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Based on kernel version 4.1. Page generated on 2015-06-28 12:15 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 1 (kernel internal): ALSR_FLAG
410		- Used internally by the compressed kernel to communicate
411		  KASLR status to kernel proper.
412		  If 1, KASLR enabled.
413		  If 0, KASLR disabled.
414	
415	  Bit 5 (write): QUIET_FLAG
416		- If 0, print early messages.
417		- If 1, suppress early messages.
418			This requests to the kernel (decompressor and early
419			kernel) to not write early messages that require
420			accessing the display hardware directly.
421	
422	  Bit 6 (write): KEEP_SEGMENTS
423		Protocol: 2.07+
424		- If 0, reload the segment registers in the 32bit entry point.
425		- If 1, do not reload the segment registers in the 32bit entry point.
426			Assume that %cs %ds %ss %es are all set to flat segments with
427			a base of 0 (or the equivalent for their environment).
428	
429	  Bit 7 (write): CAN_USE_HEAP
430		Set this bit to 1 to indicate that the value entered in the
431		heap_end_ptr is valid.  If this field is clear, some setup code
432		functionality will be disabled.
433	
434	Field name:	setup_move_size
435	Type:		modify (obligatory)
436	Offset/size:	0x212/2
437	Protocol:	2.00-2.01
438	
439	  When using protocol 2.00 or 2.01, if the real mode kernel is not
440	  loaded at 0x90000, it gets moved there later in the loading
441	  sequence.  Fill in this field if you want additional data (such as
442	  the kernel command line) moved in addition to the real-mode kernel
443	  itself.
444	
445	  The unit is bytes starting with the beginning of the boot sector.
446	  
447	  This field is can be ignored when the protocol is 2.02 or higher, or
448	  if the real-mode code is loaded at 0x90000.
449	
450	Field name:	code32_start
451	Type:		modify (optional, reloc)
452	Offset/size:	0x214/4
453	Protocol:	2.00+
454	
455	  The address to jump to in protected mode.  This defaults to the load
456	  address of the kernel, and can be used by the boot loader to
457	  determine the proper load address.
458	
459	  This field can be modified for two purposes:
460	
461	  1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
462	
463	  2. if a bootloader which does not install a hook loads a
464	     relocatable kernel at a nonstandard address it will have to modify
465	     this field to point to the load address.
466	
467	Field name:	ramdisk_image
468	Type:		write (obligatory)
469	Offset/size:	0x218/4
470	Protocol:	2.00+
471	
472	  The 32-bit linear address of the initial ramdisk or ramfs.  Leave at
473	  zero if there is no initial ramdisk/ramfs.
474	
475	Field name:	ramdisk_size
476	Type:		write (obligatory)
477	Offset/size:	0x21c/4
478	Protocol:	2.00+
479	
480	  Size of the initial ramdisk or ramfs.  Leave at zero if there is no
481	  initial ramdisk/ramfs.
482	
483	Field name:	bootsect_kludge
484	Type:		kernel internal
485	Offset/size:	0x220/4
486	Protocol:	2.00+
487	
488	  This field is obsolete.
489	
490	Field name:	heap_end_ptr
491	Type:		write (obligatory)
492	Offset/size:	0x224/2
493	Protocol:	2.01+
494	
495	  Set this field to the offset (from the beginning of the real-mode
496	  code) of the end of the setup stack/heap, minus 0x0200.
497	
498	Field name:	ext_loader_ver
499	Type:		write (optional)
500	Offset/size:	0x226/1
501	Protocol:	2.02+
502	
503	  This field is used as an extension of the version number in the
504	  type_of_loader field.  The total version number is considered to be
505	  (type_of_loader & 0x0f) + (ext_loader_ver << 4).
506	
507	  The use of this field is boot loader specific.  If not written, it
508	  is zero.
509	
510	  Kernels prior to 2.6.31 did not recognize this field, but it is safe
511	  to write for protocol version 2.02 or higher.
512	
513	Field name:	ext_loader_type
514	Type:		write (obligatory if (type_of_loader & 0xf0) == 0xe0)
515	Offset/size:	0x227/1
516	Protocol:	2.02+
517	
518	  This field is used as an extension of the type number in
519	  type_of_loader field.  If the type in type_of_loader is 0xE, then
520	  the actual type is (ext_loader_type + 0x10).
521	
522	  This field is ignored if the type in type_of_loader is not 0xE.
523	
524	  Kernels prior to 2.6.31 did not recognize this field, but it is safe
525	  to write for protocol version 2.02 or higher.
526	
527	Field name:	cmd_line_ptr
528	Type:		write (obligatory)
529	Offset/size:	0x228/4
530	Protocol:	2.02+
531	
532	  Set this field to the linear address of the kernel command line.
533	  The kernel command line can be located anywhere between the end of
534	  the setup heap and 0xA0000; it does not have to be located in the
535	  same 64K segment as the real-mode code itself.
536	
537	  Fill in this field even if your boot loader does not support a
538	  command line, in which case you can point this to an empty string
539	  (or better yet, to the string "auto".)  If this field is left at
540	  zero, the kernel will assume that your boot loader does not support
541	  the 2.02+ protocol.
542	
543	Field name:	initrd_addr_max
544	Type:		read
545	Offset/size:	0x22c/4
546	Protocol:	2.03+
547	
548	  The maximum address that may be occupied by the initial
549	  ramdisk/ramfs contents.  For boot protocols 2.02 or earlier, this
550	  field is not present, and the maximum address is 0x37FFFFFF.  (This
551	  address is defined as the address of the highest safe byte, so if
552	  your ramdisk is exactly 131072 bytes long and this field is
553	  0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
554	
555	Field name:	kernel_alignment
556	Type:		read/modify (reloc)
557	Offset/size:	0x230/4
558	Protocol:	2.05+ (read), 2.10+ (modify)
559	
560	  Alignment unit required by the kernel (if relocatable_kernel is
561	  true.)  A relocatable kernel that is loaded at an alignment
562	  incompatible with the value in this field will be realigned during
563	  kernel initialization.
564	
565	  Starting with protocol version 2.10, this reflects the kernel
566	  alignment preferred for optimal performance; it is possible for the
567	  loader to modify this field to permit a lesser alignment.  See the
568	  min_alignment and pref_address field below.
569	
570	Field name:	relocatable_kernel
571	Type:		read (reloc)
572	Offset/size:	0x234/1
573	Protocol:	2.05+
574	
575	  If this field is nonzero, the protected-mode part of the kernel can
576	  be loaded at any address that satisfies the kernel_alignment field.
577	  After loading, the boot loader must set the code32_start field to
578	  point to the loaded code, or to a boot loader hook.
579	
580	Field name:	min_alignment
581	Type:		read (reloc)
582	Offset/size:	0x235/1
583	Protocol:	2.10+
584	
585	  This field, if nonzero, indicates as a power of two the minimum
586	  alignment required, as opposed to preferred, by the kernel to boot.
587	  If a boot loader makes use of this field, it should update the
588	  kernel_alignment field with the alignment unit desired; typically:
589	
590		kernel_alignment = 1 << min_alignment
591	
592	  There may be a considerable performance cost with an excessively
593	  misaligned kernel.  Therefore, a loader should typically try each
594	  power-of-two alignment from kernel_alignment down to this alignment.
595	
596	Field name:     xloadflags
597	Type:           read
598	Offset/size:    0x236/2
599	Protocol:       2.12+
600	
601	  This field is a bitmask.
602	
603	  Bit 0 (read):	XLF_KERNEL_64
604		- If 1, this kernel has the legacy 64-bit entry point at 0x200.
605	
606	  Bit 1 (read): XLF_CAN_BE_LOADED_ABOVE_4G
607	        - If 1, kernel/boot_params/cmdline/ramdisk can be above 4G.
608	
609	  Bit 2 (read):	XLF_EFI_HANDOVER_32
610		- If 1, the kernel supports the 32-bit EFI handoff entry point
611	          given at handover_offset.
612	
613	  Bit 3 (read): XLF_EFI_HANDOVER_64
614		- If 1, the kernel supports the 64-bit EFI handoff entry point
615	          given at handover_offset + 0x200.
616	
617	  Bit 4 (read): XLF_EFI_KEXEC
618		- If 1, the kernel supports kexec EFI boot with EFI runtime support.
619	
620	Field name:	cmdline_size
621	Type:		read
622	Offset/size:	0x238/4
623	Protocol:	2.06+
624	
625	  The maximum size of the command line without the terminating
626	  zero. This means that the command line can contain at most
627	  cmdline_size characters. With protocol version 2.05 and earlier, the
628	  maximum size was 255.
629	
630	Field name:	hardware_subarch
631	Type:		write (optional, defaults to x86/PC)
632	Offset/size:	0x23c/4
633	Protocol:	2.07+
634	
635	  In a paravirtualized environment the hardware low level architectural
636	  pieces such as interrupt handling, page table handling, and
637	  accessing process control registers needs to be done differently.
638	
639	  This field allows the bootloader to inform the kernel we are in one
640	  one of those environments.
641	
642	  0x00000000	The default x86/PC environment
643	  0x00000001	lguest
644	  0x00000002	Xen
645	  0x00000003	Moorestown MID
646	  0x00000004	CE4100 TV Platform
647	
648	Field name:	hardware_subarch_data
649	Type:		write (subarch-dependent)
650	Offset/size:	0x240/8
651	Protocol:	2.07+
652	
653	  A pointer to data that is specific to hardware subarch
654	  This field is currently unused for the default x86/PC environment,
655	  do not modify.
656	
657	Field name:	payload_offset
658	Type:		read
659	Offset/size:	0x248/4
660	Protocol:	2.08+
661	
662	  If non-zero then this field contains the offset from the beginning
663	  of the protected-mode code to the payload.
664	
665	  The payload may be compressed. The format of both the compressed and
666	  uncompressed data should be determined using the standard magic
667	  numbers.  The currently supported compression formats are gzip
668	  (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA
669	  (magic number 5D 00), XZ (magic number FD 37), and LZ4 (magic number
670	  02 21).  The uncompressed payload is currently always ELF (magic
671	  number 7F 45 4C 46).
672	
673	Field name:	payload_length
674	Type:		read
675	Offset/size:	0x24c/4
676	Protocol:	2.08+
677	
678	  The length of the payload.
679	
680	Field name:	setup_data
681	Type:		write (special)
682	Offset/size:	0x250/8
683	Protocol:	2.09+
684	
685	  The 64-bit physical pointer to NULL terminated single linked list of
686	  struct setup_data. This is used to define a more extensible boot
687	  parameters passing mechanism. The definition of struct setup_data is
688	  as follow:
689	
690	  struct setup_data {
691		  u64 next;
692		  u32 type;
693		  u32 len;
694		  u8  data[0];
695	  };
696	
697	  Where, the next is a 64-bit physical pointer to the next node of
698	  linked list, the next field of the last node is 0; the type is used
699	  to identify the contents of data; the len is the length of data
700	  field; the data holds the real payload.
701	
702	  This list may be modified at a number of points during the bootup
703	  process.  Therefore, when modifying this list one should always make
704	  sure to consider the case where the linked list already contains
705	  entries.
706	
707	Field name:	pref_address
708	Type:		read (reloc)
709	Offset/size:	0x258/8
710	Protocol:	2.10+
711	
712	  This field, if nonzero, represents a preferred load address for the
713	  kernel.  A relocating bootloader should attempt to load at this
714	  address if possible.
715	
716	  A non-relocatable kernel will unconditionally move itself and to run
717	  at this address.
718	
719	Field name:	init_size
720	Type:		read
721	Offset/size:	0x260/4
722	
723	  This field indicates the amount of linear contiguous memory starting
724	  at the kernel runtime start address that the kernel needs before it
725	  is capable of examining its memory map.  This is not the same thing
726	  as the total amount of memory the kernel needs to boot, but it can
727	  be used by a relocating boot loader to help select a safe load
728	  address for the kernel.
729	
730	  The kernel runtime start address is determined by the following algorithm:
731	
732	  if (relocatable_kernel)
733		runtime_start = align_up(load_address, kernel_alignment)
734	  else
735		runtime_start = pref_address
736	
737	Field name:	handover_offset
738	Type:		read
739	Offset/size:	0x264/4
740	
741	  This field is the offset from the beginning of the kernel image to
742	  the EFI handover protocol entry point. Boot loaders using the EFI
743	  handover protocol to boot the kernel should jump to this offset.
744	
745	  See EFI HANDOVER PROTOCOL below for more details.
746	
747	
748	**** THE IMAGE CHECKSUM
749	
750	From boot protocol version 2.08 onwards the CRC-32 is calculated over
751	the entire file using the characteristic polynomial 0x04C11DB7 and an
752	initial remainder of 0xffffffff.  The checksum is appended to the
753	file; therefore the CRC of the file up to the limit specified in the
754	syssize field of the header is always 0.
755	
756	
757	**** THE KERNEL COMMAND LINE
758	
759	The kernel command line has become an important way for the boot
760	loader to communicate with the kernel.  Some of its options are also
761	relevant to the boot loader itself, see "special command line options"
762	below.
763	
764	The kernel command line is a null-terminated string. The maximum
765	length can be retrieved from the field cmdline_size.  Before protocol
766	version 2.06, the maximum was 255 characters.  A string that is too
767	long will be automatically truncated by the kernel.
768	
769	If the boot protocol version is 2.02 or later, the address of the
770	kernel command line is given by the header field cmd_line_ptr (see
771	above.)  This address can be anywhere between the end of the setup
772	heap and 0xA0000.
773	
774	If the protocol version is *not* 2.02 or higher, the kernel
775	command line is entered using the following protocol:
776	
777		At offset 0x0020 (word), "cmd_line_magic", enter the magic
778		number 0xA33F.
779	
780		At offset 0x0022 (word), "cmd_line_offset", enter the offset
781		of the kernel command line (relative to the start of the
782		real-mode kernel).
783		
784		The kernel command line *must* be within the memory region
785		covered by setup_move_size, so you may need to adjust this
786		field.
787	
788	
789	**** MEMORY LAYOUT OF THE REAL-MODE CODE
790	
791	The real-mode code requires a stack/heap to be set up, as well as
792	memory allocated for the kernel command line.  This needs to be done
793	in the real-mode accessible memory in bottom megabyte.
794	
795	It should be noted that modern machines often have a sizable Extended
796	BIOS Data Area (EBDA).  As a result, it is advisable to use as little
797	of the low megabyte as possible.
798	
799	Unfortunately, under the following circumstances the 0x90000 memory
800	segment has to be used:
801	
802		- When loading a zImage kernel ((loadflags & 0x01) == 0).
803		- When loading a 2.01 or earlier boot protocol kernel.
804	
805		  -> For the 2.00 and 2.01 boot protocols, the real-mode code
806		     can be loaded at another address, but it is internally
807		     relocated to 0x90000.  For the "old" protocol, the
808		     real-mode code must be loaded at 0x90000.
809	
810	When loading at 0x90000, avoid using memory above 0x9a000.
811	
812	For boot protocol 2.02 or higher, the command line does not have to be
813	located in the same 64K segment as the real-mode setup code; it is
814	thus permitted to give the stack/heap the full 64K segment and locate
815	the command line above it.
816	
817	The kernel command line should not be located below the real-mode
818	code, nor should it be located in high memory.
819	
820	
821	**** SAMPLE BOOT CONFIGURATION
822	
823	As a sample configuration, assume the following layout of the real
824	mode segment:
825	
826	    When loading below 0x90000, use the entire segment:
827	
828		0x0000-0x7fff	Real mode kernel
829		0x8000-0xdfff	Stack and heap
830		0xe000-0xffff	Kernel command line
831	
832	    When loading at 0x90000 OR the protocol version is 2.01 or earlier:
833	
834		0x0000-0x7fff	Real mode kernel
835		0x8000-0x97ff	Stack and heap
836		0x9800-0x9fff	Kernel command line
837	
838	Such a boot loader should enter the following fields in the header:
839	
840		unsigned long base_ptr;	/* base address for real-mode segment */
841	
842		if ( setup_sects == 0 ) {
843			setup_sects = 4;
844		}
845	
846		if ( protocol >= 0x0200 ) {
847			type_of_loader = <type code>;
848			if ( loading_initrd ) {
849				ramdisk_image = <initrd_address>;
850				ramdisk_size = <initrd_size>;
851			}
852	
853			if ( protocol >= 0x0202 && loadflags & 0x01 )
854				heap_end = 0xe000;
855			else
856				heap_end = 0x9800;
857	
858			if ( protocol >= 0x0201 ) {
859				heap_end_ptr = heap_end - 0x200;
860				loadflags |= 0x80; /* CAN_USE_HEAP */
861			}
862	
863			if ( protocol >= 0x0202 ) {
864				cmd_line_ptr = base_ptr + heap_end;
865				strcpy(cmd_line_ptr, cmdline);
866			} else {
867				cmd_line_magic	= 0xA33F;
868				cmd_line_offset = heap_end;
869				setup_move_size = heap_end + strlen(cmdline)+1;
870				strcpy(base_ptr+cmd_line_offset, cmdline);
871			}
872		} else {
873			/* Very old kernel */
874	
875			heap_end = 0x9800;
876	
877			cmd_line_magic	= 0xA33F;
878			cmd_line_offset = heap_end;
879	
880			/* A very old kernel MUST have its real-mode code
881			   loaded at 0x90000 */
882	
883			if ( base_ptr != 0x90000 ) {
884				/* Copy the real-mode kernel */
885				memcpy(0x90000, base_ptr, (setup_sects+1)*512);
886				base_ptr = 0x90000;		 /* Relocated */
887			}
888	
889			strcpy(0x90000+cmd_line_offset, cmdline);
890	
891			/* It is recommended to clear memory up to the 32K mark */
892			memset(0x90000 + (setup_sects+1)*512, 0,
893			       (64-(setup_sects+1))*512);
894		}
895	
896	
897	**** LOADING THE REST OF THE KERNEL
898	
899	The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
900	in the kernel file (again, if setup_sects == 0 the real value is 4.)
901	It should be loaded at address 0x10000 for Image/zImage kernels and
902	0x100000 for bzImage kernels.
903	
904	The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
905	bit (LOAD_HIGH) in the loadflags field is set:
906	
907		is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
908		load_address = is_bzImage ? 0x100000 : 0x10000;
909	
910	Note that Image/zImage kernels can be up to 512K in size, and thus use
911	the entire 0x10000-0x90000 range of memory.  This means it is pretty
912	much a requirement for these kernels to load the real-mode part at
913	0x90000.  bzImage kernels allow much more flexibility.
914	
915	
916	**** SPECIAL COMMAND LINE OPTIONS
917	
918	If the command line provided by the boot loader is entered by the
919	user, the user may expect the following command line options to work.
920	They should normally not be deleted from the kernel command line even
921	though not all of them are actually meaningful to the kernel.  Boot
922	loader authors who need additional command line options for the boot
923	loader itself should get them registered in
924	Documentation/kernel-parameters.txt to make sure they will not
925	conflict with actual kernel options now or in the future.
926	
927	  vga=<mode>
928		<mode> here is either an integer (in C notation, either
929		decimal, octal, or hexadecimal) or one of the strings
930		"normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
931		(meaning 0xFFFD).  This value should be entered into the
932		vid_mode field, as it is used by the kernel before the command
933		line is parsed.
934	
935	  mem=<size>
936		<size> is an integer in C notation optionally followed by
937		(case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
938		<< 30, << 40, << 50 or << 60).  This specifies the end of
939		memory to the kernel. This affects the possible placement of
940		an initrd, since an initrd should be placed near end of
941		memory.  Note that this is an option to *both* the kernel and
942		the bootloader!
943	
944	  initrd=<file>
945		An initrd should be loaded.  The meaning of <file> is
946		obviously bootloader-dependent, and some boot loaders
947		(e.g. LILO) do not have such a command.
948	
949	In addition, some boot loaders add the following options to the
950	user-specified command line:
951	
952	  BOOT_IMAGE=<file>
953		The boot image which was loaded.  Again, the meaning of <file>
954		is obviously bootloader-dependent.
955	
956	  auto
957		The kernel was booted without explicit user intervention.
958	
959	If these options are added by the boot loader, it is highly
960	recommended that they are located *first*, before the user-specified
961	or configuration-specified command line.  Otherwise, "init=/bin/sh"
962	gets confused by the "auto" option.
963	
964	
965	**** RUNNING THE KERNEL
966	
967	The kernel is started by jumping to the kernel entry point, which is
968	located at *segment* offset 0x20 from the start of the real mode
969	kernel.  This means that if you loaded your real-mode kernel code at
970	0x90000, the kernel entry point is 9020:0000.
971	
972	At entry, ds = es = ss should point to the start of the real-mode
973	kernel code (0x9000 if the code is loaded at 0x90000), sp should be
974	set up properly, normally pointing to the top of the heap, and
975	interrupts should be disabled.  Furthermore, to guard against bugs in
976	the kernel, it is recommended that the boot loader sets fs = gs = ds =
977	es = ss.
978	
979	In our example from above, we would do:
980	
981		/* Note: in the case of the "old" kernel protocol, base_ptr must
982		   be == 0x90000 at this point; see the previous sample code */
983	
984		seg = base_ptr >> 4;
985	
986		cli();	/* Enter with interrupts disabled! */
987	
988		/* Set up the real-mode kernel stack */
989		_SS = seg;
990		_SP = heap_end;
991	
992		_DS = _ES = _FS = _GS = seg;
993		jmp_far(seg+0x20, 0);	/* Run the kernel */
994	
995	If your boot sector accesses a floppy drive, it is recommended to
996	switch off the floppy motor before running the kernel, since the
997	kernel boot leaves interrupts off and thus the motor will not be
998	switched off, especially if the loaded kernel has the floppy driver as
999	a demand-loaded module!
1000	
1001	
1002	**** ADVANCED BOOT LOADER HOOKS
1003	
1004	If the boot loader runs in a particularly hostile environment (such as
1005	LOADLIN, which runs under DOS) it may be impossible to follow the
1006	standard memory location requirements.  Such a boot loader may use the
1007	following hooks that, if set, are invoked by the kernel at the
1008	appropriate time.  The use of these hooks should probably be
1009	considered an absolutely last resort!
1010	
1011	IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
1012	%edi across invocation.
1013	
1014	  realmode_swtch:
1015		A 16-bit real mode far subroutine invoked immediately before
1016		entering protected mode.  The default routine disables NMI, so
1017		your routine should probably do so, too.
1018	
1019	  code32_start:
1020		A 32-bit flat-mode routine *jumped* to immediately after the
1021		transition to protected mode, but before the kernel is
1022		uncompressed.  No segments, except CS, are guaranteed to be
1023		set up (current kernels do, but older ones do not); you should
1024		set them up to BOOT_DS (0x18) yourself.
1025	
1026		After completing your hook, you should jump to the address
1027		that was in this field before your boot loader overwrote it
1028		(relocated, if appropriate.)
1029	
1030	
1031	**** 32-bit BOOT PROTOCOL
1032	
1033	For machine with some new BIOS other than legacy BIOS, such as EFI,
1034	LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
1035	based on legacy BIOS can not be used, so a 32-bit boot protocol needs
1036	to be defined.
1037	
1038	In 32-bit boot protocol, the first step in loading a Linux kernel
1039	should be to setup the boot parameters (struct boot_params,
1040	traditionally known as "zero page"). The memory for struct boot_params
1041	should be allocated and initialized to all zero. Then the setup header
1042	from offset 0x01f1 of kernel image on should be loaded into struct
1043	boot_params and examined. The end of setup header can be calculated as
1044	follow:
1045	
1046		0x0202 + byte value at offset 0x0201
1047	
1048	In addition to read/modify/write the setup header of the struct
1049	boot_params as that of 16-bit boot protocol, the boot loader should
1050	also fill the additional fields of the struct boot_params as that
1051	described in zero-page.txt.
1052	
1053	After setting up the struct boot_params, the boot loader can load the
1054	32/64-bit kernel in the same way as that of 16-bit boot protocol.
1055	
1056	In 32-bit boot protocol, the kernel is started by jumping to the
1057	32-bit kernel entry point, which is the start address of loaded
1058	32/64-bit kernel.
1059	
1060	At entry, the CPU must be in 32-bit protected mode with paging
1061	disabled; a GDT must be loaded with the descriptors for selectors
1062	__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1063	segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1064	must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1065	must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
1066	address of the struct boot_params; %ebp, %edi and %ebx must be zero.
1067	
1068	**** 64-bit BOOT PROTOCOL
1069	
1070	For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader
1071	and we need a 64-bit boot protocol.
1072	
1073	In 64-bit boot protocol, the first step in loading a Linux kernel
1074	should be to setup the boot parameters (struct boot_params,
1075	traditionally known as "zero page"). The memory for struct boot_params
1076	could be allocated anywhere (even above 4G) and initialized to all zero.
1077	Then, the setup header at offset 0x01f1 of kernel image on should be
1078	loaded into struct boot_params and examined. The end of setup header
1079	can be calculated as follows:
1080	
1081		0x0202 + byte value at offset 0x0201
1082	
1083	In addition to read/modify/write the setup header of the struct
1084	boot_params as that of 16-bit boot protocol, the boot loader should
1085	also fill the additional fields of the struct boot_params as described
1086	in zero-page.txt.
1087	
1088	After setting up the struct boot_params, the boot loader can load
1089	64-bit kernel in the same way as that of 16-bit boot protocol, but
1090	kernel could be loaded above 4G.
1091	
1092	In 64-bit boot protocol, the kernel is started by jumping to the
1093	64-bit kernel entry point, which is the start address of loaded
1094	64-bit kernel plus 0x200.
1095	
1096	At entry, the CPU must be in 64-bit mode with paging enabled.
1097	The range with setup_header.init_size from start address of loaded
1098	kernel and zero page and command line buffer get ident mapping;
1099	a GDT must be loaded with the descriptors for selectors
1100	__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1101	segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1102	must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1103	must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base
1104	address of the struct boot_params.
1105	
1106	**** EFI HANDOVER PROTOCOL
1107	
1108	This protocol allows boot loaders to defer initialisation to the EFI
1109	boot stub. The boot loader is required to load the kernel/initrd(s)
1110	from the boot media and jump to the EFI handover protocol entry point
1111	which is hdr->handover_offset bytes from the beginning of
1112	startup_{32,64}.
1113	
1114	The function prototype for the handover entry point looks like this,
1115	
1116	    efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp)
1117	
1118	'handle' is the EFI image handle passed to the boot loader by the EFI
1119	firmware, 'table' is the EFI system table - these are the first two
1120	arguments of the "handoff state" as described in section 2.3 of the
1121	UEFI specification. 'bp' is the boot loader-allocated boot params.
1122	
1123	The boot loader *must* fill out the following fields in bp,
1124	
1125	    o hdr.code32_start
1126	    o hdr.cmd_line_ptr
1127	    o hdr.cmdline_size
1128	    o hdr.ramdisk_image (if applicable)
1129	    o hdr.ramdisk_size  (if applicable)
1130	
1131	All other fields should be zero.
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