boot.txt

linux 内核源代码

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  This field is obsolete.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:	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 (reloc)
Offset/size:	0x230/4
Protocol:	2.05+

  Alignment unit required by the kernel (if relocatable_kernel is true.)

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:	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
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

Field name:	hardware_subarch_data
Type:		write
Offset/size:	0x240/8
Protocol:	2.07+

  A pointer to data that is specific to hardware subarch


**** 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.