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1.. SPDX-License-Identifier: GPL-2.0
2
3===========================
4The Linux/x86 Boot Protocol
5===========================
6
7On the x86 platform, the Linux kernel uses a rather complicated boot
8convention. This has evolved partially due to historical aspects, as
9well as the desire in the early days to have the kernel itself be a
10bootable image, the complicated PC memory model and due to changed
11expectations in the PC industry caused by the effective demise of
12real-mode DOS as a mainstream operating system.
13
14Currently, the following versions of the Linux/x86 boot protocol exist.
15
16============= ============================================================
17Old kernels zImage/Image support only. Some very early kernels
18 may not even support a command line.
19
20Protocol 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.
25
26Protocol 2.01 (Kernel 1.3.76) Added a heap overrun warning.
27
28Protocol 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.
34
35Protocol 2.03 (Kernel 2.4.18-pre1) Explicitly makes the highest possible
36 initrd address available to the bootloader.
37
38Protocol 2.04 (Kernel 2.6.14) Extend the syssize field to four bytes.
39
40Protocol 2.05 (Kernel 2.6.20) Make protected mode kernel relocatable.
41 Introduce relocatable_kernel and kernel_alignment fields.
42
43Protocol 2.06 (Kernel 2.6.22) Added a field that contains the size of
44 the boot command line.
45
46Protocol 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.
49
50Protocol 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.
53
54Protocol 2.09 (Kernel 2.6.26) Added a field of 64-bit physical
55 pointer to single linked list of struct setup_data.
56
57Protocol 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.
60
61Protocol 2.11 (Kernel 3.6) Added a field for offset of EFI handover
62 protocol entry point.
63
64Protocol 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.
67
68Protocol 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
71============= ============================================================
72
73
74Memory Layout
75=============
76
77The traditional memory map for the kernel loader, used for Image or
78zImage kernels, typically looks like::
79
80 | |
81 0A0000 +------------------------+
82 | Reserved for BIOS | Do not use. Reserved for BIOS EBDA.
83 09A000 +------------------------+
84 | Command line |
85 | Stack/heap | For use by the kernel real-mode code.
86 098000 +------------------------+
87 | Kernel setup | The kernel real-mode code.
88 090200 +------------------------+
89 | Kernel boot sector | The kernel legacy boot sector.
90 090000 +------------------------+
91 | Protected-mode kernel | The bulk of the kernel image.
92 010000 +------------------------+
93 | Boot loader | <- Boot sector entry point 0000:7C00
94 001000 +------------------------+
95 | Reserved for MBR/BIOS |
96 000800 +------------------------+
97 | Typically used by MBR |
98 000600 +------------------------+
99 | BIOS use only |
100 000000 +------------------------+
101
102When using bzImage, the protected-mode kernel was relocated to
1030x100000 ("high memory"), and the kernel real-mode block (boot sector,
104setup, and stack/heap) was made relocatable to any address between
1050x10000 and end of low memory. Unfortunately, in protocols 2.00 and
1062.01 the 0x90000+ memory range is still used internally by the kernel;
107the 2.02 protocol resolves that problem.
108
109It is desirable to keep the "memory ceiling" -- the highest point in
110low memory touched by the boot loader -- as low as possible, since
111some newer BIOSes have begun to allocate some rather large amounts of
112memory, called the Extended BIOS Data Area, near the top of low
113memory. The boot loader should use the "INT 12h" BIOS call to verify
114how much low memory is available.
115
116Unfortunately, if INT 12h reports that the amount of memory is too
117low, there is usually nothing the boot loader can do but to report an
118error to the user. The boot loader should therefore be designed to
119take up as little space in low memory as it reasonably can. For
120zImage or old bzImage kernels, which need data written into the
1210x90000 segment, the boot loader should make sure not to use memory
122above the 0x9A000 point; too many BIOSes will break above that point.
123
124For a modern bzImage kernel with boot protocol version >= 2.02, a
125memory layout like the following is suggested::
126
127 ~ ~
128 | Protected-mode kernel |
129 100000 +------------------------+
130 | I/O memory hole |
131 0A0000 +------------------------+
132 | Reserved for BIOS | Leave as much as possible unused
133 ~ ~
134 | Command line | (Can also be below the X+10000 mark)
135 X+10000 +------------------------+
136 | Stack/heap | For use by the kernel real-mode code.
137 X+08000 +------------------------+
138 | Kernel setup | The kernel real-mode code.
139 | Kernel boot sector | The kernel legacy boot sector.
140 X +------------------------+
141 | Boot loader | <- Boot sector entry point 0000:7C00
142 001000 +------------------------+
143 | Reserved for MBR/BIOS |
144 000800 +------------------------+
145 | Typically used by MBR |
146 000600 +------------------------+
147 | BIOS use only |
148 000000 +------------------------+
149
150 ... where the address X is as low as the design of the boot loader permits.
151
152
153The Real-Mode Kernel Header
154===========================
155
156In the following text, and anywhere in the kernel boot sequence, "a
157sector" refers to 512 bytes. It is independent of the actual sector
158size of the underlying medium.
159
160The first step in loading a Linux kernel should be to load the
161real-mode code (boot sector and setup code) and then examine the
162following header at offset 0x01f1. The real-mode code can total up to
16332K, although the boot loader may choose to load only the first two
164sectors (1K) and then examine the bootup sector size.
165
166The header looks like:
167
168=========== ======== ===================== ============================================
169Offset/Size Proto Name Meaning
170=========== ======== ===================== ============================================
17101F1/1 ALL(1) setup_sects The size of the setup in sectors
17201F2/2 ALL root_flags If set, the root is mounted readonly
17301F4/4 2.04+(2) syssize The size of the 32-bit code in 16-byte paras
17401F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
17501FA/2 ALL vid_mode Video mode control
17601FC/2 ALL root_dev Default root device number
17701FE/2 ALL boot_flag 0xAA55 magic number
1780200/2 2.00+ jump Jump instruction
1790202/4 2.00+ header Magic signature "HdrS"
1800206/2 2.00+ version Boot protocol version supported
1810208/4 2.00+ realmode_swtch Boot loader hook (see below)
182020C/2 2.00+ start_sys_seg The load-low segment (0x1000) (obsolete)
183020E/2 2.00+ kernel_version Pointer to kernel version string
1840210/1 2.00+ type_of_loader Boot loader identifier
1850211/1 2.00+ loadflags Boot protocol option flags
1860212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
1870214/4 2.00+ code32_start Boot loader hook (see below)
1880218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
189021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
1900220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
1910224/2 2.01+ heap_end_ptr Free memory after setup end
1920226/1 2.02+(3) ext_loader_ver Extended boot loader version
1930227/1 2.02+(3) ext_loader_type Extended boot loader ID
1940228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
195022C/4 2.03+ initrd_addr_max Highest legal initrd address
1960230/4 2.05+ kernel_alignment Physical addr alignment required for kernel
1970234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
1980235/1 2.10+ min_alignment Minimum alignment, as a power of two
1990236/2 2.12+ xloadflags Boot protocol option flags
2000238/4 2.06+ cmdline_size Maximum size of the kernel command line
201023C/4 2.07+ hardware_subarch Hardware subarchitecture
2020240/8 2.07+ hardware_subarch_data Subarchitecture-specific data
2030248/4 2.08+ payload_offset Offset of kernel payload
204024C/4 2.08+ payload_length Length of kernel payload
2050250/8 2.09+ setup_data 64-bit physical pointer to linked list
206 of struct setup_data
2070258/8 2.10+ pref_address Preferred loading address
2080260/4 2.10+ init_size Linear memory required during initialization
2090264/4 2.11+ handover_offset Offset of handover entry point
210=========== ======== ===================== ============================================
211
212.. note::
213 (1) For backwards compatibility, if the setup_sects field contains 0, the
214 real value is 4.
215
216 (2) For boot protocol prior to 2.04, the upper two bytes of the syssize
217 field are unusable, which means the size of a bzImage kernel
218 cannot be determined.
219
220 (3) Ignored, but safe to set, for boot protocols 2.02-2.09.
221
222If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
223the boot protocol version is "old". Loading an old kernel, the
224following parameters should be assumed::
225
226 Image type = zImage
227 initrd not supported
228 Real-mode kernel must be located at 0x90000.
229
230Otherwise, the "version" field contains the protocol version,
231e.g. protocol version 2.01 will contain 0x0201 in this field. When
232setting fields in the header, you must make sure only to set fields
233supported by the protocol version in use.
234
235
236Details of Harder Fileds
237========================
238
239For each field, some are information from the kernel to the bootloader
240("read"), some are expected to be filled out by the bootloader
241("write"), and some are expected to be read and modified by the
242bootloader ("modify").
243
244All general purpose boot loaders should write the fields marked
245(obligatory). Boot loaders who want to load the kernel at a
246nonstandard address should fill in the fields marked (reloc); other
247boot loaders can ignore those fields.
248
249The byte order of all fields is littleendian (this is x86, after all.)
250
251============ ===========
252Field name: setup_sects
253Type: read
254Offset/size: 0x1f1/1
255Protocol: ALL
256============ ===========
257
258 The size of the setup code in 512-byte sectors. If this field is
259 0, the real value is 4. The real-mode code consists of the boot
260 sector (always one 512-byte sector) plus the setup code.
261
262============ =================
263Field name: root_flags
264Type: modify (optional)
265Offset/size: 0x1f2/2
266Protocol: ALL
267============ =================
268
269 If this field is nonzero, the root defaults to readonly. The use of
270 this field is deprecated; use the "ro" or "rw" options on the
271 command line instead.
272
273============ ===============================================
274Field name: syssize
275Type: read
276Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
277Protocol: 2.04+
278============ ===============================================
279
280 The size of the protected-mode code in units of 16-byte paragraphs.
281 For protocol versions older than 2.04 this field is only two bytes
282 wide, and therefore cannot be trusted for the size of a kernel if
283 the LOAD_HIGH flag is set.
284
285============ ===============
286Field name: ram_size
287Type: kernel internal
288Offset/size: 0x1f8/2
289Protocol: ALL
290============ ===============
291
292 This field is obsolete.
293
294============ ===================
295Field name: vid_mode
296Type: modify (obligatory)
297Offset/size: 0x1fa/2
298============ ===================
299
300 Please see the section on SPECIAL COMMAND LINE OPTIONS.
301
302============ =================
303Field name: root_dev
304Type: modify (optional)
305Offset/size: 0x1fc/2
306Protocol: ALL
307============ =================
308
309 The default root device device number. The use of this field is
310 deprecated, use the "root=" option on the command line instead.
311
312============ =========
313Field name: boot_flag
314Type: read
315Offset/size: 0x1fe/2
316Protocol: ALL
317============ =========
318
319 Contains 0xAA55. This is the closest thing old Linux kernels have
320 to a magic number.
321
322============ =======
323Field name: jump
324Type: read
325Offset/size: 0x200/2
326Protocol: 2.00+
327============ =======
328
329 Contains an x86 jump instruction, 0xEB followed by a signed offset
330 relative to byte 0x202. This can be used to determine the size of
331 the header.
332
333============ =======
334Field name: header
335Type: read
336Offset/size: 0x202/4
337Protocol: 2.00+
338============ =======
339
340 Contains the magic number "HdrS" (0x53726448).
341
342============ =======
343Field name: version
344Type: read
345Offset/size: 0x206/2
346Protocol: 2.00+
347============ =======
348
349 Contains the boot protocol version, in (major << 8)+minor format,
350 e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
351 10.17.
352
353============ =================
354Field name: realmode_swtch
355Type: modify (optional)
356Offset/size: 0x208/4
357Protocol: 2.00+
358============ =================
359
360 Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
361
362============ =============
363Field name: start_sys_seg
364Type: read
365Offset/size: 0x20c/2
366Protocol: 2.00+
367============ =============
368
369 The load low segment (0x1000). Obsolete.
370
371============ ==============
372Field name: kernel_version
373Type: read
374Offset/size: 0x20e/2
375Protocol: 2.00+
376============ ==============
377
378 If set to a nonzero value, contains a pointer to a NUL-terminated
379 human-readable kernel version number string, less 0x200. This can
380 be used to display the kernel version to the user. This value
381 should be less than (0x200*setup_sects).
382
383 For example, if this value is set to 0x1c00, the kernel version
384 number string can be found at offset 0x1e00 in the kernel file.
385 This is a valid value if and only if the "setup_sects" field
386 contains the value 15 or higher, as::
387
388 0x1c00 < 15*0x200 (= 0x1e00) but
389 0x1c00 >= 14*0x200 (= 0x1c00)
390
391 0x1c00 >> 9 = 14, So the minimum value for setup_secs is 15.
392
393============ ==================
394Field name: type_of_loader
395Type: write (obligatory)
396Offset/size: 0x210/1
397Protocol: 2.00+
398============ ==================
399
400 If your boot loader has an assigned id (see table below), enter
401 0xTV here, where T is an identifier for the boot loader and V is
402 a version number. Otherwise, enter 0xFF here.
403
404 For boot loader IDs above T = 0xD, write T = 0xE to this field and
405 write the extended ID minus 0x10 to the ext_loader_type field.
406 Similarly, the ext_loader_ver field can be used to provide more than
407 four bits for the bootloader version.
408
409 For example, for T = 0x15, V = 0x234, write::
410
411 type_of_loader <- 0xE4
412 ext_loader_type <- 0x05
413 ext_loader_ver <- 0x23
414
415 Assigned boot loader ids (hexadecimal):
416
417 == =======================================
418 0 LILO
419 (0x00 reserved for pre-2.00 bootloader)
420 1 Loadlin
421 2 bootsect-loader
422 (0x20, all other values reserved)
423 3 Syslinux
424 4 Etherboot/gPXE/iPXE
425 5 ELILO
426 7 GRUB
427 8 U-Boot
428 9 Xen
429 A Gujin
430 B Qemu
431 C Arcturus Networks uCbootloader
432 D kexec-tools
433 E Extended (see ext_loader_type)
434 F Special (0xFF = undefined)
435 10 Reserved
436 11 Minimal Linux Bootloader
437 <http://sebastian-plotz.blogspot.de>
438 12 OVMF UEFI virtualization stack
439 == =======================================
440
441 Please contact <hpa@zytor.com> if you need a bootloader ID value assigned.
442
443============ ===================
444Field name: loadflags
445Type: modify (obligatory)
446Offset/size: 0x211/1
447Protocol: 2.00+
448============ ===================
449
450 This field is a bitmask.
451
452 Bit 0 (read): LOADED_HIGH
453
454 - If 0, the protected-mode code is loaded at 0x10000.
455 - If 1, the protected-mode code is loaded at 0x100000.
456
457 Bit 1 (kernel internal): KASLR_FLAG
458
459 - Used internally by the compressed kernel to communicate
460 KASLR status to kernel proper.
461
462 - If 1, KASLR enabled.
463 - If 0, KASLR disabled.
464
465 Bit 5 (write): QUIET_FLAG
466
467 - If 0, print early messages.
468 - If 1, suppress early messages.
469
470 This requests to the kernel (decompressor and early
471 kernel) to not write early messages that require
472 accessing the display hardware directly.
473
474 Bit 6 (write): KEEP_SEGMENTS
475
476 Protocol: 2.07+
477
478 - If 0, reload the segment registers in the 32bit entry point.
479 - If 1, do not reload the segment registers in the 32bit entry point.
480
481 Assume that %cs %ds %ss %es are all set to flat segments with
482 a base of 0 (or the equivalent for their environment).
483
484 Bit 7 (write): CAN_USE_HEAP
485
486 Set this bit to 1 to indicate that the value entered in the
487 heap_end_ptr is valid. If this field is clear, some setup code
488 functionality will be disabled.
489
490
491============ ===================
492Field name: setup_move_size
493Type: modify (obligatory)
494Offset/size: 0x212/2
495Protocol: 2.00-2.01
496============ ===================
497
498 When using protocol 2.00 or 2.01, if the real mode kernel is not
499 loaded at 0x90000, it gets moved there later in the loading
500 sequence. Fill in this field if you want additional data (such as
501 the kernel command line) moved in addition to the real-mode kernel
502 itself.
503
504 The unit is bytes starting with the beginning of the boot sector.
505
506 This field is can be ignored when the protocol is 2.02 or higher, or
507 if the real-mode code is loaded at 0x90000.
508
509============ ========================
510Field name: code32_start
511Type: modify (optional, reloc)
512Offset/size: 0x214/4
513Protocol: 2.00+
514============ ========================
515
516 The address to jump to in protected mode. This defaults to the load
517 address of the kernel, and can be used by the boot loader to
518 determine the proper load address.
519
520 This field can be modified for two purposes:
521
522 1. as a boot loader hook (see Advanced Boot Loader Hooks below.)
523
524 2. if a bootloader which does not install a hook loads a
525 relocatable kernel at a nonstandard address it will have to modify
526 this field to point to the load address.
527
528============ ==================
529Field name: ramdisk_image
530Type: write (obligatory)
531Offset/size: 0x218/4
532Protocol: 2.00+
533============ ==================
534
535 The 32-bit linear address of the initial ramdisk or ramfs. Leave at
536 zero if there is no initial ramdisk/ramfs.
537
538============ ==================
539Field name: ramdisk_size
540Type: write (obligatory)
541Offset/size: 0x21c/4
542Protocol: 2.00+
543============ ==================
544
545 Size of the initial ramdisk or ramfs. Leave at zero if there is no
546 initial ramdisk/ramfs.
547
548============ ===============
549Field name: bootsect_kludge
550Type: kernel internal
551Offset/size: 0x220/4
552Protocol: 2.00+
553============ ===============
554
555 This field is obsolete.
556
557============ ==================
558Field name: heap_end_ptr
559Type: write (obligatory)
560Offset/size: 0x224/2
561Protocol: 2.01+
562============ ==================
563
564 Set this field to the offset (from the beginning of the real-mode
565 code) of the end of the setup stack/heap, minus 0x0200.
566
567============ ================
568Field name: ext_loader_ver
569Type: write (optional)
570Offset/size: 0x226/1
571Protocol: 2.02+
572============ ================
573
574 This field is used as an extension of the version number in the
575 type_of_loader field. The total version number is considered to be
576 (type_of_loader & 0x0f) + (ext_loader_ver << 4).
577
578 The use of this field is boot loader specific. If not written, it
579 is zero.
580
581 Kernels prior to 2.6.31 did not recognize this field, but it is safe
582 to write for protocol version 2.02 or higher.
583
584============ =====================================================
585Field name: ext_loader_type
586Type: write (obligatory if (type_of_loader & 0xf0) == 0xe0)
587Offset/size: 0x227/1
588Protocol: 2.02+
589============ =====================================================
590
591 This field is used as an extension of the type number in
592 type_of_loader field. If the type in type_of_loader is 0xE, then
593 the actual type is (ext_loader_type + 0x10).
594
595 This field is ignored if the type in type_of_loader is not 0xE.
596
597 Kernels prior to 2.6.31 did not recognize this field, but it is safe
598 to write for protocol version 2.02 or higher.
599
600============ ==================
601Field name: cmd_line_ptr
602Type: write (obligatory)
603Offset/size: 0x228/4
604Protocol: 2.02+
605============ ==================
606
607 Set this field to the linear address of the kernel command line.
608 The kernel command line can be located anywhere between the end of
609 the setup heap and 0xA0000; it does not have to be located in the
610 same 64K segment as the real-mode code itself.
611
612 Fill in this field even if your boot loader does not support a
613 command line, in which case you can point this to an empty string
614 (or better yet, to the string "auto".) If this field is left at
615 zero, the kernel will assume that your boot loader does not support
616 the 2.02+ protocol.
617
618============ ===============
619Field name: initrd_addr_max
620Type: read
621Offset/size: 0x22c/4
622Protocol: 2.03+
623============ ===============
624
625 The maximum address that may be occupied by the initial
626 ramdisk/ramfs contents. For boot protocols 2.02 or earlier, this
627 field is not present, and the maximum address is 0x37FFFFFF. (This
628 address is defined as the address of the highest safe byte, so if
629 your ramdisk is exactly 131072 bytes long and this field is
630 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
631
632============ ============================
633Field name: kernel_alignment
634Type: read/modify (reloc)
635Offset/size: 0x230/4
636Protocol: 2.05+ (read), 2.10+ (modify)
637============ ============================
638
639 Alignment unit required by the kernel (if relocatable_kernel is
640 true.) A relocatable kernel that is loaded at an alignment
641 incompatible with the value in this field will be realigned during
642 kernel initialization.
643
644 Starting with protocol version 2.10, this reflects the kernel
645 alignment preferred for optimal performance; it is possible for the
646 loader to modify this field to permit a lesser alignment. See the
647 min_alignment and pref_address field below.
648
649============ ==================
650Field name: relocatable_kernel
651Type: read (reloc)
652Offset/size: 0x234/1
653Protocol: 2.05+
654============ ==================
655
656 If this field is nonzero, the protected-mode part of the kernel can
657 be loaded at any address that satisfies the kernel_alignment field.
658 After loading, the boot loader must set the code32_start field to
659 point to the loaded code, or to a boot loader hook.
660
661============ =============
662Field name: min_alignment
663Type: read (reloc)
664Offset/size: 0x235/1
665Protocol: 2.10+
666============ =============
667
668 This field, if nonzero, indicates as a power of two the minimum
669 alignment required, as opposed to preferred, by the kernel to boot.
670 If a boot loader makes use of this field, it should update the
671 kernel_alignment field with the alignment unit desired; typically::
672
673 kernel_alignment = 1 << min_alignment
674
675 There may be a considerable performance cost with an excessively
676 misaligned kernel. Therefore, a loader should typically try each
677 power-of-two alignment from kernel_alignment down to this alignment.
678
679============ ==========
680Field name: xloadflags
681Type: read
682Offset/size: 0x236/2
683Protocol: 2.12+
684============ ==========
685
686 This field is a bitmask.
687
688 Bit 0 (read): XLF_KERNEL_64
689
690 - If 1, this kernel has the legacy 64-bit entry point at 0x200.
691
692 Bit 1 (read): XLF_CAN_BE_LOADED_ABOVE_4G
693
694 - If 1, kernel/boot_params/cmdline/ramdisk can be above 4G.
695
696 Bit 2 (read): XLF_EFI_HANDOVER_32
697
698 - If 1, the kernel supports the 32-bit EFI handoff entry point
699 given at handover_offset.
700
701 Bit 3 (read): XLF_EFI_HANDOVER_64
702
703 - If 1, the kernel supports the 64-bit EFI handoff entry point
704 given at handover_offset + 0x200.
705
706 Bit 4 (read): XLF_EFI_KEXEC
707
708 - If 1, the kernel supports kexec EFI boot with EFI runtime support.
709
710
711============ ============
712Field name: cmdline_size
713Type: read
714Offset/size: 0x238/4
715Protocol: 2.06+
716============ ============
717
718 The maximum size of the command line without the terminating
719 zero. This means that the command line can contain at most
720 cmdline_size characters. With protocol version 2.05 and earlier, the
721 maximum size was 255.
722
723============ ====================================
724Field name: hardware_subarch
725Type: write (optional, defaults to x86/PC)
726Offset/size: 0x23c/4
727Protocol: 2.07+
728============ ====================================
729
730 In a paravirtualized environment the hardware low level architectural
731 pieces such as interrupt handling, page table handling, and
732 accessing process control registers needs to be done differently.
733
734 This field allows the bootloader to inform the kernel we are in one
735 one of those environments.
736
737 ========== ==============================
738 0x00000000 The default x86/PC environment
739 0x00000001 lguest
740 0x00000002 Xen
741 0x00000003 Moorestown MID
742 0x00000004 CE4100 TV Platform
743 ========== ==============================
744
745============ =========================
746Field name: hardware_subarch_data
747Type: write (subarch-dependent)
748Offset/size: 0x240/8
749Protocol: 2.07+
750============ =========================
751
752 A pointer to data that is specific to hardware subarch
753 This field is currently unused for the default x86/PC environment,
754 do not modify.
755
756============ ==============
757Field name: payload_offset
758Type: read
759Offset/size: 0x248/4
760Protocol: 2.08+
761============ ==============
762
763 If non-zero then this field contains the offset from the beginning
764 of the protected-mode code to the payload.
765
766 The payload may be compressed. The format of both the compressed and
767 uncompressed data should be determined using the standard magic
768 numbers. The currently supported compression formats are gzip
769 (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA
770 (magic number 5D 00), XZ (magic number FD 37), and LZ4 (magic number
771 02 21). The uncompressed payload is currently always ELF (magic
772 number 7F 45 4C 46).
773
774============ ==============
775Field name: payload_length
776Type: read
777Offset/size: 0x24c/4
778Protocol: 2.08+
779============ ==============
780
781 The length of the payload.
782
783============ ===============
784Field name: setup_data
785Type: write (special)
786Offset/size: 0x250/8
787Protocol: 2.09+
788============ ===============
789
790 The 64-bit physical pointer to NULL terminated single linked list of
791 struct setup_data. This is used to define a more extensible boot
792 parameters passing mechanism. The definition of struct setup_data is
793 as follow::
794
795 struct setup_data {
796 u64 next;
797 u32 type;
798 u32 len;
799 u8 data[0];
800 };
801
802 Where, the next is a 64-bit physical pointer to the next node of
803 linked list, the next field of the last node is 0; the type is used
804 to identify the contents of data; the len is the length of data
805 field; the data holds the real payload.
806
807 This list may be modified at a number of points during the bootup
808 process. Therefore, when modifying this list one should always make
809 sure to consider the case where the linked list already contains
810 entries.
811
812============ ============
813Field name: pref_address
814Type: read (reloc)
815Offset/size: 0x258/8
816Protocol: 2.10+
817============ ============
818
819 This field, if nonzero, represents a preferred load address for the
820 kernel. A relocating bootloader should attempt to load at this
821 address if possible.
822
823 A non-relocatable kernel will unconditionally move itself and to run
824 at this address.
825
826============ =======
827Field name: init_size
828Type: read
829Offset/size: 0x260/4
830============ =======
831
832 This field indicates the amount of linear contiguous memory starting
833 at the kernel runtime start address that the kernel needs before it
834 is capable of examining its memory map. This is not the same thing
835 as the total amount of memory the kernel needs to boot, but it can
836 be used by a relocating boot loader to help select a safe load
837 address for the kernel.
838
839 The kernel runtime start address is determined by the following algorithm::
840
841 if (relocatable_kernel)
842 runtime_start = align_up(load_address, kernel_alignment)
843 else
844 runtime_start = pref_address
845
846============ ===============
847Field name: handover_offset
848Type: read
849Offset/size: 0x264/4
850============ ===============
851
852 This field is the offset from the beginning of the kernel image to
853 the EFI handover protocol entry point. Boot loaders using the EFI
854 handover protocol to boot the kernel should jump to this offset.
855
856 See EFI HANDOVER PROTOCOL below for more details.
857
858
859The Image Checksum
860==================
861
862From boot protocol version 2.08 onwards the CRC-32 is calculated over
863the entire file using the characteristic polynomial 0x04C11DB7 and an
864initial remainder of 0xffffffff. The checksum is appended to the
865file; therefore the CRC of the file up to the limit specified in the
866syssize field of the header is always 0.
867
868
869The Kernel Command Line
870=======================
871
872The kernel command line has become an important way for the boot
873loader to communicate with the kernel. Some of its options are also
874relevant to the boot loader itself, see "special command line options"
875below.
876
877The kernel command line is a null-terminated string. The maximum
878length can be retrieved from the field cmdline_size. Before protocol
879version 2.06, the maximum was 255 characters. A string that is too
880long will be automatically truncated by the kernel.
881
882If the boot protocol version is 2.02 or later, the address of the
883kernel command line is given by the header field cmd_line_ptr (see
884above.) This address can be anywhere between the end of the setup
885heap and 0xA0000.
886
887If the protocol version is *not* 2.02 or higher, the kernel
888command line is entered using the following protocol:
889
890 - At offset 0x0020 (word), "cmd_line_magic", enter the magic
891 number 0xA33F.
892
893 - At offset 0x0022 (word), "cmd_line_offset", enter the offset
894 of the kernel command line (relative to the start of the
895 real-mode kernel).
896
897 - The kernel command line *must* be within the memory region
898 covered by setup_move_size, so you may need to adjust this
899 field.
900
901
902Memory Layout of The Real-Mode Code
903===================================
904
905The real-mode code requires a stack/heap to be set up, as well as
906memory allocated for the kernel command line. This needs to be done
907in the real-mode accessible memory in bottom megabyte.
908
909It should be noted that modern machines often have a sizable Extended
910BIOS Data Area (EBDA). As a result, it is advisable to use as little
911of the low megabyte as possible.
912
913Unfortunately, under the following circumstances the 0x90000 memory
914segment has to be used:
915
916 - When loading a zImage kernel ((loadflags & 0x01) == 0).
917 - When loading a 2.01 or earlier boot protocol kernel.
918
919.. note::
920 For the 2.00 and 2.01 boot protocols, the real-mode code
921 can be loaded at another address, but it is internally
922 relocated to 0x90000. For the "old" protocol, the
923 real-mode code must be loaded at 0x90000.
924
925When loading at 0x90000, avoid using memory above 0x9a000.
926
927For boot protocol 2.02 or higher, the command line does not have to be
928located in the same 64K segment as the real-mode setup code; it is
929thus permitted to give the stack/heap the full 64K segment and locate
930the command line above it.
931
932The kernel command line should not be located below the real-mode
933code, nor should it be located in high memory.
934
935
936Sample Boot Configuartion
937=========================
938
939As a sample configuration, assume the following layout of the real
940mode segment.
941
942 When loading below 0x90000, use the entire segment:
943
944 ============= ===================
945 0x0000-0x7fff Real mode kernel
946 0x8000-0xdfff Stack and heap
947 0xe000-0xffff Kernel command line
948 ============= ===================
949
950 When loading at 0x90000 OR the protocol version is 2.01 or earlier:
951
952 ============= ===================
953 0x0000-0x7fff Real mode kernel
954 0x8000-0x97ff Stack and heap
955 0x9800-0x9fff Kernel command line
956 ============= ===================
957
958Such a boot loader should enter the following fields in the header::
959
960 unsigned long base_ptr; /* base address for real-mode segment */
961
962 if ( setup_sects == 0 ) {
963 setup_sects = 4;
964 }
965
966 if ( protocol >= 0x0200 ) {
967 type_of_loader = <type code>;
968 if ( loading_initrd ) {
969 ramdisk_image = <initrd_address>;
970 ramdisk_size = <initrd_size>;
971 }
972
973 if ( protocol >= 0x0202 && loadflags & 0x01 )
974 heap_end = 0xe000;
975 else
976 heap_end = 0x9800;
977
978 if ( protocol >= 0x0201 ) {
979 heap_end_ptr = heap_end - 0x200;
980 loadflags |= 0x80; /* CAN_USE_HEAP */
981 }
982
983 if ( protocol >= 0x0202 ) {
984 cmd_line_ptr = base_ptr + heap_end;
985 strcpy(cmd_line_ptr, cmdline);
986 } else {
987 cmd_line_magic = 0xA33F;
988 cmd_line_offset = heap_end;
989 setup_move_size = heap_end + strlen(cmdline)+1;
990 strcpy(base_ptr+cmd_line_offset, cmdline);
991 }
992 } else {
993 /* Very old kernel */
994
995 heap_end = 0x9800;
996
997 cmd_line_magic = 0xA33F;
998 cmd_line_offset = heap_end;
999
1000 /* A very old kernel MUST have its real-mode code
1001 loaded at 0x90000 */
1002
1003 if ( base_ptr != 0x90000 ) {
1004 /* Copy the real-mode kernel */
1005 memcpy(0x90000, base_ptr, (setup_sects+1)*512);
1006 base_ptr = 0x90000; /* Relocated */
1007 }
1008
1009 strcpy(0x90000+cmd_line_offset, cmdline);
1010
1011 /* It is recommended to clear memory up to the 32K mark */
1012 memset(0x90000 + (setup_sects+1)*512, 0,
1013 (64-(setup_sects+1))*512);
1014 }
1015
1016
1017Loading The Rest of The Kernel
1018==============================
1019
1020The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
1021in the kernel file (again, if setup_sects == 0 the real value is 4.)
1022It should be loaded at address 0x10000 for Image/zImage kernels and
10230x100000 for bzImage kernels.
1024
1025The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
1026bit (LOAD_HIGH) in the loadflags field is set::
1027
1028 is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
1029 load_address = is_bzImage ? 0x100000 : 0x10000;
1030
1031Note that Image/zImage kernels can be up to 512K in size, and thus use
1032the entire 0x10000-0x90000 range of memory. This means it is pretty
1033much a requirement for these kernels to load the real-mode part at
10340x90000. bzImage kernels allow much more flexibility.
1035
1036Special Command Line Options
1037============================
1038
1039If the command line provided by the boot loader is entered by the
1040user, the user may expect the following command line options to work.
1041They should normally not be deleted from the kernel command line even
1042though not all of them are actually meaningful to the kernel. Boot
1043loader authors who need additional command line options for the boot
1044loader itself should get them registered in
1045Documentation/admin-guide/kernel-parameters.rst to make sure they will not
1046conflict with actual kernel options now or in the future.
1047
1048 vga=<mode>
1049 <mode> here is either an integer (in C notation, either
1050 decimal, octal, or hexadecimal) or one of the strings
1051 "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
1052 (meaning 0xFFFD). This value should be entered into the
1053 vid_mode field, as it is used by the kernel before the command
1054 line is parsed.
1055
1056 mem=<size>
1057 <size> is an integer in C notation optionally followed by
1058 (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
1059 << 30, << 40, << 50 or << 60). This specifies the end of
1060 memory to the kernel. This affects the possible placement of
1061 an initrd, since an initrd should be placed near end of
1062 memory. Note that this is an option to *both* the kernel and
1063 the bootloader!
1064
1065 initrd=<file>
1066 An initrd should be loaded. The meaning of <file> is
1067 obviously bootloader-dependent, and some boot loaders
1068 (e.g. LILO) do not have such a command.
1069
1070In addition, some boot loaders add the following options to the
1071user-specified command line:
1072
1073 BOOT_IMAGE=<file>
1074 The boot image which was loaded. Again, the meaning of <file>
1075 is obviously bootloader-dependent.
1076
1077 auto
1078 The kernel was booted without explicit user intervention.
1079
1080If these options are added by the boot loader, it is highly
1081recommended that they are located *first*, before the user-specified
1082or configuration-specified command line. Otherwise, "init=/bin/sh"
1083gets confused by the "auto" option.
1084
1085
1086Running the Kernel
1087==================
1088
1089The kernel is started by jumping to the kernel entry point, which is
1090located at *segment* offset 0x20 from the start of the real mode
1091kernel. This means that if you loaded your real-mode kernel code at
10920x90000, the kernel entry point is 9020:0000.
1093
1094At entry, ds = es = ss should point to the start of the real-mode
1095kernel code (0x9000 if the code is loaded at 0x90000), sp should be
1096set up properly, normally pointing to the top of the heap, and
1097interrupts should be disabled. Furthermore, to guard against bugs in
1098the kernel, it is recommended that the boot loader sets fs = gs = ds =
1099es = ss.
1100
1101In our example from above, we would do::
1102
1103 /* Note: in the case of the "old" kernel protocol, base_ptr must
1104 be == 0x90000 at this point; see the previous sample code */
1105
1106 seg = base_ptr >> 4;
1107
1108 cli(); /* Enter with interrupts disabled! */
1109
1110 /* Set up the real-mode kernel stack */
1111 _SS = seg;
1112 _SP = heap_end;
1113
1114 _DS = _ES = _FS = _GS = seg;
1115 jmp_far(seg+0x20, 0); /* Run the kernel */
1116
1117If your boot sector accesses a floppy drive, it is recommended to
1118switch off the floppy motor before running the kernel, since the
1119kernel boot leaves interrupts off and thus the motor will not be
1120switched off, especially if the loaded kernel has the floppy driver as
1121a demand-loaded module!
1122
1123
1124Advanced Boot Loader Hooks
1125==========================
1126
1127If the boot loader runs in a particularly hostile environment (such as
1128LOADLIN, which runs under DOS) it may be impossible to follow the
1129standard memory location requirements. Such a boot loader may use the
1130following hooks that, if set, are invoked by the kernel at the
1131appropriate time. The use of these hooks should probably be
1132considered an absolutely last resort!
1133
1134IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
1135%edi across invocation.
1136
1137 realmode_swtch:
1138 A 16-bit real mode far subroutine invoked immediately before
1139 entering protected mode. The default routine disables NMI, so
1140 your routine should probably do so, too.
1141
1142 code32_start:
1143 A 32-bit flat-mode routine *jumped* to immediately after the
1144 transition to protected mode, but before the kernel is
1145 uncompressed. No segments, except CS, are guaranteed to be
1146 set up (current kernels do, but older ones do not); you should
1147 set them up to BOOT_DS (0x18) yourself.
1148
1149 After completing your hook, you should jump to the address
1150 that was in this field before your boot loader overwrote it
1151 (relocated, if appropriate.)
1152
1153
115432-bit Boot Protocol
1155====================
1156
1157For machine with some new BIOS other than legacy BIOS, such as EFI,
1158LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
1159based on legacy BIOS can not be used, so a 32-bit boot protocol needs
1160to be defined.
1161
1162In 32-bit boot protocol, the first step in loading a Linux kernel
1163should be to setup the boot parameters (struct boot_params,
1164traditionally known as "zero page"). The memory for struct boot_params
1165should be allocated and initialized to all zero. Then the setup header
1166from offset 0x01f1 of kernel image on should be loaded into struct
1167boot_params and examined. The end of setup header can be calculated as
1168follow::
1169
1170 0x0202 + byte value at offset 0x0201
1171
1172In addition to read/modify/write the setup header of the struct
1173boot_params as that of 16-bit boot protocol, the boot loader should
1174also fill the additional fields of the struct boot_params as that
1175described in zero-page.txt.
1176
1177After setting up the struct boot_params, the boot loader can load the
117832/64-bit kernel in the same way as that of 16-bit boot protocol.
1179
1180In 32-bit boot protocol, the kernel is started by jumping to the
118132-bit kernel entry point, which is the start address of loaded
118232/64-bit kernel.
1183
1184At entry, the CPU must be in 32-bit protected mode with paging
1185disabled; a GDT must be loaded with the descriptors for selectors
1186__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1187segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1188must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1189must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
1190address of the struct boot_params; %ebp, %edi and %ebx must be zero.
1191
119264-bit Boot Protocol
1193====================
1194
1195For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader
1196and we need a 64-bit boot protocol.
1197
1198In 64-bit boot protocol, the first step in loading a Linux kernel
1199should be to setup the boot parameters (struct boot_params,
1200traditionally known as "zero page"). The memory for struct boot_params
1201could be allocated anywhere (even above 4G) and initialized to all zero.
1202Then, the setup header at offset 0x01f1 of kernel image on should be
1203loaded into struct boot_params and examined. The end of setup header
1204can be calculated as follows::
1205
1206 0x0202 + byte value at offset 0x0201
1207
1208In addition to read/modify/write the setup header of the struct
1209boot_params as that of 16-bit boot protocol, the boot loader should
1210also fill the additional fields of the struct boot_params as described
1211in zero-page.txt.
1212
1213After setting up the struct boot_params, the boot loader can load
121464-bit kernel in the same way as that of 16-bit boot protocol, but
1215kernel could be loaded above 4G.
1216
1217In 64-bit boot protocol, the kernel is started by jumping to the
121864-bit kernel entry point, which is the start address of loaded
121964-bit kernel plus 0x200.
1220
1221At entry, the CPU must be in 64-bit mode with paging enabled.
1222The range with setup_header.init_size from start address of loaded
1223kernel and zero page and command line buffer get ident mapping;
1224a GDT must be loaded with the descriptors for selectors
1225__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1226segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1227must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1228must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base
1229address of the struct boot_params.
1230
1231EFI Handover Protocol
1232=====================
1233
1234This protocol allows boot loaders to defer initialisation to the EFI
1235boot stub. The boot loader is required to load the kernel/initrd(s)
1236from the boot media and jump to the EFI handover protocol entry point
1237which is hdr->handover_offset bytes from the beginning of
1238startup_{32,64}.
1239
1240The function prototype for the handover entry point looks like this::
1241
1242 efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp)
1243
1244'handle' is the EFI image handle passed to the boot loader by the EFI
1245firmware, 'table' is the EFI system table - these are the first two
1246arguments of the "handoff state" as described in section 2.3 of the
1247UEFI specification. 'bp' is the boot loader-allocated boot params.
1248
1249The boot loader *must* fill out the following fields in bp::
1250
1251 - hdr.code32_start
1252 - hdr.cmd_line_ptr
1253 - hdr.ramdisk_image (if applicable)
1254 - hdr.ramdisk_size (if applicable)
1255
1256All other fields should be zero.