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Commit db2668fd authored by H. Peter Anvin's avatar H. Peter Anvin Committed by Linus Torvalds
Browse files

boot documentation: clarifications 



Textual clarifications (and fix an off-by-one error) based on feedback
mostly from Jeremy Fitzhardinge.

Signed-off-by: default avatarH. Peter Anvin <hpa@zytor.com>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent af669c97

Documentation/i386/boot.txt

+22 −10
Original line number Diff line number Diff line
@@ -2,7 +2,7 @@ 
		     ----------------------------



		    H. Peter Anvin <hpa@zytor.com>

			Last update 2007-05-16

			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
@@ -202,6 +202,8 @@ All general purpose boot loaders should write the fields marked 
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_secs

Type:		read

Offset/size:	0x1f1/1
@@ -280,14 +282,16 @@ Type: read 
Offset/size:	0x206/2

Protocol:	2.00+



  Contains the boot protocol version, e.g. 0x0204 for version 2.04.

  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 separate chapter.)

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



Field name:	start_sys

Type:		read
@@ -304,10 +308,17 @@ 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 14 or higher.

  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)
@@ -377,7 +388,7 @@ Protocol: 2.00+ 


  This field can be modified for two purposes:



  1. as a boot loader hook (see separate chapter.)

  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
@@ -715,7 +726,7 @@ switched off, especially if the loaded kernel has the floppy driver as 
a demand-loaded module!





**** ADVANCED BOOT TIME HOOKS

**** 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
@@ -740,4 +751,5 @@ IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and 
	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.
 
	that was in this field before your boot loader overwrote it

	(relocated, if appropriate.)