boot.txt

《嵌入式系统设计与实例开发实验教材二源码》Linux内核移植与编译实验

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

		    H. Peter Anvin <[e-mail]>
			Last update 2002-01-01

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, four 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.


**** 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	+------------------------+
	|  Stack/heap/cmdline	 |	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 command line is still required to live in the 0x9XXXX memory
range, and that memory range is still overwritten by the early 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.


**** 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	setup_sects	The size of the setup in sectors
01F2/2	ALL	root_flags	If set, the root is mounted readonly
01F4/2	ALL	syssize		DO NOT USE - for bootsect.S use only
01F6/2	ALL	swap_dev	DO NOT USE - obsolete
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

For backwards compatibility, if the setup_sects field contains 0, the
real value is 4.

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.

The "kernel_version" field, if set to a nonzero value, contains a
pointer to a null-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
14 or higher.

Most boot loaders will simply load the kernel at its target address
directly.  Such boot loaders do not need to worry about filling in
most of the fields in the header.  The following fields should be
filled out, however:

  vid_mode:
	Please see the section on SPECIAL COMMAND LINE OPTIONS.

  type_of_loader:
	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
	1  Loadlin
	2  bootsect-loader
	3  SYSLINUX
	4  EtherBoot

	Please contact <[e-mail]> if you need a bootloader ID
	value assigned.

  loadflags, heap_end_ptr:
	If the protocol version is 2.01 or higher, enter the
	offset limit of the setup heap into heap_end_ptr and set the
	0x80 bit (CAN_USE_HEAP) of loadflags.  heap_end_ptr appears to
	be relative to the start of setup (offset 0x0200).

  setup_move_size: 
	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.

  ramdisk_image, ramdisk_size:
	If your boot loader has loaded an initial ramdisk (initrd),
	set ramdisk_image to the 32-bit pointer to the ramdisk data
	and the ramdisk_size to the size of the ramdisk data.

	The initrd should typically be located as high in memory as
	possible, as it may otherwise get overwritten by the early
	kernel initialization sequence.	 However, it must never be
	located above the address specified in the initrd_addr_max
	field.	The initrd should be at least 4K page aligned.

  cmd_line_ptr:
	If the protocol version is 2.02 or higher, this is a 32-bit
	pointer to the kernel command line.  The kernel command line
	can be located anywhere between the end of setup and 0xA0000.
	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.

  ramdisk_max:
	The maximum address that may be occupied by the initrd
	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.)