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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
57**** MEMORY LAYOUT
58
59The traditional memory map for the kernel loader, used for Image or
60zImage kernels, typically looks like:
61
62 | |
630A0000 +------------------------+
64 | Reserved for BIOS | Do not use. Reserved for BIOS EBDA.
6509A000 +------------------------+
66 | Command line |
67 | Stack/heap | For use by the kernel real-mode code.
68098000 +------------------------+
69 | Kernel setup | The kernel real-mode code.
70090200 +------------------------+
71 | Kernel boot sector | The kernel legacy boot sector.
72090000 +------------------------+
73 | Protected-mode kernel | The bulk of the kernel image.
74010000 +------------------------+
75 | Boot loader | <- Boot sector entry point 0000:7C00
76001000 +------------------------+
77 | Reserved for MBR/BIOS |
78000800 +------------------------+
79 | Typically used by MBR |
80000600 +------------------------+
81 | BIOS use only |
82000000 +------------------------+
83
84
85When using bzImage, the protected-mode kernel was relocated to
860x100000 ("high memory"), and the kernel real-mode block (boot sector,
87setup, and stack/heap) was made relocatable to any address between
880x10000 and end of low memory. Unfortunately, in protocols 2.00 and
892.01 the 0x90000+ memory range is still used internally by the kernel;
90the 2.02 protocol resolves that problem.
91
92It is desirable to keep the "memory ceiling" -- the highest point in
93low memory touched by the boot loader -- as low as possible, since
94some newer BIOSes have begun to allocate some rather large amounts of
95memory, called the Extended BIOS Data Area, near the top of low
96memory. The boot loader should use the "INT 12h" BIOS call to verify
97how much low memory is available.
98
99Unfortunately, if INT 12h reports that the amount of memory is too
100low, there is usually nothing the boot loader can do but to report an
101error to the user. The boot loader should therefore be designed to
102take up as little space in low memory as it reasonably can. For
103zImage or old bzImage kernels, which need data written into the
1040x90000 segment, the boot loader should make sure not to use memory
105above the 0x9A000 point; too many BIOSes will break above that point.
106
107For a modern bzImage kernel with boot protocol version >= 2.02, a
108memory layout like the following is suggested:
109
110 ~ ~
111 | Protected-mode kernel |
112100000 +------------------------+
113 | I/O memory hole |
1140A0000 +------------------------+
115 | Reserved for BIOS | Leave as much as possible unused
116 ~ ~
117 | Command line | (Can also be below the X+10000 mark)
118X+10000 +------------------------+
119 | Stack/heap | For use by the kernel real-mode code.
120X+08000 +------------------------+
121 | Kernel setup | The kernel real-mode code.
122 | Kernel boot sector | The kernel legacy boot sector.
123X +------------------------+
124 | Boot loader | <- Boot sector entry point 0000:7C00
125001000 +------------------------+
126 | Reserved for MBR/BIOS |
127000800 +------------------------+
128 | Typically used by MBR |
129000600 +------------------------+
130 | BIOS use only |
131000000 +------------------------+
132
133... where the address X is as low as the design of the boot loader
134permits.
135
136
137**** THE REAL-MODE KERNEL HEADER
138
139In the following text, and anywhere in the kernel boot sequence, "a
140sector" refers to 512 bytes. It is independent of the actual sector
141size of the underlying medium.
142
143The first step in loading a Linux kernel should be to load the
144real-mode code (boot sector and setup code) and then examine the
145following header at offset 0x01f1. The real-mode code can total up to
14632K, although the boot loader may choose to load only the first two
147sectors (1K) and then examine the bootup sector size.
148
149The header looks like:
150
151Offset Proto Name Meaning
152/Size
153
15401F1/1 ALL(1 setup_sects The size of the setup in sectors
15501F2/2 ALL root_flags If set, the root is mounted readonly
15601F4/4 2.04+(2 syssize The size of the 32-bit code in 16-byte paras
15701F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
15801FA/2 ALL vid_mode Video mode control
15901FC/2 ALL root_dev Default root device number
16001FE/2 ALL boot_flag 0xAA55 magic number
1610200/2 2.00+ jump Jump instruction
1620202/4 2.00+ header Magic signature "HdrS"
1630206/2 2.00+ version Boot protocol version supported
1640208/4 2.00+ realmode_swtch Boot loader hook (see below)
165020C/2 2.00+ start_sys_seg The load-low segment (0x1000) (obsolete)
166020E/2 2.00+ kernel_version Pointer to kernel version string
1670210/1 2.00+ type_of_loader Boot loader identifier
1680211/1 2.00+ loadflags Boot protocol option flags
1690212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
1700214/4 2.00+ code32_start Boot loader hook (see below)
1710218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
172021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
1730220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
1740224/2 2.01+ heap_end_ptr Free memory after setup end
1750226/1 2.02+(3 ext_loader_ver Extended boot loader version
1760227/1 2.02+(3 ext_loader_type Extended boot loader ID
1770228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
178022C/4 2.03+ ramdisk_max Highest legal initrd address
1790230/4 2.05+ kernel_alignment Physical addr alignment required for kernel
1800234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
1810235/1 2.10+ min_alignment Minimum alignment, as a power of two
1820236/2 N/A pad3 Unused
1830238/4 2.06+ cmdline_size Maximum size of the kernel command line
184023C/4 2.07+ hardware_subarch Hardware subarchitecture
1850240/8 2.07+ hardware_subarch_data Subarchitecture-specific data
1860248/4 2.08+ payload_offset Offset of kernel payload
187024C/4 2.08+ payload_length Length of kernel payload
1880250/8 2.09+ setup_data 64-bit physical pointer to linked list
189 of struct setup_data
1900258/8 2.10+ pref_address Preferred loading address
1910260/4 2.10+ init_size Linear memory required during initialization
192
193(1) For backwards compatibility, if the setup_sects field contains 0, the
194 real value is 4.
195
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.
199
200(3) Ignored, but safe to set, for boot protocols 2.02-2.09.
201
202If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
203the boot protocol version is "old". Loading an old kernel, the
204following parameters should be assumed:
205
206 Image type = zImage
207 initrd not supported
208 Real-mode kernel must be located at 0x90000.
209
210Otherwise, the "version" field contains the protocol version,
211e.g. protocol version 2.01 will contain 0x0201 in this field. When
212setting fields in the header, you must make sure only to set fields
213supported by the protocol version in use.
214
215
216**** DETAILS OF HEADER FIELDS
217
218For 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
221bootloader ("modify").
222
223All general purpose boot loaders should write the fields marked
224(obligatory). Boot loaders who want to load the kernel at a
225nonstandard address should fill in the fields marked (reloc); other
226boot loaders can ignore those fields.
227
228The byte order of all fields is littleendian (this is x86, after all.)
229
230Field name: setup_sects
231Type: read
232Offset/size: 0x1f1/1
233Protocol: ALL
234
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.
238
239Field name: root_flags
240Type: modify (optional)
241Offset/size: 0x1f2/2
242Protocol: ALL
243
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.
247
248Field name: syssize
249Type: read
250Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
251Protocol: 2.04+
252
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.
257
258Field name: ram_size
259Type: kernel internal
260Offset/size: 0x1f8/2
261Protocol: ALL
262
263 This field is obsolete.
264
265Field name: vid_mode
266Type: modify (obligatory)
267Offset/size: 0x1fa/2
268
269 Please see the section on SPECIAL COMMAND LINE OPTIONS.
270
271Field name: root_dev
272Type: modify (optional)
273Offset/size: 0x1fc/2
274Protocol: ALL
275
276 The default root device device number. The use of this field is
277 deprecated, use the "root=" option on the command line instead.
278
279Field name: boot_flag
280Type: read
281Offset/size: 0x1fe/2
282Protocol: ALL
283
284 Contains 0xAA55. This is the closest thing old Linux kernels have
285 to a magic number.
286
287Field name: jump
288Type: read
289Offset/size: 0x200/2
290Protocol: 2.00+
291
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.
295
296Field name: header
297Type: read
298Offset/size: 0x202/4
299Protocol: 2.00+
300
301 Contains the magic number "HdrS" (0x53726448).
302
303Field name: version
304Type: read
305Offset/size: 0x206/2
306Protocol: 2.00+
307
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.
311
312Field name: realmode_swtch
313Type: modify (optional)
314Offset/size: 0x208/4
315Protocol: 2.00+
316
317 Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
318
319Field name: start_sys_seg
320Type: read
321Offset/size: 0x20c/2
322Protocol: 2.00+
323
324 The load low segment (0x1000). Obsolete.
325
326Field name: kernel_version
327Type: read
328Offset/size: 0x20e/2
329Protocol: 2.00+
330
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).
335
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:
340
341 0x1c00 < 15*0x200 (= 0x1e00) but
342 0x1c00 >= 14*0x200 (= 0x1c00)
343
344 0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
345
346Field name: type_of_loader
347Type: write (obligatory)
348Offset/size: 0x210/1
349Protocol: 2.00+
350
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.
354
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.
359
360 For example, for T = 0x15, V = 0x234, write:
361
362 type_of_loader <- 0xE4
363 ext_loader_type <- 0x05
364 ext_loader_ver <- 0x23
365
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)
381
382 Please contact <hpa@zytor.com> if you need a bootloader ID
383 value assigned.
384
385Field name: loadflags
386Type: modify (obligatory)
387Offset/size: 0x211/1
388Protocol: 2.00+
389
390 This field is a bitmask.
391
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.
395
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.
402
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).
409
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.
414
415Field name: setup_move_size
416Type: modify (obligatory)
417Offset/size: 0x212/2
418Protocol: 2.00-2.01
419
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.
425
426 The unit is bytes starting with the beginning of the boot sector.
427
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.
430
431Field name: code32_start
432Type: modify (optional, reloc)
433Offset/size: 0x214/4
434Protocol: 2.00+
435
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.
439
440 This field can be modified for two purposes:
441
442 1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
443
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.
447
448Field name: ramdisk_image
449Type: write (obligatory)
450Offset/size: 0x218/4
451Protocol: 2.00+
452
453 The 32-bit linear address of the initial ramdisk or ramfs. Leave at
454 zero if there is no initial ramdisk/ramfs.
455
456Field name: ramdisk_size
457Type: write (obligatory)
458Offset/size: 0x21c/4
459Protocol: 2.00+
460
461 Size of the initial ramdisk or ramfs. Leave at zero if there is no
462 initial ramdisk/ramfs.
463
464Field name: bootsect_kludge
465Type: kernel internal
466Offset/size: 0x220/4
467Protocol: 2.00+
468
469 This field is obsolete.
470
471Field name: heap_end_ptr
472Type: write (obligatory)
473Offset/size: 0x224/2
474Protocol: 2.01+
475
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.
478
479Field name: ext_loader_ver
480Type: write (optional)
481Offset/size: 0x226/1
482Protocol: 2.02+
483
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).
487
488 The use of this field is boot loader specific. If not written, it
489 is zero.
490
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.
493
494Field name: ext_loader_type
495Type: write (obligatory if (type_of_loader & 0xf0) == 0xe0)
496Offset/size: 0x227/1
497Protocol: 2.02+
498
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).
502
503 This field is ignored if the type in type_of_loader is not 0xE.
504
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.
507
508Field name: cmd_line_ptr
509Type: write (obligatory)
510Offset/size: 0x228/4
511Protocol: 2.02+
512
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.
517
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.
523
524Field name: ramdisk_max
525Type: read
526Offset/size: 0x22c/4
527Protocol: 2.03+
528
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.)
535
536Field name: kernel_alignment
537Type: read/modify (reloc)
538Offset/size: 0x230/4
539Protocol: 2.05+ (read), 2.10+ (modify)
540
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.
545
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.
550
551Field name: relocatable_kernel
552Type: read (reloc)
553Offset/size: 0x234/1
554Protocol: 2.05+
555
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.
560
561Field name: min_alignment
562Type: read (reloc)
563Offset/size: 0x235/1
564Protocol: 2.10+
565
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:
570
571 kernel_alignment = 1 << min_alignment
572
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.
576
577Field name: cmdline_size
578Type: read
579Offset/size: 0x238/4
580Protocol: 2.06+
581
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.
586
587Field name: hardware_subarch
588Type: write (optional, defaults to x86/PC)
589Offset/size: 0x23c/4
590Protocol: 2.07+
591
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.
595
596 This field allows the bootloader to inform the kernel we are in one
597 one of those environments.
598
599 0x00000000 The default x86/PC environment
600 0x00000001 lguest
601 0x00000002 Xen
602 0x00000003 Moorestown MID
603 0x00000004 CE4100 TV Platform
604
605Field name: hardware_subarch_data
606Type: write (subarch-dependent)
607Offset/size: 0x240/8
608Protocol: 2.07+
609
610 A pointer to data that is specific to hardware subarch
611 This field is currently unused for the default x86/PC environment,
612 do not modify.
613
614Field name: payload_offset
615Type: read
616Offset/size: 0x248/4
617Protocol: 2.08+
618
619 If non-zero then this field contains the offset from the beginning
620 of the protected-mode code to the payload.
621
622 The payload may be compressed. The format of both the compressed and
623 uncompressed data should be determined using the standard magic
624 numbers. The currently supported compression formats are gzip
625 (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA
626 (magic number 5D 00), and XZ (magic number FD 37). The uncompressed
627 payload is currently always ELF (magic number 7F 45 4C 46).
628
629Field name: payload_length
630Type: read
631Offset/size: 0x24c/4
632Protocol: 2.08+
633
634 The length of the payload.
635
636Field name: setup_data
637Type: write (special)
638Offset/size: 0x250/8
639Protocol: 2.09+
640
641 The 64-bit physical pointer to NULL terminated single linked list of
642 struct setup_data. This is used to define a more extensible boot
643 parameters passing mechanism. The definition of struct setup_data is
644 as follow:
645
646 struct setup_data {
647 u64 next;
648 u32 type;
649 u32 len;
650 u8 data[0];
651 };
652
653 Where, the next is a 64-bit physical pointer to the next node of
654 linked list, the next field of the last node is 0; the type is used
655 to identify the contents of data; the len is the length of data
656 field; the data holds the real payload.
657
658 This list may be modified at a number of points during the bootup
659 process. Therefore, when modifying this list one should always make
660 sure to consider the case where the linked list already contains
661 entries.
662
663Field name: pref_address
664Type: read (reloc)
665Offset/size: 0x258/8
666Protocol: 2.10+
667
668 This field, if nonzero, represents a preferred load address for the
669 kernel. A relocating bootloader should attempt to load at this
670 address if possible.
671
672 A non-relocatable kernel will unconditionally move itself and to run
673 at this address.
674
675Field name: init_size
676Type: read
677Offset/size: 0x260/4
678
679 This field indicates the amount of linear contiguous memory starting
680 at the kernel runtime start address that the kernel needs before it
681 is capable of examining its memory map. This is not the same thing
682 as the total amount of memory the kernel needs to boot, but it can
683 be used by a relocating boot loader to help select a safe load
684 address for the kernel.
685
686 The kernel runtime start address is determined by the following algorithm:
687
688 if (relocatable_kernel)
689 runtime_start = align_up(load_address, kernel_alignment)
690 else
691 runtime_start = pref_address
692
693
694**** THE IMAGE CHECKSUM
695
696From boot protocol version 2.08 onwards the CRC-32 is calculated over
697the entire file using the characteristic polynomial 0x04C11DB7 and an
698initial remainder of 0xffffffff. The checksum is appended to the
699file; therefore the CRC of the file up to the limit specified in the
700syssize field of the header is always 0.
701
702
703**** THE KERNEL COMMAND LINE
704
705The kernel command line has become an important way for the boot
706loader to communicate with the kernel. Some of its options are also
707relevant to the boot loader itself, see "special command line options"
708below.
709
710The kernel command line is a null-terminated string. The maximum
711length can be retrieved from the field cmdline_size. Before protocol
712version 2.06, the maximum was 255 characters. A string that is too
713long will be automatically truncated by the kernel.
714
715If the boot protocol version is 2.02 or later, the address of the
716kernel command line is given by the header field cmd_line_ptr (see
717above.) This address can be anywhere between the end of the setup
718heap and 0xA0000.
719
720If the protocol version is *not* 2.02 or higher, the kernel
721command line is entered using the following protocol:
722
723 At offset 0x0020 (word), "cmd_line_magic", enter the magic
724 number 0xA33F.
725
726 At offset 0x0022 (word), "cmd_line_offset", enter the offset
727 of the kernel command line (relative to the start of the
728 real-mode kernel).
729
730 The kernel command line *must* be within the memory region
731 covered by setup_move_size, so you may need to adjust this
732 field.
733
734
735**** MEMORY LAYOUT OF THE REAL-MODE CODE
736
737The real-mode code requires a stack/heap to be set up, as well as
738memory allocated for the kernel command line. This needs to be done
739in the real-mode accessible memory in bottom megabyte.
740
741It should be noted that modern machines often have a sizable Extended
742BIOS Data Area (EBDA). As a result, it is advisable to use as little
743of the low megabyte as possible.
744
745Unfortunately, under the following circumstances the 0x90000 memory
746segment has to be used:
747
748 - When loading a zImage kernel ((loadflags & 0x01) == 0).
749 - When loading a 2.01 or earlier boot protocol kernel.
750
751 -> For the 2.00 and 2.01 boot protocols, the real-mode code
752 can be loaded at another address, but it is internally
753 relocated to 0x90000. For the "old" protocol, the
754 real-mode code must be loaded at 0x90000.
755
756When loading at 0x90000, avoid using memory above 0x9a000.
757
758For boot protocol 2.02 or higher, the command line does not have to be
759located in the same 64K segment as the real-mode setup code; it is
760thus permitted to give the stack/heap the full 64K segment and locate
761the command line above it.
762
763The kernel command line should not be located below the real-mode
764code, nor should it be located in high memory.
765
766
767**** SAMPLE BOOT CONFIGURATION
768
769As a sample configuration, assume the following layout of the real
770mode segment:
771
772 When loading below 0x90000, use the entire segment:
773
774 0x0000-0x7fff Real mode kernel
775 0x8000-0xdfff Stack and heap
776 0xe000-0xffff Kernel command line
777
778 When loading at 0x90000 OR the protocol version is 2.01 or earlier:
779
780 0x0000-0x7fff Real mode kernel
781 0x8000-0x97ff Stack and heap
782 0x9800-0x9fff Kernel command line
783
784Such a boot loader should enter the following fields in the header:
785
786 unsigned long base_ptr; /* base address for real-mode segment */
787
788 if ( setup_sects == 0 ) {
789 setup_sects = 4;
790 }
791
792 if ( protocol >= 0x0200 ) {
793 type_of_loader = <type code>;
794 if ( loading_initrd ) {
795 ramdisk_image = <initrd_address>;
796 ramdisk_size = <initrd_size>;
797 }
798
799 if ( protocol >= 0x0202 && loadflags & 0x01 )
800 heap_end = 0xe000;
801 else
802 heap_end = 0x9800;
803
804 if ( protocol >= 0x0201 ) {
805 heap_end_ptr = heap_end - 0x200;
806 loadflags |= 0x80; /* CAN_USE_HEAP */
807 }
808
809 if ( protocol >= 0x0202 ) {
810 cmd_line_ptr = base_ptr + heap_end;
811 strcpy(cmd_line_ptr, cmdline);
812 } else {
813 cmd_line_magic = 0xA33F;
814 cmd_line_offset = heap_end;
815 setup_move_size = heap_end + strlen(cmdline)+1;
816 strcpy(base_ptr+cmd_line_offset, cmdline);
817 }
818 } else {
819 /* Very old kernel */
820
821 heap_end = 0x9800;
822
823 cmd_line_magic = 0xA33F;
824 cmd_line_offset = heap_end;
825
826 /* A very old kernel MUST have its real-mode code
827 loaded at 0x90000 */
828
829 if ( base_ptr != 0x90000 ) {
830 /* Copy the real-mode kernel */
831 memcpy(0x90000, base_ptr, (setup_sects+1)*512);
832 base_ptr = 0x90000; /* Relocated */
833 }
834
835 strcpy(0x90000+cmd_line_offset, cmdline);
836
837 /* It is recommended to clear memory up to the 32K mark */
838 memset(0x90000 + (setup_sects+1)*512, 0,
839 (64-(setup_sects+1))*512);
840 }
841
842
843**** LOADING THE REST OF THE KERNEL
844
845The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
846in the kernel file (again, if setup_sects == 0 the real value is 4.)
847It should be loaded at address 0x10000 for Image/zImage kernels and
8480x100000 for bzImage kernels.
849
850The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
851bit (LOAD_HIGH) in the loadflags field is set:
852
853 is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
854 load_address = is_bzImage ? 0x100000 : 0x10000;
855
856Note that Image/zImage kernels can be up to 512K in size, and thus use
857the entire 0x10000-0x90000 range of memory. This means it is pretty
858much a requirement for these kernels to load the real-mode part at
8590x90000. bzImage kernels allow much more flexibility.
860
861
862**** SPECIAL COMMAND LINE OPTIONS
863
864If the command line provided by the boot loader is entered by the
865user, the user may expect the following command line options to work.
866They should normally not be deleted from the kernel command line even
867though not all of them are actually meaningful to the kernel. Boot
868loader authors who need additional command line options for the boot
869loader itself should get them registered in
870Documentation/kernel-parameters.txt to make sure they will not
871conflict with actual kernel options now or in the future.
872
873 vga=<mode>
874 <mode> here is either an integer (in C notation, either
875 decimal, octal, or hexadecimal) or one of the strings
876 "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
877 (meaning 0xFFFD). This value should be entered into the
878 vid_mode field, as it is used by the kernel before the command
879 line is parsed.
880
881 mem=<size>
882 <size> is an integer in C notation optionally followed by
883 (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
884 << 30, << 40, << 50 or << 60). This specifies the end of
885 memory to the kernel. This affects the possible placement of
886 an initrd, since an initrd should be placed near end of
887 memory. Note that this is an option to *both* the kernel and
888 the bootloader!
889
890 initrd=<file>
891 An initrd should be loaded. The meaning of <file> is
892 obviously bootloader-dependent, and some boot loaders
893 (e.g. LILO) do not have such a command.
894
895In addition, some boot loaders add the following options to the
896user-specified command line:
897
898 BOOT_IMAGE=<file>
899 The boot image which was loaded. Again, the meaning of <file>
900 is obviously bootloader-dependent.
901
902 auto
903 The kernel was booted without explicit user intervention.
904
905If these options are added by the boot loader, it is highly
906recommended that they are located *first*, before the user-specified
907or configuration-specified command line. Otherwise, "init=/bin/sh"
908gets confused by the "auto" option.
909
910
911**** RUNNING THE KERNEL
912
913The kernel is started by jumping to the kernel entry point, which is
914located at *segment* offset 0x20 from the start of the real mode
915kernel. This means that if you loaded your real-mode kernel code at
9160x90000, the kernel entry point is 9020:0000.
917
918At entry, ds = es = ss should point to the start of the real-mode
919kernel code (0x9000 if the code is loaded at 0x90000), sp should be
920set up properly, normally pointing to the top of the heap, and
921interrupts should be disabled. Furthermore, to guard against bugs in
922the kernel, it is recommended that the boot loader sets fs = gs = ds =
923es = ss.
924
925In our example from above, we would do:
926
927 /* Note: in the case of the "old" kernel protocol, base_ptr must
928 be == 0x90000 at this point; see the previous sample code */
929
930 seg = base_ptr >> 4;
931
932 cli(); /* Enter with interrupts disabled! */
933
934 /* Set up the real-mode kernel stack */
935 _SS = seg;
936 _SP = heap_end;
937
938 _DS = _ES = _FS = _GS = seg;
939 jmp_far(seg+0x20, 0); /* Run the kernel */
940
941If your boot sector accesses a floppy drive, it is recommended to
942switch off the floppy motor before running the kernel, since the
943kernel boot leaves interrupts off and thus the motor will not be
944switched off, especially if the loaded kernel has the floppy driver as
945a demand-loaded module!
946
947
948**** ADVANCED BOOT LOADER HOOKS
949
950If the boot loader runs in a particularly hostile environment (such as
951LOADLIN, which runs under DOS) it may be impossible to follow the
952standard memory location requirements. Such a boot loader may use the
953following hooks that, if set, are invoked by the kernel at the
954appropriate time. The use of these hooks should probably be
955considered an absolutely last resort!
956
957IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
958%edi across invocation.
959
960 realmode_swtch:
961 A 16-bit real mode far subroutine invoked immediately before
962 entering protected mode. The default routine disables NMI, so
963 your routine should probably do so, too.
964
965 code32_start:
966 A 32-bit flat-mode routine *jumped* to immediately after the
967 transition to protected mode, but before the kernel is
968 uncompressed. No segments, except CS, are guaranteed to be
969 set up (current kernels do, but older ones do not); you should
970 set them up to BOOT_DS (0x18) yourself.
971
972 After completing your hook, you should jump to the address
973 that was in this field before your boot loader overwrote it
974 (relocated, if appropriate.)
975
976
977**** 32-bit BOOT PROTOCOL
978
979For machine with some new BIOS other than legacy BIOS, such as EFI,
980LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
981based on legacy BIOS can not be used, so a 32-bit boot protocol needs
982to be defined.
983
984In 32-bit boot protocol, the first step in loading a Linux kernel
985should be to setup the boot parameters (struct boot_params,
986traditionally known as "zero page"). The memory for struct boot_params
987should be allocated and initialized to all zero. Then the setup header
988from offset 0x01f1 of kernel image on should be loaded into struct
989boot_params and examined. The end of setup header can be calculated as
990follow:
991
992 0x0202 + byte value at offset 0x0201
993
994In addition to read/modify/write the setup header of the struct
995boot_params as that of 16-bit boot protocol, the boot loader should
996also fill the additional fields of the struct boot_params as that
997described in zero-page.txt.
998
999After setupping the struct boot_params, the boot loader can load the
100032/64-bit kernel in the same way as that of 16-bit boot protocol.
1001
1002In 32-bit boot protocol, the kernel is started by jumping to the
100332-bit kernel entry point, which is the start address of loaded
100432/64-bit kernel.
1005
1006At entry, the CPU must be in 32-bit protected mode with paging
1007disabled; a GDT must be loaded with the descriptors for selectors
1008__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1009segment; __BOOS_CS must have execute/read permission, and __BOOT_DS
1010must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1011must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
1012address of the struct boot_params; %ebp, %edi and %ebx must be zero.