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Commit 4039feb5 authored by H. Peter Anvin's avatar H. Peter Anvin
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x86: update Documentation/i386/boot.txt 



Document QUIET_FLAG, correct the definition of several fields, make it
clear this applies to the entire x86 architecture, not just i386.

Signed-off-by: default avatarH. Peter Anvin <hpa@zytor.com>
parent 3b6b9293

Documentation/i386/boot.txt

+46 −33
Original line number Diff line number Diff line 
		     THE LINUX/I386 BOOT PROTOCOL

		     ----------------------------

		     THE LINUX/x86 BOOT PROTOCOL

		     ---------------------------



		    H. Peter Anvin <hpa@zytor.com>

			Last update 2007-05-23



On the i386 platform, the Linux kernel uses a rather complicated boot

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/i386 boot protocol exist.

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.
@@ -372,10 +369,17 @@ Protocol: 2.00+ 
	- 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.

	- 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).
@@ -504,7 +508,7 @@ Protocol: 2.06+ 
  maximum size was 255.



Field name:	hardware_subarch

Type:		write

Type:		write (optional, defaults to x86/PC)

Offset/size:	0x23c/4

Protocol:	2.07+
@@ -520,11 +524,13 @@ Protocol: 2.07+ 
  0x00000002	Xen



Field name:	hardware_subarch_data

Type:		write

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
@@ -545,6 +551,34 @@ 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.





**** THE IMAGE CHECKSUM



From boot protocol version 2.08 onwards the CRC-32 is calculated over
@@ -553,6 +587,7 @@ 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
@@ -584,28 +619,6 @@ command line is entered using the following protocol: 
	covered by setup_move_size, so you may need to adjust this

	field.



Field name:	setup_data

Type:		write (obligatory)

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.





**** MEMORY LAYOUT OF THE REAL-MODE CODE