boot.txt

linux 内核源代码

		     THE LINUX/I386 BOOT PROTOCOL
		     ----------------------------

		    H. Peter Anvin <[e-mail]>
			Last update 2007-05-23

On the i386 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/i386 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


**** 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	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
0226/2	N/A	pad1		Unused
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
0235/3	N/A	pad2		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

(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.

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:	readmode_swtch
Type:		modify (optional)
Offset/size:	0x208/4
Protocol:	2.00+

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

Field name:	start_sys
Type:		read
Offset/size:	0x20c/4
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.

  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
	5  ELILO
	7  GRuB
	8  U-BOOT
	9  Xen
	A  Gujin
	B  Qemu

  Please contact <[e-mail]> 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 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.