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.

  **** 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+	ramdisk_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	N/A	pad3		Unused

  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

  (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:

  	0  LILO			(0x00 reserved for pre-2.00 bootloader)

  	1  Loadlin

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

  	3  Syslinux
  	4  Etherboot/gPXE

  	5  ELILO

  	7  GRUB

  	8  U-Boot

  	9  Xen

  	A  Gujin

  	B  Qemu

  	C  Arcturus Networks uCbootloader
  	E  Extended		(see ext_loader_type)
  	F  Special		(0xFF = undefined)

    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 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:	ramdisk_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:	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), and XZ (magic number FD 37).  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

  **** 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/kernel-parameters.txt 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 setupping 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; __BOOS_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.