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Based on kernel version 3.16. Page generated on 2014-08-06 21:41 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+	initrd_addr_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:	initrd_addr_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	  Bit 4 (read): XLF_EFI_KEXEC
612		- If 1, the kernel supports kexec EFI boot with EFI runtime support.
613	
614	Field name:	cmdline_size
615	Type:		read
616	Offset/size:	0x238/4
617	Protocol:	2.06+
618	
619	  The maximum size of the command line without the terminating
620	  zero. This means that the command line can contain at most
621	  cmdline_size characters. With protocol version 2.05 and earlier, the
622	  maximum size was 255.
623	
624	Field name:	hardware_subarch
625	Type:		write (optional, defaults to x86/PC)
626	Offset/size:	0x23c/4
627	Protocol:	2.07+
628	
629	  In a paravirtualized environment the hardware low level architectural
630	  pieces such as interrupt handling, page table handling, and
631	  accessing process control registers needs to be done differently.
632	
633	  This field allows the bootloader to inform the kernel we are in one
634	  one of those environments.
635	
636	  0x00000000	The default x86/PC environment
637	  0x00000001	lguest
638	  0x00000002	Xen
639	  0x00000003	Moorestown MID
640	  0x00000004	CE4100 TV Platform
641	
642	Field name:	hardware_subarch_data
643	Type:		write (subarch-dependent)
644	Offset/size:	0x240/8
645	Protocol:	2.07+
646	
647	  A pointer to data that is specific to hardware subarch
648	  This field is currently unused for the default x86/PC environment,
649	  do not modify.
650	
651	Field name:	payload_offset
652	Type:		read
653	Offset/size:	0x248/4
654	Protocol:	2.08+
655	
656	  If non-zero then this field contains the offset from the beginning
657	  of the protected-mode code to the payload.
658	
659	  The payload may be compressed. The format of both the compressed and
660	  uncompressed data should be determined using the standard magic
661	  numbers.  The currently supported compression formats are gzip
662	  (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA
663	  (magic number 5D 00), XZ (magic number FD 37), and LZ4 (magic number
664	  02 21).  The uncompressed payload is currently always ELF (magic
665	  number 7F 45 4C 46).
666	
667	Field name:	payload_length
668	Type:		read
669	Offset/size:	0x24c/4
670	Protocol:	2.08+
671	
672	  The length of the payload.
673	
674	Field name:	setup_data
675	Type:		write (special)
676	Offset/size:	0x250/8
677	Protocol:	2.09+
678	
679	  The 64-bit physical pointer to NULL terminated single linked list of
680	  struct setup_data. This is used to define a more extensible boot
681	  parameters passing mechanism. The definition of struct setup_data is
682	  as follow:
683	
684	  struct setup_data {
685		  u64 next;
686		  u32 type;
687		  u32 len;
688		  u8  data[0];
689	  };
690	
691	  Where, the next is a 64-bit physical pointer to the next node of
692	  linked list, the next field of the last node is 0; the type is used
693	  to identify the contents of data; the len is the length of data
694	  field; the data holds the real payload.
695	
696	  This list may be modified at a number of points during the bootup
697	  process.  Therefore, when modifying this list one should always make
698	  sure to consider the case where the linked list already contains
699	  entries.
700	
701	Field name:	pref_address
702	Type:		read (reloc)
703	Offset/size:	0x258/8
704	Protocol:	2.10+
705	
706	  This field, if nonzero, represents a preferred load address for the
707	  kernel.  A relocating bootloader should attempt to load at this
708	  address if possible.
709	
710	  A non-relocatable kernel will unconditionally move itself and to run
711	  at this address.
712	
713	Field name:	init_size
714	Type:		read
715	Offset/size:	0x260/4
716	
717	  This field indicates the amount of linear contiguous memory starting
718	  at the kernel runtime start address that the kernel needs before it
719	  is capable of examining its memory map.  This is not the same thing
720	  as the total amount of memory the kernel needs to boot, but it can
721	  be used by a relocating boot loader to help select a safe load
722	  address for the kernel.
723	
724	  The kernel runtime start address is determined by the following algorithm:
725	
726	  if (relocatable_kernel)
727		runtime_start = align_up(load_address, kernel_alignment)
728	  else
729		runtime_start = pref_address
730	
731	Field name:	handover_offset
732	Type:		read
733	Offset/size:	0x264/4
734	
735	  This field is the offset from the beginning of the kernel image to
736	  the EFI handover protocol entry point. Boot loaders using the EFI
737	  handover protocol to boot the kernel should jump to this offset.
738	
739	  See EFI HANDOVER PROTOCOL below for more details.
740	
741	
742	**** THE IMAGE CHECKSUM
743	
744	From boot protocol version 2.08 onwards the CRC-32 is calculated over
745	the entire file using the characteristic polynomial 0x04C11DB7 and an
746	initial remainder of 0xffffffff.  The checksum is appended to the
747	file; therefore the CRC of the file up to the limit specified in the
748	syssize field of the header is always 0.
749	
750	
751	**** THE KERNEL COMMAND LINE
752	
753	The kernel command line has become an important way for the boot
754	loader to communicate with the kernel.  Some of its options are also
755	relevant to the boot loader itself, see "special command line options"
756	below.
757	
758	The kernel command line is a null-terminated string. The maximum
759	length can be retrieved from the field cmdline_size.  Before protocol
760	version 2.06, the maximum was 255 characters.  A string that is too
761	long will be automatically truncated by the kernel.
762	
763	If the boot protocol version is 2.02 or later, the address of the
764	kernel command line is given by the header field cmd_line_ptr (see
765	above.)  This address can be anywhere between the end of the setup
766	heap and 0xA0000.
767	
768	If the protocol version is *not* 2.02 or higher, the kernel
769	command line is entered using the following protocol:
770	
771		At offset 0x0020 (word), "cmd_line_magic", enter the magic
772		number 0xA33F.
773	
774		At offset 0x0022 (word), "cmd_line_offset", enter the offset
775		of the kernel command line (relative to the start of the
776		real-mode kernel).
777		
778		The kernel command line *must* be within the memory region
779		covered by setup_move_size, so you may need to adjust this
780		field.
781	
782	
783	**** MEMORY LAYOUT OF THE REAL-MODE CODE
784	
785	The real-mode code requires a stack/heap to be set up, as well as
786	memory allocated for the kernel command line.  This needs to be done
787	in the real-mode accessible memory in bottom megabyte.
788	
789	It should be noted that modern machines often have a sizable Extended
790	BIOS Data Area (EBDA).  As a result, it is advisable to use as little
791	of the low megabyte as possible.
792	
793	Unfortunately, under the following circumstances the 0x90000 memory
794	segment has to be used:
795	
796		- When loading a zImage kernel ((loadflags & 0x01) == 0).
797		- When loading a 2.01 or earlier boot protocol kernel.
798	
799		  -> For the 2.00 and 2.01 boot protocols, the real-mode code
800		     can be loaded at another address, but it is internally
801		     relocated to 0x90000.  For the "old" protocol, the
802		     real-mode code must be loaded at 0x90000.
803	
804	When loading at 0x90000, avoid using memory above 0x9a000.
805	
806	For boot protocol 2.02 or higher, the command line does not have to be
807	located in the same 64K segment as the real-mode setup code; it is
808	thus permitted to give the stack/heap the full 64K segment and locate
809	the command line above it.
810	
811	The kernel command line should not be located below the real-mode
812	code, nor should it be located in high memory.
813	
814	
815	**** SAMPLE BOOT CONFIGURATION
816	
817	As a sample configuration, assume the following layout of the real
818	mode segment:
819	
820	    When loading below 0x90000, use the entire segment:
821	
822		0x0000-0x7fff	Real mode kernel
823		0x8000-0xdfff	Stack and heap
824		0xe000-0xffff	Kernel command line
825	
826	    When loading at 0x90000 OR the protocol version is 2.01 or earlier:
827	
828		0x0000-0x7fff	Real mode kernel
829		0x8000-0x97ff	Stack and heap
830		0x9800-0x9fff	Kernel command line
831	
832	Such a boot loader should enter the following fields in the header:
833	
834		unsigned long base_ptr;	/* base address for real-mode segment */
835	
836		if ( setup_sects == 0 ) {
837			setup_sects = 4;
838		}
839	
840		if ( protocol >= 0x0200 ) {
841			type_of_loader = <type code>;
842			if ( loading_initrd ) {
843				ramdisk_image = <initrd_address>;
844				ramdisk_size = <initrd_size>;
845			}
846	
847			if ( protocol >= 0x0202 && loadflags & 0x01 )
848				heap_end = 0xe000;
849			else
850				heap_end = 0x9800;
851	
852			if ( protocol >= 0x0201 ) {
853				heap_end_ptr = heap_end - 0x200;
854				loadflags |= 0x80; /* CAN_USE_HEAP */
855			}
856	
857			if ( protocol >= 0x0202 ) {
858				cmd_line_ptr = base_ptr + heap_end;
859				strcpy(cmd_line_ptr, cmdline);
860			} else {
861				cmd_line_magic	= 0xA33F;
862				cmd_line_offset = heap_end;
863				setup_move_size = heap_end + strlen(cmdline)+1;
864				strcpy(base_ptr+cmd_line_offset, cmdline);
865			}
866		} else {
867			/* Very old kernel */
868	
869			heap_end = 0x9800;
870	
871			cmd_line_magic	= 0xA33F;
872			cmd_line_offset = heap_end;
873	
874			/* A very old kernel MUST have its real-mode code
875			   loaded at 0x90000 */
876	
877			if ( base_ptr != 0x90000 ) {
878				/* Copy the real-mode kernel */
879				memcpy(0x90000, base_ptr, (setup_sects+1)*512);
880				base_ptr = 0x90000;		 /* Relocated */
881			}
882	
883			strcpy(0x90000+cmd_line_offset, cmdline);
884	
885			/* It is recommended to clear memory up to the 32K mark */
886			memset(0x90000 + (setup_sects+1)*512, 0,
887			       (64-(setup_sects+1))*512);
888		}
889	
890	
891	**** LOADING THE REST OF THE KERNEL
892	
893	The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
894	in the kernel file (again, if setup_sects == 0 the real value is 4.)
895	It should be loaded at address 0x10000 for Image/zImage kernels and
896	0x100000 for bzImage kernels.
897	
898	The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
899	bit (LOAD_HIGH) in the loadflags field is set:
900	
901		is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
902		load_address = is_bzImage ? 0x100000 : 0x10000;
903	
904	Note that Image/zImage kernels can be up to 512K in size, and thus use
905	the entire 0x10000-0x90000 range of memory.  This means it is pretty
906	much a requirement for these kernels to load the real-mode part at
907	0x90000.  bzImage kernels allow much more flexibility.
908	
909	
910	**** SPECIAL COMMAND LINE OPTIONS
911	
912	If the command line provided by the boot loader is entered by the
913	user, the user may expect the following command line options to work.
914	They should normally not be deleted from the kernel command line even
915	though not all of them are actually meaningful to the kernel.  Boot
916	loader authors who need additional command line options for the boot
917	loader itself should get them registered in
918	Documentation/kernel-parameters.txt to make sure they will not
919	conflict with actual kernel options now or in the future.
920	
921	  vga=<mode>
922		<mode> here is either an integer (in C notation, either
923		decimal, octal, or hexadecimal) or one of the strings
924		"normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
925		(meaning 0xFFFD).  This value should be entered into the
926		vid_mode field, as it is used by the kernel before the command
927		line is parsed.
928	
929	  mem=<size>
930		<size> is an integer in C notation optionally followed by
931		(case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
932		<< 30, << 40, << 50 or << 60).  This specifies the end of
933		memory to the kernel. This affects the possible placement of
934		an initrd, since an initrd should be placed near end of
935		memory.  Note that this is an option to *both* the kernel and
936		the bootloader!
937	
938	  initrd=<file>
939		An initrd should be loaded.  The meaning of <file> is
940		obviously bootloader-dependent, and some boot loaders
941		(e.g. LILO) do not have such a command.
942	
943	In addition, some boot loaders add the following options to the
944	user-specified command line:
945	
946	  BOOT_IMAGE=<file>
947		The boot image which was loaded.  Again, the meaning of <file>
948		is obviously bootloader-dependent.
949	
950	  auto
951		The kernel was booted without explicit user intervention.
952	
953	If these options are added by the boot loader, it is highly
954	recommended that they are located *first*, before the user-specified
955	or configuration-specified command line.  Otherwise, "init=/bin/sh"
956	gets confused by the "auto" option.
957	
958	
959	**** RUNNING THE KERNEL
960	
961	The kernel is started by jumping to the kernel entry point, which is
962	located at *segment* offset 0x20 from the start of the real mode
963	kernel.  This means that if you loaded your real-mode kernel code at
964	0x90000, the kernel entry point is 9020:0000.
965	
966	At entry, ds = es = ss should point to the start of the real-mode
967	kernel code (0x9000 if the code is loaded at 0x90000), sp should be
968	set up properly, normally pointing to the top of the heap, and
969	interrupts should be disabled.  Furthermore, to guard against bugs in
970	the kernel, it is recommended that the boot loader sets fs = gs = ds =
971	es = ss.
972	
973	In our example from above, we would do:
974	
975		/* Note: in the case of the "old" kernel protocol, base_ptr must
976		   be == 0x90000 at this point; see the previous sample code */
977	
978		seg = base_ptr >> 4;
979	
980		cli();	/* Enter with interrupts disabled! */
981	
982		/* Set up the real-mode kernel stack */
983		_SS = seg;
984		_SP = heap_end;
985	
986		_DS = _ES = _FS = _GS = seg;
987		jmp_far(seg+0x20, 0);	/* Run the kernel */
988	
989	If your boot sector accesses a floppy drive, it is recommended to
990	switch off the floppy motor before running the kernel, since the
991	kernel boot leaves interrupts off and thus the motor will not be
992	switched off, especially if the loaded kernel has the floppy driver as
993	a demand-loaded module!
994	
995	
996	**** ADVANCED BOOT LOADER HOOKS
997	
998	If the boot loader runs in a particularly hostile environment (such as
999	LOADLIN, which runs under DOS) it may be impossible to follow the
1000	standard memory location requirements.  Such a boot loader may use the
1001	following hooks that, if set, are invoked by the kernel at the
1002	appropriate time.  The use of these hooks should probably be
1003	considered an absolutely last resort!
1004	
1005	IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
1006	%edi across invocation.
1007	
1008	  realmode_swtch:
1009		A 16-bit real mode far subroutine invoked immediately before
1010		entering protected mode.  The default routine disables NMI, so
1011		your routine should probably do so, too.
1012	
1013	  code32_start:
1014		A 32-bit flat-mode routine *jumped* to immediately after the
1015		transition to protected mode, but before the kernel is
1016		uncompressed.  No segments, except CS, are guaranteed to be
1017		set up (current kernels do, but older ones do not); you should
1018		set them up to BOOT_DS (0x18) yourself.
1019	
1020		After completing your hook, you should jump to the address
1021		that was in this field before your boot loader overwrote it
1022		(relocated, if appropriate.)
1023	
1024	
1025	**** 32-bit BOOT PROTOCOL
1026	
1027	For machine with some new BIOS other than legacy BIOS, such as EFI,
1028	LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
1029	based on legacy BIOS can not be used, so a 32-bit boot protocol needs
1030	to be defined.
1031	
1032	In 32-bit boot protocol, the first step in loading a Linux kernel
1033	should be to setup the boot parameters (struct boot_params,
1034	traditionally known as "zero page"). The memory for struct boot_params
1035	should be allocated and initialized to all zero. Then the setup header
1036	from offset 0x01f1 of kernel image on should be loaded into struct
1037	boot_params and examined. The end of setup header can be calculated as
1038	follow:
1039	
1040		0x0202 + byte value at offset 0x0201
1041	
1042	In addition to read/modify/write the setup header of the struct
1043	boot_params as that of 16-bit boot protocol, the boot loader should
1044	also fill the additional fields of the struct boot_params as that
1045	described in zero-page.txt.
1046	
1047	After setting up the struct boot_params, the boot loader can load the
1048	32/64-bit kernel in the same way as that of 16-bit boot protocol.
1049	
1050	In 32-bit boot protocol, the kernel is started by jumping to the
1051	32-bit kernel entry point, which is the start address of loaded
1052	32/64-bit kernel.
1053	
1054	At entry, the CPU must be in 32-bit protected mode with paging
1055	disabled; a GDT must be loaded with the descriptors for selectors
1056	__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1057	segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1058	must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1059	must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
1060	address of the struct boot_params; %ebp, %edi and %ebx must be zero.
1061	
1062	**** 64-bit BOOT PROTOCOL
1063	
1064	For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader
1065	and we need a 64-bit boot protocol.
1066	
1067	In 64-bit boot protocol, the first step in loading a Linux kernel
1068	should be to setup the boot parameters (struct boot_params,
1069	traditionally known as "zero page"). The memory for struct boot_params
1070	could be allocated anywhere (even above 4G) and initialized to all zero.
1071	Then, the setup header at offset 0x01f1 of kernel image on should be
1072	loaded into struct boot_params and examined. The end of setup header
1073	can be calculated as follows:
1074	
1075		0x0202 + byte value at offset 0x0201
1076	
1077	In addition to read/modify/write the setup header of the struct
1078	boot_params as that of 16-bit boot protocol, the boot loader should
1079	also fill the additional fields of the struct boot_params as described
1080	in zero-page.txt.
1081	
1082	After setting up the struct boot_params, the boot loader can load
1083	64-bit kernel in the same way as that of 16-bit boot protocol, but
1084	kernel could be loaded above 4G.
1085	
1086	In 64-bit boot protocol, the kernel is started by jumping to the
1087	64-bit kernel entry point, which is the start address of loaded
1088	64-bit kernel plus 0x200.
1089	
1090	At entry, the CPU must be in 64-bit mode with paging enabled.
1091	The range with setup_header.init_size from start address of loaded
1092	kernel and zero page and command line buffer get ident mapping;
1093	a GDT must be loaded with the descriptors for selectors
1094	__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1095	segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1096	must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1097	must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base
1098	address of the struct boot_params.
1099	
1100	**** EFI HANDOVER PROTOCOL
1101	
1102	This protocol allows boot loaders to defer initialisation to the EFI
1103	boot stub. The boot loader is required to load the kernel/initrd(s)
1104	from the boot media and jump to the EFI handover protocol entry point
1105	which is hdr->handover_offset bytes from the beginning of
1106	startup_{32,64}.
1107	
1108	The function prototype for the handover entry point looks like this,
1109	
1110	    efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp)
1111	
1112	'handle' is the EFI image handle passed to the boot loader by the EFI
1113	firmware, 'table' is the EFI system table - these are the first two
1114	arguments of the "handoff state" as described in section 2.3 of the
1115	UEFI specification. 'bp' is the boot loader-allocated boot params.
1116	
1117	The boot loader *must* fill out the following fields in bp,
1118	
1119	    o hdr.code32_start
1120	    o hdr.cmd_line_ptr
1121	    o hdr.cmdline_size
1122	    o hdr.ramdisk_image (if applicable)
1123	    o hdr.ramdisk_size  (if applicable)
1124	
1125	All other fields should be zero.
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