| blob | c9c201596c3e5bac1a9f8c1d75f84e89c3e27501 |
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 (write): KEEP_SEGMENTS
495 Protocol: 2.07+
497 - If 0, reload the segment registers in the 32bit entry point.
498 - If 1, do not reload the segment registers in the 32bit entry point.
500 Assume that %cs %ds %ss %es are all set to flat segments with
501 a base of 0 (or the equivalent for their environment).
503 Bit 7 (write): CAN_USE_HEAP
505 Set this bit to 1 to indicate that the value entered in the
506 heap_end_ptr is valid. If this field is clear, some setup code
507 functionality will be disabled.
510 ============ ===================
511 Field name: setup_move_size
512 Type: modify (obligatory)
513 Offset/size: 0x212/2
514 Protocol: 2.00-2.01
515 ============ ===================
517 When using protocol 2.00 or 2.01, if the real mode kernel is not
518 loaded at 0x90000, it gets moved there later in the loading
519 sequence. Fill in this field if you want additional data (such as
520 the kernel command line) moved in addition to the real-mode kernel
521 itself.
523 The unit is bytes starting with the beginning of the boot sector.
525 This field is can be ignored when the protocol is 2.02 or higher, or
526 if the real-mode code is loaded at 0x90000.
528 ============ ========================
529 Field name: code32_start
530 Type: modify (optional, reloc)
531 Offset/size: 0x214/4
532 Protocol: 2.00+
533 ============ ========================
535 The address to jump to in protected mode. This defaults to the load
536 address of the kernel, and can be used by the boot loader to
537 determine the proper load address.
539 This field can be modified for two purposes:
541 1. as a boot loader hook (see Advanced Boot Loader Hooks below.)
543 2. if a bootloader which does not install a hook loads a
544 relocatable kernel at a nonstandard address it will have to modify
545 this field to point to the load address.
547 ============ ==================
548 Field name: ramdisk_image
549 Type: write (obligatory)
550 Offset/size: 0x218/4
551 Protocol: 2.00+
552 ============ ==================
554 The 32-bit linear address of the initial ramdisk or ramfs. Leave at
555 zero if there is no initial ramdisk/ramfs.
557 ============ ==================
558 Field name: ramdisk_size
559 Type: write (obligatory)
560 Offset/size: 0x21c/4
561 Protocol: 2.00+
562 ============ ==================
564 Size of the initial ramdisk or ramfs. Leave at zero if there is no
565 initial ramdisk/ramfs.
567 ============ ===============
568 Field name: bootsect_kludge
569 Type: kernel internal
570 Offset/size: 0x220/4
571 Protocol: 2.00+
572 ============ ===============
574 This field is obsolete.
576 ============ ==================
577 Field name: heap_end_ptr
578 Type: write (obligatory)
579 Offset/size: 0x224/2
580 Protocol: 2.01+
581 ============ ==================
583 Set this field to the offset (from the beginning of the real-mode
584 code) of the end of the setup stack/heap, minus 0x0200.
586 ============ ================
587 Field name: ext_loader_ver
588 Type: write (optional)
589 Offset/size: 0x226/1
590 Protocol: 2.02+
591 ============ ================
593 This field is used as an extension of the version number in the
594 type_of_loader field. The total version number is considered to be
595 (type_of_loader & 0x0f) + (ext_loader_ver << 4).
597 The use of this field is boot loader specific. If not written, it
598 is zero.
600 Kernels prior to 2.6.31 did not recognize this field, but it is safe
601 to write for protocol version 2.02 or higher.
603 ============ =====================================================
604 Field name: ext_loader_type
605 Type: write (obligatory if (type_of_loader & 0xf0) == 0xe0)
606 Offset/size: 0x227/1
607 Protocol: 2.02+
608 ============ =====================================================
610 This field is used as an extension of the type number in
611 type_of_loader field. If the type in type_of_loader is 0xE, then
612 the actual type is (ext_loader_type + 0x10).
614 This field is ignored if the type in type_of_loader is not 0xE.
616 Kernels prior to 2.6.31 did not recognize this field, but it is safe
617 to write for protocol version 2.02 or higher.
619 ============ ==================
620 Field name: cmd_line_ptr
621 Type: write (obligatory)
622 Offset/size: 0x228/4
623 Protocol: 2.02+
624 ============ ==================
626 Set this field to the linear address of the kernel command line.
627 The kernel command line can be located anywhere between the end of
628 the setup heap and 0xA0000; it does not have to be located in the
629 same 64K segment as the real-mode code itself.
631 Fill in this field even if your boot loader does not support a
632 command line, in which case you can point this to an empty string
633 (or better yet, to the string "auto".) If this field is left at
634 zero, the kernel will assume that your boot loader does not support
635 the 2.02+ protocol.
637 ============ ===============
638 Field name: initrd_addr_max
639 Type: read
640 Offset/size: 0x22c/4
641 Protocol: 2.03+
642 ============ ===============
644 The maximum address that may be occupied by the initial
645 ramdisk/ramfs contents. For boot protocols 2.02 or earlier, this
646 field is not present, and the maximum address is 0x37FFFFFF. (This
647 address is defined as the address of the highest safe byte, so if
648 your ramdisk is exactly 131072 bytes long and this field is
649 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
651 ============ ============================
652 Field name: kernel_alignment
653 Type: read/modify (reloc)
654 Offset/size: 0x230/4
655 Protocol: 2.05+ (read), 2.10+ (modify)
656 ============ ============================
658 Alignment unit required by the kernel (if relocatable_kernel is
659 true.) A relocatable kernel that is loaded at an alignment
660 incompatible with the value in this field will be realigned during
661 kernel initialization.
663 Starting with protocol version 2.10, this reflects the kernel
664 alignment preferred for optimal performance; it is possible for the
665 loader to modify this field to permit a lesser alignment. See the
666 min_alignment and pref_address field below.
668 ============ ==================
669 Field name: relocatable_kernel
670 Type: read (reloc)
671 Offset/size: 0x234/1
672 Protocol: 2.05+
673 ============ ==================
675 If this field is nonzero, the protected-mode part of the kernel can
676 be loaded at any address that satisfies the kernel_alignment field.
677 After loading, the boot loader must set the code32_start field to
678 point to the loaded code, or to a boot loader hook.
680 ============ =============
681 Field name: min_alignment
682 Type: read (reloc)
683 Offset/size: 0x235/1
684 Protocol: 2.10+
685 ============ =============
687 This field, if nonzero, indicates as a power of two the minimum
688 alignment required, as opposed to preferred, by the kernel to boot.
689 If a boot loader makes use of this field, it should update the
690 kernel_alignment field with the alignment unit desired; typically::
692 kernel_alignment = 1 << min_alignment
694 There may be a considerable performance cost with an excessively
695 misaligned kernel. Therefore, a loader should typically try each
696 power-of-two alignment from kernel_alignment down to this alignment.
698 ============ ==========
699 Field name: xloadflags
700 Type: read
701 Offset/size: 0x236/2
702 Protocol: 2.12+
703 ============ ==========
705 This field is a bitmask.
707 Bit 0 (read): XLF_KERNEL_64
709 - If 1, this kernel has the legacy 64-bit entry point at 0x200.
711 Bit 1 (read): XLF_CAN_BE_LOADED_ABOVE_4G
713 - If 1, kernel/boot_params/cmdline/ramdisk can be above 4G.
715 Bit 2 (read): XLF_EFI_HANDOVER_32
717 - If 1, the kernel supports the 32-bit EFI handoff entry point
718 given at handover_offset.
720 Bit 3 (read): XLF_EFI_HANDOVER_64
722 - If 1, the kernel supports the 64-bit EFI handoff entry point
723 given at handover_offset + 0x200.
725 Bit 4 (read): XLF_EFI_KEXEC
727 - If 1, the kernel supports kexec EFI boot with EFI runtime support.
730 ============ ============
731 Field name: cmdline_size
732 Type: read
733 Offset/size: 0x238/4
734 Protocol: 2.06+
735 ============ ============
737 The maximum size of the command line without the terminating
738 zero. This means that the command line can contain at most
739 cmdline_size characters. With protocol version 2.05 and earlier, the
740 maximum size was 255.
742 ============ ====================================
743 Field name: hardware_subarch
744 Type: write (optional, defaults to x86/PC)
745 Offset/size: 0x23c/4
746 Protocol: 2.07+
747 ============ ====================================
749 In a paravirtualized environment the hardware low level architectural
750 pieces such as interrupt handling, page table handling, and
751 accessing process control registers needs to be done differently.
753 This field allows the bootloader to inform the kernel we are in one
754 one of those environments.
756 ========== ==============================
757 0x00000000 The default x86/PC environment
758 0x00000001 lguest
759 0x00000002 Xen
760 0x00000003 Moorestown MID
761 0x00000004 CE4100 TV Platform
762 ========== ==============================
764 ============ =========================
765 Field name: hardware_subarch_data
766 Type: write (subarch-dependent)
767 Offset/size: 0x240/8
768 Protocol: 2.07+
769 ============ =========================
771 A pointer to data that is specific to hardware subarch
772 This field is currently unused for the default x86/PC environment,
773 do not modify.
775 ============ ==============
776 Field name: payload_offset
777 Type: read
778 Offset/size: 0x248/4
779 Protocol: 2.08+
780 ============ ==============
782 If non-zero then this field contains the offset from the beginning
783 of the protected-mode code to the payload.
785 The payload may be compressed. The format of both the compressed and
786 uncompressed data should be determined using the standard magic
787 numbers. The currently supported compression formats are gzip
788 (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA
789 (magic number 5D 00), XZ (magic number FD 37), and LZ4 (magic number
790 02 21). The uncompressed payload is currently always ELF (magic
791 number 7F 45 4C 46).
793 ============ ==============
794 Field name: payload_length
795 Type: read
796 Offset/size: 0x24c/4
797 Protocol: 2.08+
798 ============ ==============
800 The length of the payload.
802 ============ ===============
803 Field name: setup_data
804 Type: write (special)
805 Offset/size: 0x250/8
806 Protocol: 2.09+
807 ============ ===============
809 The 64-bit physical pointer to NULL terminated single linked list of
810 struct setup_data. This is used to define a more extensible boot
811 parameters passing mechanism. The definition of struct setup_data is
812 as follow::
814 struct setup_data {
815 u64 next;
816 u32 type;
817 u32 len;
818 u8 data[0];
819 };
821 Where, the next is a 64-bit physical pointer to the next node of
822 linked list, the next field of the last node is 0; the type is used
823 to identify the contents of data; the len is the length of data
824 field; the data holds the real payload.
826 This list may be modified at a number of points during the bootup
827 process. Therefore, when modifying this list one should always make
828 sure to consider the case where the linked list already contains
829 entries.
831 The setup_data is a bit awkward to use for extremely large data objects,
832 both because the setup_data header has to be adjacent to the data object
833 and because it has a 32-bit length field. However, it is important that
834 intermediate stages of the boot process have a way to identify which
835 chunks of memory are occupied by kernel data.
837 Thus setup_indirect struct and SETUP_INDIRECT type were introduced in
838 protocol 2.15::
840 struct setup_indirect {
841 __u32 type;
842 __u32 reserved; /* Reserved, must be set to zero. */
843 __u64 len;
844 __u64 addr;
845 };
847 The type member is a SETUP_INDIRECT | SETUP_* type. However, it cannot be
848 SETUP_INDIRECT itself since making the setup_indirect a tree structure
849 could require a lot of stack space in something that needs to parse it
850 and stack space can be limited in boot contexts.
852 Let's give an example how to point to SETUP_E820_EXT data using setup_indirect.
853 In this case setup_data and setup_indirect will look like this::
855 struct setup_data {
856 __u64 next = 0 or <addr_of_next_setup_data_struct>;
857 __u32 type = SETUP_INDIRECT;
858 __u32 len = sizeof(setup_data);
859 __u8 data[sizeof(setup_indirect)] = struct setup_indirect {
860 __u32 type = SETUP_INDIRECT | SETUP_E820_EXT;
861 __u32 reserved = 0;
862 __u64 len = <len_of_SETUP_E820_EXT_data>;
863 __u64 addr = <addr_of_SETUP_E820_EXT_data>;
864 }
865 }
867 .. note::
868 SETUP_INDIRECT | SETUP_NONE objects cannot be properly distinguished
869 from SETUP_INDIRECT itself. So, this kind of objects cannot be provided
870 by the bootloaders.
872 ============ ============
873 Field name: pref_address
874 Type: read (reloc)
875 Offset/size: 0x258/8
876 Protocol: 2.10+
877 ============ ============
879 This field, if nonzero, represents a preferred load address for the
880 kernel. A relocating bootloader should attempt to load at this
881 address if possible.
883 A non-relocatable kernel will unconditionally move itself and to run
884 at this address.
886 ============ =======
887 Field name: init_size
888 Type: read
889 Offset/size: 0x260/4
890 ============ =======
892 This field indicates the amount of linear contiguous memory starting
893 at the kernel runtime start address that the kernel needs before it
894 is capable of examining its memory map. This is not the same thing
895 as the total amount of memory the kernel needs to boot, but it can
896 be used by a relocating boot loader to help select a safe load
897 address for the kernel.
899 The kernel runtime start address is determined by the following algorithm::
901 if (relocatable_kernel)
902 runtime_start = align_up(load_address, kernel_alignment)
903 else
904 runtime_start = pref_address
906 ============ ===============
907 Field name: handover_offset
908 Type: read
909 Offset/size: 0x264/4
910 ============ ===============
912 This field is the offset from the beginning of the kernel image to
913 the EFI handover protocol entry point. Boot loaders using the EFI
914 handover protocol to boot the kernel should jump to this offset.
916 See EFI HANDOVER PROTOCOL below for more details.
918 ============ ==================
919 Field name: kernel_info_offset
920 Type: read
921 Offset/size: 0x268/4
922 Protocol: 2.15+
923 ============ ==================
925 This field is the offset from the beginning of the kernel image to the
926 kernel_info. The kernel_info structure is embedded in the Linux image
927 in the uncompressed protected mode region.
930 The kernel_info
931 ===============
933 The relationships between the headers are analogous to the various data
934 sections:
936 setup_header = .data
937 boot_params/setup_data = .bss
939 What is missing from the above list? That's right:
941 kernel_info = .rodata
943 We have been (ab)using .data for things that could go into .rodata or .bss for
944 a long time, for lack of alternatives and -- especially early on -- inertia.
945 Also, the BIOS stub is responsible for creating boot_params, so it isn't
946 available to a BIOS-based loader (setup_data is, though).
948 setup_header is permanently limited to 144 bytes due to the reach of the
949 2-byte jump field, which doubles as a length field for the structure, combined
950 with the size of the "hole" in struct boot_params that a protected-mode loader
951 or the BIOS stub has to copy it into. It is currently 119 bytes long, which
952 leaves us with 25 very precious bytes. This isn't something that can be fixed
953 without revising the boot protocol entirely, breaking backwards compatibility.
955 boot_params proper is limited to 4096 bytes, but can be arbitrarily extended
956 by adding setup_data entries. It cannot be used to communicate properties of
957 the kernel image, because it is .bss and has no image-provided content.
959 kernel_info solves this by providing an extensible place for information about
960 the kernel image. It is readonly, because the kernel cannot rely on a
961 bootloader copying its contents anywhere, but that is OK; if it becomes
962 necessary it can still contain data items that an enabled bootloader would be
963 expected to copy into a setup_data chunk.
965 All kernel_info data should be part of this structure. Fixed size data have to
966 be put before kernel_info_var_len_data label. Variable size data have to be put
967 after kernel_info_var_len_data label. Each chunk of variable size data has to
968 be prefixed with header/magic and its size, e.g.::
970 kernel_info:
971 .ascii "LToP" /* Header, Linux top (structure). */
972 .long kernel_info_var_len_data - kernel_info
973 .long kernel_info_end - kernel_info
974 .long 0x01234567 /* Some fixed size data for the bootloaders. */
975 kernel_info_var_len_data:
976 example_struct: /* Some variable size data for the bootloaders. */
977 .ascii "0123" /* Header/Magic. */
978 .long example_struct_end - example_struct
979 .ascii "Struct"
980 .long 0x89012345
981 example_struct_end:
982 example_strings: /* Some variable size data for the bootloaders. */
983 .ascii "ABCD" /* Header/Magic. */
984 .long example_strings_end - example_strings
985 .asciz "String_0"
986 .asciz "String_1"
987 example_strings_end:
988 kernel_info_end:
990 This way the kernel_info is self-contained blob.
992 .. note::
993 Each variable size data header/magic can be any 4-character string,
994 without \0 at the end of the string, which does not collide with
995 existing variable length data headers/magics.
998 Details of the kernel_info Fields
999 =================================
1001 ============ ========
1002 Field name: header
1003 Offset/size: 0x0000/4
1004 ============ ========
1006 Contains the magic number "LToP" (0x506f544c).
1008 ============ ========
1009 Field name: size
1010 Offset/size: 0x0004/4
1011 ============ ========
1013 This field contains the size of the kernel_info including kernel_info.header.
1014 It does not count kernel_info.kernel_info_var_len_data size. This field should be
1015 used by the bootloaders to detect supported fixed size fields in the kernel_info
1016 and beginning of kernel_info.kernel_info_var_len_data.
1018 ============ ========
1019 Field name: size_total
1020 Offset/size: 0x0008/4
1021 ============ ========
1023 This field contains the size of the kernel_info including kernel_info.header
1024 and kernel_info.kernel_info_var_len_data.
1026 ============ ==============
1027 Field name: setup_type_max
1028 Offset/size: 0x000c/4
1029 ============ ==============
1031 This field contains maximal allowed type for setup_data and setup_indirect structs.
1034 The Image Checksum
1035 ==================
1037 From boot protocol version 2.08 onwards the CRC-32 is calculated over
1038 the entire file using the characteristic polynomial 0x04C11DB7 and an
1039 initial remainder of 0xffffffff. The checksum is appended to the
1040 file; therefore the CRC of the file up to the limit specified in the
1041 syssize field of the header is always 0.
1044 The Kernel Command Line
1045 =======================
1047 The kernel command line has become an important way for the boot
1048 loader to communicate with the kernel. Some of its options are also
1049 relevant to the boot loader itself, see "special command line options"
1050 below.
1052 The kernel command line is a null-terminated string. The maximum
1053 length can be retrieved from the field cmdline_size. Before protocol
1054 version 2.06, the maximum was 255 characters. A string that is too
1055 long will be automatically truncated by the kernel.
1057 If the boot protocol version is 2.02 or later, the address of the
1058 kernel command line is given by the header field cmd_line_ptr (see
1059 above.) This address can be anywhere between the end of the setup
1060 heap and 0xA0000.
1062 If the protocol version is *not* 2.02 or higher, the kernel
1063 command line is entered using the following protocol:
1065 - At offset 0x0020 (word), "cmd_line_magic", enter the magic
1066 number 0xA33F.
1068 - At offset 0x0022 (word), "cmd_line_offset", enter the offset
1069 of the kernel command line (relative to the start of the
1070 real-mode kernel).
1072 - The kernel command line *must* be within the memory region
1073 covered by setup_move_size, so you may need to adjust this
1074 field.
1077 Memory Layout of The Real-Mode Code
1078 ===================================
1080 The real-mode code requires a stack/heap to be set up, as well as
1081 memory allocated for the kernel command line. This needs to be done
1082 in the real-mode accessible memory in bottom megabyte.
1084 It should be noted that modern machines often have a sizable Extended
1085 BIOS Data Area (EBDA). As a result, it is advisable to use as little
1086 of the low megabyte as possible.
1088 Unfortunately, under the following circumstances the 0x90000 memory
1089 segment has to be used:
1091 - When loading a zImage kernel ((loadflags & 0x01) == 0).
1092 - When loading a 2.01 or earlier boot protocol kernel.
1094 .. note::
1095 For the 2.00 and 2.01 boot protocols, the real-mode code
1096 can be loaded at another address, but it is internally
1097 relocated to 0x90000. For the "old" protocol, the
1098 real-mode code must be loaded at 0x90000.
1100 When loading at 0x90000, avoid using memory above 0x9a000.
1102 For boot protocol 2.02 or higher, the command line does not have to be
1103 located in the same 64K segment as the real-mode setup code; it is
1104 thus permitted to give the stack/heap the full 64K segment and locate
1105 the command line above it.
1107 The kernel command line should not be located below the real-mode
1108 code, nor should it be located in high memory.
1111 Sample Boot Configuartion
1112 =========================
1114 As a sample configuration, assume the following layout of the real
1115 mode segment.
1117 When loading below 0x90000, use the entire segment:
1119 ============= ===================
1120 0x0000-0x7fff Real mode kernel
1121 0x8000-0xdfff Stack and heap
1122 0xe000-0xffff Kernel command line
1123 ============= ===================
1125 When loading at 0x90000 OR the protocol version is 2.01 or earlier:
1127 ============= ===================
1128 0x0000-0x7fff Real mode kernel
1129 0x8000-0x97ff Stack and heap
1130 0x9800-0x9fff Kernel command line
1131 ============= ===================
1133 Such a boot loader should enter the following fields in the header::
1135 unsigned long base_ptr; /* base address for real-mode segment */
1137 if ( setup_sects == 0 ) {
1138 setup_sects = 4;
1139 }
1141 if ( protocol >= 0x0200 ) {
1142 type_of_loader = <type code>;
1143 if ( loading_initrd ) {
1144 ramdisk_image = <initrd_address>;
1145 ramdisk_size = <initrd_size>;
1146 }
1148 if ( protocol >= 0x0202 && loadflags & 0x01 )
1149 heap_end = 0xe000;
1150 else
1151 heap_end = 0x9800;
1153 if ( protocol >= 0x0201 ) {
1154 heap_end_ptr = heap_end - 0x200;
1155 loadflags |= 0x80; /* CAN_USE_HEAP */
1156 }
1158 if ( protocol >= 0x0202 ) {
1159 cmd_line_ptr = base_ptr + heap_end;
1160 strcpy(cmd_line_ptr, cmdline);
1161 } else {
1162 cmd_line_magic = 0xA33F;
1163 cmd_line_offset = heap_end;
1164 setup_move_size = heap_end + strlen(cmdline)+1;
1165 strcpy(base_ptr+cmd_line_offset, cmdline);
1166 }
1167 } else {
1168 /* Very old kernel */
1170 heap_end = 0x9800;
1172 cmd_line_magic = 0xA33F;
1173 cmd_line_offset = heap_end;
1175 /* A very old kernel MUST have its real-mode code
1176 loaded at 0x90000 */
1178 if ( base_ptr != 0x90000 ) {
1179 /* Copy the real-mode kernel */
1180 memcpy(0x90000, base_ptr, (setup_sects+1)*512);
1181 base_ptr = 0x90000; /* Relocated */
1182 }
1184 strcpy(0x90000+cmd_line_offset, cmdline);
1186 /* It is recommended to clear memory up to the 32K mark */
1187 memset(0x90000 + (setup_sects+1)*512, 0,
1188 (64-(setup_sects+1))*512);
1189 }
1192 Loading The Rest of The Kernel
1193 ==============================
1195 The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
1196 in the kernel file (again, if setup_sects == 0 the real value is 4.)
1197 It should be loaded at address 0x10000 for Image/zImage kernels and
1198 0x100000 for bzImage kernels.
1200 The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
1201 bit (LOAD_HIGH) in the loadflags field is set::
1203 is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
1204 load_address = is_bzImage ? 0x100000 : 0x10000;
1206 Note that Image/zImage kernels can be up to 512K in size, and thus use
1207 the entire 0x10000-0x90000 range of memory. This means it is pretty
1208 much a requirement for these kernels to load the real-mode part at
1209 0x90000. bzImage kernels allow much more flexibility.
1211 Special Command Line Options
1212 ============================
1214 If the command line provided by the boot loader is entered by the
1215 user, the user may expect the following command line options to work.
1216 They should normally not be deleted from the kernel command line even
1217 though not all of them are actually meaningful to the kernel. Boot
1218 loader authors who need additional command line options for the boot
1219 loader itself should get them registered in
1220 Documentation/admin-guide/kernel-parameters.rst to make sure they will not
1221 conflict with actual kernel options now or in the future.
1223 vga=<mode>
1224 <mode> here is either an integer (in C notation, either
1225 decimal, octal, or hexadecimal) or one of the strings
1226 "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
1227 (meaning 0xFFFD). This value should be entered into the
1228 vid_mode field, as it is used by the kernel before the command
1229 line is parsed.
1231 mem=<size>
1232 <size> is an integer in C notation optionally followed by
1233 (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
1234 << 30, << 40, << 50 or << 60). This specifies the end of
1235 memory to the kernel. This affects the possible placement of
1236 an initrd, since an initrd should be placed near end of
1237 memory. Note that this is an option to *both* the kernel and
1238 the bootloader!
1240 initrd=<file>
1241 An initrd should be loaded. The meaning of <file> is
1242 obviously bootloader-dependent, and some boot loaders
1243 (e.g. LILO) do not have such a command.
1245 In addition, some boot loaders add the following options to the
1246 user-specified command line:
1248 BOOT_IMAGE=<file>
1249 The boot image which was loaded. Again, the meaning of <file>
1250 is obviously bootloader-dependent.
1252 auto
1253 The kernel was booted without explicit user intervention.
1255 If these options are added by the boot loader, it is highly
1256 recommended that they are located *first*, before the user-specified
1257 or configuration-specified command line. Otherwise, "init=/bin/sh"
1258 gets confused by the "auto" option.
1261 Running the Kernel
1262 ==================
1264 The kernel is started by jumping to the kernel entry point, which is
1265 located at *segment* offset 0x20 from the start of the real mode
1266 kernel. This means that if you loaded your real-mode kernel code at
1267 0x90000, the kernel entry point is 9020:0000.
1269 At entry, ds = es = ss should point to the start of the real-mode
1270 kernel code (0x9000 if the code is loaded at 0x90000), sp should be
1271 set up properly, normally pointing to the top of the heap, and
1272 interrupts should be disabled. Furthermore, to guard against bugs in
1273 the kernel, it is recommended that the boot loader sets fs = gs = ds =
1274 es = ss.
1276 In our example from above, we would do::
1278 /* Note: in the case of the "old" kernel protocol, base_ptr must
1279 be == 0x90000 at this point; see the previous sample code */
1281 seg = base_ptr >> 4;
1283 cli(); /* Enter with interrupts disabled! */
1285 /* Set up the real-mode kernel stack */
1286 _SS = seg;
1287 _SP = heap_end;
1289 _DS = _ES = _FS = _GS = seg;
1290 jmp_far(seg+0x20, 0); /* Run the kernel */
1292 If your boot sector accesses a floppy drive, it is recommended to
1293 switch off the floppy motor before running the kernel, since the
1294 kernel boot leaves interrupts off and thus the motor will not be
1295 switched off, especially if the loaded kernel has the floppy driver as
1296 a demand-loaded module!
1299 Advanced Boot Loader Hooks
1300 ==========================
1302 If the boot loader runs in a particularly hostile environment (such as
1303 LOADLIN, which runs under DOS) it may be impossible to follow the
1304 standard memory location requirements. Such a boot loader may use the
1305 following hooks that, if set, are invoked by the kernel at the
1306 appropriate time. The use of these hooks should probably be
1307 considered an absolutely last resort!
1309 IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
1310 %edi across invocation.
1312 realmode_swtch:
1313 A 16-bit real mode far subroutine invoked immediately before
1314 entering protected mode. The default routine disables NMI, so
1315 your routine should probably do so, too.
1317 code32_start:
1318 A 32-bit flat-mode routine *jumped* to immediately after the
1319 transition to protected mode, but before the kernel is
1320 uncompressed. No segments, except CS, are guaranteed to be
1321 set up (current kernels do, but older ones do not); you should
1322 set them up to BOOT_DS (0x18) yourself.
1324 After completing your hook, you should jump to the address
1325 that was in this field before your boot loader overwrote it
1326 (relocated, if appropriate.)
1329 32-bit Boot Protocol
1330 ====================
1332 For machine with some new BIOS other than legacy BIOS, such as EFI,
1333 LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
1334 based on legacy BIOS can not be used, so a 32-bit boot protocol needs
1335 to be defined.
1337 In 32-bit boot protocol, the first step in loading a Linux kernel
1338 should be to setup the boot parameters (struct boot_params,
1339 traditionally known as "zero page"). The memory for struct boot_params
1340 should be allocated and initialized to all zero. Then the setup header
1341 from offset 0x01f1 of kernel image on should be loaded into struct
1342 boot_params and examined. The end of setup header can be calculated as
1343 follow::
1345 0x0202 + byte value at offset 0x0201
1347 In addition to read/modify/write the setup header of the struct
1348 boot_params as that of 16-bit boot protocol, the boot loader should
1349 also fill the additional fields of the struct boot_params as that
1350 described in zero-page.txt.
1352 After setting up the struct boot_params, the boot loader can load the
1353 32/64-bit kernel in the same way as that of 16-bit boot protocol.
1355 In 32-bit boot protocol, the kernel is started by jumping to the
1356 32-bit kernel entry point, which is the start address of loaded
1357 32/64-bit kernel.
1359 At entry, the CPU must be in 32-bit protected mode with paging
1360 disabled; a GDT must be loaded with the descriptors for selectors
1361 __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1362 segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1363 must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1364 must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
1365 address of the struct boot_params; %ebp, %edi and %ebx must be zero.
1367 64-bit Boot Protocol
1368 ====================
1370 For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader
1371 and we need a 64-bit boot protocol.
1373 In 64-bit boot protocol, the first step in loading a Linux kernel
1374 should be to setup the boot parameters (struct boot_params,
1375 traditionally known as "zero page"). The memory for struct boot_params
1376 could be allocated anywhere (even above 4G) and initialized to all zero.
1377 Then, the setup header at offset 0x01f1 of kernel image on should be
1378 loaded into struct boot_params and examined. The end of setup header
1379 can be calculated as follows::
1381 0x0202 + byte value at offset 0x0201
1383 In addition to read/modify/write the setup header of the struct
1384 boot_params as that of 16-bit boot protocol, the boot loader should
1385 also fill the additional fields of the struct boot_params as described
1386 in zero-page.txt.
1388 After setting up the struct boot_params, the boot loader can load
1389 64-bit kernel in the same way as that of 16-bit boot protocol, but
1390 kernel could be loaded above 4G.
1392 In 64-bit boot protocol, the kernel is started by jumping to the
1393 64-bit kernel entry point, which is the start address of loaded
1394 64-bit kernel plus 0x200.
1396 At entry, the CPU must be in 64-bit mode with paging enabled.
1397 The range with setup_header.init_size from start address of loaded
1398 kernel and zero page and command line buffer get ident mapping;
1399 a GDT must be loaded with the descriptors for selectors
1400 __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1401 segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1402 must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1403 must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base
1404 address of the struct boot_params.
1406 EFI Handover Protocol
1407 =====================
1409 This protocol allows boot loaders to defer initialisation to the EFI
1410 boot stub. The boot loader is required to load the kernel/initrd(s)
1411 from the boot media and jump to the EFI handover protocol entry point
1412 which is hdr->handover_offset bytes from the beginning of
1413 startup_{32,64}.
1415 The function prototype for the handover entry point looks like this::
1417 efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp)
1419 'handle' is the EFI image handle passed to the boot loader by the EFI
1420 firmware, 'table' is the EFI system table - these are the first two
1421 arguments of the "handoff state" as described in section 2.3 of the
1422 UEFI specification. 'bp' is the boot loader-allocated boot params.
1424 The boot loader *must* fill out the following fields in bp::
1426 - hdr.code32_start
1427 - hdr.cmd_line_ptr
1428 - hdr.ramdisk_image (if applicable)
1429 - hdr.ramdisk_size (if applicable)
1431 All other fields should be zero.