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1 THE LINUX/x86 BOOT PROTOCOL
2 ---------------------------
3
4On the x86 platform, the Linux kernel uses a rather complicated boot
5convention. This has evolved partially due to historical aspects, as
6well as the desire in the early days to have the kernel itself be a
7bootable image, the complicated PC memory model and due to changed
8expectations in the PC industry caused by the effective demise of
9real-mode DOS as a mainstream operating system.
10
11Currently, the following versions of the Linux/x86 boot protocol exist.
12
13Old kernels: zImage/Image support only. Some very early kernels
14 may not even support a command line.
15
16Protocol 2.00: (Kernel 1.3.73) Added bzImage and initrd support, as
17 well as a formalized way to communicate between the
18 boot loader and the kernel. setup.S made relocatable,
19 although the traditional setup area still assumed
20 writable.
21
22Protocol 2.01: (Kernel 1.3.76) Added a heap overrun warning.
23
24Protocol 2.02: (Kernel 2.4.0-test3-pre3) New command line protocol.
25 Lower the conventional memory ceiling. No overwrite
26 of the traditional setup area, thus making booting
27 safe for systems which use the EBDA from SMM or 32-bit
28 BIOS entry points. zImage deprecated but still
29 supported.
30
31Protocol 2.03: (Kernel 2.4.18-pre1) Explicitly makes the highest possible
32 initrd address available to the bootloader.
33
34Protocol 2.04: (Kernel 2.6.14) Extend the syssize field to four bytes.
35
36Protocol 2.05: (Kernel 2.6.20) Make protected mode kernel relocatable.
37 Introduce relocatable_kernel and kernel_alignment fields.
38
39Protocol 2.06: (Kernel 2.6.22) Added a field that contains the size of
40 the boot command line.
41
42Protocol 2.07: (Kernel 2.6.24) Added paravirtualised boot protocol.
43 Introduced hardware_subarch and hardware_subarch_data
44 and KEEP_SEGMENTS flag in load_flags.
45
46Protocol 2.08: (Kernel 2.6.26) Added crc32 checksum and ELF format
47 payload. Introduced payload_offset and payload_length
48 fields to aid in locating the payload.
49
50Protocol 2.09: (Kernel 2.6.26) Added a field of 64-bit physical
51 pointer to single linked list of struct setup_data.
52
53Protocol 2.10: (Kernel 2.6.31) Added a protocol for relaxed alignment
54 beyond the kernel_alignment added, new init_size and
55 pref_address fields. Added extended boot loader IDs.
56
57Protocol 2.11: (Kernel 3.6) Added a field for offset of EFI handover
58 protocol entry point.
59
60Protocol 2.12: (Kernel 3.8) Added the xloadflags field and extension fields
61 to struct boot_params for loading bzImage and ramdisk
62 above 4G in 64bit.
63
64**** MEMORY LAYOUT
65
66The traditional memory map for the kernel loader, used for Image or
67zImage kernels, typically looks like:
68
69 | |
700A0000 +------------------------+
71 | Reserved for BIOS | Do not use. Reserved for BIOS EBDA.
7209A000 +------------------------+
73 | Command line |
74 | Stack/heap | For use by the kernel real-mode code.
75098000 +------------------------+
76 | Kernel setup | The kernel real-mode code.
77090200 +------------------------+
78 | Kernel boot sector | The kernel legacy boot sector.
79090000 +------------------------+
80 | Protected-mode kernel | The bulk of the kernel image.
81010000 +------------------------+
82 | Boot loader | <- Boot sector entry point 0000:7C00
83001000 +------------------------+
84 | Reserved for MBR/BIOS |
85000800 +------------------------+
86 | Typically used by MBR |
87000600 +------------------------+
88 | BIOS use only |
89000000 +------------------------+
90
91
92When using bzImage, the protected-mode kernel was relocated to
930x100000 ("high memory"), and the kernel real-mode block (boot sector,
94setup, and stack/heap) was made relocatable to any address between
950x10000 and end of low memory. Unfortunately, in protocols 2.00 and
962.01 the 0x90000+ memory range is still used internally by the kernel;
97the 2.02 protocol resolves that problem.
98
99It is desirable to keep the "memory ceiling" -- the highest point in
100low memory touched by the boot loader -- as low as possible, since
101some newer BIOSes have begun to allocate some rather large amounts of
102memory, called the Extended BIOS Data Area, near the top of low
103memory. The boot loader should use the "INT 12h" BIOS call to verify
104how much low memory is available.
105
106Unfortunately, if INT 12h reports that the amount of memory is too
107low, there is usually nothing the boot loader can do but to report an
108error to the user. The boot loader should therefore be designed to
109take up as little space in low memory as it reasonably can. For
110zImage or old bzImage kernels, which need data written into the
1110x90000 segment, the boot loader should make sure not to use memory
112above the 0x9A000 point; too many BIOSes will break above that point.
113
114For a modern bzImage kernel with boot protocol version >= 2.02, a
115memory layout like the following is suggested:
116
117 ~ ~
118 | Protected-mode kernel |
119100000 +------------------------+
120 | I/O memory hole |
1210A0000 +------------------------+
122 | Reserved for BIOS | Leave as much as possible unused
123 ~ ~
124 | Command line | (Can also be below the X+10000 mark)
125X+10000 +------------------------+
126 | Stack/heap | For use by the kernel real-mode code.
127X+08000 +------------------------+
128 | Kernel setup | The kernel real-mode code.
129 | Kernel boot sector | The kernel legacy boot sector.
130X +------------------------+
131 | Boot loader | <- Boot sector entry point 0000:7C00
132001000 +------------------------+
133 | Reserved for MBR/BIOS |
134000800 +------------------------+
135 | Typically used by MBR |
136000600 +------------------------+
137 | BIOS use only |
138000000 +------------------------+
139
140... where the address X is as low as the design of the boot loader
141permits.
142
143
144**** THE REAL-MODE KERNEL HEADER
145
146In the following text, and anywhere in the kernel boot sequence, "a
147sector" refers to 512 bytes. It is independent of the actual sector
148size of the underlying medium.
149
150The first step in loading a Linux kernel should be to load the
151real-mode code (boot sector and setup code) and then examine the
152following header at offset 0x01f1. The real-mode code can total up to
15332K, although the boot loader may choose to load only the first two
154sectors (1K) and then examine the bootup sector size.
155
156The header looks like:
157
158Offset Proto Name Meaning
159/Size
160
16101F1/1 ALL(1 setup_sects The size of the setup in sectors
16201F2/2 ALL root_flags If set, the root is mounted readonly
16301F4/4 2.04+(2 syssize The size of the 32-bit code in 16-byte paras
16401F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
16501FA/2 ALL vid_mode Video mode control
16601FC/2 ALL root_dev Default root device number
16701FE/2 ALL boot_flag 0xAA55 magic number
1680200/2 2.00+ jump Jump instruction
1690202/4 2.00+ header Magic signature "HdrS"
1700206/2 2.00+ version Boot protocol version supported
1710208/4 2.00+ realmode_swtch Boot loader hook (see below)
172020C/2 2.00+ start_sys_seg The load-low segment (0x1000) (obsolete)
173020E/2 2.00+ kernel_version Pointer to kernel version string
1740210/1 2.00+ type_of_loader Boot loader identifier
1750211/1 2.00+ loadflags Boot protocol option flags
1760212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
1770214/4 2.00+ code32_start Boot loader hook (see below)
1780218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
179021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
1800220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
1810224/2 2.01+ heap_end_ptr Free memory after setup end
1820226/1 2.02+(3 ext_loader_ver Extended boot loader version
1830227/1 2.02+(3 ext_loader_type Extended boot loader ID
1840228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
185022C/4 2.03+ initrd_addr_max Highest legal initrd address
1860230/4 2.05+ kernel_alignment Physical addr alignment required for kernel
1870234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
1880235/1 2.10+ min_alignment Minimum alignment, as a power of two
1890236/2 2.12+ xloadflags Boot protocol option flags
1900238/4 2.06+ cmdline_size Maximum size of the kernel command line
191023C/4 2.07+ hardware_subarch Hardware subarchitecture
1920240/8 2.07+ hardware_subarch_data Subarchitecture-specific data
1930248/4 2.08+ payload_offset Offset of kernel payload
194024C/4 2.08+ payload_length Length of kernel payload
1950250/8 2.09+ setup_data 64-bit physical pointer to linked list
196 of struct setup_data
1970258/8 2.10+ pref_address Preferred loading address
1980260/4 2.10+ init_size Linear memory required during initialization
1990264/4 2.11+ handover_offset Offset of handover entry point
200
201(1) For backwards compatibility, if the setup_sects field contains 0, the
202 real value is 4.
203
204(2) For boot protocol prior to 2.04, the upper two bytes of the syssize
205 field are unusable, which means the size of a bzImage kernel
206 cannot be determined.
207
208(3) Ignored, but safe to set, for boot protocols 2.02-2.09.
209
210If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
211the boot protocol version is "old". Loading an old kernel, the
212following parameters should be assumed:
213
214 Image type = zImage
215 initrd not supported
216 Real-mode kernel must be located at 0x90000.
217
218Otherwise, the "version" field contains the protocol version,
219e.g. protocol version 2.01 will contain 0x0201 in this field. When
220setting fields in the header, you must make sure only to set fields
221supported by the protocol version in use.
222
223
224**** DETAILS OF HEADER FIELDS
225
226For each field, some are information from the kernel to the bootloader
227("read"), some are expected to be filled out by the bootloader
228("write"), and some are expected to be read and modified by the
229bootloader ("modify").
230
231All general purpose boot loaders should write the fields marked
232(obligatory). Boot loaders who want to load the kernel at a
233nonstandard address should fill in the fields marked (reloc); other
234boot loaders can ignore those fields.
235
236The byte order of all fields is littleendian (this is x86, after all.)
237
238Field name: setup_sects
239Type: read
240Offset/size: 0x1f1/1
241Protocol: ALL
242
243 The size of the setup code in 512-byte sectors. If this field is
244 0, the real value is 4. The real-mode code consists of the boot
245 sector (always one 512-byte sector) plus the setup code.
246
247Field name: root_flags
248Type: modify (optional)
249Offset/size: 0x1f2/2
250Protocol: ALL
251
252 If this field is nonzero, the root defaults to readonly. The use of
253 this field is deprecated; use the "ro" or "rw" options on the
254 command line instead.
255
256Field name: syssize
257Type: read
258Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
259Protocol: 2.04+
260
261 The size of the protected-mode code in units of 16-byte paragraphs.
262 For protocol versions older than 2.04 this field is only two bytes
263 wide, and therefore cannot be trusted for the size of a kernel if
264 the LOAD_HIGH flag is set.
265
266Field name: ram_size
267Type: kernel internal
268Offset/size: 0x1f8/2
269Protocol: ALL
270
271 This field is obsolete.
272
273Field name: vid_mode
274Type: modify (obligatory)
275Offset/size: 0x1fa/2
276
277 Please see the section on SPECIAL COMMAND LINE OPTIONS.
278
279Field name: root_dev
280Type: modify (optional)
281Offset/size: 0x1fc/2
282Protocol: ALL
283
284 The default root device device number. The use of this field is
285 deprecated, use the "root=" option on the command line instead.
286
287Field name: boot_flag
288Type: read
289Offset/size: 0x1fe/2
290Protocol: ALL
291
292 Contains 0xAA55. This is the closest thing old Linux kernels have
293 to a magic number.
294
295Field name: jump
296Type: read
297Offset/size: 0x200/2
298Protocol: 2.00+
299
300 Contains an x86 jump instruction, 0xEB followed by a signed offset
301 relative to byte 0x202. This can be used to determine the size of
302 the header.
303
304Field name: header
305Type: read
306Offset/size: 0x202/4
307Protocol: 2.00+
308
309 Contains the magic number "HdrS" (0x53726448).
310
311Field name: version
312Type: read
313Offset/size: 0x206/2
314Protocol: 2.00+
315
316 Contains the boot protocol version, in (major << 8)+minor format,
317 e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
318 10.17.
319
320Field name: realmode_swtch
321Type: modify (optional)
322Offset/size: 0x208/4
323Protocol: 2.00+
324
325 Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
326
327Field name: start_sys_seg
328Type: read
329Offset/size: 0x20c/2
330Protocol: 2.00+
331
332 The load low segment (0x1000). Obsolete.
333
334Field name: kernel_version
335Type: read
336Offset/size: 0x20e/2
337Protocol: 2.00+
338
339 If set to a nonzero value, contains a pointer to a NUL-terminated
340 human-readable kernel version number string, less 0x200. This can
341 be used to display the kernel version to the user. This value
342 should be less than (0x200*setup_sects).
343
344 For example, if this value is set to 0x1c00, the kernel version
345 number string can be found at offset 0x1e00 in the kernel file.
346 This is a valid value if and only if the "setup_sects" field
347 contains the value 15 or higher, as:
348
349 0x1c00 < 15*0x200 (= 0x1e00) but
350 0x1c00 >= 14*0x200 (= 0x1c00)
351
352 0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
353
354Field name: type_of_loader
355Type: write (obligatory)
356Offset/size: 0x210/1
357Protocol: 2.00+
358
359 If your boot loader has an assigned id (see table below), enter
360 0xTV here, where T is an identifier for the boot loader and V is
361 a version number. Otherwise, enter 0xFF here.
362
363 For boot loader IDs above T = 0xD, write T = 0xE to this field and
364 write the extended ID minus 0x10 to the ext_loader_type field.
365 Similarly, the ext_loader_ver field can be used to provide more than
366 four bits for the bootloader version.
367
368 For example, for T = 0x15, V = 0x234, write:
369
370 type_of_loader <- 0xE4
371 ext_loader_type <- 0x05
372 ext_loader_ver <- 0x23
373
374 Assigned boot loader ids (hexadecimal):
375
376 0 LILO (0x00 reserved for pre-2.00 bootloader)
377 1 Loadlin
378 2 bootsect-loader (0x20, all other values reserved)
379 3 Syslinux
380 4 Etherboot/gPXE/iPXE
381 5 ELILO
382 7 GRUB
383 8 U-Boot
384 9 Xen
385 A Gujin
386 B Qemu
387 C Arcturus Networks uCbootloader
388 D kexec-tools
389 E Extended (see ext_loader_type)
390 F Special (0xFF = undefined)
391 10 Reserved
392 11 Minimal Linux Bootloader <http://sebastian-plotz.blogspot.de>
393 12 OVMF UEFI virtualization stack
394
395 Please contact <hpa@zytor.com> if you need a bootloader ID
396 value assigned.
397
398Field name: loadflags
399Type: modify (obligatory)
400Offset/size: 0x211/1
401Protocol: 2.00+
402
403 This field is a bitmask.
404
405 Bit 0 (read): LOADED_HIGH
406 - If 0, the protected-mode code is loaded at 0x10000.
407 - If 1, the protected-mode code is loaded at 0x100000.
408
409 Bit 1 (kernel internal): KASLR_FLAG
410 - Used internally by the compressed kernel to communicate
411 KASLR status to kernel proper.
412 If 1, KASLR enabled.
413 If 0, KASLR disabled.
414
415 Bit 5 (write): QUIET_FLAG
416 - If 0, print early messages.
417 - If 1, suppress early messages.
418 This requests to the kernel (decompressor and early
419 kernel) to not write early messages that require
420 accessing the display hardware directly.
421
422 Bit 6 (write): KEEP_SEGMENTS
423 Protocol: 2.07+
424 - If 0, reload the segment registers in the 32bit entry point.
425 - If 1, do not reload the segment registers in the 32bit entry point.
426 Assume that %cs %ds %ss %es are all set to flat segments with
427 a base of 0 (or the equivalent for their environment).
428
429 Bit 7 (write): CAN_USE_HEAP
430 Set this bit to 1 to indicate that the value entered in the
431 heap_end_ptr is valid. If this field is clear, some setup code
432 functionality will be disabled.
433
434Field name: setup_move_size
435Type: modify (obligatory)
436Offset/size: 0x212/2
437Protocol: 2.00-2.01
438
439 When using protocol 2.00 or 2.01, if the real mode kernel is not
440 loaded at 0x90000, it gets moved there later in the loading
441 sequence. Fill in this field if you want additional data (such as
442 the kernel command line) moved in addition to the real-mode kernel
443 itself.
444
445 The unit is bytes starting with the beginning of the boot sector.
446
447 This field is can be ignored when the protocol is 2.02 or higher, or
448 if the real-mode code is loaded at 0x90000.
449
450Field name: code32_start
451Type: modify (optional, reloc)
452Offset/size: 0x214/4
453Protocol: 2.00+
454
455 The address to jump to in protected mode. This defaults to the load
456 address of the kernel, and can be used by the boot loader to
457 determine the proper load address.
458
459 This field can be modified for two purposes:
460
461 1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
462
463 2. if a bootloader which does not install a hook loads a
464 relocatable kernel at a nonstandard address it will have to modify
465 this field to point to the load address.
466
467Field name: ramdisk_image
468Type: write (obligatory)
469Offset/size: 0x218/4
470Protocol: 2.00+
471
472 The 32-bit linear address of the initial ramdisk or ramfs. Leave at
473 zero if there is no initial ramdisk/ramfs.
474
475Field name: ramdisk_size
476Type: write (obligatory)
477Offset/size: 0x21c/4
478Protocol: 2.00+
479
480 Size of the initial ramdisk or ramfs. Leave at zero if there is no
481 initial ramdisk/ramfs.
482
483Field name: bootsect_kludge
484Type: kernel internal
485Offset/size: 0x220/4
486Protocol: 2.00+
487
488 This field is obsolete.
489
490Field name: heap_end_ptr
491Type: write (obligatory)
492Offset/size: 0x224/2
493Protocol: 2.01+
494
495 Set this field to the offset (from the beginning of the real-mode
496 code) of the end of the setup stack/heap, minus 0x0200.
497
498Field name: ext_loader_ver
499Type: write (optional)
500Offset/size: 0x226/1
501Protocol: 2.02+
502
503 This field is used as an extension of the version number in the
504 type_of_loader field. The total version number is considered to be
505 (type_of_loader & 0x0f) + (ext_loader_ver << 4).
506
507 The use of this field is boot loader specific. If not written, it
508 is zero.
509
510 Kernels prior to 2.6.31 did not recognize this field, but it is safe
511 to write for protocol version 2.02 or higher.
512
513Field name: ext_loader_type
514Type: write (obligatory if (type_of_loader & 0xf0) == 0xe0)
515Offset/size: 0x227/1
516Protocol: 2.02+
517
518 This field is used as an extension of the type number in
519 type_of_loader field. If the type in type_of_loader is 0xE, then
520 the actual type is (ext_loader_type + 0x10).
521
522 This field is ignored if the type in type_of_loader is not 0xE.
523
524 Kernels prior to 2.6.31 did not recognize this field, but it is safe
525 to write for protocol version 2.02 or higher.
526
527Field name: cmd_line_ptr
528Type: write (obligatory)
529Offset/size: 0x228/4
530Protocol: 2.02+
531
532 Set this field to the linear address of the kernel command line.
533 The kernel command line can be located anywhere between the end of
534 the setup heap and 0xA0000; it does not have to be located in the
535 same 64K segment as the real-mode code itself.
536
537 Fill in this field even if your boot loader does not support a
538 command line, in which case you can point this to an empty string
539 (or better yet, to the string "auto".) If this field is left at
540 zero, the kernel will assume that your boot loader does not support
541 the 2.02+ protocol.
542
543Field name: initrd_addr_max
544Type: read
545Offset/size: 0x22c/4
546Protocol: 2.03+
547
548 The maximum address that may be occupied by the initial
549 ramdisk/ramfs contents. For boot protocols 2.02 or earlier, this
550 field is not present, and the maximum address is 0x37FFFFFF. (This
551 address is defined as the address of the highest safe byte, so if
552 your ramdisk is exactly 131072 bytes long and this field is
553 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
554
555Field name: kernel_alignment
556Type: read/modify (reloc)
557Offset/size: 0x230/4
558Protocol: 2.05+ (read), 2.10+ (modify)
559
560 Alignment unit required by the kernel (if relocatable_kernel is
561 true.) A relocatable kernel that is loaded at an alignment
562 incompatible with the value in this field will be realigned during
563 kernel initialization.
564
565 Starting with protocol version 2.10, this reflects the kernel
566 alignment preferred for optimal performance; it is possible for the
567 loader to modify this field to permit a lesser alignment. See the
568 min_alignment and pref_address field below.
569
570Field name: relocatable_kernel
571Type: read (reloc)
572Offset/size: 0x234/1
573Protocol: 2.05+
574
575 If this field is nonzero, the protected-mode part of the kernel can
576 be loaded at any address that satisfies the kernel_alignment field.
577 After loading, the boot loader must set the code32_start field to
578 point to the loaded code, or to a boot loader hook.
579
580Field name: min_alignment
581Type: read (reloc)
582Offset/size: 0x235/1
583Protocol: 2.10+
584
585 This field, if nonzero, indicates as a power of two the minimum
586 alignment required, as opposed to preferred, by the kernel to boot.
587 If a boot loader makes use of this field, it should update the
588 kernel_alignment field with the alignment unit desired; typically:
589
590 kernel_alignment = 1 << min_alignment
591
592 There may be a considerable performance cost with an excessively
593 misaligned kernel. Therefore, a loader should typically try each
594 power-of-two alignment from kernel_alignment down to this alignment.
595
596Field name: xloadflags
597Type: read
598Offset/size: 0x236/2
599Protocol: 2.12+
600
601 This field is a bitmask.
602
603 Bit 0 (read): XLF_KERNEL_64
604 - If 1, this kernel has the legacy 64-bit entry point at 0x200.
605
606 Bit 1 (read): XLF_CAN_BE_LOADED_ABOVE_4G
607 - If 1, kernel/boot_params/cmdline/ramdisk can be above 4G.
608
609 Bit 2 (read): XLF_EFI_HANDOVER_32
610 - If 1, the kernel supports the 32-bit EFI handoff entry point
611 given at handover_offset.
612
613 Bit 3 (read): XLF_EFI_HANDOVER_64
614 - If 1, the kernel supports the 64-bit EFI handoff entry point
615 given at handover_offset + 0x200.
616
617 Bit 4 (read): XLF_EFI_KEXEC
618 - If 1, the kernel supports kexec EFI boot with EFI runtime support.
619
620Field name: cmdline_size
621Type: read
622Offset/size: 0x238/4
623Protocol: 2.06+
624
625 The maximum size of the command line without the terminating
626 zero. This means that the command line can contain at most
627 cmdline_size characters. With protocol version 2.05 and earlier, the
628 maximum size was 255.
629
630Field name: hardware_subarch
631Type: write (optional, defaults to x86/PC)
632Offset/size: 0x23c/4
633Protocol: 2.07+
634
635 In a paravirtualized environment the hardware low level architectural
636 pieces such as interrupt handling, page table handling, and
637 accessing process control registers needs to be done differently.
638
639 This field allows the bootloader to inform the kernel we are in one
640 one of those environments.
641
642 0x00000000 The default x86/PC environment
643 0x00000001 lguest
644 0x00000002 Xen
645 0x00000003 Moorestown MID
646 0x00000004 CE4100 TV Platform
647
648Field name: hardware_subarch_data
649Type: write (subarch-dependent)
650Offset/size: 0x240/8
651Protocol: 2.07+
652
653 A pointer to data that is specific to hardware subarch
654 This field is currently unused for the default x86/PC environment,
655 do not modify.
656
657Field name: payload_offset
658Type: read
659Offset/size: 0x248/4
660Protocol: 2.08+
661
662 If non-zero then this field contains the offset from the beginning
663 of the protected-mode code to the payload.
664
665 The payload may be compressed. The format of both the compressed and
666 uncompressed data should be determined using the standard magic
667 numbers. The currently supported compression formats are gzip
668 (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA
669 (magic number 5D 00), XZ (magic number FD 37), and LZ4 (magic number
670 02 21). The uncompressed payload is currently always ELF (magic
671 number 7F 45 4C 46).
672
673Field name: payload_length
674Type: read
675Offset/size: 0x24c/4
676Protocol: 2.08+
677
678 The length of the payload.
679
680Field name: setup_data
681Type: write (special)
682Offset/size: 0x250/8
683Protocol: 2.09+
684
685 The 64-bit physical pointer to NULL terminated single linked list of
686 struct setup_data. This is used to define a more extensible boot
687 parameters passing mechanism. The definition of struct setup_data is
688 as follow:
689
690 struct setup_data {
691 u64 next;
692 u32 type;
693 u32 len;
694 u8 data[0];
695 };
696
697 Where, the next is a 64-bit physical pointer to the next node of
698 linked list, the next field of the last node is 0; the type is used
699 to identify the contents of data; the len is the length of data
700 field; the data holds the real payload.
701
702 This list may be modified at a number of points during the bootup
703 process. Therefore, when modifying this list one should always make
704 sure to consider the case where the linked list already contains
705 entries.
706
707Field name: pref_address
708Type: read (reloc)
709Offset/size: 0x258/8
710Protocol: 2.10+
711
712 This field, if nonzero, represents a preferred load address for the
713 kernel. A relocating bootloader should attempt to load at this
714 address if possible.
715
716 A non-relocatable kernel will unconditionally move itself and to run
717 at this address.
718
719Field name: init_size
720Type: read
721Offset/size: 0x260/4
722
723 This field indicates the amount of linear contiguous memory starting
724 at the kernel runtime start address that the kernel needs before it
725 is capable of examining its memory map. This is not the same thing
726 as the total amount of memory the kernel needs to boot, but it can
727 be used by a relocating boot loader to help select a safe load
728 address for the kernel.
729
730 The kernel runtime start address is determined by the following algorithm:
731
732 if (relocatable_kernel)
733 runtime_start = align_up(load_address, kernel_alignment)
734 else
735 runtime_start = pref_address
736
737Field name: handover_offset
738Type: read
739Offset/size: 0x264/4
740
741 This field is the offset from the beginning of the kernel image to
742 the EFI handover protocol entry point. Boot loaders using the EFI
743 handover protocol to boot the kernel should jump to this offset.
744
745 See EFI HANDOVER PROTOCOL below for more details.
746
747
748**** THE IMAGE CHECKSUM
749
750From boot protocol version 2.08 onwards the CRC-32 is calculated over
751the entire file using the characteristic polynomial 0x04C11DB7 and an
752initial remainder of 0xffffffff. The checksum is appended to the
753file; therefore the CRC of the file up to the limit specified in the
754syssize field of the header is always 0.
755
756
757**** THE KERNEL COMMAND LINE
758
759The kernel command line has become an important way for the boot
760loader to communicate with the kernel. Some of its options are also
761relevant to the boot loader itself, see "special command line options"
762below.
763
764The kernel command line is a null-terminated string. The maximum
765length can be retrieved from the field cmdline_size. Before protocol
766version 2.06, the maximum was 255 characters. A string that is too
767long will be automatically truncated by the kernel.
768
769If the boot protocol version is 2.02 or later, the address of the
770kernel command line is given by the header field cmd_line_ptr (see
771above.) This address can be anywhere between the end of the setup
772heap and 0xA0000.
773
774If the protocol version is *not* 2.02 or higher, the kernel
775command line is entered using the following protocol:
776
777 At offset 0x0020 (word), "cmd_line_magic", enter the magic
778 number 0xA33F.
779
780 At offset 0x0022 (word), "cmd_line_offset", enter the offset
781 of the kernel command line (relative to the start of the
782 real-mode kernel).
783
784 The kernel command line *must* be within the memory region
785 covered by setup_move_size, so you may need to adjust this
786 field.
787
788
789**** MEMORY LAYOUT OF THE REAL-MODE CODE
790
791The real-mode code requires a stack/heap to be set up, as well as
792memory allocated for the kernel command line. This needs to be done
793in the real-mode accessible memory in bottom megabyte.
794
795It should be noted that modern machines often have a sizable Extended
796BIOS Data Area (EBDA). As a result, it is advisable to use as little
797of the low megabyte as possible.
798
799Unfortunately, under the following circumstances the 0x90000 memory
800segment has to be used:
801
802 - When loading a zImage kernel ((loadflags & 0x01) == 0).
803 - When loading a 2.01 or earlier boot protocol kernel.
804
805 -> For the 2.00 and 2.01 boot protocols, the real-mode code
806 can be loaded at another address, but it is internally
807 relocated to 0x90000. For the "old" protocol, the
808 real-mode code must be loaded at 0x90000.
809
810When loading at 0x90000, avoid using memory above 0x9a000.
811
812For boot protocol 2.02 or higher, the command line does not have to be
813located in the same 64K segment as the real-mode setup code; it is
814thus permitted to give the stack/heap the full 64K segment and locate
815the command line above it.
816
817The kernel command line should not be located below the real-mode
818code, nor should it be located in high memory.
819
820
821**** SAMPLE BOOT CONFIGURATION
822
823As a sample configuration, assume the following layout of the real
824mode segment:
825
826 When loading below 0x90000, use the entire segment:
827
828 0x0000-0x7fff Real mode kernel
829 0x8000-0xdfff Stack and heap
830 0xe000-0xffff Kernel command line
831
832 When loading at 0x90000 OR the protocol version is 2.01 or earlier:
833
834 0x0000-0x7fff Real mode kernel
835 0x8000-0x97ff Stack and heap
836 0x9800-0x9fff Kernel command line
837
838Such a boot loader should enter the following fields in the header:
839
840 unsigned long base_ptr; /* base address for real-mode segment */
841
842 if ( setup_sects == 0 ) {
843 setup_sects = 4;
844 }
845
846 if ( protocol >= 0x0200 ) {
847 type_of_loader = <type code>;
848 if ( loading_initrd ) {
849 ramdisk_image = <initrd_address>;
850 ramdisk_size = <initrd_size>;
851 }
852
853 if ( protocol >= 0x0202 && loadflags & 0x01 )
854 heap_end = 0xe000;
855 else
856 heap_end = 0x9800;
857
858 if ( protocol >= 0x0201 ) {
859 heap_end_ptr = heap_end - 0x200;
860 loadflags |= 0x80; /* CAN_USE_HEAP */
861 }
862
863 if ( protocol >= 0x0202 ) {
864 cmd_line_ptr = base_ptr + heap_end;
865 strcpy(cmd_line_ptr, cmdline);
866 } else {
867 cmd_line_magic = 0xA33F;
868 cmd_line_offset = heap_end;
869 setup_move_size = heap_end + strlen(cmdline)+1;
870 strcpy(base_ptr+cmd_line_offset, cmdline);
871 }
872 } else {
873 /* Very old kernel */
874
875 heap_end = 0x9800;
876
877 cmd_line_magic = 0xA33F;
878 cmd_line_offset = heap_end;
879
880 /* A very old kernel MUST have its real-mode code
881 loaded at 0x90000 */
882
883 if ( base_ptr != 0x90000 ) {
884 /* Copy the real-mode kernel */
885 memcpy(0x90000, base_ptr, (setup_sects+1)*512);
886 base_ptr = 0x90000; /* Relocated */
887 }
888
889 strcpy(0x90000+cmd_line_offset, cmdline);
890
891 /* It is recommended to clear memory up to the 32K mark */
892 memset(0x90000 + (setup_sects+1)*512, 0,
893 (64-(setup_sects+1))*512);
894 }
895
896
897**** LOADING THE REST OF THE KERNEL
898
899The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
900in the kernel file (again, if setup_sects == 0 the real value is 4.)
901It should be loaded at address 0x10000 for Image/zImage kernels and
9020x100000 for bzImage kernels.
903
904The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
905bit (LOAD_HIGH) in the loadflags field is set:
906
907 is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
908 load_address = is_bzImage ? 0x100000 : 0x10000;
909
910Note that Image/zImage kernels can be up to 512K in size, and thus use
911the entire 0x10000-0x90000 range of memory. This means it is pretty
912much a requirement for these kernels to load the real-mode part at
9130x90000. bzImage kernels allow much more flexibility.
914
915
916**** SPECIAL COMMAND LINE OPTIONS
917
918If the command line provided by the boot loader is entered by the
919user, the user may expect the following command line options to work.
920They should normally not be deleted from the kernel command line even
921though not all of them are actually meaningful to the kernel. Boot
922loader authors who need additional command line options for the boot
923loader itself should get them registered in
924Documentation/kernel-parameters.txt to make sure they will not
925conflict with actual kernel options now or in the future.
926
927 vga=<mode>
928 <mode> here is either an integer (in C notation, either
929 decimal, octal, or hexadecimal) or one of the strings
930 "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
931 (meaning 0xFFFD). This value should be entered into the
932 vid_mode field, as it is used by the kernel before the command
933 line is parsed.
934
935 mem=<size>
936 <size> is an integer in C notation optionally followed by
937 (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
938 << 30, << 40, << 50 or << 60). This specifies the end of
939 memory to the kernel. This affects the possible placement of
940 an initrd, since an initrd should be placed near end of
941 memory. Note that this is an option to *both* the kernel and
942 the bootloader!
943
944 initrd=<file>
945 An initrd should be loaded. The meaning of <file> is
946 obviously bootloader-dependent, and some boot loaders
947 (e.g. LILO) do not have such a command.
948
949In addition, some boot loaders add the following options to the
950user-specified command line:
951
952 BOOT_IMAGE=<file>
953 The boot image which was loaded. Again, the meaning of <file>
954 is obviously bootloader-dependent.
955
956 auto
957 The kernel was booted without explicit user intervention.
958
959If these options are added by the boot loader, it is highly
960recommended that they are located *first*, before the user-specified
961or configuration-specified command line. Otherwise, "init=/bin/sh"
962gets confused by the "auto" option.
963
964
965**** RUNNING THE KERNEL
966
967The kernel is started by jumping to the kernel entry point, which is
968located at *segment* offset 0x20 from the start of the real mode
969kernel. This means that if you loaded your real-mode kernel code at
9700x90000, the kernel entry point is 9020:0000.
971
972At entry, ds = es = ss should point to the start of the real-mode
973kernel code (0x9000 if the code is loaded at 0x90000), sp should be
974set up properly, normally pointing to the top of the heap, and
975interrupts should be disabled. Furthermore, to guard against bugs in
976the kernel, it is recommended that the boot loader sets fs = gs = ds =
977es = ss.
978
979In our example from above, we would do:
980
981 /* Note: in the case of the "old" kernel protocol, base_ptr must
982 be == 0x90000 at this point; see the previous sample code */
983
984 seg = base_ptr >> 4;
985
986 cli(); /* Enter with interrupts disabled! */
987
988 /* Set up the real-mode kernel stack */
989 _SS = seg;
990 _SP = heap_end;
991
992 _DS = _ES = _FS = _GS = seg;
993 jmp_far(seg+0x20, 0); /* Run the kernel */
994
995If your boot sector accesses a floppy drive, it is recommended to
996switch off the floppy motor before running the kernel, since the
997kernel boot leaves interrupts off and thus the motor will not be
998switched off, especially if the loaded kernel has the floppy driver as
999a demand-loaded module!
1000
1001
1002**** ADVANCED BOOT LOADER HOOKS
1003
1004If the boot loader runs in a particularly hostile environment (such as
1005LOADLIN, which runs under DOS) it may be impossible to follow the
1006standard memory location requirements. Such a boot loader may use the
1007following hooks that, if set, are invoked by the kernel at the
1008appropriate time. The use of these hooks should probably be
1009considered an absolutely last resort!
1010
1011IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
1012%edi across invocation.
1013
1014 realmode_swtch:
1015 A 16-bit real mode far subroutine invoked immediately before
1016 entering protected mode. The default routine disables NMI, so
1017 your routine should probably do so, too.
1018
1019 code32_start:
1020 A 32-bit flat-mode routine *jumped* to immediately after the
1021 transition to protected mode, but before the kernel is
1022 uncompressed. No segments, except CS, are guaranteed to be
1023 set up (current kernels do, but older ones do not); you should
1024 set them up to BOOT_DS (0x18) yourself.
1025
1026 After completing your hook, you should jump to the address
1027 that was in this field before your boot loader overwrote it
1028 (relocated, if appropriate.)
1029
1030
1031**** 32-bit BOOT PROTOCOL
1032
1033For machine with some new BIOS other than legacy BIOS, such as EFI,
1034LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
1035based on legacy BIOS can not be used, so a 32-bit boot protocol needs
1036to be defined.
1037
1038In 32-bit boot protocol, the first step in loading a Linux kernel
1039should be to setup the boot parameters (struct boot_params,
1040traditionally known as "zero page"). The memory for struct boot_params
1041should be allocated and initialized to all zero. Then the setup header
1042from offset 0x01f1 of kernel image on should be loaded into struct
1043boot_params and examined. The end of setup header can be calculated as
1044follow:
1045
1046 0x0202 + byte value at offset 0x0201
1047
1048In addition to read/modify/write the setup header of the struct
1049boot_params as that of 16-bit boot protocol, the boot loader should
1050also fill the additional fields of the struct boot_params as that
1051described in zero-page.txt.
1052
1053After setting up the struct boot_params, the boot loader can load the
105432/64-bit kernel in the same way as that of 16-bit boot protocol.
1055
1056In 32-bit boot protocol, the kernel is started by jumping to the
105732-bit kernel entry point, which is the start address of loaded
105832/64-bit kernel.
1059
1060At entry, the CPU must be in 32-bit protected mode with paging
1061disabled; a GDT must be loaded with the descriptors for selectors
1062__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1063segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1064must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1065must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
1066address of the struct boot_params; %ebp, %edi and %ebx must be zero.
1067
1068**** 64-bit BOOT PROTOCOL
1069
1070For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader
1071and we need a 64-bit boot protocol.
1072
1073In 64-bit boot protocol, the first step in loading a Linux kernel
1074should be to setup the boot parameters (struct boot_params,
1075traditionally known as "zero page"). The memory for struct boot_params
1076could be allocated anywhere (even above 4G) and initialized to all zero.
1077Then, the setup header at offset 0x01f1 of kernel image on should be
1078loaded into struct boot_params and examined. The end of setup header
1079can be calculated as follows:
1080
1081 0x0202 + byte value at offset 0x0201
1082
1083In addition to read/modify/write the setup header of the struct
1084boot_params as that of 16-bit boot protocol, the boot loader should
1085also fill the additional fields of the struct boot_params as described
1086in zero-page.txt.
1087
1088After setting up the struct boot_params, the boot loader can load
108964-bit kernel in the same way as that of 16-bit boot protocol, but
1090kernel could be loaded above 4G.
1091
1092In 64-bit boot protocol, the kernel is started by jumping to the
109364-bit kernel entry point, which is the start address of loaded
109464-bit kernel plus 0x200.
1095
1096At entry, the CPU must be in 64-bit mode with paging enabled.
1097The range with setup_header.init_size from start address of loaded
1098kernel and zero page and command line buffer get ident mapping;
1099a GDT must be loaded with the descriptors for selectors
1100__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1101segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1102must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1103must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base
1104address of the struct boot_params.
1105
1106**** EFI HANDOVER PROTOCOL
1107
1108This protocol allows boot loaders to defer initialisation to the EFI
1109boot stub. The boot loader is required to load the kernel/initrd(s)
1110from the boot media and jump to the EFI handover protocol entry point
1111which is hdr->handover_offset bytes from the beginning of
1112startup_{32,64}.
1113
1114The function prototype for the handover entry point looks like this,
1115
1116 efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp)
1117
1118'handle' is the EFI image handle passed to the boot loader by the EFI
1119firmware, 'table' is the EFI system table - these are the first two
1120arguments of the "handoff state" as described in section 2.3 of the
1121UEFI specification. 'bp' is the boot loader-allocated boot params.
1122
1123The boot loader *must* fill out the following fields in bp,
1124
1125 o hdr.code32_start
1126 o hdr.cmd_line_ptr
1127 o hdr.ramdisk_image (if applicable)
1128 o hdr.ramdisk_size (if applicable)
1129
1130All other fields should be zero.