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Based on kernel version 3.13. Page generated on 2014-01-20 22:05 EST.

1			     THE LINUX/x86 BOOT PROTOCOL
2			     ---------------------------
3	
4	On the x86 platform, the Linux kernel uses a rather complicated boot
5	convention.  This has evolved partially due to historical aspects, as
6	well as the desire in the early days to have the kernel itself be a
7	bootable image, the complicated PC memory model and due to changed
8	expectations in the PC industry caused by the effective demise of
9	real-mode DOS as a mainstream operating system.
10	
11	Currently, the following versions of the Linux/x86 boot protocol exist.
12	
13	Old kernels:	zImage/Image support only.  Some very early kernels
14			may not even support a command line.
15	
16	Protocol 2.00:	(Kernel 1.3.73) Added bzImage and initrd support, as
17			well as a formalized way to communicate between the
18			boot loader and the kernel.  setup.S made relocatable,
19			although the traditional setup area still assumed
20			writable.
21	
22	Protocol 2.01:	(Kernel 1.3.76) Added a heap overrun warning.
23	
24	Protocol 2.02:	(Kernel 2.4.0-test3-pre3) New command line protocol.
25			Lower the conventional memory ceiling.	No overwrite
26			of the traditional setup area, thus making booting
27			safe for systems which use the EBDA from SMM or 32-bit
28			BIOS entry points.  zImage deprecated but still
29			supported.
30	
31	Protocol 2.03:	(Kernel 2.4.18-pre1) Explicitly makes the highest possible
32			initrd address available to the bootloader.
33	
34	Protocol 2.04:	(Kernel 2.6.14) Extend the syssize field to four bytes.
35	
36	Protocol 2.05:	(Kernel 2.6.20) Make protected mode kernel relocatable.
37			Introduce relocatable_kernel and kernel_alignment fields.
38	
39	Protocol 2.06:	(Kernel 2.6.22) Added a field that contains the size of
40			the boot command line.
41	
42	Protocol 2.07:	(Kernel 2.6.24) Added paravirtualised boot protocol.
43			Introduced hardware_subarch and hardware_subarch_data
44			and KEEP_SEGMENTS flag in load_flags.
45	
46	Protocol 2.08:	(Kernel 2.6.26) Added crc32 checksum and ELF format
47			payload. Introduced payload_offset and payload_length
48			fields to aid in locating the payload.
49	
50	Protocol 2.09:	(Kernel 2.6.26) Added a field of 64-bit physical
51			pointer to single linked list of struct	setup_data.
52	
53	Protocol 2.10:	(Kernel 2.6.31) Added a protocol for relaxed alignment
54			beyond the kernel_alignment added, new init_size and
55			pref_address fields.  Added extended boot loader IDs.
56	
57	Protocol 2.11:	(Kernel 3.6) Added a field for offset of EFI handover
58			protocol entry point.
59	
60	Protocol 2.12:	(Kernel 3.8) Added the xloadflags field and extension fields
61		 	to struct boot_params for loading bzImage and ramdisk
62			above 4G in 64bit.
63	
64	**** MEMORY LAYOUT
65	
66	The traditional memory map for the kernel loader, used for Image or
67	zImage kernels, typically looks like:
68	
69		|			 |
70	0A0000	+------------------------+
71		|  Reserved for BIOS	 |	Do not use.  Reserved for BIOS EBDA.
72	09A000	+------------------------+
73		|  Command line		 |
74		|  Stack/heap		 |	For use by the kernel real-mode code.
75	098000	+------------------------+	
76		|  Kernel setup		 |	The kernel real-mode code.
77	090200	+------------------------+
78		|  Kernel boot sector	 |	The kernel legacy boot sector.
79	090000	+------------------------+
80		|  Protected-mode kernel |	The bulk of the kernel image.
81	010000	+------------------------+
82		|  Boot loader		 |	<- Boot sector entry point 0000:7C00
83	001000	+------------------------+
84		|  Reserved for MBR/BIOS |
85	000800	+------------------------+
86		|  Typically used by MBR |
87	000600	+------------------------+ 
88		|  BIOS use only	 |
89	000000	+------------------------+
90	
91	
92	When using bzImage, the protected-mode kernel was relocated to
93	0x100000 ("high memory"), and the kernel real-mode block (boot sector,
94	setup, and stack/heap) was made relocatable to any address between
95	0x10000 and end of low memory. Unfortunately, in protocols 2.00 and
96	2.01 the 0x90000+ memory range is still used internally by the kernel;
97	the 2.02 protocol resolves that problem.
98	
99	It is desirable to keep the "memory ceiling" -- the highest point in
100	low memory touched by the boot loader -- as low as possible, since
101	some newer BIOSes have begun to allocate some rather large amounts of
102	memory, called the Extended BIOS Data Area, near the top of low
103	memory.	 The boot loader should use the "INT 12h" BIOS call to verify
104	how much low memory is available.
105	
106	Unfortunately, if INT 12h reports that the amount of memory is too
107	low, there is usually nothing the boot loader can do but to report an
108	error to the user.  The boot loader should therefore be designed to
109	take up as little space in low memory as it reasonably can.  For
110	zImage or old bzImage kernels, which need data written into the
111	0x90000 segment, the boot loader should make sure not to use memory
112	above the 0x9A000 point; too many BIOSes will break above that point.
113	
114	For a modern bzImage kernel with boot protocol version >= 2.02, a
115	memory layout like the following is suggested:
116	
117		~                        ~
118	        |  Protected-mode kernel |
119	100000  +------------------------+
120		|  I/O memory hole	 |
121	0A0000	+------------------------+
122		|  Reserved for BIOS	 |	Leave as much as possible unused
123		~                        ~
124		|  Command line		 |	(Can also be below the X+10000 mark)
125	X+10000	+------------------------+
126		|  Stack/heap		 |	For use by the kernel real-mode code.
127	X+08000	+------------------------+	
128		|  Kernel setup		 |	The kernel real-mode code.
129		|  Kernel boot sector	 |	The kernel legacy boot sector.
130	X       +------------------------+
131		|  Boot loader		 |	<- Boot sector entry point 0000:7C00
132	001000	+------------------------+
133		|  Reserved for MBR/BIOS |
134	000800	+------------------------+
135		|  Typically used by MBR |
136	000600	+------------------------+ 
137		|  BIOS use only	 |
138	000000	+------------------------+
139	
140	... where the address X is as low as the design of the boot loader
141	permits.
142	
143	
144	**** THE REAL-MODE KERNEL HEADER
145	
146	In the following text, and anywhere in the kernel boot sequence, "a
147	sector" refers to 512 bytes.  It is independent of the actual sector
148	size of the underlying medium.
149	
150	The first step in loading a Linux kernel should be to load the
151	real-mode code (boot sector and setup code) and then examine the
152	following header at offset 0x01f1.  The real-mode code can total up to
153	32K, although the boot loader may choose to load only the first two
154	sectors (1K) and then examine the bootup sector size.
155	
156	The header looks like:
157	
158	Offset	Proto	Name		Meaning
159	/Size
160	
161	01F1/1	ALL(1	setup_sects	The size of the setup in sectors
162	01F2/2	ALL	root_flags	If set, the root is mounted readonly
163	01F4/4	2.04+(2	syssize		The size of the 32-bit code in 16-byte paras
164	01F8/2	ALL	ram_size	DO NOT USE - for bootsect.S use only
165	01FA/2	ALL	vid_mode	Video mode control
166	01FC/2	ALL	root_dev	Default root device number
167	01FE/2	ALL	boot_flag	0xAA55 magic number
168	0200/2	2.00+	jump		Jump instruction
169	0202/4	2.00+	header		Magic signature "HdrS"
170	0206/2	2.00+	version		Boot protocol version supported
171	0208/4	2.00+	realmode_swtch	Boot loader hook (see below)
172	020C/2	2.00+	start_sys_seg	The load-low segment (0x1000) (obsolete)
173	020E/2	2.00+	kernel_version	Pointer to kernel version string
174	0210/1	2.00+	type_of_loader	Boot loader identifier
175	0211/1	2.00+	loadflags	Boot protocol option flags
176	0212/2	2.00+	setup_move_size	Move to high memory size (used with hooks)
177	0214/4	2.00+	code32_start	Boot loader hook (see below)
178	0218/4	2.00+	ramdisk_image	initrd load address (set by boot loader)
179	021C/4	2.00+	ramdisk_size	initrd size (set by boot loader)
180	0220/4	2.00+	bootsect_kludge	DO NOT USE - for bootsect.S use only
181	0224/2	2.01+	heap_end_ptr	Free memory after setup end
182	0226/1	2.02+(3 ext_loader_ver	Extended boot loader version
183	0227/1	2.02+(3	ext_loader_type	Extended boot loader ID
184	0228/4	2.02+	cmd_line_ptr	32-bit pointer to the kernel command line
185	022C/4	2.03+	ramdisk_max	Highest legal initrd address
186	0230/4	2.05+	kernel_alignment Physical addr alignment required for kernel
187	0234/1	2.05+	relocatable_kernel Whether kernel is relocatable or not
188	0235/1	2.10+	min_alignment	Minimum alignment, as a power of two
189	0236/2	2.12+	xloadflags	Boot protocol option flags
190	0238/4	2.06+	cmdline_size	Maximum size of the kernel command line
191	023C/4	2.07+	hardware_subarch Hardware subarchitecture
192	0240/8	2.07+	hardware_subarch_data Subarchitecture-specific data
193	0248/4	2.08+	payload_offset	Offset of kernel payload
194	024C/4	2.08+	payload_length	Length of kernel payload
195	0250/8	2.09+	setup_data	64-bit physical pointer to linked list
196					of struct setup_data
197	0258/8	2.10+	pref_address	Preferred loading address
198	0260/4	2.10+	init_size	Linear memory required during initialization
199	0264/4	2.11+	handover_offset	Offset of handover entry point
200	
201	(1) For backwards compatibility, if the setup_sects field contains 0, the
202	    real value is 4.
203	
204	(2) For boot protocol prior to 2.04, the upper two bytes of the syssize
205	    field are unusable, which means the size of a bzImage kernel
206	    cannot be determined.
207	
208	(3) Ignored, but safe to set, for boot protocols 2.02-2.09.
209	
210	If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
211	the boot protocol version is "old".  Loading an old kernel, the
212	following parameters should be assumed:
213	
214		Image type = zImage
215		initrd not supported
216		Real-mode kernel must be located at 0x90000.
217	
218	Otherwise, the "version" field contains the protocol version,
219	e.g. protocol version 2.01 will contain 0x0201 in this field.  When
220	setting fields in the header, you must make sure only to set fields
221	supported by the protocol version in use.
222	
223	
224	**** DETAILS OF HEADER FIELDS
225	
226	For each field, some are information from the kernel to the bootloader
227	("read"), some are expected to be filled out by the bootloader
228	("write"), and some are expected to be read and modified by the
229	bootloader ("modify").
230	
231	All general purpose boot loaders should write the fields marked
232	(obligatory).  Boot loaders who want to load the kernel at a
233	nonstandard address should fill in the fields marked (reloc); other
234	boot loaders can ignore those fields.
235	
236	The byte order of all fields is littleendian (this is x86, after all.)
237	
238	Field name:	setup_sects
239	Type:		read
240	Offset/size:	0x1f1/1
241	Protocol:	ALL
242	
243	  The size of the setup code in 512-byte sectors.  If this field is
244	  0, the real value is 4.  The real-mode code consists of the boot
245	  sector (always one 512-byte sector) plus the setup code.
246	
247	Field name:	 root_flags
248	Type:		 modify (optional)
249	Offset/size:	 0x1f2/2
250	Protocol:	 ALL
251	
252	  If this field is nonzero, the root defaults to readonly.  The use of
253	  this field is deprecated; use the "ro" or "rw" options on the
254	  command line instead.
255	
256	Field name:	syssize
257	Type:		read
258	Offset/size:	0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
259	Protocol:	2.04+
260	
261	  The size of the protected-mode code in units of 16-byte paragraphs.
262	  For protocol versions older than 2.04 this field is only two bytes
263	  wide, and therefore cannot be trusted for the size of a kernel if
264	  the LOAD_HIGH flag is set.
265	
266	Field name:	ram_size
267	Type:		kernel internal
268	Offset/size:	0x1f8/2
269	Protocol:	ALL
270	
271	  This field is obsolete.
272	
273	Field name:	vid_mode
274	Type:		modify (obligatory)
275	Offset/size:	0x1fa/2
276	
277	  Please see the section on SPECIAL COMMAND LINE OPTIONS.
278	
279	Field name:	root_dev
280	Type:		modify (optional)
281	Offset/size:	0x1fc/2
282	Protocol:	ALL
283	
284	  The default root device device number.  The use of this field is
285	  deprecated, use the "root=" option on the command line instead.
286	
287	Field name:	boot_flag
288	Type:		read
289	Offset/size:	0x1fe/2
290	Protocol:	ALL
291	
292	  Contains 0xAA55.  This is the closest thing old Linux kernels have
293	  to a magic number.
294	
295	Field name:	jump
296	Type:		read
297	Offset/size:	0x200/2
298	Protocol:	2.00+
299	
300	  Contains an x86 jump instruction, 0xEB followed by a signed offset
301	  relative to byte 0x202.  This can be used to determine the size of
302	  the header.
303	
304	Field name:	header
305	Type:		read
306	Offset/size:	0x202/4
307	Protocol:	2.00+
308	
309	  Contains the magic number "HdrS" (0x53726448).
310	
311	Field name:	version
312	Type:		read
313	Offset/size:	0x206/2
314	Protocol:	2.00+
315	
316	  Contains the boot protocol version, in (major << 8)+minor format,
317	  e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
318	  10.17.
319	
320	Field name:	realmode_swtch
321	Type:		modify (optional)
322	Offset/size:	0x208/4
323	Protocol:	2.00+
324	
325	  Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
326	
327	Field name:	start_sys_seg
328	Type:		read
329	Offset/size:	0x20c/2
330	Protocol:	2.00+
331	
332	  The load low segment (0x1000).  Obsolete.
333	
334	Field name:	kernel_version
335	Type:		read
336	Offset/size:	0x20e/2
337	Protocol:	2.00+
338	
339	  If set to a nonzero value, contains a pointer to a NUL-terminated
340	  human-readable kernel version number string, less 0x200.  This can
341	  be used to display the kernel version to the user.  This value
342	  should be less than (0x200*setup_sects).
343	
344	  For example, if this value is set to 0x1c00, the kernel version
345	  number string can be found at offset 0x1e00 in the kernel file.
346	  This is a valid value if and only if the "setup_sects" field
347	  contains the value 15 or higher, as:
348	
349		0x1c00  < 15*0x200 (= 0x1e00) but
350		0x1c00 >= 14*0x200 (= 0x1c00)
351	
352		0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
353	
354	Field name:	type_of_loader
355	Type:		write (obligatory)
356	Offset/size:	0x210/1
357	Protocol:	2.00+
358	
359	  If your boot loader has an assigned id (see table below), enter
360	  0xTV here, where T is an identifier for the boot loader and V is
361	  a version number.  Otherwise, enter 0xFF here.
362	
363	  For boot loader IDs above T = 0xD, write T = 0xE to this field and
364	  write the extended ID minus 0x10 to the ext_loader_type field.
365	  Similarly, the ext_loader_ver field can be used to provide more than
366	  four bits for the bootloader version.
367	
368	  For example, for T = 0x15, V = 0x234, write:
369	
370	  type_of_loader  <- 0xE4
371	  ext_loader_type <- 0x05
372	  ext_loader_ver  <- 0x23
373	
374	  Assigned boot loader ids (hexadecimal):
375	
376		0  LILO			(0x00 reserved for pre-2.00 bootloader)
377		1  Loadlin
378		2  bootsect-loader	(0x20, all other values reserved)
379		3  Syslinux
380		4  Etherboot/gPXE/iPXE
381		5  ELILO
382		7  GRUB
383		8  U-Boot
384		9  Xen
385		A  Gujin
386		B  Qemu
387		C  Arcturus Networks uCbootloader
388		D  kexec-tools
389		E  Extended		(see ext_loader_type)
390		F  Special		(0xFF = undefined)
391	       10  Reserved
392	       11  Minimal Linux Bootloader <http://sebastian-plotz.blogspot.de>
393	       12  OVMF UEFI virtualization stack
394	
395	  Please contact <hpa@zytor.com> if you need a bootloader ID
396	  value assigned.
397	
398	Field name:	loadflags
399	Type:		modify (obligatory)
400	Offset/size:	0x211/1
401	Protocol:	2.00+
402	
403	  This field is a bitmask.
404	
405	  Bit 0 (read):	LOADED_HIGH
406		- If 0, the protected-mode code is loaded at 0x10000.
407		- If 1, the protected-mode code is loaded at 0x100000.
408	
409	  Bit 5 (write): QUIET_FLAG
410		- If 0, print early messages.
411		- If 1, suppress early messages.
412			This requests to the kernel (decompressor and early
413			kernel) to not write early messages that require
414			accessing the display hardware directly.
415	
416	  Bit 6 (write): KEEP_SEGMENTS
417		Protocol: 2.07+
418		- If 0, reload the segment registers in the 32bit entry point.
419		- If 1, do not reload the segment registers in the 32bit entry point.
420			Assume that %cs %ds %ss %es are all set to flat segments with
421			a base of 0 (or the equivalent for their environment).
422	
423	  Bit 7 (write): CAN_USE_HEAP
424		Set this bit to 1 to indicate that the value entered in the
425		heap_end_ptr is valid.  If this field is clear, some setup code
426		functionality will be disabled.
427	
428	Field name:	setup_move_size
429	Type:		modify (obligatory)
430	Offset/size:	0x212/2
431	Protocol:	2.00-2.01
432	
433	  When using protocol 2.00 or 2.01, if the real mode kernel is not
434	  loaded at 0x90000, it gets moved there later in the loading
435	  sequence.  Fill in this field if you want additional data (such as
436	  the kernel command line) moved in addition to the real-mode kernel
437	  itself.
438	
439	  The unit is bytes starting with the beginning of the boot sector.
440	  
441	  This field is can be ignored when the protocol is 2.02 or higher, or
442	  if the real-mode code is loaded at 0x90000.
443	
444	Field name:	code32_start
445	Type:		modify (optional, reloc)
446	Offset/size:	0x214/4
447	Protocol:	2.00+
448	
449	  The address to jump to in protected mode.  This defaults to the load
450	  address of the kernel, and can be used by the boot loader to
451	  determine the proper load address.
452	
453	  This field can be modified for two purposes:
454	
455	  1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
456	
457	  2. if a bootloader which does not install a hook loads a
458	     relocatable kernel at a nonstandard address it will have to modify
459	     this field to point to the load address.
460	
461	Field name:	ramdisk_image
462	Type:		write (obligatory)
463	Offset/size:	0x218/4
464	Protocol:	2.00+
465	
466	  The 32-bit linear address of the initial ramdisk or ramfs.  Leave at
467	  zero if there is no initial ramdisk/ramfs.
468	
469	Field name:	ramdisk_size
470	Type:		write (obligatory)
471	Offset/size:	0x21c/4
472	Protocol:	2.00+
473	
474	  Size of the initial ramdisk or ramfs.  Leave at zero if there is no
475	  initial ramdisk/ramfs.
476	
477	Field name:	bootsect_kludge
478	Type:		kernel internal
479	Offset/size:	0x220/4
480	Protocol:	2.00+
481	
482	  This field is obsolete.
483	
484	Field name:	heap_end_ptr
485	Type:		write (obligatory)
486	Offset/size:	0x224/2
487	Protocol:	2.01+
488	
489	  Set this field to the offset (from the beginning of the real-mode
490	  code) of the end of the setup stack/heap, minus 0x0200.
491	
492	Field name:	ext_loader_ver
493	Type:		write (optional)
494	Offset/size:	0x226/1
495	Protocol:	2.02+
496	
497	  This field is used as an extension of the version number in the
498	  type_of_loader field.  The total version number is considered to be
499	  (type_of_loader & 0x0f) + (ext_loader_ver << 4).
500	
501	  The use of this field is boot loader specific.  If not written, it
502	  is zero.
503	
504	  Kernels prior to 2.6.31 did not recognize this field, but it is safe
505	  to write for protocol version 2.02 or higher.
506	
507	Field name:	ext_loader_type
508	Type:		write (obligatory if (type_of_loader & 0xf0) == 0xe0)
509	Offset/size:	0x227/1
510	Protocol:	2.02+
511	
512	  This field is used as an extension of the type number in
513	  type_of_loader field.  If the type in type_of_loader is 0xE, then
514	  the actual type is (ext_loader_type + 0x10).
515	
516	  This field is ignored if the type in type_of_loader is not 0xE.
517	
518	  Kernels prior to 2.6.31 did not recognize this field, but it is safe
519	  to write for protocol version 2.02 or higher.
520	
521	Field name:	cmd_line_ptr
522	Type:		write (obligatory)
523	Offset/size:	0x228/4
524	Protocol:	2.02+
525	
526	  Set this field to the linear address of the kernel command line.
527	  The kernel command line can be located anywhere between the end of
528	  the setup heap and 0xA0000; it does not have to be located in the
529	  same 64K segment as the real-mode code itself.
530	
531	  Fill in this field even if your boot loader does not support a
532	  command line, in which case you can point this to an empty string
533	  (or better yet, to the string "auto".)  If this field is left at
534	  zero, the kernel will assume that your boot loader does not support
535	  the 2.02+ protocol.
536	
537	Field name:	ramdisk_max
538	Type:		read
539	Offset/size:	0x22c/4
540	Protocol:	2.03+
541	
542	  The maximum address that may be occupied by the initial
543	  ramdisk/ramfs contents.  For boot protocols 2.02 or earlier, this
544	  field is not present, and the maximum address is 0x37FFFFFF.  (This
545	  address is defined as the address of the highest safe byte, so if
546	  your ramdisk is exactly 131072 bytes long and this field is
547	  0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
548	
549	Field name:	kernel_alignment
550	Type:		read/modify (reloc)
551	Offset/size:	0x230/4
552	Protocol:	2.05+ (read), 2.10+ (modify)
553	
554	  Alignment unit required by the kernel (if relocatable_kernel is
555	  true.)  A relocatable kernel that is loaded at an alignment
556	  incompatible with the value in this field will be realigned during
557	  kernel initialization.
558	
559	  Starting with protocol version 2.10, this reflects the kernel
560	  alignment preferred for optimal performance; it is possible for the
561	  loader to modify this field to permit a lesser alignment.  See the
562	  min_alignment and pref_address field below.
563	
564	Field name:	relocatable_kernel
565	Type:		read (reloc)
566	Offset/size:	0x234/1
567	Protocol:	2.05+
568	
569	  If this field is nonzero, the protected-mode part of the kernel can
570	  be loaded at any address that satisfies the kernel_alignment field.
571	  After loading, the boot loader must set the code32_start field to
572	  point to the loaded code, or to a boot loader hook.
573	
574	Field name:	min_alignment
575	Type:		read (reloc)
576	Offset/size:	0x235/1
577	Protocol:	2.10+
578	
579	  This field, if nonzero, indicates as a power of two the minimum
580	  alignment required, as opposed to preferred, by the kernel to boot.
581	  If a boot loader makes use of this field, it should update the
582	  kernel_alignment field with the alignment unit desired; typically:
583	
584		kernel_alignment = 1 << min_alignment
585	
586	  There may be a considerable performance cost with an excessively
587	  misaligned kernel.  Therefore, a loader should typically try each
588	  power-of-two alignment from kernel_alignment down to this alignment.
589	
590	Field name:     xloadflags
591	Type:           read
592	Offset/size:    0x236/2
593	Protocol:       2.12+
594	
595	  This field is a bitmask.
596	
597	  Bit 0 (read):	XLF_KERNEL_64
598		- If 1, this kernel has the legacy 64-bit entry point at 0x200.
599	
600	  Bit 1 (read): XLF_CAN_BE_LOADED_ABOVE_4G
601	        - If 1, kernel/boot_params/cmdline/ramdisk can be above 4G.
602	
603	  Bit 2 (read):	XLF_EFI_HANDOVER_32
604		- If 1, the kernel supports the 32-bit EFI handoff entry point
605	          given at handover_offset.
606	
607	  Bit 3 (read): XLF_EFI_HANDOVER_64
608		- If 1, the kernel supports the 64-bit EFI handoff entry point
609	          given at handover_offset + 0x200.
610	
611	Field name:	cmdline_size
612	Type:		read
613	Offset/size:	0x238/4
614	Protocol:	2.06+
615	
616	  The maximum size of the command line without the terminating
617	  zero. This means that the command line can contain at most
618	  cmdline_size characters. With protocol version 2.05 and earlier, the
619	  maximum size was 255.
620	
621	Field name:	hardware_subarch
622	Type:		write (optional, defaults to x86/PC)
623	Offset/size:	0x23c/4
624	Protocol:	2.07+
625	
626	  In a paravirtualized environment the hardware low level architectural
627	  pieces such as interrupt handling, page table handling, and
628	  accessing process control registers needs to be done differently.
629	
630	  This field allows the bootloader to inform the kernel we are in one
631	  one of those environments.
632	
633	  0x00000000	The default x86/PC environment
634	  0x00000001	lguest
635	  0x00000002	Xen
636	  0x00000003	Moorestown MID
637	  0x00000004	CE4100 TV Platform
638	
639	Field name:	hardware_subarch_data
640	Type:		write (subarch-dependent)
641	Offset/size:	0x240/8
642	Protocol:	2.07+
643	
644	  A pointer to data that is specific to hardware subarch
645	  This field is currently unused for the default x86/PC environment,
646	  do not modify.
647	
648	Field name:	payload_offset
649	Type:		read
650	Offset/size:	0x248/4
651	Protocol:	2.08+
652	
653	  If non-zero then this field contains the offset from the beginning
654	  of the protected-mode code to the payload.
655	
656	  The payload may be compressed. The format of both the compressed and
657	  uncompressed data should be determined using the standard magic
658	  numbers.  The currently supported compression formats are gzip
659	  (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA
660	  (magic number 5D 00), XZ (magic number FD 37), and LZ4 (magic number
661	  02 21).  The uncompressed payload is currently always ELF (magic
662	  number 7F 45 4C 46).
663	
664	Field name:	payload_length
665	Type:		read
666	Offset/size:	0x24c/4
667	Protocol:	2.08+
668	
669	  The length of the payload.
670	
671	Field name:	setup_data
672	Type:		write (special)
673	Offset/size:	0x250/8
674	Protocol:	2.09+
675	
676	  The 64-bit physical pointer to NULL terminated single linked list of
677	  struct setup_data. This is used to define a more extensible boot
678	  parameters passing mechanism. The definition of struct setup_data is
679	  as follow:
680	
681	  struct setup_data {
682		  u64 next;
683		  u32 type;
684		  u32 len;
685		  u8  data[0];
686	  };
687	
688	  Where, the next is a 64-bit physical pointer to the next node of
689	  linked list, the next field of the last node is 0; the type is used
690	  to identify the contents of data; the len is the length of data
691	  field; the data holds the real payload.
692	
693	  This list may be modified at a number of points during the bootup
694	  process.  Therefore, when modifying this list one should always make
695	  sure to consider the case where the linked list already contains
696	  entries.
697	
698	Field name:	pref_address
699	Type:		read (reloc)
700	Offset/size:	0x258/8
701	Protocol:	2.10+
702	
703	  This field, if nonzero, represents a preferred load address for the
704	  kernel.  A relocating bootloader should attempt to load at this
705	  address if possible.
706	
707	  A non-relocatable kernel will unconditionally move itself and to run
708	  at this address.
709	
710	Field name:	init_size
711	Type:		read
712	Offset/size:	0x260/4
713	
714	  This field indicates the amount of linear contiguous memory starting
715	  at the kernel runtime start address that the kernel needs before it
716	  is capable of examining its memory map.  This is not the same thing
717	  as the total amount of memory the kernel needs to boot, but it can
718	  be used by a relocating boot loader to help select a safe load
719	  address for the kernel.
720	
721	  The kernel runtime start address is determined by the following algorithm:
722	
723	  if (relocatable_kernel)
724		runtime_start = align_up(load_address, kernel_alignment)
725	  else
726		runtime_start = pref_address
727	
728	Field name:	handover_offset
729	Type:		read
730	Offset/size:	0x264/4
731	
732	  This field is the offset from the beginning of the kernel image to
733	  the EFI handover protocol entry point. Boot loaders using the EFI
734	  handover protocol to boot the kernel should jump to this offset.
735	
736	  See EFI HANDOVER PROTOCOL below for more details.
737	
738	
739	**** THE IMAGE CHECKSUM
740	
741	From boot protocol version 2.08 onwards the CRC-32 is calculated over
742	the entire file using the characteristic polynomial 0x04C11DB7 and an
743	initial remainder of 0xffffffff.  The checksum is appended to the
744	file; therefore the CRC of the file up to the limit specified in the
745	syssize field of the header is always 0.
746	
747	
748	**** THE KERNEL COMMAND LINE
749	
750	The kernel command line has become an important way for the boot
751	loader to communicate with the kernel.  Some of its options are also
752	relevant to the boot loader itself, see "special command line options"
753	below.
754	
755	The kernel command line is a null-terminated string. The maximum
756	length can be retrieved from the field cmdline_size.  Before protocol
757	version 2.06, the maximum was 255 characters.  A string that is too
758	long will be automatically truncated by the kernel.
759	
760	If the boot protocol version is 2.02 or later, the address of the
761	kernel command line is given by the header field cmd_line_ptr (see
762	above.)  This address can be anywhere between the end of the setup
763	heap and 0xA0000.
764	
765	If the protocol version is *not* 2.02 or higher, the kernel
766	command line is entered using the following protocol:
767	
768		At offset 0x0020 (word), "cmd_line_magic", enter the magic
769		number 0xA33F.
770	
771		At offset 0x0022 (word), "cmd_line_offset", enter the offset
772		of the kernel command line (relative to the start of the
773		real-mode kernel).
774		
775		The kernel command line *must* be within the memory region
776		covered by setup_move_size, so you may need to adjust this
777		field.
778	
779	
780	**** MEMORY LAYOUT OF THE REAL-MODE CODE
781	
782	The real-mode code requires a stack/heap to be set up, as well as
783	memory allocated for the kernel command line.  This needs to be done
784	in the real-mode accessible memory in bottom megabyte.
785	
786	It should be noted that modern machines often have a sizable Extended
787	BIOS Data Area (EBDA).  As a result, it is advisable to use as little
788	of the low megabyte as possible.
789	
790	Unfortunately, under the following circumstances the 0x90000 memory
791	segment has to be used:
792	
793		- When loading a zImage kernel ((loadflags & 0x01) == 0).
794		- When loading a 2.01 or earlier boot protocol kernel.
795	
796		  -> For the 2.00 and 2.01 boot protocols, the real-mode code
797		     can be loaded at another address, but it is internally
798		     relocated to 0x90000.  For the "old" protocol, the
799		     real-mode code must be loaded at 0x90000.
800	
801	When loading at 0x90000, avoid using memory above 0x9a000.
802	
803	For boot protocol 2.02 or higher, the command line does not have to be
804	located in the same 64K segment as the real-mode setup code; it is
805	thus permitted to give the stack/heap the full 64K segment and locate
806	the command line above it.
807	
808	The kernel command line should not be located below the real-mode
809	code, nor should it be located in high memory.
810	
811	
812	**** SAMPLE BOOT CONFIGURATION
813	
814	As a sample configuration, assume the following layout of the real
815	mode segment:
816	
817	    When loading below 0x90000, use the entire segment:
818	
819		0x0000-0x7fff	Real mode kernel
820		0x8000-0xdfff	Stack and heap
821		0xe000-0xffff	Kernel command line
822	
823	    When loading at 0x90000 OR the protocol version is 2.01 or earlier:
824	
825		0x0000-0x7fff	Real mode kernel
826		0x8000-0x97ff	Stack and heap
827		0x9800-0x9fff	Kernel command line
828	
829	Such a boot loader should enter the following fields in the header:
830	
831		unsigned long base_ptr;	/* base address for real-mode segment */
832	
833		if ( setup_sects == 0 ) {
834			setup_sects = 4;
835		}
836	
837		if ( protocol >= 0x0200 ) {
838			type_of_loader = <type code>;
839			if ( loading_initrd ) {
840				ramdisk_image = <initrd_address>;
841				ramdisk_size = <initrd_size>;
842			}
843	
844			if ( protocol >= 0x0202 && loadflags & 0x01 )
845				heap_end = 0xe000;
846			else
847				heap_end = 0x9800;
848	
849			if ( protocol >= 0x0201 ) {
850				heap_end_ptr = heap_end - 0x200;
851				loadflags |= 0x80; /* CAN_USE_HEAP */
852			}
853	
854			if ( protocol >= 0x0202 ) {
855				cmd_line_ptr = base_ptr + heap_end;
856				strcpy(cmd_line_ptr, cmdline);
857			} else {
858				cmd_line_magic	= 0xA33F;
859				cmd_line_offset = heap_end;
860				setup_move_size = heap_end + strlen(cmdline)+1;
861				strcpy(base_ptr+cmd_line_offset, cmdline);
862			}
863		} else {
864			/* Very old kernel */
865	
866			heap_end = 0x9800;
867	
868			cmd_line_magic	= 0xA33F;
869			cmd_line_offset = heap_end;
870	
871			/* A very old kernel MUST have its real-mode code
872			   loaded at 0x90000 */
873	
874			if ( base_ptr != 0x90000 ) {
875				/* Copy the real-mode kernel */
876				memcpy(0x90000, base_ptr, (setup_sects+1)*512);
877				base_ptr = 0x90000;		 /* Relocated */
878			}
879	
880			strcpy(0x90000+cmd_line_offset, cmdline);
881	
882			/* It is recommended to clear memory up to the 32K mark */
883			memset(0x90000 + (setup_sects+1)*512, 0,
884			       (64-(setup_sects+1))*512);
885		}
886	
887	
888	**** LOADING THE REST OF THE KERNEL
889	
890	The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
891	in the kernel file (again, if setup_sects == 0 the real value is 4.)
892	It should be loaded at address 0x10000 for Image/zImage kernels and
893	0x100000 for bzImage kernels.
894	
895	The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
896	bit (LOAD_HIGH) in the loadflags field is set:
897	
898		is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
899		load_address = is_bzImage ? 0x100000 : 0x10000;
900	
901	Note that Image/zImage kernels can be up to 512K in size, and thus use
902	the entire 0x10000-0x90000 range of memory.  This means it is pretty
903	much a requirement for these kernels to load the real-mode part at
904	0x90000.  bzImage kernels allow much more flexibility.
905	
906	
907	**** SPECIAL COMMAND LINE OPTIONS
908	
909	If the command line provided by the boot loader is entered by the
910	user, the user may expect the following command line options to work.
911	They should normally not be deleted from the kernel command line even
912	though not all of them are actually meaningful to the kernel.  Boot
913	loader authors who need additional command line options for the boot
914	loader itself should get them registered in
915	Documentation/kernel-parameters.txt to make sure they will not
916	conflict with actual kernel options now or in the future.
917	
918	  vga=<mode>
919		<mode> here is either an integer (in C notation, either
920		decimal, octal, or hexadecimal) or one of the strings
921		"normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
922		(meaning 0xFFFD).  This value should be entered into the
923		vid_mode field, as it is used by the kernel before the command
924		line is parsed.
925	
926	  mem=<size>
927		<size> is an integer in C notation optionally followed by
928		(case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
929		<< 30, << 40, << 50 or << 60).  This specifies the end of
930		memory to the kernel. This affects the possible placement of
931		an initrd, since an initrd should be placed near end of
932		memory.  Note that this is an option to *both* the kernel and
933		the bootloader!
934	
935	  initrd=<file>
936		An initrd should be loaded.  The meaning of <file> is
937		obviously bootloader-dependent, and some boot loaders
938		(e.g. LILO) do not have such a command.
939	
940	In addition, some boot loaders add the following options to the
941	user-specified command line:
942	
943	  BOOT_IMAGE=<file>
944		The boot image which was loaded.  Again, the meaning of <file>
945		is obviously bootloader-dependent.
946	
947	  auto
948		The kernel was booted without explicit user intervention.
949	
950	If these options are added by the boot loader, it is highly
951	recommended that they are located *first*, before the user-specified
952	or configuration-specified command line.  Otherwise, "init=/bin/sh"
953	gets confused by the "auto" option.
954	
955	
956	**** RUNNING THE KERNEL
957	
958	The kernel is started by jumping to the kernel entry point, which is
959	located at *segment* offset 0x20 from the start of the real mode
960	kernel.  This means that if you loaded your real-mode kernel code at
961	0x90000, the kernel entry point is 9020:0000.
962	
963	At entry, ds = es = ss should point to the start of the real-mode
964	kernel code (0x9000 if the code is loaded at 0x90000), sp should be
965	set up properly, normally pointing to the top of the heap, and
966	interrupts should be disabled.  Furthermore, to guard against bugs in
967	the kernel, it is recommended that the boot loader sets fs = gs = ds =
968	es = ss.
969	
970	In our example from above, we would do:
971	
972		/* Note: in the case of the "old" kernel protocol, base_ptr must
973		   be == 0x90000 at this point; see the previous sample code */
974	
975		seg = base_ptr >> 4;
976	
977		cli();	/* Enter with interrupts disabled! */
978	
979		/* Set up the real-mode kernel stack */
980		_SS = seg;
981		_SP = heap_end;
982	
983		_DS = _ES = _FS = _GS = seg;
984		jmp_far(seg+0x20, 0);	/* Run the kernel */
985	
986	If your boot sector accesses a floppy drive, it is recommended to
987	switch off the floppy motor before running the kernel, since the
988	kernel boot leaves interrupts off and thus the motor will not be
989	switched off, especially if the loaded kernel has the floppy driver as
990	a demand-loaded module!
991	
992	
993	**** ADVANCED BOOT LOADER HOOKS
994	
995	If the boot loader runs in a particularly hostile environment (such as
996	LOADLIN, which runs under DOS) it may be impossible to follow the
997	standard memory location requirements.  Such a boot loader may use the
998	following hooks that, if set, are invoked by the kernel at the
999	appropriate time.  The use of these hooks should probably be
1000	considered an absolutely last resort!
1001	
1002	IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
1003	%edi across invocation.
1004	
1005	  realmode_swtch:
1006		A 16-bit real mode far subroutine invoked immediately before
1007		entering protected mode.  The default routine disables NMI, so
1008		your routine should probably do so, too.
1009	
1010	  code32_start:
1011		A 32-bit flat-mode routine *jumped* to immediately after the
1012		transition to protected mode, but before the kernel is
1013		uncompressed.  No segments, except CS, are guaranteed to be
1014		set up (current kernels do, but older ones do not); you should
1015		set them up to BOOT_DS (0x18) yourself.
1016	
1017		After completing your hook, you should jump to the address
1018		that was in this field before your boot loader overwrote it
1019		(relocated, if appropriate.)
1020	
1021	
1022	**** 32-bit BOOT PROTOCOL
1023	
1024	For machine with some new BIOS other than legacy BIOS, such as EFI,
1025	LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
1026	based on legacy BIOS can not be used, so a 32-bit boot protocol needs
1027	to be defined.
1028	
1029	In 32-bit boot protocol, the first step in loading a Linux kernel
1030	should be to setup the boot parameters (struct boot_params,
1031	traditionally known as "zero page"). The memory for struct boot_params
1032	should be allocated and initialized to all zero. Then the setup header
1033	from offset 0x01f1 of kernel image on should be loaded into struct
1034	boot_params and examined. The end of setup header can be calculated as
1035	follow:
1036	
1037		0x0202 + byte value at offset 0x0201
1038	
1039	In addition to read/modify/write the setup header of the struct
1040	boot_params as that of 16-bit boot protocol, the boot loader should
1041	also fill the additional fields of the struct boot_params as that
1042	described in zero-page.txt.
1043	
1044	After setting up the struct boot_params, the boot loader can load the
1045	32/64-bit kernel in the same way as that of 16-bit boot protocol.
1046	
1047	In 32-bit boot protocol, the kernel is started by jumping to the
1048	32-bit kernel entry point, which is the start address of loaded
1049	32/64-bit kernel.
1050	
1051	At entry, the CPU must be in 32-bit protected mode with paging
1052	disabled; a GDT must be loaded with the descriptors for selectors
1053	__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1054	segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1055	must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1056	must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
1057	address of the struct boot_params; %ebp, %edi and %ebx must be zero.
1058	
1059	**** 64-bit BOOT PROTOCOL
1060	
1061	For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader
1062	and we need a 64-bit boot protocol.
1063	
1064	In 64-bit boot protocol, the first step in loading a Linux kernel
1065	should be to setup the boot parameters (struct boot_params,
1066	traditionally known as "zero page"). The memory for struct boot_params
1067	could be allocated anywhere (even above 4G) and initialized to all zero.
1068	Then, the setup header at offset 0x01f1 of kernel image on should be
1069	loaded into struct boot_params and examined. The end of setup header
1070	can be calculated as follows:
1071	
1072		0x0202 + byte value at offset 0x0201
1073	
1074	In addition to read/modify/write the setup header of the struct
1075	boot_params as that of 16-bit boot protocol, the boot loader should
1076	also fill the additional fields of the struct boot_params as described
1077	in zero-page.txt.
1078	
1079	After setting up the struct boot_params, the boot loader can load
1080	64-bit kernel in the same way as that of 16-bit boot protocol, but
1081	kernel could be loaded above 4G.
1082	
1083	In 64-bit boot protocol, the kernel is started by jumping to the
1084	64-bit kernel entry point, which is the start address of loaded
1085	64-bit kernel plus 0x200.
1086	
1087	At entry, the CPU must be in 64-bit mode with paging enabled.
1088	The range with setup_header.init_size from start address of loaded
1089	kernel and zero page and command line buffer get ident mapping;
1090	a GDT must be loaded with the descriptors for selectors
1091	__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1092	segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1093	must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1094	must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base
1095	address of the struct boot_params.
1096	
1097	**** EFI HANDOVER PROTOCOL
1098	
1099	This protocol allows boot loaders to defer initialisation to the EFI
1100	boot stub. The boot loader is required to load the kernel/initrd(s)
1101	from the boot media and jump to the EFI handover protocol entry point
1102	which is hdr->handover_offset bytes from the beginning of
1103	startup_{32,64}.
1104	
1105	The function prototype for the handover entry point looks like this,
1106	
1107	    efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp)
1108	
1109	'handle' is the EFI image handle passed to the boot loader by the EFI
1110	firmware, 'table' is the EFI system table - these are the first two
1111	arguments of the "handoff state" as described in section 2.3 of the
1112	UEFI specification. 'bp' is the boot loader-allocated boot params.
1113	
1114	The boot loader *must* fill out the following fields in bp,
1115	
1116	    o hdr.code32_start
1117	    o hdr.cmd_line_ptr
1118	    o hdr.cmdline_size
1119	    o hdr.ramdisk_image (if applicable)
1120	    o hdr.ramdisk_size  (if applicable)
1121	
1122	All other fields should be zero.
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