| blob | 08a2f100c0e6ac3a402f9a81b69eeac7d3c0d270 |
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
71 ============= ============================================================
74 Memory Layout
75 =============
77 The traditional memory map for the kernel loader, used for Image or
78 zImage kernels, typically looks like::
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 +------------------------+
102 When using bzImage, the protected-mode kernel was relocated to
103 0x100000 ("high memory"), and the kernel real-mode block (boot sector,
104 setup, and stack/heap) was made relocatable to any address between
105 0x10000 and end of low memory. Unfortunately, in protocols 2.00 and
106 2.01 the 0x90000+ memory range is still used internally by the kernel;
107 the 2.02 protocol resolves that problem.
109 It is desirable to keep the "memory ceiling" -- the highest point in
110 low memory touched by the boot loader -- as low as possible, since
111 some newer BIOSes have begun to allocate some rather large amounts of
112 memory, called the Extended BIOS Data Area, near the top of low
113 memory. The boot loader should use the "INT 12h" BIOS call to verify
114 how much low memory is available.
116 Unfortunately, if INT 12h reports that the amount of memory is too
117 low, there is usually nothing the boot loader can do but to report an
118 error to the user. The boot loader should therefore be designed to
119 take up as little space in low memory as it reasonably can. For
120 zImage or old bzImage kernels, which need data written into the
121 0x90000 segment, the boot loader should make sure not to use memory
122 above the 0x9A000 point; too many BIOSes will break above that point.
124 For a modern bzImage kernel with boot protocol version >= 2.02, a
125 memory layout like the following is suggested::
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 +------------------------+
150 ... where the address X is as low as the design of the boot loader permits.
153 The Real-Mode Kernel Header
154 ===========================
156 In the following text, and anywhere in the kernel boot sequence, "a
157 sector" refers to 512 bytes. It is independent of the actual sector
158 size of the underlying medium.
160 The first step in loading a Linux kernel should be to load the
161 real-mode code (boot sector and setup code) and then examine the
162 following header at offset 0x01f1. The real-mode code can total up to
163 32K, although the boot loader may choose to load only the first two
164 sectors (1K) and then examine the bootup sector size.
166 The header looks like:
168 =========== ======== ===================== ============================================
169 Offset/Size Proto Name Meaning
170 =========== ======== ===================== ============================================
171 01F1/1 ALL(1) setup_sects The size of the setup in sectors
172 01F2/2 ALL root_flags If set, the root is mounted readonly
173 01F4/4 2.04+(2) syssize The size of the 32-bit code in 16-byte paras
174 01F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
175 01FA/2 ALL vid_mode Video mode control
176 01FC/2 ALL root_dev Default root device number
177 01FE/2 ALL boot_flag 0xAA55 magic number
178 0200/2 2.00+ jump Jump instruction
179 0202/4 2.00+ header Magic signature "HdrS"
180 0206/2 2.00+ version Boot protocol version supported
181 0208/4 2.00+ realmode_swtch Boot loader hook (see below)
182 020C/2 2.00+ start_sys_seg The load-low segment (0x1000) (obsolete)
183 020E/2 2.00+ kernel_version Pointer to kernel version string
184 0210/1 2.00+ type_of_loader Boot loader identifier
185 0211/1 2.00+ loadflags Boot protocol option flags
186 0212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
187 0214/4 2.00+ code32_start Boot loader hook (see below)
188 0218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
189 021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
190 0220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
191 0224/2 2.01+ heap_end_ptr Free memory after setup end
192 0226/1 2.02+(3) ext_loader_ver Extended boot loader version
193 0227/1 2.02+(3) ext_loader_type Extended boot loader ID
194 0228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
195 022C/4 2.03+ initrd_addr_max Highest legal initrd address
196 0230/4 2.05+ kernel_alignment Physical addr alignment required for kernel
197 0234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
198 0235/1 2.10+ min_alignment Minimum alignment, as a power of two
199 0236/2 2.12+ xloadflags Boot protocol option flags
200 0238/4 2.06+ cmdline_size Maximum size of the kernel command line
201 023C/4 2.07+ hardware_subarch Hardware subarchitecture
202 0240/8 2.07+ hardware_subarch_data Subarchitecture-specific data
203 0248/4 2.08+ payload_offset Offset of kernel payload
204 024C/4 2.08+ payload_length Length of kernel payload
205 0250/8 2.09+ setup_data 64-bit physical pointer to linked list
206 of struct setup_data
207 0258/8 2.10+ pref_address Preferred loading address
208 0260/4 2.10+ init_size Linear memory required during initialization
209 0264/4 2.11+ handover_offset Offset of handover entry point
210 =========== ======== ===================== ============================================
212 .. note::
213 (1) For backwards compatibility, if the setup_sects field contains 0, the
214 real value is 4.
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.
220 (3) Ignored, but safe to set, for boot protocols 2.02-2.09.
222 If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
223 the boot protocol version is "old". Loading an old kernel, the
224 following parameters should be assumed::
226 Image type = zImage
227 initrd not supported
228 Real-mode kernel must be located at 0x90000.
230 Otherwise, the "version" field contains the protocol version,
231 e.g. protocol version 2.01 will contain 0x0201 in this field. When
232 setting fields in the header, you must make sure only to set fields
233 supported by the protocol version in use.
236 Details of Harder Fileds
237 ========================
239 For 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
242 bootloader ("modify").
244 All general purpose boot loaders should write the fields marked
245 (obligatory). Boot loaders who want to load the kernel at a
246 nonstandard address should fill in the fields marked (reloc); other
247 boot loaders can ignore those fields.
249 The byte order of all fields is littleendian (this is x86, after all.)
251 ============ ===========
252 Field name: setup_sects
253 Type: read
254 Offset/size: 0x1f1/1
255 Protocol: ALL
256 ============ ===========
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.
262 ============ =================
263 Field name: root_flags
264 Type: modify (optional)
265 Offset/size: 0x1f2/2
266 Protocol: ALL
267 ============ =================
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.
273 ============ ===============================================
274 Field name: syssize
275 Type: read
276 Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
277 Protocol: 2.04+
278 ============ ===============================================
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.
285 ============ ===============
286 Field name: ram_size
287 Type: kernel internal
288 Offset/size: 0x1f8/2
289 Protocol: ALL
290 ============ ===============
292 This field is obsolete.
294 ============ ===================
295 Field name: vid_mode
296 Type: modify (obligatory)
297 Offset/size: 0x1fa/2
298 ============ ===================
300 Please see the section on SPECIAL COMMAND LINE OPTIONS.
302 ============ =================
303 Field name: root_dev
304 Type: modify (optional)
305 Offset/size: 0x1fc/2
306 Protocol: ALL
307 ============ =================
309 The default root device device number. The use of this field is
310 deprecated, use the "root=" option on the command line instead.
312 ============ =========
313 Field name: boot_flag
314 Type: read
315 Offset/size: 0x1fe/2
316 Protocol: ALL
317 ============ =========
319 Contains 0xAA55. This is the closest thing old Linux kernels have
320 to a magic number.
322 ============ =======
323 Field name: jump
324 Type: read
325 Offset/size: 0x200/2
326 Protocol: 2.00+
327 ============ =======
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.
333 ============ =======
334 Field name: header
335 Type: read
336 Offset/size: 0x202/4
337 Protocol: 2.00+
338 ============ =======
340 Contains the magic number "HdrS" (0x53726448).
342 ============ =======
343 Field name: version
344 Type: read
345 Offset/size: 0x206/2
346 Protocol: 2.00+
347 ============ =======
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.
353 ============ =================
354 Field name: realmode_swtch
355 Type: modify (optional)
356 Offset/size: 0x208/4
357 Protocol: 2.00+
358 ============ =================
360 Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
362 ============ =============
363 Field name: start_sys_seg
364 Type: read
365 Offset/size: 0x20c/2
366 Protocol: 2.00+
367 ============ =============
369 The load low segment (0x1000). Obsolete.
371 ============ ==============
372 Field name: kernel_version
373 Type: read
374 Offset/size: 0x20e/2
375 Protocol: 2.00+
376 ============ ==============
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).
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::
388 0x1c00 < 15*0x200 (= 0x1e00) but
389 0x1c00 >= 14*0x200 (= 0x1c00)
391 0x1c00 >> 9 = 14, So the minimum value for setup_secs is 15.
393 ============ ==================
394 Field name: type_of_loader
395 Type: write (obligatory)
396 Offset/size: 0x210/1
397 Protocol: 2.00+
398 ============ ==================
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.
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.
409 For example, for T = 0x15, V = 0x234, write::
411 type_of_loader <- 0xE4
412 ext_loader_type <- 0x05
413 ext_loader_ver <- 0x23
415 Assigned boot loader ids (hexadecimal):
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 == =======================================
441 Please contact <hpa@zytor.com> if you need a bootloader ID value assigned.
443 ============ ===================
444 Field name: loadflags
445 Type: modify (obligatory)
446 Offset/size: 0x211/1
447 Protocol: 2.00+
448 ============ ===================
450 This field is a bitmask.
452 Bit 0 (read): LOADED_HIGH
454 - If 0, the protected-mode code is loaded at 0x10000.
455 - If 1, the protected-mode code is loaded at 0x100000.
457 Bit 1 (kernel internal): KASLR_FLAG
459 - Used internally by the compressed kernel to communicate
460 KASLR status to kernel proper.
462 - If 1, KASLR enabled.
463 - If 0, KASLR disabled.
465 Bit 5 (write): QUIET_FLAG
467 - If 0, print early messages.
468 - If 1, suppress early messages.
470 This requests to the kernel (decompressor and early
471 kernel) to not write early messages that require
472 accessing the display hardware directly.
474 Bit 6 (write): KEEP_SEGMENTS
476 Protocol: 2.07+
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.
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).
484 Bit 7 (write): CAN_USE_HEAP
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.
491 ============ ===================
492 Field name: setup_move_size
493 Type: modify (obligatory)
494 Offset/size: 0x212/2
495 Protocol: 2.00-2.01
496 ============ ===================
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.
504 The unit is bytes starting with the beginning of the boot sector.
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.
509 ============ ========================
510 Field name: code32_start
511 Type: modify (optional, reloc)
512 Offset/size: 0x214/4
513 Protocol: 2.00+
514 ============ ========================
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.
520 This field can be modified for two purposes:
522 1. as a boot loader hook (see Advanced Boot Loader Hooks below.)
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.
528 ============ ==================
529 Field name: ramdisk_image
530 Type: write (obligatory)
531 Offset/size: 0x218/4
532 Protocol: 2.00+
533 ============ ==================
535 The 32-bit linear address of the initial ramdisk or ramfs. Leave at
536 zero if there is no initial ramdisk/ramfs.
538 ============ ==================
539 Field name: ramdisk_size
540 Type: write (obligatory)
541 Offset/size: 0x21c/4
542 Protocol: 2.00+
543 ============ ==================
545 Size of the initial ramdisk or ramfs. Leave at zero if there is no
546 initial ramdisk/ramfs.
548 ============ ===============
549 Field name: bootsect_kludge
550 Type: kernel internal
551 Offset/size: 0x220/4
552 Protocol: 2.00+
553 ============ ===============
555 This field is obsolete.
557 ============ ==================
558 Field name: heap_end_ptr
559 Type: write (obligatory)
560 Offset/size: 0x224/2
561 Protocol: 2.01+
562 ============ ==================
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.
567 ============ ================
568 Field name: ext_loader_ver
569 Type: write (optional)
570 Offset/size: 0x226/1
571 Protocol: 2.02+
572 ============ ================
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).
578 The use of this field is boot loader specific. If not written, it
579 is zero.
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.
584 ============ =====================================================
585 Field name: ext_loader_type
586 Type: write (obligatory if (type_of_loader & 0xf0) == 0xe0)
587 Offset/size: 0x227/1
588 Protocol: 2.02+
589 ============ =====================================================
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).
595 This field is ignored if the type in type_of_loader is not 0xE.
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.
600 ============ ==================
601 Field name: cmd_line_ptr
602 Type: write (obligatory)
603 Offset/size: 0x228/4
604 Protocol: 2.02+
605 ============ ==================
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.
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.
618 ============ ===============
619 Field name: initrd_addr_max
620 Type: read
621 Offset/size: 0x22c/4
622 Protocol: 2.03+
623 ============ ===============
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.)
632 ============ ============================
633 Field name: kernel_alignment
634 Type: read/modify (reloc)
635 Offset/size: 0x230/4
636 Protocol: 2.05+ (read), 2.10+ (modify)
637 ============ ============================
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.
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.
649 ============ ==================
650 Field name: relocatable_kernel
651 Type: read (reloc)
652 Offset/size: 0x234/1
653 Protocol: 2.05+
654 ============ ==================
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.
661 ============ =============
662 Field name: min_alignment
663 Type: read (reloc)
664 Offset/size: 0x235/1
665 Protocol: 2.10+
666 ============ =============
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::
673 kernel_alignment = 1 << min_alignment
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.
679 ============ ==========
680 Field name: xloadflags
681 Type: read
682 Offset/size: 0x236/2
683 Protocol: 2.12+
684 ============ ==========
686 This field is a bitmask.
688 Bit 0 (read): XLF_KERNEL_64
690 - If 1, this kernel has the legacy 64-bit entry point at 0x200.
692 Bit 1 (read): XLF_CAN_BE_LOADED_ABOVE_4G
694 - If 1, kernel/boot_params/cmdline/ramdisk can be above 4G.
696 Bit 2 (read): XLF_EFI_HANDOVER_32
698 - If 1, the kernel supports the 32-bit EFI handoff entry point
699 given at handover_offset.
701 Bit 3 (read): XLF_EFI_HANDOVER_64
703 - If 1, the kernel supports the 64-bit EFI handoff entry point
704 given at handover_offset + 0x200.
706 Bit 4 (read): XLF_EFI_KEXEC
708 - If 1, the kernel supports kexec EFI boot with EFI runtime support.
711 ============ ============
712 Field name: cmdline_size
713 Type: read
714 Offset/size: 0x238/4
715 Protocol: 2.06+
716 ============ ============
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.
723 ============ ====================================
724 Field name: hardware_subarch
725 Type: write (optional, defaults to x86/PC)
726 Offset/size: 0x23c/4
727 Protocol: 2.07+
728 ============ ====================================
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.
734 This field allows the bootloader to inform the kernel we are in one
735 one of those environments.
737 ========== ==============================
738 0x00000000 The default x86/PC environment
739 0x00000001 lguest
740 0x00000002 Xen
741 0x00000003 Moorestown MID
742 0x00000004 CE4100 TV Platform
743 ========== ==============================
745 ============ =========================
746 Field name: hardware_subarch_data
747 Type: write (subarch-dependent)
748 Offset/size: 0x240/8
749 Protocol: 2.07+
750 ============ =========================
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.
756 ============ ==============
757 Field name: payload_offset
758 Type: read
759 Offset/size: 0x248/4
760 Protocol: 2.08+
761 ============ ==============
763 If non-zero then this field contains the offset from the beginning
764 of the protected-mode code to the payload.
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).
774 ============ ==============
775 Field name: payload_length
776 Type: read
777 Offset/size: 0x24c/4
778 Protocol: 2.08+
779 ============ ==============
781 The length of the payload.
783 ============ ===============
784 Field name: setup_data
785 Type: write (special)
786 Offset/size: 0x250/8
787 Protocol: 2.09+
788 ============ ===============
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::
795 struct setup_data {
796 u64 next;
797 u32 type;
798 u32 len;
799 u8 data[0];
800 };
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.
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.
812 ============ ============
813 Field name: pref_address
814 Type: read (reloc)
815 Offset/size: 0x258/8
816 Protocol: 2.10+
817 ============ ============
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.
823 A non-relocatable kernel will unconditionally move itself and to run
824 at this address.
826 ============ =======
827 Field name: init_size
828 Type: read
829 Offset/size: 0x260/4
830 ============ =======
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.
839 The kernel runtime start address is determined by the following algorithm::
841 if (relocatable_kernel)
842 runtime_start = align_up(load_address, kernel_alignment)
843 else
844 runtime_start = pref_address
846 ============ ===============
847 Field name: handover_offset
848 Type: read
849 Offset/size: 0x264/4
850 ============ ===============
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.
856 See EFI HANDOVER PROTOCOL below for more details.
859 The Image Checksum
860 ==================
862 From boot protocol version 2.08 onwards the CRC-32 is calculated over
863 the entire file using the characteristic polynomial 0x04C11DB7 and an
864 initial remainder of 0xffffffff. The checksum is appended to the
865 file; therefore the CRC of the file up to the limit specified in the
866 syssize field of the header is always 0.
869 The Kernel Command Line
870 =======================
872 The kernel command line has become an important way for the boot
873 loader to communicate with the kernel. Some of its options are also
874 relevant to the boot loader itself, see "special command line options"
875 below.
877 The kernel command line is a null-terminated string. The maximum
878 length can be retrieved from the field cmdline_size. Before protocol
879 version 2.06, the maximum was 255 characters. A string that is too
880 long will be automatically truncated by the kernel.
882 If the boot protocol version is 2.02 or later, the address of the
883 kernel command line is given by the header field cmd_line_ptr (see
884 above.) This address can be anywhere between the end of the setup
885 heap and 0xA0000.
887 If the protocol version is *not* 2.02 or higher, the kernel
888 command line is entered using the following protocol:
890 - At offset 0x0020 (word), "cmd_line_magic", enter the magic
891 number 0xA33F.
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).
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.
902 Memory Layout of The Real-Mode Code
903 ===================================
905 The real-mode code requires a stack/heap to be set up, as well as
906 memory allocated for the kernel command line. This needs to be done
907 in the real-mode accessible memory in bottom megabyte.
909 It should be noted that modern machines often have a sizable Extended
910 BIOS Data Area (EBDA). As a result, it is advisable to use as little
911 of the low megabyte as possible.
913 Unfortunately, under the following circumstances the 0x90000 memory
914 segment has to be used:
916 - When loading a zImage kernel ((loadflags & 0x01) == 0).
917 - When loading a 2.01 or earlier boot protocol kernel.
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.
925 When loading at 0x90000, avoid using memory above 0x9a000.
927 For boot protocol 2.02 or higher, the command line does not have to be
928 located in the same 64K segment as the real-mode setup code; it is
929 thus permitted to give the stack/heap the full 64K segment and locate
930 the command line above it.
932 The kernel command line should not be located below the real-mode
933 code, nor should it be located in high memory.
936 Sample Boot Configuartion
937 =========================
939 As a sample configuration, assume the following layout of the real
940 mode segment.
942 When loading below 0x90000, use the entire segment:
944 ============= ===================
945 0x0000-0x7fff Real mode kernel
946 0x8000-0xdfff Stack and heap
947 0xe000-0xffff Kernel command line
948 ============= ===================
950 When loading at 0x90000 OR the protocol version is 2.01 or earlier:
952 ============= ===================
953 0x0000-0x7fff Real mode kernel
954 0x8000-0x97ff Stack and heap
955 0x9800-0x9fff Kernel command line
956 ============= ===================
958 Such a boot loader should enter the following fields in the header::
960 unsigned long base_ptr; /* base address for real-mode segment */
962 if ( setup_sects == 0 ) {
963 setup_sects = 4;
964 }
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 }
973 if ( protocol >= 0x0202 && loadflags & 0x01 )
974 heap_end = 0xe000;
975 else
976 heap_end = 0x9800;
978 if ( protocol >= 0x0201 ) {
979 heap_end_ptr = heap_end - 0x200;
980 loadflags |= 0x80; /* CAN_USE_HEAP */
981 }
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 */
995 heap_end = 0x9800;
997 cmd_line_magic = 0xA33F;
998 cmd_line_offset = heap_end;
1000 /* A very old kernel MUST have its real-mode code
1001 loaded at 0x90000 */
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 }
1009 strcpy(0x90000+cmd_line_offset, cmdline);
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 }
1017 Loading The Rest of The Kernel
1018 ==============================
1020 The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
1021 in the kernel file (again, if setup_sects == 0 the real value is 4.)
1022 It should be loaded at address 0x10000 for Image/zImage kernels and
1023 0x100000 for bzImage kernels.
1025 The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
1026 bit (LOAD_HIGH) in the loadflags field is set::
1028 is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
1029 load_address = is_bzImage ? 0x100000 : 0x10000;
1031 Note that Image/zImage kernels can be up to 512K in size, and thus use
1032 the entire 0x10000-0x90000 range of memory. This means it is pretty
1033 much a requirement for these kernels to load the real-mode part at
1034 0x90000. bzImage kernels allow much more flexibility.
1036 Special Command Line Options
1037 ============================
1039 If the command line provided by the boot loader is entered by the
1040 user, the user may expect the following command line options to work.
1041 They should normally not be deleted from the kernel command line even
1042 though not all of them are actually meaningful to the kernel. Boot
1043 loader authors who need additional command line options for the boot
1044 loader itself should get them registered in
1045 Documentation/admin-guide/kernel-parameters.rst to make sure they will not
1046 conflict with actual kernel options now or in the future.
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.
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!
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.
1070 In addition, some boot loaders add the following options to the
1071 user-specified command line:
1073 BOOT_IMAGE=<file>
1074 The boot image which was loaded. Again, the meaning of <file>
1075 is obviously bootloader-dependent.
1077 auto
1078 The kernel was booted without explicit user intervention.
1080 If these options are added by the boot loader, it is highly
1081 recommended that they are located *first*, before the user-specified
1082 or configuration-specified command line. Otherwise, "init=/bin/sh"
1083 gets confused by the "auto" option.
1086 Running the Kernel
1087 ==================
1089 The kernel is started by jumping to the kernel entry point, which is
1090 located at *segment* offset 0x20 from the start of the real mode
1091 kernel. This means that if you loaded your real-mode kernel code at
1092 0x90000, the kernel entry point is 9020:0000.
1094 At entry, ds = es = ss should point to the start of the real-mode
1095 kernel code (0x9000 if the code is loaded at 0x90000), sp should be
1096 set up properly, normally pointing to the top of the heap, and
1097 interrupts should be disabled. Furthermore, to guard against bugs in
1098 the kernel, it is recommended that the boot loader sets fs = gs = ds =
1099 es = ss.
1101 In our example from above, we would do::
1103 /* Note: in the case of the "old" kernel protocol, base_ptr must
1104 be == 0x90000 at this point; see the previous sample code */
1106 seg = base_ptr >> 4;
1108 cli(); /* Enter with interrupts disabled! */
1110 /* Set up the real-mode kernel stack */
1111 _SS = seg;
1112 _SP = heap_end;
1114 _DS = _ES = _FS = _GS = seg;
1115 jmp_far(seg+0x20, 0); /* Run the kernel */
1117 If your boot sector accesses a floppy drive, it is recommended to
1118 switch off the floppy motor before running the kernel, since the
1119 kernel boot leaves interrupts off and thus the motor will not be
1120 switched off, especially if the loaded kernel has the floppy driver as
1121 a demand-loaded module!
1124 Advanced Boot Loader Hooks
1125 ==========================
1127 If the boot loader runs in a particularly hostile environment (such as
1128 LOADLIN, which runs under DOS) it may be impossible to follow the
1129 standard memory location requirements. Such a boot loader may use the
1130 following hooks that, if set, are invoked by the kernel at the
1131 appropriate time. The use of these hooks should probably be
1132 considered an absolutely last resort!
1134 IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
1135 %edi across invocation.
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.
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.
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.)
1154 32-bit Boot Protocol
1155 ====================
1157 For machine with some new BIOS other than legacy BIOS, such as EFI,
1158 LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
1159 based on legacy BIOS can not be used, so a 32-bit boot protocol needs
1160 to be defined.
1162 In 32-bit boot protocol, the first step in loading a Linux kernel
1163 should be to setup the boot parameters (struct boot_params,
1164 traditionally known as "zero page"). The memory for struct boot_params
1165 should be allocated and initialized to all zero. Then the setup header
1166 from offset 0x01f1 of kernel image on should be loaded into struct
1167 boot_params and examined. The end of setup header can be calculated as
1168 follow::
1170 0x0202 + byte value at offset 0x0201
1172 In addition to read/modify/write the setup header of the struct
1173 boot_params as that of 16-bit boot protocol, the boot loader should
1174 also fill the additional fields of the struct boot_params as that
1175 described in zero-page.txt.
1177 After setting up the struct boot_params, the boot loader can load the
1178 32/64-bit kernel in the same way as that of 16-bit boot protocol.
1180 In 32-bit boot protocol, the kernel is started by jumping to the
1181 32-bit kernel entry point, which is the start address of loaded
1182 32/64-bit kernel.
1184 At entry, the CPU must be in 32-bit protected mode with paging
1185 disabled; a GDT must be loaded with the descriptors for selectors
1186 __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1187 segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1188 must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1189 must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
1190 address of the struct boot_params; %ebp, %edi and %ebx must be zero.
1192 64-bit Boot Protocol
1193 ====================
1195 For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader
1196 and we need a 64-bit boot protocol.
1198 In 64-bit boot protocol, the first step in loading a Linux kernel
1199 should be to setup the boot parameters (struct boot_params,
1200 traditionally known as "zero page"). The memory for struct boot_params
1201 could be allocated anywhere (even above 4G) and initialized to all zero.
1202 Then, the setup header at offset 0x01f1 of kernel image on should be
1203 loaded into struct boot_params and examined. The end of setup header
1204 can be calculated as follows::
1206 0x0202 + byte value at offset 0x0201
1208 In addition to read/modify/write the setup header of the struct
1209 boot_params as that of 16-bit boot protocol, the boot loader should
1210 also fill the additional fields of the struct boot_params as described
1211 in zero-page.txt.
1213 After setting up the struct boot_params, the boot loader can load
1214 64-bit kernel in the same way as that of 16-bit boot protocol, but
1215 kernel could be loaded above 4G.
1217 In 64-bit boot protocol, the kernel is started by jumping to the
1218 64-bit kernel entry point, which is the start address of loaded
1219 64-bit kernel plus 0x200.
1221 At entry, the CPU must be in 64-bit mode with paging enabled.
1222 The range with setup_header.init_size from start address of loaded
1223 kernel and zero page and command line buffer get ident mapping;
1224 a GDT must be loaded with the descriptors for selectors
1225 __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1226 segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1227 must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1228 must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base
1229 address of the struct boot_params.
1231 EFI Handover Protocol
1232 =====================
1234 This protocol allows boot loaders to defer initialisation to the EFI
1235 boot stub. The boot loader is required to load the kernel/initrd(s)
1236 from the boot media and jump to the EFI handover protocol entry point
1237 which is hdr->handover_offset bytes from the beginning of
1238 startup_{32,64}.
1240 The function prototype for the handover entry point looks like this::
1242 efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp)
1244 'handle' is the EFI image handle passed to the boot loader by the EFI
1245 firmware, 'table' is the EFI system table - these are the first two
1246 arguments of the "handoff state" as described in section 2.3 of the
1247 UEFI specification. 'bp' is the boot loader-allocated boot params.
1249 The boot loader *must* fill out the following fields in bp::
1251 - hdr.code32_start
1252 - hdr.cmd_line_ptr
1253 - hdr.ramdisk_image (if applicable)
1254 - hdr.ramdisk_size (if applicable)
1256 All other fields should be zero.