| blob | 30b43e1b26979cee024aa7636e250520b8ba235f |
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 **** MEMORY LAYOUT
59 The traditional memory map for the kernel loader, used for Image or
60 zImage kernels, typically looks like:
62 | |
63 0A0000 +------------------------+
64 | Reserved for BIOS | Do not use. Reserved for BIOS EBDA.
65 09A000 +------------------------+
66 | Command line |
67 | Stack/heap | For use by the kernel real-mode code.
68 098000 +------------------------+
69 | Kernel setup | The kernel real-mode code.
70 090200 +------------------------+
71 | Kernel boot sector | The kernel legacy boot sector.
72 090000 +------------------------+
73 | Protected-mode kernel | The bulk of the kernel image.
74 010000 +------------------------+
75 | Boot loader | <- Boot sector entry point 0000:7C00
76 001000 +------------------------+
77 | Reserved for MBR/BIOS |
78 000800 +------------------------+
79 | Typically used by MBR |
80 000600 +------------------------+
81 | BIOS use only |
82 000000 +------------------------+
85 When using bzImage, the protected-mode kernel was relocated to
86 0x100000 ("high memory"), and the kernel real-mode block (boot sector,
87 setup, and stack/heap) was made relocatable to any address between
88 0x10000 and end of low memory. Unfortunately, in protocols 2.00 and
89 2.01 the 0x90000+ memory range is still used internally by the kernel;
90 the 2.02 protocol resolves that problem.
92 It is desirable to keep the "memory ceiling" -- the highest point in
93 low memory touched by the boot loader -- as low as possible, since
94 some newer BIOSes have begun to allocate some rather large amounts of
95 memory, called the Extended BIOS Data Area, near the top of low
96 memory. The boot loader should use the "INT 12h" BIOS call to verify
97 how much low memory is available.
99 Unfortunately, if INT 12h reports that the amount of memory is too
100 low, there is usually nothing the boot loader can do but to report an
101 error to the user. The boot loader should therefore be designed to
102 take up as little space in low memory as it reasonably can. For
103 zImage or old bzImage kernels, which need data written into the
104 0x90000 segment, the boot loader should make sure not to use memory
105 above the 0x9A000 point; too many BIOSes will break above that point.
107 For a modern bzImage kernel with boot protocol version >= 2.02, a
108 memory layout like the following is suggested:
110 ~ ~
111 | Protected-mode kernel |
112 100000 +------------------------+
113 | I/O memory hole |
114 0A0000 +------------------------+
115 | Reserved for BIOS | Leave as much as possible unused
116 ~ ~
117 | Command line | (Can also be below the X+10000 mark)
118 X+10000 +------------------------+
119 | Stack/heap | For use by the kernel real-mode code.
120 X+08000 +------------------------+
121 | Kernel setup | The kernel real-mode code.
122 | Kernel boot sector | The kernel legacy boot sector.
123 X +------------------------+
124 | Boot loader | <- Boot sector entry point 0000:7C00
125 001000 +------------------------+
126 | Reserved for MBR/BIOS |
127 000800 +------------------------+
128 | Typically used by MBR |
129 000600 +------------------------+
130 | BIOS use only |
131 000000 +------------------------+
133 ... where the address X is as low as the design of the boot loader
134 permits.
137 **** THE REAL-MODE KERNEL HEADER
139 In the following text, and anywhere in the kernel boot sequence, "a
140 sector" refers to 512 bytes. It is independent of the actual sector
141 size of the underlying medium.
143 The first step in loading a Linux kernel should be to load the
144 real-mode code (boot sector and setup code) and then examine the
145 following header at offset 0x01f1. The real-mode code can total up to
146 32K, although the boot loader may choose to load only the first two
147 sectors (1K) and then examine the bootup sector size.
149 The header looks like:
151 Offset Proto Name Meaning
152 /Size
154 01F1/1 ALL(1 setup_sects The size of the setup in sectors
155 01F2/2 ALL root_flags If set, the root is mounted readonly
156 01F4/4 2.04+(2 syssize The size of the 32-bit code in 16-byte paras
157 01F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
158 01FA/2 ALL vid_mode Video mode control
159 01FC/2 ALL root_dev Default root device number
160 01FE/2 ALL boot_flag 0xAA55 magic number
161 0200/2 2.00+ jump Jump instruction
162 0202/4 2.00+ header Magic signature "HdrS"
163 0206/2 2.00+ version Boot protocol version supported
164 0208/4 2.00+ realmode_swtch Boot loader hook (see below)
165 020C/2 2.00+ start_sys_seg The load-low segment (0x1000) (obsolete)
166 020E/2 2.00+ kernel_version Pointer to kernel version string
167 0210/1 2.00+ type_of_loader Boot loader identifier
168 0211/1 2.00+ loadflags Boot protocol option flags
169 0212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
170 0214/4 2.00+ code32_start Boot loader hook (see below)
171 0218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
172 021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
173 0220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
174 0224/2 2.01+ heap_end_ptr Free memory after setup end
175 0226/1 2.02+(3 ext_loader_ver Extended boot loader version
176 0227/1 2.02+(3 ext_loader_type Extended boot loader ID
177 0228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
178 022C/4 2.03+ ramdisk_max Highest legal initrd address
179 0230/4 2.05+ kernel_alignment Physical addr alignment required for kernel
180 0234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
181 0235/1 2.10+ min_alignment Minimum alignment, as a power of two
182 0236/2 N/A pad3 Unused
183 0238/4 2.06+ cmdline_size Maximum size of the kernel command line
184 023C/4 2.07+ hardware_subarch Hardware subarchitecture
185 0240/8 2.07+ hardware_subarch_data Subarchitecture-specific data
186 0248/4 2.08+ payload_offset Offset of kernel payload
187 024C/4 2.08+ payload_length Length of kernel payload
188 0250/8 2.09+ setup_data 64-bit physical pointer to linked list
189 of struct setup_data
190 0258/8 2.10+ pref_address Preferred loading address
191 0260/4 2.10+ init_size Linear memory required during initialization
193 (1) For backwards compatibility, if the setup_sects field contains 0, the
194 real value is 4.
196 (2) For boot protocol prior to 2.04, the upper two bytes of the syssize
197 field are unusable, which means the size of a bzImage kernel
198 cannot be determined.
200 (3) Ignored, but safe to set, for boot protocols 2.02-2.09.
202 If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
203 the boot protocol version is "old". Loading an old kernel, the
204 following parameters should be assumed:
206 Image type = zImage
207 initrd not supported
208 Real-mode kernel must be located at 0x90000.
210 Otherwise, the "version" field contains the protocol version,
211 e.g. protocol version 2.01 will contain 0x0201 in this field. When
212 setting fields in the header, you must make sure only to set fields
213 supported by the protocol version in use.
216 **** DETAILS OF HEADER FIELDS
218 For each field, some are information from the kernel to the bootloader
219 ("read"), some are expected to be filled out by the bootloader
220 ("write"), and some are expected to be read and modified by the
221 bootloader ("modify").
223 All general purpose boot loaders should write the fields marked
224 (obligatory). Boot loaders who want to load the kernel at a
225 nonstandard address should fill in the fields marked (reloc); other
226 boot loaders can ignore those fields.
228 The byte order of all fields is littleendian (this is x86, after all.)
230 Field name: setup_sects
231 Type: read
232 Offset/size: 0x1f1/1
233 Protocol: ALL
235 The size of the setup code in 512-byte sectors. If this field is
236 0, the real value is 4. The real-mode code consists of the boot
237 sector (always one 512-byte sector) plus the setup code.
239 Field name: root_flags
240 Type: modify (optional)
241 Offset/size: 0x1f2/2
242 Protocol: ALL
244 If this field is nonzero, the root defaults to readonly. The use of
245 this field is deprecated; use the "ro" or "rw" options on the
246 command line instead.
248 Field name: syssize
249 Type: read
250 Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
251 Protocol: 2.04+
253 The size of the protected-mode code in units of 16-byte paragraphs.
254 For protocol versions older than 2.04 this field is only two bytes
255 wide, and therefore cannot be trusted for the size of a kernel if
256 the LOAD_HIGH flag is set.
258 Field name: ram_size
259 Type: kernel internal
260 Offset/size: 0x1f8/2
261 Protocol: ALL
263 This field is obsolete.
265 Field name: vid_mode
266 Type: modify (obligatory)
267 Offset/size: 0x1fa/2
269 Please see the section on SPECIAL COMMAND LINE OPTIONS.
271 Field name: root_dev
272 Type: modify (optional)
273 Offset/size: 0x1fc/2
274 Protocol: ALL
276 The default root device device number. The use of this field is
277 deprecated, use the "root=" option on the command line instead.
279 Field name: boot_flag
280 Type: read
281 Offset/size: 0x1fe/2
282 Protocol: ALL
284 Contains 0xAA55. This is the closest thing old Linux kernels have
285 to a magic number.
287 Field name: jump
288 Type: read
289 Offset/size: 0x200/2
290 Protocol: 2.00+
292 Contains an x86 jump instruction, 0xEB followed by a signed offset
293 relative to byte 0x202. This can be used to determine the size of
294 the header.
296 Field name: header
297 Type: read
298 Offset/size: 0x202/4
299 Protocol: 2.00+
301 Contains the magic number "HdrS" (0x53726448).
303 Field name: version
304 Type: read
305 Offset/size: 0x206/2
306 Protocol: 2.00+
308 Contains the boot protocol version, in (major << 8)+minor format,
309 e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
310 10.17.
312 Field name: realmode_swtch
313 Type: modify (optional)
314 Offset/size: 0x208/4
315 Protocol: 2.00+
317 Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
319 Field name: start_sys_seg
320 Type: read
321 Offset/size: 0x20c/2
322 Protocol: 2.00+
324 The load low segment (0x1000). Obsolete.
326 Field name: kernel_version
327 Type: read
328 Offset/size: 0x20e/2
329 Protocol: 2.00+
331 If set to a nonzero value, contains a pointer to a NUL-terminated
332 human-readable kernel version number string, less 0x200. This can
333 be used to display the kernel version to the user. This value
334 should be less than (0x200*setup_sects).
336 For example, if this value is set to 0x1c00, the kernel version
337 number string can be found at offset 0x1e00 in the kernel file.
338 This is a valid value if and only if the "setup_sects" field
339 contains the value 15 or higher, as:
341 0x1c00 < 15*0x200 (= 0x1e00) but
342 0x1c00 >= 14*0x200 (= 0x1c00)
344 0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
346 Field name: type_of_loader
347 Type: write (obligatory)
348 Offset/size: 0x210/1
349 Protocol: 2.00+
351 If your boot loader has an assigned id (see table below), enter
352 0xTV here, where T is an identifier for the boot loader and V is
353 a version number. Otherwise, enter 0xFF here.
355 For boot loader IDs above T = 0xD, write T = 0xE to this field and
356 write the extended ID minus 0x10 to the ext_loader_type field.
357 Similarly, the ext_loader_ver field can be used to provide more than
358 four bits for the bootloader version.
360 For example, for T = 0x15, V = 0x234, write:
362 type_of_loader <- 0xE4
363 ext_loader_type <- 0x05
364 ext_loader_ver <- 0x23
366 Assigned boot loader ids:
367 0 LILO (0x00 reserved for pre-2.00 bootloader)
368 1 Loadlin
369 2 bootsect-loader (0x20, all other values reserved)
370 3 Syslinux
371 4 Etherboot/gPXE
372 5 ELILO
373 7 GRUB
374 8 U-Boot
375 9 Xen
376 A Gujin
377 B Qemu
378 C Arcturus Networks uCbootloader
379 E Extended (see ext_loader_type)
380 F Special (0xFF = undefined)
382 Please contact <hpa@zytor.com> if you need a bootloader ID
383 value assigned.
385 Field name: loadflags
386 Type: modify (obligatory)
387 Offset/size: 0x211/1
388 Protocol: 2.00+
390 This field is a bitmask.
392 Bit 0 (read): LOADED_HIGH
393 - If 0, the protected-mode code is loaded at 0x10000.
394 - If 1, the protected-mode code is loaded at 0x100000.
396 Bit 5 (write): QUIET_FLAG
397 - If 0, print early messages.
398 - If 1, suppress early messages.
399 This requests to the kernel (decompressor and early
400 kernel) to not write early messages that require
401 accessing the display hardware directly.
403 Bit 6 (write): KEEP_SEGMENTS
404 Protocol: 2.07+
405 - If 0, reload the segment registers in the 32bit entry point.
406 - If 1, do not reload the segment registers in the 32bit entry point.
407 Assume that %cs %ds %ss %es are all set to flat segments with
408 a base of 0 (or the equivalent for their environment).
410 Bit 7 (write): CAN_USE_HEAP
411 Set this bit to 1 to indicate that the value entered in the
412 heap_end_ptr is valid. If this field is clear, some setup code
413 functionality will be disabled.
415 Field name: setup_move_size
416 Type: modify (obligatory)
417 Offset/size: 0x212/2
418 Protocol: 2.00-2.01
420 When using protocol 2.00 or 2.01, if the real mode kernel is not
421 loaded at 0x90000, it gets moved there later in the loading
422 sequence. Fill in this field if you want additional data (such as
423 the kernel command line) moved in addition to the real-mode kernel
424 itself.
426 The unit is bytes starting with the beginning of the boot sector.
428 This field is can be ignored when the protocol is 2.02 or higher, or
429 if the real-mode code is loaded at 0x90000.
431 Field name: code32_start
432 Type: modify (optional, reloc)
433 Offset/size: 0x214/4
434 Protocol: 2.00+
436 The address to jump to in protected mode. This defaults to the load
437 address of the kernel, and can be used by the boot loader to
438 determine the proper load address.
440 This field can be modified for two purposes:
442 1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
444 2. if a bootloader which does not install a hook loads a
445 relocatable kernel at a nonstandard address it will have to modify
446 this field to point to the load address.
448 Field name: ramdisk_image
449 Type: write (obligatory)
450 Offset/size: 0x218/4
451 Protocol: 2.00+
453 The 32-bit linear address of the initial ramdisk or ramfs. Leave at
454 zero if there is no initial ramdisk/ramfs.
456 Field name: ramdisk_size
457 Type: write (obligatory)
458 Offset/size: 0x21c/4
459 Protocol: 2.00+
461 Size of the initial ramdisk or ramfs. Leave at zero if there is no
462 initial ramdisk/ramfs.
464 Field name: bootsect_kludge
465 Type: kernel internal
466 Offset/size: 0x220/4
467 Protocol: 2.00+
469 This field is obsolete.
471 Field name: heap_end_ptr
472 Type: write (obligatory)
473 Offset/size: 0x224/2
474 Protocol: 2.01+
476 Set this field to the offset (from the beginning of the real-mode
477 code) of the end of the setup stack/heap, minus 0x0200.
479 Field name: ext_loader_ver
480 Type: write (optional)
481 Offset/size: 0x226/1
482 Protocol: 2.02+
484 This field is used as an extension of the version number in the
485 type_of_loader field. The total version number is considered to be
486 (type_of_loader & 0x0f) + (ext_loader_ver << 4).
488 The use of this field is boot loader specific. If not written, it
489 is zero.
491 Kernels prior to 2.6.31 did not recognize this field, but it is safe
492 to write for protocol version 2.02 or higher.
494 Field name: ext_loader_type
495 Type: write (obligatory if (type_of_loader & 0xf0) == 0xe0)
496 Offset/size: 0x227/1
497 Protocol: 2.02+
499 This field is used as an extension of the type number in
500 type_of_loader field. If the type in type_of_loader is 0xE, then
501 the actual type is (ext_loader_type + 0x10).
503 This field is ignored if the type in type_of_loader is not 0xE.
505 Kernels prior to 2.6.31 did not recognize this field, but it is safe
506 to write for protocol version 2.02 or higher.
508 Field name: cmd_line_ptr
509 Type: write (obligatory)
510 Offset/size: 0x228/4
511 Protocol: 2.02+
513 Set this field to the linear address of the kernel command line.
514 The kernel command line can be located anywhere between the end of
515 the setup heap and 0xA0000; it does not have to be located in the
516 same 64K segment as the real-mode code itself.
518 Fill in this field even if your boot loader does not support a
519 command line, in which case you can point this to an empty string
520 (or better yet, to the string "auto".) If this field is left at
521 zero, the kernel will assume that your boot loader does not support
522 the 2.02+ protocol.
524 Field name: ramdisk_max
525 Type: read
526 Offset/size: 0x22c/4
527 Protocol: 2.03+
529 The maximum address that may be occupied by the initial
530 ramdisk/ramfs contents. For boot protocols 2.02 or earlier, this
531 field is not present, and the maximum address is 0x37FFFFFF. (This
532 address is defined as the address of the highest safe byte, so if
533 your ramdisk is exactly 131072 bytes long and this field is
534 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
536 Field name: kernel_alignment
537 Type: read/modify (reloc)
538 Offset/size: 0x230/4
539 Protocol: 2.05+ (read), 2.10+ (modify)
541 Alignment unit required by the kernel (if relocatable_kernel is
542 true.) A relocatable kernel that is loaded at an alignment
543 incompatible with the value in this field will be realigned during
544 kernel initialization.
546 Starting with protocol version 2.10, this reflects the kernel
547 alignment preferred for optimal performance; it is possible for the
548 loader to modify this field to permit a lesser alignment. See the
549 min_alignment and pref_address field below.
551 Field name: relocatable_kernel
552 Type: read (reloc)
553 Offset/size: 0x234/1
554 Protocol: 2.05+
556 If this field is nonzero, the protected-mode part of the kernel can
557 be loaded at any address that satisfies the kernel_alignment field.
558 After loading, the boot loader must set the code32_start field to
559 point to the loaded code, or to a boot loader hook.
561 Field name: min_alignment
562 Type: read (reloc)
563 Offset/size: 0x235/1
564 Protocol: 2.10+
566 This field, if nonzero, indicates as a power of two the minimum
567 alignment required, as opposed to preferred, by the kernel to boot.
568 If a boot loader makes use of this field, it should update the
569 kernel_alignment field with the alignment unit desired; typically:
571 kernel_alignment = 1 << min_alignment
573 There may be a considerable performance cost with an excessively
574 misaligned kernel. Therefore, a loader should typically try each
575 power-of-two alignment from kernel_alignment down to this alignment.
577 Field name: cmdline_size
578 Type: read
579 Offset/size: 0x238/4
580 Protocol: 2.06+
582 The maximum size of the command line without the terminating
583 zero. This means that the command line can contain at most
584 cmdline_size characters. With protocol version 2.05 and earlier, the
585 maximum size was 255.
587 Field name: hardware_subarch
588 Type: write (optional, defaults to x86/PC)
589 Offset/size: 0x23c/4
590 Protocol: 2.07+
592 In a paravirtualized environment the hardware low level architectural
593 pieces such as interrupt handling, page table handling, and
594 accessing process control registers needs to be done differently.
596 This field allows the bootloader to inform the kernel we are in one
597 one of those environments.
599 0x00000000 The default x86/PC environment
600 0x00000001 lguest
601 0x00000002 Xen
602 0x00000003 Moorestown MID
604 Field name: hardware_subarch_data
605 Type: write (subarch-dependent)
606 Offset/size: 0x240/8
607 Protocol: 2.07+
609 A pointer to data that is specific to hardware subarch
610 This field is currently unused for the default x86/PC environment,
611 do not modify.
613 Field name: payload_offset
614 Type: read
615 Offset/size: 0x248/4
616 Protocol: 2.08+
618 If non-zero then this field contains the offset from the beginning
619 of the protected-mode code to the payload.
621 The payload may be compressed. The format of both the compressed and
622 uncompressed data should be determined using the standard magic
623 numbers. The currently supported compression formats are gzip
624 (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A) and LZMA
625 (magic number 5D 00). The uncompressed payload is currently always ELF
626 (magic number 7F 45 4C 46).
628 Field name: payload_length
629 Type: read
630 Offset/size: 0x24c/4
631 Protocol: 2.08+
633 The length of the payload.
635 Field name: setup_data
636 Type: write (special)
637 Offset/size: 0x250/8
638 Protocol: 2.09+
640 The 64-bit physical pointer to NULL terminated single linked list of
641 struct setup_data. This is used to define a more extensible boot
642 parameters passing mechanism. The definition of struct setup_data is
643 as follow:
645 struct setup_data {
646 u64 next;
647 u32 type;
648 u32 len;
649 u8 data[0];
650 };
652 Where, the next is a 64-bit physical pointer to the next node of
653 linked list, the next field of the last node is 0; the type is used
654 to identify the contents of data; the len is the length of data
655 field; the data holds the real payload.
657 This list may be modified at a number of points during the bootup
658 process. Therefore, when modifying this list one should always make
659 sure to consider the case where the linked list already contains
660 entries.
662 Field name: pref_address
663 Type: read (reloc)
664 Offset/size: 0x258/8
665 Protocol: 2.10+
667 This field, if nonzero, represents a preferred load address for the
668 kernel. A relocating bootloader should attempt to load at this
669 address if possible.
671 A non-relocatable kernel will unconditionally move itself and to run
672 at this address.
674 Field name: init_size
675 Type: read
676 Offset/size: 0x25c/4
678 This field indicates the amount of linear contiguous memory starting
679 at the kernel runtime start address that the kernel needs before it
680 is capable of examining its memory map. This is not the same thing
681 as the total amount of memory the kernel needs to boot, but it can
682 be used by a relocating boot loader to help select a safe load
683 address for the kernel.
685 The kernel runtime start address is determined by the following algorithm:
687 if (relocatable_kernel)
688 runtime_start = align_up(load_address, kernel_alignment)
689 else
690 runtime_start = pref_address
693 **** THE IMAGE CHECKSUM
695 From boot protocol version 2.08 onwards the CRC-32 is calculated over
696 the entire file using the characteristic polynomial 0x04C11DB7 and an
697 initial remainder of 0xffffffff. The checksum is appended to the
698 file; therefore the CRC of the file up to the limit specified in the
699 syssize field of the header is always 0.
702 **** THE KERNEL COMMAND LINE
704 The kernel command line has become an important way for the boot
705 loader to communicate with the kernel. Some of its options are also
706 relevant to the boot loader itself, see "special command line options"
707 below.
709 The kernel command line is a null-terminated string. The maximum
710 length can be retrieved from the field cmdline_size. Before protocol
711 version 2.06, the maximum was 255 characters. A string that is too
712 long will be automatically truncated by the kernel.
714 If the boot protocol version is 2.02 or later, the address of the
715 kernel command line is given by the header field cmd_line_ptr (see
716 above.) This address can be anywhere between the end of the setup
717 heap and 0xA0000.
719 If the protocol version is *not* 2.02 or higher, the kernel
720 command line is entered using the following protocol:
722 At offset 0x0020 (word), "cmd_line_magic", enter the magic
723 number 0xA33F.
725 At offset 0x0022 (word), "cmd_line_offset", enter the offset
726 of the kernel command line (relative to the start of the
727 real-mode kernel).
729 The kernel command line *must* be within the memory region
730 covered by setup_move_size, so you may need to adjust this
731 field.
734 **** MEMORY LAYOUT OF THE REAL-MODE CODE
736 The real-mode code requires a stack/heap to be set up, as well as
737 memory allocated for the kernel command line. This needs to be done
738 in the real-mode accessible memory in bottom megabyte.
740 It should be noted that modern machines often have a sizable Extended
741 BIOS Data Area (EBDA). As a result, it is advisable to use as little
742 of the low megabyte as possible.
744 Unfortunately, under the following circumstances the 0x90000 memory
745 segment has to be used:
747 - When loading a zImage kernel ((loadflags & 0x01) == 0).
748 - When loading a 2.01 or earlier boot protocol kernel.
750 -> For the 2.00 and 2.01 boot protocols, the real-mode code
751 can be loaded at another address, but it is internally
752 relocated to 0x90000. For the "old" protocol, the
753 real-mode code must be loaded at 0x90000.
755 When loading at 0x90000, avoid using memory above 0x9a000.
757 For boot protocol 2.02 or higher, the command line does not have to be
758 located in the same 64K segment as the real-mode setup code; it is
759 thus permitted to give the stack/heap the full 64K segment and locate
760 the command line above it.
762 The kernel command line should not be located below the real-mode
763 code, nor should it be located in high memory.
766 **** SAMPLE BOOT CONFIGURATION
768 As a sample configuration, assume the following layout of the real
769 mode segment:
771 When loading below 0x90000, use the entire segment:
773 0x0000-0x7fff Real mode kernel
774 0x8000-0xdfff Stack and heap
775 0xe000-0xffff Kernel command line
777 When loading at 0x90000 OR the protocol version is 2.01 or earlier:
779 0x0000-0x7fff Real mode kernel
780 0x8000-0x97ff Stack and heap
781 0x9800-0x9fff Kernel command line
783 Such a boot loader should enter the following fields in the header:
785 unsigned long base_ptr; /* base address for real-mode segment */
787 if ( setup_sects == 0 ) {
788 setup_sects = 4;
789 }
791 if ( protocol >= 0x0200 ) {
792 type_of_loader = <type code>;
793 if ( loading_initrd ) {
794 ramdisk_image = <initrd_address>;
795 ramdisk_size = <initrd_size>;
796 }
798 if ( protocol >= 0x0202 && loadflags & 0x01 )
799 heap_end = 0xe000;
800 else
801 heap_end = 0x9800;
803 if ( protocol >= 0x0201 ) {
804 heap_end_ptr = heap_end - 0x200;
805 loadflags |= 0x80; /* CAN_USE_HEAP */
806 }
808 if ( protocol >= 0x0202 ) {
809 cmd_line_ptr = base_ptr + heap_end;
810 strcpy(cmd_line_ptr, cmdline);
811 } else {
812 cmd_line_magic = 0xA33F;
813 cmd_line_offset = heap_end;
814 setup_move_size = heap_end + strlen(cmdline)+1;
815 strcpy(base_ptr+cmd_line_offset, cmdline);
816 }
817 } else {
818 /* Very old kernel */
820 heap_end = 0x9800;
822 cmd_line_magic = 0xA33F;
823 cmd_line_offset = heap_end;
825 /* A very old kernel MUST have its real-mode code
826 loaded at 0x90000 */
828 if ( base_ptr != 0x90000 ) {
829 /* Copy the real-mode kernel */
830 memcpy(0x90000, base_ptr, (setup_sects+1)*512);
831 base_ptr = 0x90000; /* Relocated */
832 }
834 strcpy(0x90000+cmd_line_offset, cmdline);
836 /* It is recommended to clear memory up to the 32K mark */
837 memset(0x90000 + (setup_sects+1)*512, 0,
838 (64-(setup_sects+1))*512);
839 }
842 **** LOADING THE REST OF THE KERNEL
844 The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
845 in the kernel file (again, if setup_sects == 0 the real value is 4.)
846 It should be loaded at address 0x10000 for Image/zImage kernels and
847 0x100000 for bzImage kernels.
849 The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
850 bit (LOAD_HIGH) in the loadflags field is set:
852 is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
853 load_address = is_bzImage ? 0x100000 : 0x10000;
855 Note that Image/zImage kernels can be up to 512K in size, and thus use
856 the entire 0x10000-0x90000 range of memory. This means it is pretty
857 much a requirement for these kernels to load the real-mode part at
858 0x90000. bzImage kernels allow much more flexibility.
861 **** SPECIAL COMMAND LINE OPTIONS
863 If the command line provided by the boot loader is entered by the
864 user, the user may expect the following command line options to work.
865 They should normally not be deleted from the kernel command line even
866 though not all of them are actually meaningful to the kernel. Boot
867 loader authors who need additional command line options for the boot
868 loader itself should get them registered in
869 Documentation/kernel-parameters.txt to make sure they will not
870 conflict with actual kernel options now or in the future.
872 vga=<mode>
873 <mode> here is either an integer (in C notation, either
874 decimal, octal, or hexadecimal) or one of the strings
875 "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
876 (meaning 0xFFFD). This value should be entered into the
877 vid_mode field, as it is used by the kernel before the command
878 line is parsed.
880 mem=<size>
881 <size> is an integer in C notation optionally followed by
882 (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
883 << 30, << 40, << 50 or << 60). This specifies the end of
884 memory to the kernel. This affects the possible placement of
885 an initrd, since an initrd should be placed near end of
886 memory. Note that this is an option to *both* the kernel and
887 the bootloader!
889 initrd=<file>
890 An initrd should be loaded. The meaning of <file> is
891 obviously bootloader-dependent, and some boot loaders
892 (e.g. LILO) do not have such a command.
894 In addition, some boot loaders add the following options to the
895 user-specified command line:
897 BOOT_IMAGE=<file>
898 The boot image which was loaded. Again, the meaning of <file>
899 is obviously bootloader-dependent.
901 auto
902 The kernel was booted without explicit user intervention.
904 If these options are added by the boot loader, it is highly
905 recommended that they are located *first*, before the user-specified
906 or configuration-specified command line. Otherwise, "init=/bin/sh"
907 gets confused by the "auto" option.
910 **** RUNNING THE KERNEL
912 The kernel is started by jumping to the kernel entry point, which is
913 located at *segment* offset 0x20 from the start of the real mode
914 kernel. This means that if you loaded your real-mode kernel code at
915 0x90000, the kernel entry point is 9020:0000.
917 At entry, ds = es = ss should point to the start of the real-mode
918 kernel code (0x9000 if the code is loaded at 0x90000), sp should be
919 set up properly, normally pointing to the top of the heap, and
920 interrupts should be disabled. Furthermore, to guard against bugs in
921 the kernel, it is recommended that the boot loader sets fs = gs = ds =
922 es = ss.
924 In our example from above, we would do:
926 /* Note: in the case of the "old" kernel protocol, base_ptr must
927 be == 0x90000 at this point; see the previous sample code */
929 seg = base_ptr >> 4;
931 cli(); /* Enter with interrupts disabled! */
933 /* Set up the real-mode kernel stack */
934 _SS = seg;
935 _SP = heap_end;
937 _DS = _ES = _FS = _GS = seg;
938 jmp_far(seg+0x20, 0); /* Run the kernel */
940 If your boot sector accesses a floppy drive, it is recommended to
941 switch off the floppy motor before running the kernel, since the
942 kernel boot leaves interrupts off and thus the motor will not be
943 switched off, especially if the loaded kernel has the floppy driver as
944 a demand-loaded module!
947 **** ADVANCED BOOT LOADER HOOKS
949 If the boot loader runs in a particularly hostile environment (such as
950 LOADLIN, which runs under DOS) it may be impossible to follow the
951 standard memory location requirements. Such a boot loader may use the
952 following hooks that, if set, are invoked by the kernel at the
953 appropriate time. The use of these hooks should probably be
954 considered an absolutely last resort!
956 IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
957 %edi across invocation.
959 realmode_swtch:
960 A 16-bit real mode far subroutine invoked immediately before
961 entering protected mode. The default routine disables NMI, so
962 your routine should probably do so, too.
964 code32_start:
965 A 32-bit flat-mode routine *jumped* to immediately after the
966 transition to protected mode, but before the kernel is
967 uncompressed. No segments, except CS, are guaranteed to be
968 set up (current kernels do, but older ones do not); you should
969 set them up to BOOT_DS (0x18) yourself.
971 After completing your hook, you should jump to the address
972 that was in this field before your boot loader overwrote it
973 (relocated, if appropriate.)
976 **** 32-bit BOOT PROTOCOL
978 For machine with some new BIOS other than legacy BIOS, such as EFI,
979 LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
980 based on legacy BIOS can not be used, so a 32-bit boot protocol needs
981 to be defined.
983 In 32-bit boot protocol, the first step in loading a Linux kernel
984 should be to setup the boot parameters (struct boot_params,
985 traditionally known as "zero page"). The memory for struct boot_params
986 should be allocated and initialized to all zero. Then the setup header
987 from offset 0x01f1 of kernel image on should be loaded into struct
988 boot_params and examined. The end of setup header can be calculated as
989 follow:
991 0x0202 + byte value at offset 0x0201
993 In addition to read/modify/write the setup header of the struct
994 boot_params as that of 16-bit boot protocol, the boot loader should
995 also fill the additional fields of the struct boot_params as that
996 described in zero-page.txt.
998 After setupping the struct boot_params, the boot loader can load the
999 32/64-bit kernel in the same way as that of 16-bit boot protocol.
1001 In 32-bit boot protocol, the kernel is started by jumping to the
1002 32-bit kernel entry point, which is the start address of loaded
1003 32/64-bit kernel.
1005 At entry, the CPU must be in 32-bit protected mode with paging
1006 disabled; a GDT must be loaded with the descriptors for selectors
1007 __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1008 segment; __BOOS_CS must have execute/read permission, and __BOOT_DS
1009 must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1010 must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
1011 address of the struct boot_params; %ebp, %edi and %ebx must be zero.