| blob | a75e3adaa39da277fb89150fb1d31daf8d1296ef |
1 THE LINUX/x86 BOOT PROTOCOL
2 ---------------------------
4 On the x86 platform, the Linux kernel uses a rather complicated boot
5 convention. This has evolved partially due to historical aspects, as
6 well as the desire in the early days to have the kernel itself be a
7 bootable image, the complicated PC memory model and due to changed
8 expectations in the PC industry caused by the effective demise of
9 real-mode DOS as a mainstream operating system.
11 Currently, the following versions of the Linux/x86 boot protocol exist.
13 Old kernels: zImage/Image support only. Some very early kernels
14 may not even support a command line.
16 Protocol 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.
22 Protocol 2.01: (Kernel 1.3.76) Added a heap overrun warning.
24 Protocol 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.
31 Protocol 2.03: (Kernel 2.4.18-pre1) Explicitly makes the highest possible
32 initrd address available to the bootloader.
34 Protocol 2.04: (Kernel 2.6.14) Extend the syssize field to four bytes.
36 Protocol 2.05: (Kernel 2.6.20) Make protected mode kernel relocatable.
37 Introduce relocatable_kernel and kernel_alignment fields.
39 Protocol 2.06: (Kernel 2.6.22) Added a field that contains the size of
40 the boot command line.
42 Protocol 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.
46 Protocol 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.
50 Protocol 2.09: (Kernel 2.6.26) Added a field of 64-bit physical
51 pointer to single linked list of struct setup_data.
53 Protocol 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.
57 Protocol 2.11: (Kernel 3.6) Added a field for offset of EFI handover
58 protocol entry point.
60 Protocol 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.
64 **** MEMORY LAYOUT
66 The traditional memory map for the kernel loader, used for Image or
67 zImage kernels, typically looks like:
69 | |
70 0A0000 +------------------------+
71 | Reserved for BIOS | Do not use. Reserved for BIOS EBDA.
72 09A000 +------------------------+
73 | Command line |
74 | Stack/heap | For use by the kernel real-mode code.
75 098000 +------------------------+
76 | Kernel setup | The kernel real-mode code.
77 090200 +------------------------+
78 | Kernel boot sector | The kernel legacy boot sector.
79 090000 +------------------------+
80 | Protected-mode kernel | The bulk of the kernel image.
81 010000 +------------------------+
82 | Boot loader | <- Boot sector entry point 0000:7C00
83 001000 +------------------------+
84 | Reserved for MBR/BIOS |
85 000800 +------------------------+
86 | Typically used by MBR |
87 000600 +------------------------+
88 | BIOS use only |
89 000000 +------------------------+
92 When using bzImage, the protected-mode kernel was relocated to
93 0x100000 ("high memory"), and the kernel real-mode block (boot sector,
94 setup, and stack/heap) was made relocatable to any address between
95 0x10000 and end of low memory. Unfortunately, in protocols 2.00 and
96 2.01 the 0x90000+ memory range is still used internally by the kernel;
97 the 2.02 protocol resolves that problem.
99 It is desirable to keep the "memory ceiling" -- the highest point in
100 low memory touched by the boot loader -- as low as possible, since
101 some newer BIOSes have begun to allocate some rather large amounts of
102 memory, called the Extended BIOS Data Area, near the top of low
103 memory. The boot loader should use the "INT 12h" BIOS call to verify
104 how much low memory is available.
106 Unfortunately, if INT 12h reports that the amount of memory is too
107 low, there is usually nothing the boot loader can do but to report an
108 error to the user. The boot loader should therefore be designed to
109 take up as little space in low memory as it reasonably can. For
110 zImage or old bzImage kernels, which need data written into the
111 0x90000 segment, the boot loader should make sure not to use memory
112 above the 0x9A000 point; too many BIOSes will break above that point.
114 For a modern bzImage kernel with boot protocol version >= 2.02, a
115 memory layout like the following is suggested:
117 ~ ~
118 | Protected-mode kernel |
119 100000 +------------------------+
120 | I/O memory hole |
121 0A0000 +------------------------+
122 | Reserved for BIOS | Leave as much as possible unused
123 ~ ~
124 | Command line | (Can also be below the X+10000 mark)
125 X+10000 +------------------------+
126 | Stack/heap | For use by the kernel real-mode code.
127 X+08000 +------------------------+
128 | Kernel setup | The kernel real-mode code.
129 | Kernel boot sector | The kernel legacy boot sector.
130 X +------------------------+
131 | Boot loader | <- Boot sector entry point 0000:7C00
132 001000 +------------------------+
133 | Reserved for MBR/BIOS |
134 000800 +------------------------+
135 | Typically used by MBR |
136 000600 +------------------------+
137 | BIOS use only |
138 000000 +------------------------+
140 ... where the address X is as low as the design of the boot loader
141 permits.
144 **** THE REAL-MODE KERNEL HEADER
146 In the following text, and anywhere in the kernel boot sequence, "a
147 sector" refers to 512 bytes. It is independent of the actual sector
148 size of the underlying medium.
150 The first step in loading a Linux kernel should be to load the
151 real-mode code (boot sector and setup code) and then examine the
152 following header at offset 0x01f1. The real-mode code can total up to
153 32K, although the boot loader may choose to load only the first two
154 sectors (1K) and then examine the bootup sector size.
156 The header looks like:
158 Offset Proto Name Meaning
159 /Size
161 01F1/1 ALL(1 setup_sects The size of the setup in sectors
162 01F2/2 ALL root_flags If set, the root is mounted readonly
163 01F4/4 2.04+(2 syssize The size of the 32-bit code in 16-byte paras
164 01F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
165 01FA/2 ALL vid_mode Video mode control
166 01FC/2 ALL root_dev Default root device number
167 01FE/2 ALL boot_flag 0xAA55 magic number
168 0200/2 2.00+ jump Jump instruction
169 0202/4 2.00+ header Magic signature "HdrS"
170 0206/2 2.00+ version Boot protocol version supported
171 0208/4 2.00+ realmode_swtch Boot loader hook (see below)
172 020C/2 2.00+ start_sys_seg The load-low segment (0x1000) (obsolete)
173 020E/2 2.00+ kernel_version Pointer to kernel version string
174 0210/1 2.00+ type_of_loader Boot loader identifier
175 0211/1 2.00+ loadflags Boot protocol option flags
176 0212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
177 0214/4 2.00+ code32_start Boot loader hook (see below)
178 0218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
179 021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
180 0220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
181 0224/2 2.01+ heap_end_ptr Free memory after setup end
182 0226/1 2.02+(3 ext_loader_ver Extended boot loader version
183 0227/1 2.02+(3 ext_loader_type Extended boot loader ID
184 0228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
185 022C/4 2.03+ initrd_addr_max Highest legal initrd address
186 0230/4 2.05+ kernel_alignment Physical addr alignment required for kernel
187 0234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
188 0235/1 2.10+ min_alignment Minimum alignment, as a power of two
189 0236/2 2.12+ xloadflags Boot protocol option flags
190 0238/4 2.06+ cmdline_size Maximum size of the kernel command line
191 023C/4 2.07+ hardware_subarch Hardware subarchitecture
192 0240/8 2.07+ hardware_subarch_data Subarchitecture-specific data
193 0248/4 2.08+ payload_offset Offset of kernel payload
194 024C/4 2.08+ payload_length Length of kernel payload
195 0250/8 2.09+ setup_data 64-bit physical pointer to linked list
196 of struct setup_data
197 0258/8 2.10+ pref_address Preferred loading address
198 0260/4 2.10+ init_size Linear memory required during initialization
199 0264/4 2.11+ handover_offset Offset of handover entry point
201 (1) For backwards compatibility, if the setup_sects field contains 0, the
202 real value is 4.
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.
208 (3) Ignored, but safe to set, for boot protocols 2.02-2.09.
210 If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
211 the boot protocol version is "old". Loading an old kernel, the
212 following parameters should be assumed:
214 Image type = zImage
215 initrd not supported
216 Real-mode kernel must be located at 0x90000.
218 Otherwise, the "version" field contains the protocol version,
219 e.g. protocol version 2.01 will contain 0x0201 in this field. When
220 setting fields in the header, you must make sure only to set fields
221 supported by the protocol version in use.
224 **** DETAILS OF HEADER FIELDS
226 For 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
229 bootloader ("modify").
231 All general purpose boot loaders should write the fields marked
232 (obligatory). Boot loaders who want to load the kernel at a
233 nonstandard address should fill in the fields marked (reloc); other
234 boot loaders can ignore those fields.
236 The byte order of all fields is littleendian (this is x86, after all.)
238 Field name: setup_sects
239 Type: read
240 Offset/size: 0x1f1/1
241 Protocol: ALL
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.
247 Field name: root_flags
248 Type: modify (optional)
249 Offset/size: 0x1f2/2
250 Protocol: ALL
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.
256 Field name: syssize
257 Type: read
258 Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
259 Protocol: 2.04+
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.
266 Field name: ram_size
267 Type: kernel internal
268 Offset/size: 0x1f8/2
269 Protocol: ALL
271 This field is obsolete.
273 Field name: vid_mode
274 Type: modify (obligatory)
275 Offset/size: 0x1fa/2
277 Please see the section on SPECIAL COMMAND LINE OPTIONS.
279 Field name: root_dev
280 Type: modify (optional)
281 Offset/size: 0x1fc/2
282 Protocol: ALL
284 The default root device device number. The use of this field is
285 deprecated, use the "root=" option on the command line instead.
287 Field name: boot_flag
288 Type: read
289 Offset/size: 0x1fe/2
290 Protocol: ALL
292 Contains 0xAA55. This is the closest thing old Linux kernels have
293 to a magic number.
295 Field name: jump
296 Type: read
297 Offset/size: 0x200/2
298 Protocol: 2.00+
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.
304 Field name: header
305 Type: read
306 Offset/size: 0x202/4
307 Protocol: 2.00+
309 Contains the magic number "HdrS" (0x53726448).
311 Field name: version
312 Type: read
313 Offset/size: 0x206/2
314 Protocol: 2.00+
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.
320 Field name: realmode_swtch
321 Type: modify (optional)
322 Offset/size: 0x208/4
323 Protocol: 2.00+
325 Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
327 Field name: start_sys_seg
328 Type: read
329 Offset/size: 0x20c/2
330 Protocol: 2.00+
332 The load low segment (0x1000). Obsolete.
334 Field name: kernel_version
335 Type: read
336 Offset/size: 0x20e/2
337 Protocol: 2.00+
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).
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:
349 0x1c00 < 15*0x200 (= 0x1e00) but
350 0x1c00 >= 14*0x200 (= 0x1c00)
352 0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
354 Field name: type_of_loader
355 Type: write (obligatory)
356 Offset/size: 0x210/1
357 Protocol: 2.00+
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.
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.
368 For example, for T = 0x15, V = 0x234, write:
370 type_of_loader <- 0xE4
371 ext_loader_type <- 0x05
372 ext_loader_ver <- 0x23
374 Assigned boot loader ids (hexadecimal):
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
395 Please contact <hpa@zytor.com> if you need a bootloader ID
396 value assigned.
398 Field name: loadflags
399 Type: modify (obligatory)
400 Offset/size: 0x211/1
401 Protocol: 2.00+
403 This field is a bitmask.
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.
409 Bit 5 (write): QUIET_FLAG
410 - If 0, print early messages.
411 - If 1, suppress early messages.
412 This requests to the kernel (decompressor and early
413 kernel) to not write early messages that require
414 accessing the display hardware directly.
416 Bit 6 (write): KEEP_SEGMENTS
417 Protocol: 2.07+
418 - If 0, reload the segment registers in the 32bit entry point.
419 - If 1, do not reload the segment registers in the 32bit entry point.
420 Assume that %cs %ds %ss %es are all set to flat segments with
421 a base of 0 (or the equivalent for their environment).
423 Bit 7 (write): CAN_USE_HEAP
424 Set this bit to 1 to indicate that the value entered in the
425 heap_end_ptr is valid. If this field is clear, some setup code
426 functionality will be disabled.
428 Field name: setup_move_size
429 Type: modify (obligatory)
430 Offset/size: 0x212/2
431 Protocol: 2.00-2.01
433 When using protocol 2.00 or 2.01, if the real mode kernel is not
434 loaded at 0x90000, it gets moved there later in the loading
435 sequence. Fill in this field if you want additional data (such as
436 the kernel command line) moved in addition to the real-mode kernel
437 itself.
439 The unit is bytes starting with the beginning of the boot sector.
441 This field is can be ignored when the protocol is 2.02 or higher, or
442 if the real-mode code is loaded at 0x90000.
444 Field name: code32_start
445 Type: modify (optional, reloc)
446 Offset/size: 0x214/4
447 Protocol: 2.00+
449 The address to jump to in protected mode. This defaults to the load
450 address of the kernel, and can be used by the boot loader to
451 determine the proper load address.
453 This field can be modified for two purposes:
455 1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
457 2. if a bootloader which does not install a hook loads a
458 relocatable kernel at a nonstandard address it will have to modify
459 this field to point to the load address.
461 Field name: ramdisk_image
462 Type: write (obligatory)
463 Offset/size: 0x218/4
464 Protocol: 2.00+
466 The 32-bit linear address of the initial ramdisk or ramfs. Leave at
467 zero if there is no initial ramdisk/ramfs.
469 Field name: ramdisk_size
470 Type: write (obligatory)
471 Offset/size: 0x21c/4
472 Protocol: 2.00+
474 Size of the initial ramdisk or ramfs. Leave at zero if there is no
475 initial ramdisk/ramfs.
477 Field name: bootsect_kludge
478 Type: kernel internal
479 Offset/size: 0x220/4
480 Protocol: 2.00+
482 This field is obsolete.
484 Field name: heap_end_ptr
485 Type: write (obligatory)
486 Offset/size: 0x224/2
487 Protocol: 2.01+
489 Set this field to the offset (from the beginning of the real-mode
490 code) of the end of the setup stack/heap, minus 0x0200.
492 Field name: ext_loader_ver
493 Type: write (optional)
494 Offset/size: 0x226/1
495 Protocol: 2.02+
497 This field is used as an extension of the version number in the
498 type_of_loader field. The total version number is considered to be
499 (type_of_loader & 0x0f) + (ext_loader_ver << 4).
501 The use of this field is boot loader specific. If not written, it
502 is zero.
504 Kernels prior to 2.6.31 did not recognize this field, but it is safe
505 to write for protocol version 2.02 or higher.
507 Field name: ext_loader_type
508 Type: write (obligatory if (type_of_loader & 0xf0) == 0xe0)
509 Offset/size: 0x227/1
510 Protocol: 2.02+
512 This field is used as an extension of the type number in
513 type_of_loader field. If the type in type_of_loader is 0xE, then
514 the actual type is (ext_loader_type + 0x10).
516 This field is ignored if the type in type_of_loader is not 0xE.
518 Kernels prior to 2.6.31 did not recognize this field, but it is safe
519 to write for protocol version 2.02 or higher.
521 Field name: cmd_line_ptr
522 Type: write (obligatory)
523 Offset/size: 0x228/4
524 Protocol: 2.02+
526 Set this field to the linear address of the kernel command line.
527 The kernel command line can be located anywhere between the end of
528 the setup heap and 0xA0000; it does not have to be located in the
529 same 64K segment as the real-mode code itself.
531 Fill in this field even if your boot loader does not support a
532 command line, in which case you can point this to an empty string
533 (or better yet, to the string "auto".) If this field is left at
534 zero, the kernel will assume that your boot loader does not support
535 the 2.02+ protocol.
537 Field name: initrd_addr_max
538 Type: read
539 Offset/size: 0x22c/4
540 Protocol: 2.03+
542 The maximum address that may be occupied by the initial
543 ramdisk/ramfs contents. For boot protocols 2.02 or earlier, this
544 field is not present, and the maximum address is 0x37FFFFFF. (This
545 address is defined as the address of the highest safe byte, so if
546 your ramdisk is exactly 131072 bytes long and this field is
547 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
549 Field name: kernel_alignment
550 Type: read/modify (reloc)
551 Offset/size: 0x230/4
552 Protocol: 2.05+ (read), 2.10+ (modify)
554 Alignment unit required by the kernel (if relocatable_kernel is
555 true.) A relocatable kernel that is loaded at an alignment
556 incompatible with the value in this field will be realigned during
557 kernel initialization.
559 Starting with protocol version 2.10, this reflects the kernel
560 alignment preferred for optimal performance; it is possible for the
561 loader to modify this field to permit a lesser alignment. See the
562 min_alignment and pref_address field below.
564 Field name: relocatable_kernel
565 Type: read (reloc)
566 Offset/size: 0x234/1
567 Protocol: 2.05+
569 If this field is nonzero, the protected-mode part of the kernel can
570 be loaded at any address that satisfies the kernel_alignment field.
571 After loading, the boot loader must set the code32_start field to
572 point to the loaded code, or to a boot loader hook.
574 Field name: min_alignment
575 Type: read (reloc)
576 Offset/size: 0x235/1
577 Protocol: 2.10+
579 This field, if nonzero, indicates as a power of two the minimum
580 alignment required, as opposed to preferred, by the kernel to boot.
581 If a boot loader makes use of this field, it should update the
582 kernel_alignment field with the alignment unit desired; typically:
584 kernel_alignment = 1 << min_alignment
586 There may be a considerable performance cost with an excessively
587 misaligned kernel. Therefore, a loader should typically try each
588 power-of-two alignment from kernel_alignment down to this alignment.
590 Field name: xloadflags
591 Type: read
592 Offset/size: 0x236/2
593 Protocol: 2.12+
595 This field is a bitmask.
597 Bit 0 (read): XLF_KERNEL_64
598 - If 1, this kernel has the legacy 64-bit entry point at 0x200.
600 Bit 1 (read): XLF_CAN_BE_LOADED_ABOVE_4G
601 - If 1, kernel/boot_params/cmdline/ramdisk can be above 4G.
603 Bit 2 (read): XLF_EFI_HANDOVER_32
604 - If 1, the kernel supports the 32-bit EFI handoff entry point
605 given at handover_offset.
607 Bit 3 (read): XLF_EFI_HANDOVER_64
608 - If 1, the kernel supports the 64-bit EFI handoff entry point
609 given at handover_offset + 0x200.
611 Bit 4 (read): XLF_EFI_KEXEC
612 - If 1, the kernel supports kexec EFI boot with EFI runtime support.
614 Field name: cmdline_size
615 Type: read
616 Offset/size: 0x238/4
617 Protocol: 2.06+
619 The maximum size of the command line without the terminating
620 zero. This means that the command line can contain at most
621 cmdline_size characters. With protocol version 2.05 and earlier, the
622 maximum size was 255.
624 Field name: hardware_subarch
625 Type: write (optional, defaults to x86/PC)
626 Offset/size: 0x23c/4
627 Protocol: 2.07+
629 In a paravirtualized environment the hardware low level architectural
630 pieces such as interrupt handling, page table handling, and
631 accessing process control registers needs to be done differently.
633 This field allows the bootloader to inform the kernel we are in one
634 one of those environments.
636 0x00000000 The default x86/PC environment
637 0x00000001 lguest
638 0x00000002 Xen
639 0x00000003 Moorestown MID
640 0x00000004 CE4100 TV Platform
642 Field name: hardware_subarch_data
643 Type: write (subarch-dependent)
644 Offset/size: 0x240/8
645 Protocol: 2.07+
647 A pointer to data that is specific to hardware subarch
648 This field is currently unused for the default x86/PC environment,
649 do not modify.
651 Field name: payload_offset
652 Type: read
653 Offset/size: 0x248/4
654 Protocol: 2.08+
656 If non-zero then this field contains the offset from the beginning
657 of the protected-mode code to the payload.
659 The payload may be compressed. The format of both the compressed and
660 uncompressed data should be determined using the standard magic
661 numbers. The currently supported compression formats are gzip
662 (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA
663 (magic number 5D 00), XZ (magic number FD 37), and LZ4 (magic number
664 02 21). The uncompressed payload is currently always ELF (magic
665 number 7F 45 4C 46).
667 Field name: payload_length
668 Type: read
669 Offset/size: 0x24c/4
670 Protocol: 2.08+
672 The length of the payload.
674 Field name: setup_data
675 Type: write (special)
676 Offset/size: 0x250/8
677 Protocol: 2.09+
679 The 64-bit physical pointer to NULL terminated single linked list of
680 struct setup_data. This is used to define a more extensible boot
681 parameters passing mechanism. The definition of struct setup_data is
682 as follow:
684 struct setup_data {
685 u64 next;
686 u32 type;
687 u32 len;
688 u8 data[0];
689 };
691 Where, the next is a 64-bit physical pointer to the next node of
692 linked list, the next field of the last node is 0; the type is used
693 to identify the contents of data; the len is the length of data
694 field; the data holds the real payload.
696 This list may be modified at a number of points during the bootup
697 process. Therefore, when modifying this list one should always make
698 sure to consider the case where the linked list already contains
699 entries.
701 Field name: pref_address
702 Type: read (reloc)
703 Offset/size: 0x258/8
704 Protocol: 2.10+
706 This field, if nonzero, represents a preferred load address for the
707 kernel. A relocating bootloader should attempt to load at this
708 address if possible.
710 A non-relocatable kernel will unconditionally move itself and to run
711 at this address.
713 Field name: init_size
714 Type: read
715 Offset/size: 0x260/4
717 This field indicates the amount of linear contiguous memory starting
718 at the kernel runtime start address that the kernel needs before it
719 is capable of examining its memory map. This is not the same thing
720 as the total amount of memory the kernel needs to boot, but it can
721 be used by a relocating boot loader to help select a safe load
722 address for the kernel.
724 The kernel runtime start address is determined by the following algorithm:
726 if (relocatable_kernel)
727 runtime_start = align_up(load_address, kernel_alignment)
728 else
729 runtime_start = pref_address
731 Field name: handover_offset
732 Type: read
733 Offset/size: 0x264/4
735 This field is the offset from the beginning of the kernel image to
736 the EFI handover protocol entry point. Boot loaders using the EFI
737 handover protocol to boot the kernel should jump to this offset.
739 See EFI HANDOVER PROTOCOL below for more details.
742 **** THE IMAGE CHECKSUM
744 From boot protocol version 2.08 onwards the CRC-32 is calculated over
745 the entire file using the characteristic polynomial 0x04C11DB7 and an
746 initial remainder of 0xffffffff. The checksum is appended to the
747 file; therefore the CRC of the file up to the limit specified in the
748 syssize field of the header is always 0.
751 **** THE KERNEL COMMAND LINE
753 The kernel command line has become an important way for the boot
754 loader to communicate with the kernel. Some of its options are also
755 relevant to the boot loader itself, see "special command line options"
756 below.
758 The kernel command line is a null-terminated string. The maximum
759 length can be retrieved from the field cmdline_size. Before protocol
760 version 2.06, the maximum was 255 characters. A string that is too
761 long will be automatically truncated by the kernel.
763 If the boot protocol version is 2.02 or later, the address of the
764 kernel command line is given by the header field cmd_line_ptr (see
765 above.) This address can be anywhere between the end of the setup
766 heap and 0xA0000.
768 If the protocol version is *not* 2.02 or higher, the kernel
769 command line is entered using the following protocol:
771 At offset 0x0020 (word), "cmd_line_magic", enter the magic
772 number 0xA33F.
774 At offset 0x0022 (word), "cmd_line_offset", enter the offset
775 of the kernel command line (relative to the start of the
776 real-mode kernel).
778 The kernel command line *must* be within the memory region
779 covered by setup_move_size, so you may need to adjust this
780 field.
783 **** MEMORY LAYOUT OF THE REAL-MODE CODE
785 The real-mode code requires a stack/heap to be set up, as well as
786 memory allocated for the kernel command line. This needs to be done
787 in the real-mode accessible memory in bottom megabyte.
789 It should be noted that modern machines often have a sizable Extended
790 BIOS Data Area (EBDA). As a result, it is advisable to use as little
791 of the low megabyte as possible.
793 Unfortunately, under the following circumstances the 0x90000 memory
794 segment has to be used:
796 - When loading a zImage kernel ((loadflags & 0x01) == 0).
797 - When loading a 2.01 or earlier boot protocol kernel.
799 -> For the 2.00 and 2.01 boot protocols, the real-mode code
800 can be loaded at another address, but it is internally
801 relocated to 0x90000. For the "old" protocol, the
802 real-mode code must be loaded at 0x90000.
804 When loading at 0x90000, avoid using memory above 0x9a000.
806 For boot protocol 2.02 or higher, the command line does not have to be
807 located in the same 64K segment as the real-mode setup code; it is
808 thus permitted to give the stack/heap the full 64K segment and locate
809 the command line above it.
811 The kernel command line should not be located below the real-mode
812 code, nor should it be located in high memory.
815 **** SAMPLE BOOT CONFIGURATION
817 As a sample configuration, assume the following layout of the real
818 mode segment:
820 When loading below 0x90000, use the entire segment:
822 0x0000-0x7fff Real mode kernel
823 0x8000-0xdfff Stack and heap
824 0xe000-0xffff Kernel command line
826 When loading at 0x90000 OR the protocol version is 2.01 or earlier:
828 0x0000-0x7fff Real mode kernel
829 0x8000-0x97ff Stack and heap
830 0x9800-0x9fff Kernel command line
832 Such a boot loader should enter the following fields in the header:
834 unsigned long base_ptr; /* base address for real-mode segment */
836 if ( setup_sects == 0 ) {
837 setup_sects = 4;
838 }
840 if ( protocol >= 0x0200 ) {
841 type_of_loader = <type code>;
842 if ( loading_initrd ) {
843 ramdisk_image = <initrd_address>;
844 ramdisk_size = <initrd_size>;
845 }
847 if ( protocol >= 0x0202 && loadflags & 0x01 )
848 heap_end = 0xe000;
849 else
850 heap_end = 0x9800;
852 if ( protocol >= 0x0201 ) {
853 heap_end_ptr = heap_end - 0x200;
854 loadflags |= 0x80; /* CAN_USE_HEAP */
855 }
857 if ( protocol >= 0x0202 ) {
858 cmd_line_ptr = base_ptr + heap_end;
859 strcpy(cmd_line_ptr, cmdline);
860 } else {
861 cmd_line_magic = 0xA33F;
862 cmd_line_offset = heap_end;
863 setup_move_size = heap_end + strlen(cmdline)+1;
864 strcpy(base_ptr+cmd_line_offset, cmdline);
865 }
866 } else {
867 /* Very old kernel */
869 heap_end = 0x9800;
871 cmd_line_magic = 0xA33F;
872 cmd_line_offset = heap_end;
874 /* A very old kernel MUST have its real-mode code
875 loaded at 0x90000 */
877 if ( base_ptr != 0x90000 ) {
878 /* Copy the real-mode kernel */
879 memcpy(0x90000, base_ptr, (setup_sects+1)*512);
880 base_ptr = 0x90000; /* Relocated */
881 }
883 strcpy(0x90000+cmd_line_offset, cmdline);
885 /* It is recommended to clear memory up to the 32K mark */
886 memset(0x90000 + (setup_sects+1)*512, 0,
887 (64-(setup_sects+1))*512);
888 }
891 **** LOADING THE REST OF THE KERNEL
893 The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
894 in the kernel file (again, if setup_sects == 0 the real value is 4.)
895 It should be loaded at address 0x10000 for Image/zImage kernels and
896 0x100000 for bzImage kernels.
898 The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
899 bit (LOAD_HIGH) in the loadflags field is set:
901 is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
902 load_address = is_bzImage ? 0x100000 : 0x10000;
904 Note that Image/zImage kernels can be up to 512K in size, and thus use
905 the entire 0x10000-0x90000 range of memory. This means it is pretty
906 much a requirement for these kernels to load the real-mode part at
907 0x90000. bzImage kernels allow much more flexibility.
910 **** SPECIAL COMMAND LINE OPTIONS
912 If the command line provided by the boot loader is entered by the
913 user, the user may expect the following command line options to work.
914 They should normally not be deleted from the kernel command line even
915 though not all of them are actually meaningful to the kernel. Boot
916 loader authors who need additional command line options for the boot
917 loader itself should get them registered in
918 Documentation/kernel-parameters.txt to make sure they will not
919 conflict with actual kernel options now or in the future.
921 vga=<mode>
922 <mode> here is either an integer (in C notation, either
923 decimal, octal, or hexadecimal) or one of the strings
924 "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
925 (meaning 0xFFFD). This value should be entered into the
926 vid_mode field, as it is used by the kernel before the command
927 line is parsed.
929 mem=<size>
930 <size> is an integer in C notation optionally followed by
931 (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
932 << 30, << 40, << 50 or << 60). This specifies the end of
933 memory to the kernel. This affects the possible placement of
934 an initrd, since an initrd should be placed near end of
935 memory. Note that this is an option to *both* the kernel and
936 the bootloader!
938 initrd=<file>
939 An initrd should be loaded. The meaning of <file> is
940 obviously bootloader-dependent, and some boot loaders
941 (e.g. LILO) do not have such a command.
943 In addition, some boot loaders add the following options to the
944 user-specified command line:
946 BOOT_IMAGE=<file>
947 The boot image which was loaded. Again, the meaning of <file>
948 is obviously bootloader-dependent.
950 auto
951 The kernel was booted without explicit user intervention.
953 If these options are added by the boot loader, it is highly
954 recommended that they are located *first*, before the user-specified
955 or configuration-specified command line. Otherwise, "init=/bin/sh"
956 gets confused by the "auto" option.
959 **** RUNNING THE KERNEL
961 The kernel is started by jumping to the kernel entry point, which is
962 located at *segment* offset 0x20 from the start of the real mode
963 kernel. This means that if you loaded your real-mode kernel code at
964 0x90000, the kernel entry point is 9020:0000.
966 At entry, ds = es = ss should point to the start of the real-mode
967 kernel code (0x9000 if the code is loaded at 0x90000), sp should be
968 set up properly, normally pointing to the top of the heap, and
969 interrupts should be disabled. Furthermore, to guard against bugs in
970 the kernel, it is recommended that the boot loader sets fs = gs = ds =
971 es = ss.
973 In our example from above, we would do:
975 /* Note: in the case of the "old" kernel protocol, base_ptr must
976 be == 0x90000 at this point; see the previous sample code */
978 seg = base_ptr >> 4;
980 cli(); /* Enter with interrupts disabled! */
982 /* Set up the real-mode kernel stack */
983 _SS = seg;
984 _SP = heap_end;
986 _DS = _ES = _FS = _GS = seg;
987 jmp_far(seg+0x20, 0); /* Run the kernel */
989 If your boot sector accesses a floppy drive, it is recommended to
990 switch off the floppy motor before running the kernel, since the
991 kernel boot leaves interrupts off and thus the motor will not be
992 switched off, especially if the loaded kernel has the floppy driver as
993 a demand-loaded module!
996 **** ADVANCED BOOT LOADER HOOKS
998 If the boot loader runs in a particularly hostile environment (such as
999 LOADLIN, which runs under DOS) it may be impossible to follow the
1000 standard memory location requirements. Such a boot loader may use the
1001 following hooks that, if set, are invoked by the kernel at the
1002 appropriate time. The use of these hooks should probably be
1003 considered an absolutely last resort!
1005 IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
1006 %edi across invocation.
1008 realmode_swtch:
1009 A 16-bit real mode far subroutine invoked immediately before
1010 entering protected mode. The default routine disables NMI, so
1011 your routine should probably do so, too.
1013 code32_start:
1014 A 32-bit flat-mode routine *jumped* to immediately after the
1015 transition to protected mode, but before the kernel is
1016 uncompressed. No segments, except CS, are guaranteed to be
1017 set up (current kernels do, but older ones do not); you should
1018 set them up to BOOT_DS (0x18) yourself.
1020 After completing your hook, you should jump to the address
1021 that was in this field before your boot loader overwrote it
1022 (relocated, if appropriate.)
1025 **** 32-bit BOOT PROTOCOL
1027 For machine with some new BIOS other than legacy BIOS, such as EFI,
1028 LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
1029 based on legacy BIOS can not be used, so a 32-bit boot protocol needs
1030 to be defined.
1032 In 32-bit boot protocol, the first step in loading a Linux kernel
1033 should be to setup the boot parameters (struct boot_params,
1034 traditionally known as "zero page"). The memory for struct boot_params
1035 should be allocated and initialized to all zero. Then the setup header
1036 from offset 0x01f1 of kernel image on should be loaded into struct
1037 boot_params and examined. The end of setup header can be calculated as
1038 follow:
1040 0x0202 + byte value at offset 0x0201
1042 In addition to read/modify/write the setup header of the struct
1043 boot_params as that of 16-bit boot protocol, the boot loader should
1044 also fill the additional fields of the struct boot_params as that
1045 described in zero-page.txt.
1047 After setting up the struct boot_params, the boot loader can load the
1048 32/64-bit kernel in the same way as that of 16-bit boot protocol.
1050 In 32-bit boot protocol, the kernel is started by jumping to the
1051 32-bit kernel entry point, which is the start address of loaded
1052 32/64-bit kernel.
1054 At entry, the CPU must be in 32-bit protected mode with paging
1055 disabled; a GDT must be loaded with the descriptors for selectors
1056 __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1057 segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1058 must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1059 must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
1060 address of the struct boot_params; %ebp, %edi and %ebx must be zero.
1062 **** 64-bit BOOT PROTOCOL
1064 For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader
1065 and we need a 64-bit boot protocol.
1067 In 64-bit boot protocol, the first step in loading a Linux kernel
1068 should be to setup the boot parameters (struct boot_params,
1069 traditionally known as "zero page"). The memory for struct boot_params
1070 could be allocated anywhere (even above 4G) and initialized to all zero.
1071 Then, the setup header at offset 0x01f1 of kernel image on should be
1072 loaded into struct boot_params and examined. The end of setup header
1073 can be calculated as follows:
1075 0x0202 + byte value at offset 0x0201
1077 In addition to read/modify/write the setup header of the struct
1078 boot_params as that of 16-bit boot protocol, the boot loader should
1079 also fill the additional fields of the struct boot_params as described
1080 in zero-page.txt.
1082 After setting up the struct boot_params, the boot loader can load
1083 64-bit kernel in the same way as that of 16-bit boot protocol, but
1084 kernel could be loaded above 4G.
1086 In 64-bit boot protocol, the kernel is started by jumping to the
1087 64-bit kernel entry point, which is the start address of loaded
1088 64-bit kernel plus 0x200.
1090 At entry, the CPU must be in 64-bit mode with paging enabled.
1091 The range with setup_header.init_size from start address of loaded
1092 kernel and zero page and command line buffer get ident mapping;
1093 a GDT must be loaded with the descriptors for selectors
1094 __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1095 segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1096 must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1097 must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base
1098 address of the struct boot_params.
1100 **** EFI HANDOVER PROTOCOL
1102 This protocol allows boot loaders to defer initialisation to the EFI
1103 boot stub. The boot loader is required to load the kernel/initrd(s)
1104 from the boot media and jump to the EFI handover protocol entry point
1105 which is hdr->handover_offset bytes from the beginning of
1106 startup_{32,64}.
1108 The function prototype for the handover entry point looks like this,
1110 efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp)
1112 'handle' is the EFI image handle passed to the boot loader by the EFI
1113 firmware, 'table' is the EFI system table - these are the first two
1114 arguments of the "handoff state" as described in section 2.3 of the
1115 UEFI specification. 'bp' is the boot loader-allocated boot params.
1117 The boot loader *must* fill out the following fields in bp,
1119 o hdr.code32_start
1120 o hdr.cmd_line_ptr
1121 o hdr.cmdline_size
1122 o hdr.ramdisk_image (if applicable)
1123 o hdr.ramdisk_size (if applicable)
1125 All other fields should be zero.