_mpx.txt
• intel_rdt_ui.txt
• kernel-stacks
• microcode.txt
• mtrr.txt
• orc-unwinder.txt
• pat.txt
• protection-keys.txt
• pti.txt
• tlb.txt
• topology.txt
• usb-legacy-support.txt
• [ x86_64 ]
• zero-page.txt
•  

---