| blob | abb9fc164657a7caa98effa0a6ba4b0b8ebd764b |
1 .. SPDX-License-Identifier: GPL-2.0
3 ===========================
4 The Linux/x86 Boot Protocol
5 ===========================
7 On the x86 platform, the Linux kernel uses a rather complicated boot
8 convention. This has evolved partially due to historical aspects, as
9 well as the desire in the early days to have the kernel itself be a
10 bootable image, the complicated PC memory model and due to changed
11 expectations in the PC industry caused by the effective demise of
12 real-mode DOS as a mainstream operating system.
14 Currently, the following versions of the Linux/x86 boot protocol exist.
16 ============= ============================================================
17 Old kernels zImage/Image support only. Some very early kernels
18 may not even support a command line.
20 Protocol 2.00 (Kernel 1.3.73) Added bzImage and initrd support, as
21 well as a formalized way to communicate between the
22 boot loader and the kernel. setup.S made relocatable,
23 although the traditional setup area still assumed
24 writable.
26 Protocol 2.01 (Kernel 1.3.76) Added a heap overrun warning.
28 Protocol 2.02 (Kernel 2.4.0-test3-pre3) New command line protocol.
29 Lower the conventional memory ceiling. No overwrite
30 of the traditional setup area, thus making booting
31 safe for systems which use the EBDA from SMM or 32-bit
32 BIOS entry points. zImage deprecated but still
33 supported.
35 Protocol 2.03 (Kernel 2.4.18-pre1) Explicitly makes the highest possible
36 initrd address available to the bootloader.
38 Protocol 2.04 (Kernel 2.6.14) Extend the syssize field to four bytes.
40 Protocol 2.05 (Kernel 2.6.20) Make protected mode kernel relocatable.
41 Introduce relocatable_kernel and kernel_alignment fields.
43 Protocol 2.06 (Kernel 2.6.22) Added a field that contains the size of
44 the boot command line.
46 Protocol 2.07 (Kernel 2.6.24) Added paravirtualised boot protocol.
47 Introduced hardware_subarch and hardware_subarch_data
48 and KEEP_SEGMENTS flag in load_flags.
50 Protocol 2.08 (Kernel 2.6.26) Added crc32 checksum and ELF format
51 payload. Introduced payload_offset and payload_length
52 fields to aid in locating the payload.
54 Protocol 2.09 (Kernel 2.6.26) Added a field of 64-bit physical
55 pointer to single linked list of struct setup_data.
57 Protocol 2.10 (Kernel 2.6.31) Added a protocol for relaxed alignment
58 beyond the kernel_alignment added, new init_size and
59 pref_address fields. Added extended boot loader IDs.
61 Protocol 2.11 (Kernel 3.6) Added a field for offset of EFI handover
62 protocol entry point.
64 Protocol 2.12 (Kernel 3.8) Added the xloadflags field and extension fields
65 to struct boot_params for loading bzImage and ramdisk
66 above 4G in 64bit.
68 Protocol 2.13 (Kernel 3.14) Support 32- and 64-bit flags being set in
69 xloadflags to support booting a 64-bit kernel from 32-bit
70 EFI
72 Protocol 2.14 BURNT BY INCORRECT COMMIT
73 ae7e1238e68f2a472a125673ab506d49158c1889
74 (x86/boot: Add ACPI RSDP address to setup_header)
75 DO NOT USE!!! ASSUME SAME AS 2.13.
77 Protocol 2.15 (Kernel 5.5) Added the kernel_info and kernel_info.setup_type_max.
78 ============= ============================================================
80 .. note::
81 The protocol version number should be changed only if the setup header
82 is changed. There is no need to update the version number if boot_params
83 or kernel_info are changed. Additionally, it is recommended to use
84 xloadflags (in this case the protocol version number should not be
85 updated either) or kernel_info to communicate supported Linux kernel
86 features to the boot loader. Due to very limited space available in
87 the original setup header every update to it should be considered
88 with great care. Starting from the protocol 2.15 the primary way to
89 communicate things to the boot loader is the kernel_info.
92 Memory Layout
93 =============
95 The traditional memory map for the kernel loader, used for Image or
96 zImage kernels, typically looks like::
98 | |
99 0A0000 +------------------------+
100 | Reserved for BIOS | Do not use. Reserved for BIOS EBDA.
101 09A000 +------------------------+
102 | Command line |
103 | Stack/heap | For use by the kernel real-mode code.
104 098000 +------------------------+
105 | Kernel setup | The kernel real-mode code.
106 090200 +------------------------+
107 | Kernel boot sector | The kernel legacy boot sector.
108 090000 +------------------------+
109 | Protected-mode kernel | The bulk of the kernel image.
110 010000 +------------------------+
111 | Boot loader | <- Boot sector entry point 0000:7C00
112 001000 +------------------------+
113 | Reserved for MBR/BIOS |
114 000800 +------------------------+
115 | Typically used by MBR |
116 000600 +------------------------+
117 | BIOS use only |
118 000000 +------------------------+
120 When using bzImage, the protected-mode kernel was relocated to
121 0x100000 ("high memory"), and the kernel real-mode block (boot sector,
122 setup, and stack/heap) was made relocatable to any address between
123 0x10000 and end of low memory. Unfortunately, in protocols 2.00 and
124 2.01 the 0x90000+ memory range is still used internally by the kernel;
125 the 2.02 protocol resolves that problem.
127 It is desirable to keep the "memory ceiling" -- the highest point in
128 low memory touched by the boot loader -- as low as possible, since
129 some newer BIOSes have begun to allocate some rather large amounts of
130 memory, called the Extended BIOS Data Area, near the top of low
131 memory. The boot loader should use the "INT 12h" BIOS call to verify
132 how much low memory is available.
134 Unfortunately, if INT 12h reports that the amount of memory is too
135 low, there is usually nothing the boot loader can do but to report an
136 error to the user. The boot loader should therefore be designed to
137 take up as little space in low memory as it reasonably can. For
138 zImage or old bzImage kernels, which need data written into the
139 0x90000 segment, the boot loader should make sure not to use memory
140 above the 0x9A000 point; too many BIOSes will break above that point.
142 For a modern bzImage kernel with boot protocol version >= 2.02, a
143 memory layout like the following is suggested::
145 ~ ~
146 | Protected-mode kernel |
147 100000 +------------------------+
148 | I/O memory hole |
149 0A0000 +------------------------+
150 | Reserved for BIOS | Leave as much as possible unused
151 ~ ~
152 | Command line | (Can also be below the X+10000 mark)
153 X+10000 +------------------------+
154 | Stack/heap | For use by the kernel real-mode code.
155 X+08000 +------------------------+
156 | Kernel setup | The kernel real-mode code.
157 | Kernel boot sector | The kernel legacy boot sector.
158 X +------------------------+
159 | Boot loader | <- Boot sector entry point 0000:7C00
160 001000 +------------------------+
161 | Reserved for MBR/BIOS |
162 000800 +------------------------+
163 | Typically used by MBR |
164 000600 +------------------------+
165 | BIOS use only |
166 000000 +------------------------+
168 ... where the address X is as low as the design of the boot loader permits.
171 The Real-Mode Kernel Header
172 ===========================
174 In the following text, and anywhere in the kernel boot sequence, "a
175 sector" refers to 512 bytes. It is independent of the actual sector
176 size of the underlying medium.
178 The first step in loading a Linux kernel should be to load the
179 real-mode code (boot sector and setup code) and then examine the
180 following header at offset 0x01f1. The real-mode code can total up to
181 32K, although the boot loader may choose to load only the first two
182 sectors (1K) and then examine the bootup sector size.
184 The header looks like:
186 =========== ======== ===================== ============================================
187 Offset/Size Proto Name Meaning
188 =========== ======== ===================== ============================================
189 01F1/1 ALL(1) setup_sects The size of the setup in sectors
190 01F2/2 ALL root_flags If set, the root is mounted readonly
191 01F4/4 2.04+(2) syssize The size of the 32-bit code in 16-byte paras
192 01F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
193 01FA/2 ALL vid_mode Video mode control
194 01FC/2 ALL root_dev Default root device number
195 01FE/2 ALL boot_flag 0xAA55 magic number
196 0200/2 2.00+ jump Jump instruction
197 0202/4 2.00+ header Magic signature "HdrS"
198 0206/2 2.00+ version Boot protocol version supported
199 0208/4 2.00+ realmode_swtch Boot loader hook (see below)
200 020C/2 2.00+ start_sys_seg The load-low segment (0x1000) (obsolete)
201 020E/2 2.00+ kernel_version Pointer to kernel version string
202 0210/1 2.00+ type_of_loader Boot loader identifier
203 0211/1 2.00+ loadflags Boot protocol option flags
204 0212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
205 0214/4 2.00+ code32_start Boot loader hook (see below)
206 0218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
207 021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
208 0220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
209 0224/2 2.01+ heap_end_ptr Free memory after setup end
210 0226/1 2.02+(3) ext_loader_ver Extended boot loader version
211 0227/1 2.02+(3) ext_loader_type Extended boot loader ID
212 0228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
213 022C/4 2.03+ initrd_addr_max Highest legal initrd address
214 0230/4 2.05+ kernel_alignment Physical addr alignment required for kernel
215 0234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
216 0235/1 2.10+ min_alignment Minimum alignment, as a power of two
217 0236/2 2.12+ xloadflags Boot protocol option flags
218 0238/4 2.06+ cmdline_size Maximum size of the kernel command line
219 023C/4 2.07+ hardware_subarch Hardware subarchitecture
220 0240/8 2.07+ hardware_subarch_data Subarchitecture-specific data
221 0248/4 2.08+ payload_offset Offset of kernel payload
222 024C/4 2.08+ payload_length Length of kernel payload
223 0250/8 2.09+ setup_data 64-bit physical pointer to linked list
224 of struct setup_data
225 0258/8 2.10+ pref_address Preferred loading address
226 0260/4 2.10+ init_size Linear memory required during initialization
227 0264/4 2.11+ handover_offset Offset of handover entry point
228 0268/4 2.15+ kernel_info_offset Offset of the kernel_info
229 =========== ======== ===================== ============================================
231 .. note::
232 (1) For backwards compatibility, if the setup_sects field contains 0, the
233 real value is 4.
235 (2) For boot protocol prior to 2.04, the upper two bytes of the syssize
236 field are unusable, which means the size of a bzImage kernel
237 cannot be determined.
239 (3) Ignored, but safe to set, for boot protocols 2.02-2.09.
241 If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
242 the boot protocol version is "old". Loading an old kernel, the
243 following parameters should be assumed::
245 Image type = zImage
246 initrd not supported
247 Real-mode kernel must be located at 0x90000.
249 Otherwise, the "version" field contains the protocol version,
250 e.g. protocol version 2.01 will contain 0x0201 in this field. When
251 setting fields in the header, you must make sure only to set fields
252 supported by the protocol version in use.
255 Details of Header Fields
256 ========================
258 For each field, some are information from the kernel to the bootloader
259 ("read"), some are expected to be filled out by the bootloader
260 ("write"), and some are expected to be read and modified by the
261 bootloader ("modify").
263 All general purpose boot loaders should write the fields marked
264 (obligatory). Boot loaders who want to load the kernel at a
265 nonstandard address should fill in the fields marked (reloc); other
266 boot loaders can ignore those fields.
268 The byte order of all fields is littleendian (this is x86, after all.)
270 ============ ===========
271 Field name: setup_sects
272 Type: read
273 Offset/size: 0x1f1/1
274 Protocol: ALL
275 ============ ===========
277 The size of the setup code in 512-byte sectors. If this field is
278 0, the real value is 4. The real-mode code consists of the boot
279 sector (always one 512-byte sector) plus the setup code.
281 ============ =================
282 Field name: root_flags
283 Type: modify (optional)
284 Offset/size: 0x1f2/2
285 Protocol: ALL
286 ============ =================
288 If this field is nonzero, the root defaults to readonly. The use of
289 this field is deprecated; use the "ro" or "rw" options on the
290 command line instead.
292 ============ ===============================================
293 Field name: syssize
294 Type: read
295 Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
296 Protocol: 2.04+
297 ============ ===============================================
299 The size of the protected-mode code in units of 16-byte paragraphs.
300 For protocol versions older than 2.04 this field is only two bytes
301 wide, and therefore cannot be trusted for the size of a kernel if
302 the LOAD_HIGH flag is set.
304 ============ ===============
305 Field name: ram_size
306 Type: kernel internal
307 Offset/size: 0x1f8/2
308 Protocol: ALL
309 ============ ===============
311 This field is obsolete.
313 ============ ===================
314 Field name: vid_mode
315 Type: modify (obligatory)
316 Offset/size: 0x1fa/2
317 ============ ===================
319 Please see the section on SPECIAL COMMAND LINE OPTIONS.
321 ============ =================
322 Field name: root_dev
323 Type: modify (optional)
324 Offset/size: 0x1fc/2
325 Protocol: ALL
326 ============ =================
328 The default root device device number. The use of this field is
329 deprecated, use the "root=" option on the command line instead.
331 ============ =========
332 Field name: boot_flag
333 Type: read
334 Offset/size: 0x1fe/2
335 Protocol: ALL
336 ============ =========
338 Contains 0xAA55. This is the closest thing old Linux kernels have
339 to a magic number.
341 ============ =======
342 Field name: jump
343 Type: read
344 Offset/size: 0x200/2
345 Protocol: 2.00+
346 ============ =======
348 Contains an x86 jump instruction, 0xEB followed by a signed offset
349 relative to byte 0x202. This can be used to determine the size of
350 the header.
352 ============ =======
353 Field name: header
354 Type: read
355 Offset/size: 0x202/4
356 Protocol: 2.00+
357 ============ =======
359 Contains the magic number "HdrS" (0x53726448).
361 ============ =======
362 Field name: version
363 Type: read
364 Offset/size: 0x206/2
365 Protocol: 2.00+
366 ============ =======
368 Contains the boot protocol version, in (major << 8)+minor format,
369 e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
370 10.17.
372 ============ =================
373 Field name: realmode_swtch
374 Type: modify (optional)
375 Offset/size: 0x208/4
376 Protocol: 2.00+
377 ============ =================
379 Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
381 ============ =============
382 Field name: start_sys_seg
383 Type: read
384 Offset/size: 0x20c/2
385 Protocol: 2.00+
386 ============ =============
388 The load low segment (0x1000). Obsolete.
390 ============ ==============
391 Field name: kernel_version
392 Type: read
393 Offset/size: 0x20e/2
394 Protocol: 2.00+
395 ============ ==============
397 If set to a nonzero value, contains a pointer to a NUL-terminated
398 human-readable kernel version number string, less 0x200. This can
399 be used to display the kernel version to the user. This value
400 should be less than (0x200*setup_sects).
402 For example, if this value is set to 0x1c00, the kernel version
403 number string can be found at offset 0x1e00 in the kernel file.
404 This is a valid value if and only if the "setup_sects" field
405 contains the value 15 or higher, as::
407 0x1c00 < 15*0x200 (= 0x1e00) but
408 0x1c00 >= 14*0x200 (= 0x1c00)
410 0x1c00 >> 9 = 14, So the minimum value for setup_secs is 15.
412 ============ ==================
413 Field name: type_of_loader
414 Type: write (obligatory)
415 Offset/size: 0x210/1
416 Protocol: 2.00+
417 ============ ==================
419 If your boot loader has an assigned id (see table below), enter
420 0xTV here, where T is an identifier for the boot loader and V is
421 a version number. Otherwise, enter 0xFF here.
423 For boot loader IDs above T = 0xD, write T = 0xE to this field and
424 write the extended ID minus 0x10 to the ext_loader_type field.
425 Similarly, the ext_loader_ver field can be used to provide more than
426 four bits for the bootloader version.
428 For example, for T = 0x15, V = 0x234, write::
430 type_of_loader <- 0xE4
431 ext_loader_type <- 0x05
432 ext_loader_ver <- 0x23
434 Assigned boot loader ids (hexadecimal):
436 == =======================================
437 0 LILO
438 (0x00 reserved for pre-2.00 bootloader)
439 1 Loadlin
440 2 bootsect-loader
441 (0x20, all other values reserved)
442 3 Syslinux
443 4 Etherboot/gPXE/iPXE
444 5 ELILO
445 7 GRUB
446 8 U-Boot
447 9 Xen
448 A Gujin
449 B Qemu
450 C Arcturus Networks uCbootloader
451 D kexec-tools
452 E Extended (see ext_loader_type)
453 F Special (0xFF = undefined)
454 10 Reserved
455 11 Minimal Linux Bootloader
456 <http://sebastian-plotz.blogspot.de>
457 12 OVMF UEFI virtualization stack
458 == =======================================
460 Please contact <hpa@zytor.com> if you need a bootloader ID value assigned.
462 ============ ===================
463 Field name: loadflags
464 Type: modify (obligatory)
465 Offset/size: 0x211/1
466 Protocol: 2.00+
467 ============ ===================
469 This field is a bitmask.
471 Bit 0 (read): LOADED_HIGH
473 - If 0, the protected-mode code is loaded at 0x10000.
474 - If 1, the protected-mode code is loaded at 0x100000.
476 Bit 1 (kernel internal): KASLR_FLAG
478 - Used internally by the compressed kernel to communicate
479 KASLR status to kernel proper.
481 - If 1, KASLR enabled.
482 - If 0, KASLR disabled.
484 Bit 5 (write): QUIET_FLAG
486 - If 0, print early messages.
487 - If 1, suppress early messages.
489 This requests to the kernel (decompressor and early
490 kernel) to not write early messages that require
491 accessing the display hardware directly.
493 Bit 6 (obsolete): KEEP_SEGMENTS
495 Protocol: 2.07+
497 - This flag is obsolete.
499 Bit 7 (write): CAN_USE_HEAP
501 Set this bit to 1 to indicate that the value entered in the
502 heap_end_ptr is valid. If this field is clear, some setup code
503 functionality will be disabled.
506 ============ ===================
507 Field name: setup_move_size
508 Type: modify (obligatory)
509 Offset/size: 0x212/2
510 Protocol: 2.00-2.01
511 ============ ===================
513 When using protocol 2.00 or 2.01, if the real mode kernel is not
514 loaded at 0x90000, it gets moved there later in the loading
515 sequence. Fill in this field if you want additional data (such as
516 the kernel command line) moved in addition to the real-mode kernel
517 itself.
519 The unit is bytes starting with the beginning of the boot sector.
521 This field is can be ignored when the protocol is 2.02 or higher, or
522 if the real-mode code is loaded at 0x90000.
524 ============ ========================
525 Field name: code32_start
526 Type: modify (optional, reloc)
527 Offset/size: 0x214/4
528 Protocol: 2.00+
529 ============ ========================
531 The address to jump to in protected mode. This defaults to the load
532 address of the kernel, and can be used by the boot loader to
533 determine the proper load address.
535 This field can be modified for two purposes:
537 1. as a boot loader hook (see Advanced Boot Loader Hooks below.)
539 2. if a bootloader which does not install a hook loads a
540 relocatable kernel at a nonstandard address it will have to modify
541 this field to point to the load address.
543 ============ ==================
544 Field name: ramdisk_image
545 Type: write (obligatory)
546 Offset/size: 0x218/4
547 Protocol: 2.00+
548 ============ ==================
550 The 32-bit linear address of the initial ramdisk or ramfs. Leave at
551 zero if there is no initial ramdisk/ramfs.
553 ============ ==================
554 Field name: ramdisk_size
555 Type: write (obligatory)
556 Offset/size: 0x21c/4
557 Protocol: 2.00+
558 ============ ==================
560 Size of the initial ramdisk or ramfs. Leave at zero if there is no
561 initial ramdisk/ramfs.
563 ============ ===============
564 Field name: bootsect_kludge
565 Type: kernel internal
566 Offset/size: 0x220/4
567 Protocol: 2.00+
568 ============ ===============
570 This field is obsolete.
572 ============ ==================
573 Field name: heap_end_ptr
574 Type: write (obligatory)
575 Offset/size: 0x224/2
576 Protocol: 2.01+
577 ============ ==================
579 Set this field to the offset (from the beginning of the real-mode
580 code) of the end of the setup stack/heap, minus 0x0200.
582 ============ ================
583 Field name: ext_loader_ver
584 Type: write (optional)
585 Offset/size: 0x226/1
586 Protocol: 2.02+
587 ============ ================
589 This field is used as an extension of the version number in the
590 type_of_loader field. The total version number is considered to be
591 (type_of_loader & 0x0f) + (ext_loader_ver << 4).
593 The use of this field is boot loader specific. If not written, it
594 is zero.
596 Kernels prior to 2.6.31 did not recognize this field, but it is safe
597 to write for protocol version 2.02 or higher.
599 ============ =====================================================
600 Field name: ext_loader_type
601 Type: write (obligatory if (type_of_loader & 0xf0) == 0xe0)
602 Offset/size: 0x227/1
603 Protocol: 2.02+
604 ============ =====================================================
606 This field is used as an extension of the type number in
607 type_of_loader field. If the type in type_of_loader is 0xE, then
608 the actual type is (ext_loader_type + 0x10).
610 This field is ignored if the type in type_of_loader is not 0xE.
612 Kernels prior to 2.6.31 did not recognize this field, but it is safe
613 to write for protocol version 2.02 or higher.
615 ============ ==================
616 Field name: cmd_line_ptr
617 Type: write (obligatory)
618 Offset/size: 0x228/4
619 Protocol: 2.02+
620 ============ ==================
622 Set this field to the linear address of the kernel command line.
623 The kernel command line can be located anywhere between the end of
624 the setup heap and 0xA0000; it does not have to be located in the
625 same 64K segment as the real-mode code itself.
627 Fill in this field even if your boot loader does not support a
628 command line, in which case you can point this to an empty string
629 (or better yet, to the string "auto".) If this field is left at
630 zero, the kernel will assume that your boot loader does not support
631 the 2.02+ protocol.
633 ============ ===============
634 Field name: initrd_addr_max
635 Type: read
636 Offset/size: 0x22c/4
637 Protocol: 2.03+
638 ============ ===============
640 The maximum address that may be occupied by the initial
641 ramdisk/ramfs contents. For boot protocols 2.02 or earlier, this
642 field is not present, and the maximum address is 0x37FFFFFF. (This
643 address is defined as the address of the highest safe byte, so if
644 your ramdisk is exactly 131072 bytes long and this field is
645 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
647 ============ ============================
648 Field name: kernel_alignment
649 Type: read/modify (reloc)
650 Offset/size: 0x230/4
651 Protocol: 2.05+ (read), 2.10+ (modify)
652 ============ ============================
654 Alignment unit required by the kernel (if relocatable_kernel is
655 true.) A relocatable kernel that is loaded at an alignment
656 incompatible with the value in this field will be realigned during
657 kernel initialization.
659 Starting with protocol version 2.10, this reflects the kernel
660 alignment preferred for optimal performance; it is possible for the
661 loader to modify this field to permit a lesser alignment. See the
662 min_alignment and pref_address field below.
664 ============ ==================
665 Field name: relocatable_kernel
666 Type: read (reloc)
667 Offset/size: 0x234/1
668 Protocol: 2.05+
669 ============ ==================
671 If this field is nonzero, the protected-mode part of the kernel can
672 be loaded at any address that satisfies the kernel_alignment field.
673 After loading, the boot loader must set the code32_start field to
674 point to the loaded code, or to a boot loader hook.
676 ============ =============
677 Field name: min_alignment
678 Type: read (reloc)
679 Offset/size: 0x235/1
680 Protocol: 2.10+
681 ============ =============
683 This field, if nonzero, indicates as a power of two the minimum
684 alignment required, as opposed to preferred, by the kernel to boot.
685 If a boot loader makes use of this field, it should update the
686 kernel_alignment field with the alignment unit desired; typically::
688 kernel_alignment = 1 << min_alignment
690 There may be a considerable performance cost with an excessively
691 misaligned kernel. Therefore, a loader should typically try each
692 power-of-two alignment from kernel_alignment down to this alignment.
694 ============ ==========
695 Field name: xloadflags
696 Type: read
697 Offset/size: 0x236/2
698 Protocol: 2.12+
699 ============ ==========
701 This field is a bitmask.
703 Bit 0 (read): XLF_KERNEL_64
705 - If 1, this kernel has the legacy 64-bit entry point at 0x200.
707 Bit 1 (read): XLF_CAN_BE_LOADED_ABOVE_4G
709 - If 1, kernel/boot_params/cmdline/ramdisk can be above 4G.
711 Bit 2 (read): XLF_EFI_HANDOVER_32
713 - If 1, the kernel supports the 32-bit EFI handoff entry point
714 given at handover_offset.
716 Bit 3 (read): XLF_EFI_HANDOVER_64
718 - If 1, the kernel supports the 64-bit EFI handoff entry point
719 given at handover_offset + 0x200.
721 Bit 4 (read): XLF_EFI_KEXEC
723 - If 1, the kernel supports kexec EFI boot with EFI runtime support.
726 ============ ============
727 Field name: cmdline_size
728 Type: read
729 Offset/size: 0x238/4
730 Protocol: 2.06+
731 ============ ============
733 The maximum size of the command line without the terminating
734 zero. This means that the command line can contain at most
735 cmdline_size characters. With protocol version 2.05 and earlier, the
736 maximum size was 255.
738 ============ ====================================
739 Field name: hardware_subarch
740 Type: write (optional, defaults to x86/PC)
741 Offset/size: 0x23c/4
742 Protocol: 2.07+
743 ============ ====================================
745 In a paravirtualized environment the hardware low level architectural
746 pieces such as interrupt handling, page table handling, and
747 accessing process control registers needs to be done differently.
749 This field allows the bootloader to inform the kernel we are in one
750 one of those environments.
752 ========== ==============================
753 0x00000000 The default x86/PC environment
754 0x00000001 lguest
755 0x00000002 Xen
756 0x00000003 Moorestown MID
757 0x00000004 CE4100 TV Platform
758 ========== ==============================
760 ============ =========================
761 Field name: hardware_subarch_data
762 Type: write (subarch-dependent)
763 Offset/size: 0x240/8
764 Protocol: 2.07+
765 ============ =========================
767 A pointer to data that is specific to hardware subarch
768 This field is currently unused for the default x86/PC environment,
769 do not modify.
771 ============ ==============
772 Field name: payload_offset
773 Type: read
774 Offset/size: 0x248/4
775 Protocol: 2.08+
776 ============ ==============
778 If non-zero then this field contains the offset from the beginning
779 of the protected-mode code to the payload.
781 The payload may be compressed. The format of both the compressed and
782 uncompressed data should be determined using the standard magic
783 numbers. The currently supported compression formats are gzip
784 (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA
785 (magic number 5D 00), XZ (magic number FD 37), LZ4 (magic number
786 02 21) and ZSTD (magic number 28 B5). The uncompressed payload is
787 currently always ELF (magic number 7F 45 4C 46).
789 ============ ==============
790 Field name: payload_length
791 Type: read
792 Offset/size: 0x24c/4
793 Protocol: 2.08+
794 ============ ==============
796 The length of the payload.
798 ============ ===============
799 Field name: setup_data
800 Type: write (special)
801 Offset/size: 0x250/8
802 Protocol: 2.09+
803 ============ ===============
805 The 64-bit physical pointer to NULL terminated single linked list of
806 struct setup_data. This is used to define a more extensible boot
807 parameters passing mechanism. The definition of struct setup_data is
808 as follow::
810 struct setup_data {
811 u64 next;
812 u32 type;
813 u32 len;
814 u8 data[0];
815 };
817 Where, the next is a 64-bit physical pointer to the next node of
818 linked list, the next field of the last node is 0; the type is used
819 to identify the contents of data; the len is the length of data
820 field; the data holds the real payload.
822 This list may be modified at a number of points during the bootup
823 process. Therefore, when modifying this list one should always make
824 sure to consider the case where the linked list already contains
825 entries.
827 The setup_data is a bit awkward to use for extremely large data objects,
828 both because the setup_data header has to be adjacent to the data object
829 and because it has a 32-bit length field. However, it is important that
830 intermediate stages of the boot process have a way to identify which
831 chunks of memory are occupied by kernel data.
833 Thus setup_indirect struct and SETUP_INDIRECT type were introduced in
834 protocol 2.15::
836 struct setup_indirect {
837 __u32 type;
838 __u32 reserved; /* Reserved, must be set to zero. */
839 __u64 len;
840 __u64 addr;
841 };
843 The type member is a SETUP_INDIRECT | SETUP_* type. However, it cannot be
844 SETUP_INDIRECT itself since making the setup_indirect a tree structure
845 could require a lot of stack space in something that needs to parse it
846 and stack space can be limited in boot contexts.
848 Let's give an example how to point to SETUP_E820_EXT data using setup_indirect.
849 In this case setup_data and setup_indirect will look like this::
851 struct setup_data {
852 __u64 next = 0 or <addr_of_next_setup_data_struct>;
853 __u32 type = SETUP_INDIRECT;
854 __u32 len = sizeof(setup_data);
855 __u8 data[sizeof(setup_indirect)] = struct setup_indirect {
856 __u32 type = SETUP_INDIRECT | SETUP_E820_EXT;
857 __u32 reserved = 0;
858 __u64 len = <len_of_SETUP_E820_EXT_data>;
859 __u64 addr = <addr_of_SETUP_E820_EXT_data>;
860 }
861 }
863 .. note::
864 SETUP_INDIRECT | SETUP_NONE objects cannot be properly distinguished
865 from SETUP_INDIRECT itself. So, this kind of objects cannot be provided
866 by the bootloaders.
868 ============ ============
869 Field name: pref_address
870 Type: read (reloc)
871 Offset/size: 0x258/8
872 Protocol: 2.10+
873 ============ ============
875 This field, if nonzero, represents a preferred load address for the
876 kernel. A relocating bootloader should attempt to load at this
877 address if possible.
879 A non-relocatable kernel will unconditionally move itself and to run
880 at this address.
882 ============ =======
883 Field name: init_size
884 Type: read
885 Offset/size: 0x260/4
886 ============ =======
888 This field indicates the amount of linear contiguous memory starting
889 at the kernel runtime start address that the kernel needs before it
890 is capable of examining its memory map. This is not the same thing
891 as the total amount of memory the kernel needs to boot, but it can
892 be used by a relocating boot loader to help select a safe load
893 address for the kernel.
895 The kernel runtime start address is determined by the following algorithm::
897 if (relocatable_kernel)
898 runtime_start = align_up(load_address, kernel_alignment)
899 else
900 runtime_start = pref_address
902 ============ ===============
903 Field name: handover_offset
904 Type: read
905 Offset/size: 0x264/4
906 ============ ===============
908 This field is the offset from the beginning of the kernel image to
909 the EFI handover protocol entry point. Boot loaders using the EFI
910 handover protocol to boot the kernel should jump to this offset.
912 See EFI HANDOVER PROTOCOL below for more details.
914 ============ ==================
915 Field name: kernel_info_offset
916 Type: read
917 Offset/size: 0x268/4
918 Protocol: 2.15+
919 ============ ==================
921 This field is the offset from the beginning of the kernel image to the
922 kernel_info. The kernel_info structure is embedded in the Linux image
923 in the uncompressed protected mode region.
926 The kernel_info
927 ===============
929 The relationships between the headers are analogous to the various data
930 sections:
932 setup_header = .data
933 boot_params/setup_data = .bss
935 What is missing from the above list? That's right:
937 kernel_info = .rodata
939 We have been (ab)using .data for things that could go into .rodata or .bss for
940 a long time, for lack of alternatives and -- especially early on -- inertia.
941 Also, the BIOS stub is responsible for creating boot_params, so it isn't
942 available to a BIOS-based loader (setup_data is, though).
944 setup_header is permanently limited to 144 bytes due to the reach of the
945 2-byte jump field, which doubles as a length field for the structure, combined
946 with the size of the "hole" in struct boot_params that a protected-mode loader
947 or the BIOS stub has to copy it into. It is currently 119 bytes long, which
948 leaves us with 25 very precious bytes. This isn't something that can be fixed
949 without revising the boot protocol entirely, breaking backwards compatibility.
951 boot_params proper is limited to 4096 bytes, but can be arbitrarily extended
952 by adding setup_data entries. It cannot be used to communicate properties of
953 the kernel image, because it is .bss and has no image-provided content.
955 kernel_info solves this by providing an extensible place for information about
956 the kernel image. It is readonly, because the kernel cannot rely on a
957 bootloader copying its contents anywhere, but that is OK; if it becomes
958 necessary it can still contain data items that an enabled bootloader would be
959 expected to copy into a setup_data chunk.
961 All kernel_info data should be part of this structure. Fixed size data have to
962 be put before kernel_info_var_len_data label. Variable size data have to be put
963 after kernel_info_var_len_data label. Each chunk of variable size data has to
964 be prefixed with header/magic and its size, e.g.::
966 kernel_info:
967 .ascii "LToP" /* Header, Linux top (structure). */
968 .long kernel_info_var_len_data - kernel_info
969 .long kernel_info_end - kernel_info
970 .long 0x01234567 /* Some fixed size data for the bootloaders. */
971 kernel_info_var_len_data:
972 example_struct: /* Some variable size data for the bootloaders. */
973 .ascii "0123" /* Header/Magic. */
974 .long example_struct_end - example_struct
975 .ascii "Struct"
976 .long 0x89012345
977 example_struct_end:
978 example_strings: /* Some variable size data for the bootloaders. */
979 .ascii "ABCD" /* Header/Magic. */
980 .long example_strings_end - example_strings
981 .asciz "String_0"
982 .asciz "String_1"
983 example_strings_end:
984 kernel_info_end:
986 This way the kernel_info is self-contained blob.
988 .. note::
989 Each variable size data header/magic can be any 4-character string,
990 without \0 at the end of the string, which does not collide with
991 existing variable length data headers/magics.
994 Details of the kernel_info Fields
995 =================================
997 ============ ========
998 Field name: header
999 Offset/size: 0x0000/4
1000 ============ ========
1002 Contains the magic number "LToP" (0x506f544c).
1004 ============ ========
1005 Field name: size
1006 Offset/size: 0x0004/4
1007 ============ ========
1009 This field contains the size of the kernel_info including kernel_info.header.
1010 It does not count kernel_info.kernel_info_var_len_data size. This field should be
1011 used by the bootloaders to detect supported fixed size fields in the kernel_info
1012 and beginning of kernel_info.kernel_info_var_len_data.
1014 ============ ========
1015 Field name: size_total
1016 Offset/size: 0x0008/4
1017 ============ ========
1019 This field contains the size of the kernel_info including kernel_info.header
1020 and kernel_info.kernel_info_var_len_data.
1022 ============ ==============
1023 Field name: setup_type_max
1024 Offset/size: 0x000c/4
1025 ============ ==============
1027 This field contains maximal allowed type for setup_data and setup_indirect structs.
1030 The Image Checksum
1031 ==================
1033 From boot protocol version 2.08 onwards the CRC-32 is calculated over
1034 the entire file using the characteristic polynomial 0x04C11DB7 and an
1035 initial remainder of 0xffffffff. The checksum is appended to the
1036 file; therefore the CRC of the file up to the limit specified in the
1037 syssize field of the header is always 0.
1040 The Kernel Command Line
1041 =======================
1043 The kernel command line has become an important way for the boot
1044 loader to communicate with the kernel. Some of its options are also
1045 relevant to the boot loader itself, see "special command line options"
1046 below.
1048 The kernel command line is a null-terminated string. The maximum
1049 length can be retrieved from the field cmdline_size. Before protocol
1050 version 2.06, the maximum was 255 characters. A string that is too
1051 long will be automatically truncated by the kernel.
1053 If the boot protocol version is 2.02 or later, the address of the
1054 kernel command line is given by the header field cmd_line_ptr (see
1055 above.) This address can be anywhere between the end of the setup
1056 heap and 0xA0000.
1058 If the protocol version is *not* 2.02 or higher, the kernel
1059 command line is entered using the following protocol:
1061 - At offset 0x0020 (word), "cmd_line_magic", enter the magic
1062 number 0xA33F.
1064 - At offset 0x0022 (word), "cmd_line_offset", enter the offset
1065 of the kernel command line (relative to the start of the
1066 real-mode kernel).
1068 - The kernel command line *must* be within the memory region
1069 covered by setup_move_size, so you may need to adjust this
1070 field.
1073 Memory Layout of The Real-Mode Code
1074 ===================================
1076 The real-mode code requires a stack/heap to be set up, as well as
1077 memory allocated for the kernel command line. This needs to be done
1078 in the real-mode accessible memory in bottom megabyte.
1080 It should be noted that modern machines often have a sizable Extended
1081 BIOS Data Area (EBDA). As a result, it is advisable to use as little
1082 of the low megabyte as possible.
1084 Unfortunately, under the following circumstances the 0x90000 memory
1085 segment has to be used:
1087 - When loading a zImage kernel ((loadflags & 0x01) == 0).
1088 - When loading a 2.01 or earlier boot protocol kernel.
1090 .. note::
1091 For the 2.00 and 2.01 boot protocols, the real-mode code
1092 can be loaded at another address, but it is internally
1093 relocated to 0x90000. For the "old" protocol, the
1094 real-mode code must be loaded at 0x90000.
1096 When loading at 0x90000, avoid using memory above 0x9a000.
1098 For boot protocol 2.02 or higher, the command line does not have to be
1099 located in the same 64K segment as the real-mode setup code; it is
1100 thus permitted to give the stack/heap the full 64K segment and locate
1101 the command line above it.
1103 The kernel command line should not be located below the real-mode
1104 code, nor should it be located in high memory.
1107 Sample Boot Configuartion
1108 =========================
1110 As a sample configuration, assume the following layout of the real
1111 mode segment.
1113 When loading below 0x90000, use the entire segment:
1115 ============= ===================
1116 0x0000-0x7fff Real mode kernel
1117 0x8000-0xdfff Stack and heap
1118 0xe000-0xffff Kernel command line
1119 ============= ===================
1121 When loading at 0x90000 OR the protocol version is 2.01 or earlier:
1123 ============= ===================
1124 0x0000-0x7fff Real mode kernel
1125 0x8000-0x97ff Stack and heap
1126 0x9800-0x9fff Kernel command line
1127 ============= ===================
1129 Such a boot loader should enter the following fields in the header::
1131 unsigned long base_ptr; /* base address for real-mode segment */
1133 if ( setup_sects == 0 ) {
1134 setup_sects = 4;
1135 }
1137 if ( protocol >= 0x0200 ) {
1138 type_of_loader = <type code>;
1139 if ( loading_initrd ) {
1140 ramdisk_image = <initrd_address>;
1141 ramdisk_size = <initrd_size>;
1142 }
1144 if ( protocol >= 0x0202 && loadflags & 0x01 )
1145 heap_end = 0xe000;
1146 else
1147 heap_end = 0x9800;
1149 if ( protocol >= 0x0201 ) {
1150 heap_end_ptr = heap_end - 0x200;
1151 loadflags |= 0x80; /* CAN_USE_HEAP */
1152 }
1154 if ( protocol >= 0x0202 ) {
1155 cmd_line_ptr = base_ptr + heap_end;
1156 strcpy(cmd_line_ptr, cmdline);
1157 } else {
1158 cmd_line_magic = 0xA33F;
1159 cmd_line_offset = heap_end;
1160 setup_move_size = heap_end + strlen(cmdline)+1;
1161 strcpy(base_ptr+cmd_line_offset, cmdline);
1162 }
1163 } else {
1164 /* Very old kernel */
1166 heap_end = 0x9800;
1168 cmd_line_magic = 0xA33F;
1169 cmd_line_offset = heap_end;
1171 /* A very old kernel MUST have its real-mode code
1172 loaded at 0x90000 */
1174 if ( base_ptr != 0x90000 ) {
1175 /* Copy the real-mode kernel */
1176 memcpy(0x90000, base_ptr, (setup_sects+1)*512);
1177 base_ptr = 0x90000; /* Relocated */
1178 }
1180 strcpy(0x90000+cmd_line_offset, cmdline);
1182 /* It is recommended to clear memory up to the 32K mark */
1183 memset(0x90000 + (setup_sects+1)*512, 0,
1184 (64-(setup_sects+1))*512);
1185 }
1188 Loading The Rest of The Kernel
1189 ==============================
1191 The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
1192 in the kernel file (again, if setup_sects == 0 the real value is 4.)
1193 It should be loaded at address 0x10000 for Image/zImage kernels and
1194 0x100000 for bzImage kernels.
1196 The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
1197 bit (LOAD_HIGH) in the loadflags field is set::
1199 is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
1200 load_address = is_bzImage ? 0x100000 : 0x10000;
1202 Note that Image/zImage kernels can be up to 512K in size, and thus use
1203 the entire 0x10000-0x90000 range of memory. This means it is pretty
1204 much a requirement for these kernels to load the real-mode part at
1205 0x90000. bzImage kernels allow much more flexibility.
1207 Special Command Line Options
1208 ============================
1210 If the command line provided by the boot loader is entered by the
1211 user, the user may expect the following command line options to work.
1212 They should normally not be deleted from the kernel command line even
1213 though not all of them are actually meaningful to the kernel. Boot
1214 loader authors who need additional command line options for the boot
1215 loader itself should get them registered in
1216 Documentation/admin-guide/kernel-parameters.rst to make sure they will not
1217 conflict with actual kernel options now or in the future.
1219 vga=<mode>
1220 <mode> here is either an integer (in C notation, either
1221 decimal, octal, or hexadecimal) or one of the strings
1222 "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
1223 (meaning 0xFFFD). This value should be entered into the
1224 vid_mode field, as it is used by the kernel before the command
1225 line is parsed.
1227 mem=<size>
1228 <size> is an integer in C notation optionally followed by
1229 (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
1230 << 30, << 40, << 50 or << 60). This specifies the end of
1231 memory to the kernel. This affects the possible placement of
1232 an initrd, since an initrd should be placed near end of
1233 memory. Note that this is an option to *both* the kernel and
1234 the bootloader!
1236 initrd=<file>
1237 An initrd should be loaded. The meaning of <file> is
1238 obviously bootloader-dependent, and some boot loaders
1239 (e.g. LILO) do not have such a command.
1241 In addition, some boot loaders add the following options to the
1242 user-specified command line:
1244 BOOT_IMAGE=<file>
1245 The boot image which was loaded. Again, the meaning of <file>
1246 is obviously bootloader-dependent.
1248 auto
1249 The kernel was booted without explicit user intervention.
1251 If these options are added by the boot loader, it is highly
1252 recommended that they are located *first*, before the user-specified
1253 or configuration-specified command line. Otherwise, "init=/bin/sh"
1254 gets confused by the "auto" option.
1257 Running the Kernel
1258 ==================
1260 The kernel is started by jumping to the kernel entry point, which is
1261 located at *segment* offset 0x20 from the start of the real mode
1262 kernel. This means that if you loaded your real-mode kernel code at
1263 0x90000, the kernel entry point is 9020:0000.
1265 At entry, ds = es = ss should point to the start of the real-mode
1266 kernel code (0x9000 if the code is loaded at 0x90000), sp should be
1267 set up properly, normally pointing to the top of the heap, and
1268 interrupts should be disabled. Furthermore, to guard against bugs in
1269 the kernel, it is recommended that the boot loader sets fs = gs = ds =
1270 es = ss.
1272 In our example from above, we would do::
1274 /* Note: in the case of the "old" kernel protocol, base_ptr must
1275 be == 0x90000 at this point; see the previous sample code */
1277 seg = base_ptr >> 4;
1279 cli(); /* Enter with interrupts disabled! */
1281 /* Set up the real-mode kernel stack */
1282 _SS = seg;
1283 _SP = heap_end;
1285 _DS = _ES = _FS = _GS = seg;
1286 jmp_far(seg+0x20, 0); /* Run the kernel */
1288 If your boot sector accesses a floppy drive, it is recommended to
1289 switch off the floppy motor before running the kernel, since the
1290 kernel boot leaves interrupts off and thus the motor will not be
1291 switched off, especially if the loaded kernel has the floppy driver as
1292 a demand-loaded module!
1295 Advanced Boot Loader Hooks
1296 ==========================
1298 If the boot loader runs in a particularly hostile environment (such as
1299 LOADLIN, which runs under DOS) it may be impossible to follow the
1300 standard memory location requirements. Such a boot loader may use the
1301 following hooks that, if set, are invoked by the kernel at the
1302 appropriate time. The use of these hooks should probably be
1303 considered an absolutely last resort!
1305 IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
1306 %edi across invocation.
1308 realmode_swtch:
1309 A 16-bit real mode far subroutine invoked immediately before
1310 entering protected mode. The default routine disables NMI, so
1311 your routine should probably do so, too.
1313 code32_start:
1314 A 32-bit flat-mode routine *jumped* to immediately after the
1315 transition to protected mode, but before the kernel is
1316 uncompressed. No segments, except CS, are guaranteed to be
1317 set up (current kernels do, but older ones do not); you should
1318 set them up to BOOT_DS (0x18) yourself.
1320 After completing your hook, you should jump to the address
1321 that was in this field before your boot loader overwrote it
1322 (relocated, if appropriate.)
1325 32-bit Boot Protocol
1326 ====================
1328 For machine with some new BIOS other than legacy BIOS, such as EFI,
1329 LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
1330 based on legacy BIOS can not be used, so a 32-bit boot protocol needs
1331 to be defined.
1333 In 32-bit boot protocol, the first step in loading a Linux kernel
1334 should be to setup the boot parameters (struct boot_params,
1335 traditionally known as "zero page"). The memory for struct boot_params
1336 should be allocated and initialized to all zero. Then the setup header
1337 from offset 0x01f1 of kernel image on should be loaded into struct
1338 boot_params and examined. The end of setup header can be calculated as
1339 follow::
1341 0x0202 + byte value at offset 0x0201
1343 In addition to read/modify/write the setup header of the struct
1344 boot_params as that of 16-bit boot protocol, the boot loader should
1345 also fill the additional fields of the struct boot_params as
1346 described in chapter :doc:`zero-page`.
1348 After setting up the struct boot_params, the boot loader can load the
1349 32/64-bit kernel in the same way as that of 16-bit boot protocol.
1351 In 32-bit boot protocol, the kernel is started by jumping to the
1352 32-bit kernel entry point, which is the start address of loaded
1353 32/64-bit kernel.
1355 At entry, the CPU must be in 32-bit protected mode with paging
1356 disabled; a GDT must be loaded with the descriptors for selectors
1357 __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1358 segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1359 must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1360 must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
1361 address of the struct boot_params; %ebp, %edi and %ebx must be zero.
1363 64-bit Boot Protocol
1364 ====================
1366 For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader
1367 and we need a 64-bit boot protocol.
1369 In 64-bit boot protocol, the first step in loading a Linux kernel
1370 should be to setup the boot parameters (struct boot_params,
1371 traditionally known as "zero page"). The memory for struct boot_params
1372 could be allocated anywhere (even above 4G) and initialized to all zero.
1373 Then, the setup header at offset 0x01f1 of kernel image on should be
1374 loaded into struct boot_params and examined. The end of setup header
1375 can be calculated as follows::
1377 0x0202 + byte value at offset 0x0201
1379 In addition to read/modify/write the setup header of the struct
1380 boot_params as that of 16-bit boot protocol, the boot loader should
1381 also fill the additional fields of the struct boot_params as described
1382 in chapter :doc:`zero-page`.
1384 After setting up the struct boot_params, the boot loader can load
1385 64-bit kernel in the same way as that of 16-bit boot protocol, but
1386 kernel could be loaded above 4G.
1388 In 64-bit boot protocol, the kernel is started by jumping to the
1389 64-bit kernel entry point, which is the start address of loaded
1390 64-bit kernel plus 0x200.
1392 At entry, the CPU must be in 64-bit mode with paging enabled.
1393 The range with setup_header.init_size from start address of loaded
1394 kernel and zero page and command line buffer get ident mapping;
1395 a GDT must be loaded with the descriptors for selectors
1396 __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1397 segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1398 must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1399 must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base
1400 address of the struct boot_params.
1402 EFI Handover Protocol (deprecated)
1403 ==================================
1405 This protocol allows boot loaders to defer initialisation to the EFI
1406 boot stub. The boot loader is required to load the kernel/initrd(s)
1407 from the boot media and jump to the EFI handover protocol entry point
1408 which is hdr->handover_offset bytes from the beginning of
1409 startup_{32,64}.
1411 The boot loader MUST respect the kernel's PE/COFF metadata when it comes
1412 to section alignment, the memory footprint of the executable image beyond
1413 the size of the file itself, and any other aspect of the PE/COFF header
1414 that may affect correct operation of the image as a PE/COFF binary in the
1415 execution context provided by the EFI firmware.
1417 The function prototype for the handover entry point looks like this::
1419 efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp)
1421 'handle' is the EFI image handle passed to the boot loader by the EFI
1422 firmware, 'table' is the EFI system table - these are the first two
1423 arguments of the "handoff state" as described in section 2.3 of the
1424 UEFI specification. 'bp' is the boot loader-allocated boot params.
1426 The boot loader *must* fill out the following fields in bp::
1428 - hdr.cmd_line_ptr
1429 - hdr.ramdisk_image (if applicable)
1430 - hdr.ramdisk_size (if applicable)
1432 All other fields should be zero.
1434 NOTE: The EFI Handover Protocol is deprecated in favour of the ordinary PE/COFF
1435 entry point, combined with the LINUX_EFI_INITRD_MEDIA_GUID based initrd
1436 loading protocol (refer to [0] for an example of the bootloader side of
1437 this), which removes the need for any knowledge on the part of the EFI
1438 bootloader regarding the internal representation of boot_params or any
1439 requirements/limitations regarding the placement of the command line
1440 and ramdisk in memory, or the placement of the kernel image itself.
1442 [0] https://github.com/u-boot/u-boot/commit/ec80b4735a593961fe701cc3a5d717d4739b0fd0