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H. Peter Anvin4039feb2008-05-30 17:16:20 -07001 THE LINUX/x86 BOOT PROTOCOL
2 ---------------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -07003
H. Peter Anvin4039feb2008-05-30 17:16:20 -07004On the x86 platform, the Linux kernel uses a rather complicated boot
Linus Torvalds1da177e2005-04-16 15:20:36 -07005convention. 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
H. Peter Anvin4039feb2008-05-30 17:16:20 -070011Currently, the following versions of the Linux/x86 boot protocol exist.
Linus Torvalds1da177e2005-04-16 15:20:36 -070012
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
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -070034Protocol 2.04: (Kernel 2.6.14) Extend the syssize field to four bytes.
Bernhard Walle8f9aeca2007-05-02 19:27:10 +020035
Vivek Goyalbe274ee2006-12-07 02:14:04 +010036Protocol 2.05: (Kernel 2.6.20) Make protected mode kernel relocatable.
37 Introduce relocatable_kernel and kernel_alignment fields.
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -070038
Bernhard Walle8f9aeca2007-05-02 19:27:10 +020039Protocol 2.06: (Kernel 2.6.22) Added a field that contains the size of
Ian Campbell4c0587e2008-04-27 12:21:11 +010040 the boot command line.
Bernhard Walle8f9aeca2007-05-02 19:27:10 +020041
Ian Campbell4c0587e2008-04-27 12:21:11 +010042Protocol 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
Baodong Chen2f6de3a2009-01-03 12:37:06 +080047 payload. Introduced payload_offset and payload_length
Ian Campbell4c0587e2008-04-27 12:21:11 +010048 fields to aid in locating the payload.
49
50Protocol 2.09: (Kernel 2.6.26) Added a field of 64-bit physical
Huang, Yingfb884382008-03-28 10:49:50 +080051 pointer to single linked list of struct setup_data.
Linus Torvalds1da177e2005-04-16 15:20:36 -070052
53**** MEMORY LAYOUT
54
55The traditional memory map for the kernel loader, used for Image or
56zImage kernels, typically looks like:
57
58 | |
590A0000 +------------------------+
60 | Reserved for BIOS | Do not use. Reserved for BIOS EBDA.
6109A000 +------------------------+
H. Peter Anvindec04cf2007-05-17 15:50:47 -070062 | Command line |
63 | Stack/heap | For use by the kernel real-mode code.
Linus Torvalds1da177e2005-04-16 15:20:36 -070064098000 +------------------------+
65 | Kernel setup | The kernel real-mode code.
66090200 +------------------------+
67 | Kernel boot sector | The kernel legacy boot sector.
68090000 +------------------------+
69 | Protected-mode kernel | The bulk of the kernel image.
70010000 +------------------------+
71 | Boot loader | <- Boot sector entry point 0000:7C00
72001000 +------------------------+
73 | Reserved for MBR/BIOS |
74000800 +------------------------+
75 | Typically used by MBR |
76000600 +------------------------+
77 | BIOS use only |
78000000 +------------------------+
79
80
81When using bzImage, the protected-mode kernel was relocated to
820x100000 ("high memory"), and the kernel real-mode block (boot sector,
83setup, and stack/heap) was made relocatable to any address between
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700840x10000 and end of low memory. Unfortunately, in protocols 2.00 and
852.01 the 0x90000+ memory range is still used internally by the kernel;
86the 2.02 protocol resolves that problem.
Linus Torvalds1da177e2005-04-16 15:20:36 -070087
88It is desirable to keep the "memory ceiling" -- the highest point in
89low memory touched by the boot loader -- as low as possible, since
90some newer BIOSes have begun to allocate some rather large amounts of
91memory, called the Extended BIOS Data Area, near the top of low
92memory. The boot loader should use the "INT 12h" BIOS call to verify
93how much low memory is available.
94
95Unfortunately, if INT 12h reports that the amount of memory is too
96low, there is usually nothing the boot loader can do but to report an
97error to the user. The boot loader should therefore be designed to
98take up as little space in low memory as it reasonably can. For
99zImage or old bzImage kernels, which need data written into the
1000x90000 segment, the boot loader should make sure not to use memory
101above the 0x9A000 point; too many BIOSes will break above that point.
102
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700103For a modern bzImage kernel with boot protocol version >= 2.02, a
104memory layout like the following is suggested:
105
106 ~ ~
107 | Protected-mode kernel |
108100000 +------------------------+
109 | I/O memory hole |
1100A0000 +------------------------+
111 | Reserved for BIOS | Leave as much as possible unused
112 ~ ~
113 | Command line | (Can also be below the X+10000 mark)
114X+10000 +------------------------+
115 | Stack/heap | For use by the kernel real-mode code.
116X+08000 +------------------------+
117 | Kernel setup | The kernel real-mode code.
118 | Kernel boot sector | The kernel legacy boot sector.
119X +------------------------+
120 | Boot loader | <- Boot sector entry point 0000:7C00
121001000 +------------------------+
122 | Reserved for MBR/BIOS |
123000800 +------------------------+
124 | Typically used by MBR |
125000600 +------------------------+
126 | BIOS use only |
127000000 +------------------------+
128
129... where the address X is as low as the design of the boot loader
130permits.
131
Linus Torvalds1da177e2005-04-16 15:20:36 -0700132
133**** THE REAL-MODE KERNEL HEADER
134
135In the following text, and anywhere in the kernel boot sequence, "a
136sector" refers to 512 bytes. It is independent of the actual sector
137size of the underlying medium.
138
139The first step in loading a Linux kernel should be to load the
140real-mode code (boot sector and setup code) and then examine the
141following header at offset 0x01f1. The real-mode code can total up to
14232K, although the boot loader may choose to load only the first two
143sectors (1K) and then examine the bootup sector size.
144
145The header looks like:
146
147Offset Proto Name Meaning
148/Size
149
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -070015001F1/1 ALL(1 setup_sects The size of the setup in sectors
Linus Torvalds1da177e2005-04-16 15:20:36 -070015101F2/2 ALL root_flags If set, the root is mounted readonly
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -070015201F4/4 2.04+(2 syssize The size of the 32-bit code in 16-byte paras
Linus Torvalds1da177e2005-04-16 15:20:36 -070015301F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
15401FA/2 ALL vid_mode Video mode control
15501FC/2 ALL root_dev Default root device number
15601FE/2 ALL boot_flag 0xAA55 magic number
1570200/2 2.00+ jump Jump instruction
1580202/4 2.00+ header Magic signature "HdrS"
1590206/2 2.00+ version Boot protocol version supported
1600208/4 2.00+ realmode_swtch Boot loader hook (see below)
161020C/2 2.00+ start_sys The load-low segment (0x1000) (obsolete)
162020E/2 2.00+ kernel_version Pointer to kernel version string
1630210/1 2.00+ type_of_loader Boot loader identifier
1640211/1 2.00+ loadflags Boot protocol option flags
1650212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
1660214/4 2.00+ code32_start Boot loader hook (see below)
1670218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
168021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
1690220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
1700224/2 2.01+ heap_end_ptr Free memory after setup end
1710226/2 N/A pad1 Unused
1720228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
173022C/4 2.03+ initrd_addr_max Highest legal initrd address
Vivek Goyald263b212006-12-07 02:14:13 +01001740230/4 2.05+ kernel_alignment Physical addr alignment required for kernel
1750234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
Bernhard Walle8f9aeca2007-05-02 19:27:10 +02001760235/3 N/A pad2 Unused
1770238/4 2.06+ cmdline_size Maximum size of the kernel command line
Rusty Russelle5371ac2007-10-21 16:41:33 -0700178023C/4 2.07+ hardware_subarch Hardware subarchitecture
1790240/8 2.07+ hardware_subarch_data Subarchitecture-specific data
Ian Campbell87253d12008-02-19 11:12:30 +00001800248/4 2.08+ payload_offset Offset of kernel payload
181024C/4 2.08+ payload_length Length of kernel payload
Huang, Yingfb884382008-03-28 10:49:50 +08001820250/8 2.09+ setup_data 64-bit physical pointer to linked list
183 of struct setup_data
Linus Torvalds1da177e2005-04-16 15:20:36 -0700184
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -0700185(1) For backwards compatibility, if the setup_sects field contains 0, the
186 real value is 4.
187
188(2) For boot protocol prior to 2.04, the upper two bytes of the syssize
189 field are unusable, which means the size of a bzImage kernel
190 cannot be determined.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700191
192If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
193the boot protocol version is "old". Loading an old kernel, the
194following parameters should be assumed:
195
196 Image type = zImage
197 initrd not supported
198 Real-mode kernel must be located at 0x90000.
199
200Otherwise, the "version" field contains the protocol version,
201e.g. protocol version 2.01 will contain 0x0201 in this field. When
202setting fields in the header, you must make sure only to set fields
203supported by the protocol version in use.
204
Linus Torvalds1da177e2005-04-16 15:20:36 -0700205
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700206**** DETAILS OF HEADER FIELDS
Linus Torvalds1da177e2005-04-16 15:20:36 -0700207
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700208For each field, some are information from the kernel to the bootloader
209("read"), some are expected to be filled out by the bootloader
210("write"), and some are expected to be read and modified by the
211bootloader ("modify").
Linus Torvalds1da177e2005-04-16 15:20:36 -0700212
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700213All general purpose boot loaders should write the fields marked
214(obligatory). Boot loaders who want to load the kernel at a
215nonstandard address should fill in the fields marked (reloc); other
216boot loaders can ignore those fields.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700217
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700218The byte order of all fields is littleendian (this is x86, after all.)
219
Rusty Russelle5371ac2007-10-21 16:41:33 -0700220Field name: setup_sects
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700221Type: read
222Offset/size: 0x1f1/1
223Protocol: ALL
224
225 The size of the setup code in 512-byte sectors. If this field is
226 0, the real value is 4. The real-mode code consists of the boot
227 sector (always one 512-byte sector) plus the setup code.
228
229Field name: root_flags
230Type: modify (optional)
231Offset/size: 0x1f2/2
232Protocol: ALL
233
234 If this field is nonzero, the root defaults to readonly. The use of
235 this field is deprecated; use the "ro" or "rw" options on the
236 command line instead.
237
238Field name: syssize
239Type: read
240Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
241Protocol: 2.04+
242
243 The size of the protected-mode code in units of 16-byte paragraphs.
244 For protocol versions older than 2.04 this field is only two bytes
245 wide, and therefore cannot be trusted for the size of a kernel if
246 the LOAD_HIGH flag is set.
247
248Field name: ram_size
249Type: kernel internal
250Offset/size: 0x1f8/2
251Protocol: ALL
252
253 This field is obsolete.
254
255Field name: vid_mode
256Type: modify (obligatory)
257Offset/size: 0x1fa/2
258
259 Please see the section on SPECIAL COMMAND LINE OPTIONS.
260
261Field name: root_dev
262Type: modify (optional)
263Offset/size: 0x1fc/2
264Protocol: ALL
265
266 The default root device device number. The use of this field is
267 deprecated, use the "root=" option on the command line instead.
268
269Field name: boot_flag
270Type: read
271Offset/size: 0x1fe/2
272Protocol: ALL
273
274 Contains 0xAA55. This is the closest thing old Linux kernels have
275 to a magic number.
276
277Field name: jump
278Type: read
279Offset/size: 0x200/2
280Protocol: 2.00+
281
282 Contains an x86 jump instruction, 0xEB followed by a signed offset
283 relative to byte 0x202. This can be used to determine the size of
284 the header.
285
286Field name: header
287Type: read
288Offset/size: 0x202/4
289Protocol: 2.00+
290
291 Contains the magic number "HdrS" (0x53726448).
292
293Field name: version
294Type: read
295Offset/size: 0x206/2
296Protocol: 2.00+
297
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700298 Contains the boot protocol version, in (major << 8)+minor format,
299 e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
300 10.17.
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700301
302Field name: readmode_swtch
303Type: modify (optional)
304Offset/size: 0x208/4
305Protocol: 2.00+
306
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700307 Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700308
309Field name: start_sys
310Type: read
H. Peter Anvina021e512008-07-22 15:33:57 -0400311Offset/size: 0x20c/2
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700312Protocol: 2.00+
313
314 The load low segment (0x1000). Obsolete.
315
316Field name: kernel_version
317Type: read
318Offset/size: 0x20e/2
319Protocol: 2.00+
320
321 If set to a nonzero value, contains a pointer to a NUL-terminated
322 human-readable kernel version number string, less 0x200. This can
323 be used to display the kernel version to the user. This value
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700324 should be less than (0x200*setup_sects).
325
326 For example, if this value is set to 0x1c00, the kernel version
327 number string can be found at offset 0x1e00 in the kernel file.
328 This is a valid value if and only if the "setup_sects" field
329 contains the value 15 or higher, as:
330
331 0x1c00 < 15*0x200 (= 0x1e00) but
332 0x1c00 >= 14*0x200 (= 0x1c00)
333
334 0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700335
336Field name: type_of_loader
337Type: write (obligatory)
338Offset/size: 0x210/1
339Protocol: 2.00+
340
341 If your boot loader has an assigned id (see table below), enter
342 0xTV here, where T is an identifier for the boot loader and V is
343 a version number. Otherwise, enter 0xFF here.
344
345 Assigned boot loader ids:
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700346 0 LILO (0x00 reserved for pre-2.00 bootloader)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700347 1 Loadlin
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700348 2 bootsect-loader (0x20, all other values reserved)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700349 3 SYSLINUX
350 4 EtherBoot
351 5 ELILO
KOSAKI Motohiro9ee670f2008-11-11 11:49:01 +0900352 7 GRUB
Linus Torvalds1da177e2005-04-16 15:20:36 -0700353 8 U-BOOT
Jeremy Fitzhardinge354332e2006-09-12 20:35:57 -0700354 9 Xen
H. Peter Anvinc229ec52007-01-26 09:15:47 -0800355 A Gujin
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700356 B Qemu
Linus Torvalds1da177e2005-04-16 15:20:36 -0700357
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700358 Please contact <hpa@zytor.com> if you need a bootloader ID
359 value assigned.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700360
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700361Field name: loadflags
362Type: modify (obligatory)
363Offset/size: 0x211/1
364Protocol: 2.00+
Linus Torvalds1da177e2005-04-16 15:20:36 -0700365
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700366 This field is a bitmask.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700367
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700368 Bit 0 (read): LOADED_HIGH
369 - If 0, the protected-mode code is loaded at 0x10000.
370 - If 1, the protected-mode code is loaded at 0x100000.
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700371
H. Peter Anvin4039feb2008-05-30 17:16:20 -0700372 Bit 5 (write): QUIET_FLAG
373 - If 0, print early messages.
374 - If 1, suppress early messages.
375 This requests to the kernel (decompressor and early
376 kernel) to not write early messages that require
377 accessing the display hardware directly.
378
Rusty Russelle5371ac2007-10-21 16:41:33 -0700379 Bit 6 (write): KEEP_SEGMENTS
380 Protocol: 2.07+
H. Peter Anvin4039feb2008-05-30 17:16:20 -0700381 - If 0, reload the segment registers in the 32bit entry point.
382 - If 1, do not reload the segment registers in the 32bit entry point.
Rusty Russelle5371ac2007-10-21 16:41:33 -0700383 Assume that %cs %ds %ss %es are all set to flat segments with
384 a base of 0 (or the equivalent for their environment).
385
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700386 Bit 7 (write): CAN_USE_HEAP
387 Set this bit to 1 to indicate that the value entered in the
388 heap_end_ptr is valid. If this field is clear, some setup code
389 functionality will be disabled.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700390
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700391Field name: setup_move_size
392Type: modify (obligatory)
393Offset/size: 0x212/2
394Protocol: 2.00-2.01
Linus Torvalds1da177e2005-04-16 15:20:36 -0700395
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700396 When using protocol 2.00 or 2.01, if the real mode kernel is not
397 loaded at 0x90000, it gets moved there later in the loading
398 sequence. Fill in this field if you want additional data (such as
399 the kernel command line) moved in addition to the real-mode kernel
400 itself.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700401
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700402 The unit is bytes starting with the beginning of the boot sector.
403
404 This field is can be ignored when the protocol is 2.02 or higher, or
405 if the real-mode code is loaded at 0x90000.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700406
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700407Field name: code32_start
408Type: modify (optional, reloc)
409Offset/size: 0x214/4
410Protocol: 2.00+
411
412 The address to jump to in protected mode. This defaults to the load
413 address of the kernel, and can be used by the boot loader to
414 determine the proper load address.
415
416 This field can be modified for two purposes:
417
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700418 1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700419
420 2. if a bootloader which does not install a hook loads a
421 relocatable kernel at a nonstandard address it will have to modify
422 this field to point to the load address.
423
424Field name: ramdisk_image
425Type: write (obligatory)
426Offset/size: 0x218/4
427Protocol: 2.00+
428
429 The 32-bit linear address of the initial ramdisk or ramfs. Leave at
430 zero if there is no initial ramdisk/ramfs.
431
432Field name: ramdisk_size
433Type: write (obligatory)
434Offset/size: 0x21c/4
435Protocol: 2.00+
436
437 Size of the initial ramdisk or ramfs. Leave at zero if there is no
438 initial ramdisk/ramfs.
439
440Field name: bootsect_kludge
441Type: kernel internal
442Offset/size: 0x220/4
443Protocol: 2.00+
444
445 This field is obsolete.
446
447Field name: heap_end_ptr
448Type: write (obligatory)
449Offset/size: 0x224/2
450Protocol: 2.01+
451
452 Set this field to the offset (from the beginning of the real-mode
453 code) of the end of the setup stack/heap, minus 0x0200.
454
455Field name: cmd_line_ptr
456Type: write (obligatory)
457Offset/size: 0x228/4
458Protocol: 2.02+
459
460 Set this field to the linear address of the kernel command line.
461 The kernel command line can be located anywhere between the end of
462 the setup heap and 0xA0000; it does not have to be located in the
463 same 64K segment as the real-mode code itself.
464
465 Fill in this field even if your boot loader does not support a
466 command line, in which case you can point this to an empty string
467 (or better yet, to the string "auto".) If this field is left at
468 zero, the kernel will assume that your boot loader does not support
469 the 2.02+ protocol.
470
471Field name: initrd_addr_max
472Type: read
473Offset/size: 0x22c/4
474Protocol: 2.03+
475
476 The maximum address that may be occupied by the initial
477 ramdisk/ramfs contents. For boot protocols 2.02 or earlier, this
478 field is not present, and the maximum address is 0x37FFFFFF. (This
479 address is defined as the address of the highest safe byte, so if
480 your ramdisk is exactly 131072 bytes long and this field is
481 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
482
483Field name: kernel_alignment
484Type: read (reloc)
485Offset/size: 0x230/4
486Protocol: 2.05+
487
488 Alignment unit required by the kernel (if relocatable_kernel is true.)
489
490Field name: relocatable_kernel
491Type: read (reloc)
492Offset/size: 0x234/1
493Protocol: 2.05+
494
495 If this field is nonzero, the protected-mode part of the kernel can
496 be loaded at any address that satisfies the kernel_alignment field.
497 After loading, the boot loader must set the code32_start field to
498 point to the loaded code, or to a boot loader hook.
499
500Field name: cmdline_size
501Type: read
502Offset/size: 0x238/4
503Protocol: 2.06+
504
505 The maximum size of the command line without the terminating
506 zero. This means that the command line can contain at most
507 cmdline_size characters. With protocol version 2.05 and earlier, the
508 maximum size was 255.
Bernhard Walle8f9aeca2007-05-02 19:27:10 +0200509
Rusty Russelle5371ac2007-10-21 16:41:33 -0700510Field name: hardware_subarch
H. Peter Anvin4039feb2008-05-30 17:16:20 -0700511Type: write (optional, defaults to x86/PC)
Rusty Russelle5371ac2007-10-21 16:41:33 -0700512Offset/size: 0x23c/4
513Protocol: 2.07+
514
515 In a paravirtualized environment the hardware low level architectural
516 pieces such as interrupt handling, page table handling, and
517 accessing process control registers needs to be done differently.
518
519 This field allows the bootloader to inform the kernel we are in one
520 one of those environments.
521
522 0x00000000 The default x86/PC environment
523 0x00000001 lguest
524 0x00000002 Xen
525
526Field name: hardware_subarch_data
H. Peter Anvin4039feb2008-05-30 17:16:20 -0700527Type: write (subarch-dependent)
Rusty Russelle5371ac2007-10-21 16:41:33 -0700528Offset/size: 0x240/8
529Protocol: 2.07+
530
531 A pointer to data that is specific to hardware subarch
H. Peter Anvin4039feb2008-05-30 17:16:20 -0700532 This field is currently unused for the default x86/PC environment,
533 do not modify.
Rusty Russelle5371ac2007-10-21 16:41:33 -0700534
Ian Campbell87253d12008-02-19 11:12:30 +0000535Field name: payload_offset
Ian Campbell099e1372008-02-13 20:54:58 +0000536Type: read
537Offset/size: 0x248/4
538Protocol: 2.08+
539
Philipp Kohlbechere1286f22008-12-14 18:50:01 +0100540 If non-zero then this field contains the offset from the beginning
541 of the protected-mode code to the payload.
Ian Campbell87253d12008-02-19 11:12:30 +0000542
543 The payload may be compressed. The format of both the compressed and
544 uncompressed data should be determined using the standard magic
545 numbers. Currently only gzip compressed ELF is used.
Ian Campbell099e1372008-02-13 20:54:58 +0000546
Ian Campbell87253d12008-02-19 11:12:30 +0000547Field name: payload_length
Ian Campbell099e1372008-02-13 20:54:58 +0000548Type: read
549Offset/size: 0x24c/4
550Protocol: 2.08+
551
Ian Campbell87253d12008-02-19 11:12:30 +0000552 The length of the payload.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700553
H. Peter Anvin4039feb2008-05-30 17:16:20 -0700554Field name: setup_data
555Type: write (special)
556Offset/size: 0x250/8
557Protocol: 2.09+
558
559 The 64-bit physical pointer to NULL terminated single linked list of
560 struct setup_data. This is used to define a more extensible boot
561 parameters passing mechanism. The definition of struct setup_data is
562 as follow:
563
564 struct setup_data {
565 u64 next;
566 u32 type;
567 u32 len;
568 u8 data[0];
569 };
570
571 Where, the next is a 64-bit physical pointer to the next node of
572 linked list, the next field of the last node is 0; the type is used
573 to identify the contents of data; the len is the length of data
574 field; the data holds the real payload.
575
576 This list may be modified at a number of points during the bootup
577 process. Therefore, when modifying this list one should always make
578 sure to consider the case where the linked list already contains
579 entries.
580
581
Ian Campbell7d6e7372008-02-17 20:06:35 +0100582**** THE IMAGE CHECKSUM
583
584From boot protocol version 2.08 onwards the CRC-32 is calculated over
585the entire file using the characteristic polynomial 0x04C11DB7 and an
586initial remainder of 0xffffffff. The checksum is appended to the
587file; therefore the CRC of the file up to the limit specified in the
588syssize field of the header is always 0.
589
H. Peter Anvin4039feb2008-05-30 17:16:20 -0700590
Linus Torvalds1da177e2005-04-16 15:20:36 -0700591**** THE KERNEL COMMAND LINE
592
593The kernel command line has become an important way for the boot
594loader to communicate with the kernel. Some of its options are also
595relevant to the boot loader itself, see "special command line options"
596below.
597
Bernhard Walle8f9aeca2007-05-02 19:27:10 +0200598The kernel command line is a null-terminated string. The maximum
599length can be retrieved from the field cmdline_size. Before protocol
600version 2.06, the maximum was 255 characters. A string that is too
601long will be automatically truncated by the kernel.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700602
603If the boot protocol version is 2.02 or later, the address of the
604kernel command line is given by the header field cmd_line_ptr (see
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -0700605above.) This address can be anywhere between the end of the setup
606heap and 0xA0000.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700607
608If the protocol version is *not* 2.02 or higher, the kernel
609command line is entered using the following protocol:
610
611 At offset 0x0020 (word), "cmd_line_magic", enter the magic
612 number 0xA33F.
613
614 At offset 0x0022 (word), "cmd_line_offset", enter the offset
615 of the kernel command line (relative to the start of the
616 real-mode kernel).
617
618 The kernel command line *must* be within the memory region
619 covered by setup_move_size, so you may need to adjust this
620 field.
621
622
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700623**** MEMORY LAYOUT OF THE REAL-MODE CODE
624
625The real-mode code requires a stack/heap to be set up, as well as
626memory allocated for the kernel command line. This needs to be done
627in the real-mode accessible memory in bottom megabyte.
628
629It should be noted that modern machines often have a sizable Extended
630BIOS Data Area (EBDA). As a result, it is advisable to use as little
631of the low megabyte as possible.
632
633Unfortunately, under the following circumstances the 0x90000 memory
634segment has to be used:
635
636 - When loading a zImage kernel ((loadflags & 0x01) == 0).
637 - When loading a 2.01 or earlier boot protocol kernel.
638
639 -> For the 2.00 and 2.01 boot protocols, the real-mode code
640 can be loaded at another address, but it is internally
641 relocated to 0x90000. For the "old" protocol, the
642 real-mode code must be loaded at 0x90000.
643
644When loading at 0x90000, avoid using memory above 0x9a000.
645
646For boot protocol 2.02 or higher, the command line does not have to be
647located in the same 64K segment as the real-mode setup code; it is
648thus permitted to give the stack/heap the full 64K segment and locate
649the command line above it.
650
651The kernel command line should not be located below the real-mode
652code, nor should it be located in high memory.
653
654
Linus Torvalds1da177e2005-04-16 15:20:36 -0700655**** SAMPLE BOOT CONFIGURATION
656
657As a sample configuration, assume the following layout of the real
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700658mode segment:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700659
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700660 When loading below 0x90000, use the entire segment:
661
662 0x0000-0x7fff Real mode kernel
663 0x8000-0xdfff Stack and heap
664 0xe000-0xffff Kernel command line
665
666 When loading at 0x90000 OR the protocol version is 2.01 or earlier:
667
668 0x0000-0x7fff Real mode kernel
669 0x8000-0x97ff Stack and heap
670 0x9800-0x9fff Kernel command line
Linus Torvalds1da177e2005-04-16 15:20:36 -0700671
672Such a boot loader should enter the following fields in the header:
673
674 unsigned long base_ptr; /* base address for real-mode segment */
675
676 if ( setup_sects == 0 ) {
677 setup_sects = 4;
678 }
679
680 if ( protocol >= 0x0200 ) {
681 type_of_loader = <type code>;
682 if ( loading_initrd ) {
683 ramdisk_image = <initrd_address>;
684 ramdisk_size = <initrd_size>;
685 }
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700686
687 if ( protocol >= 0x0202 && loadflags & 0x01 )
688 heap_end = 0xe000;
689 else
690 heap_end = 0x9800;
691
Linus Torvalds1da177e2005-04-16 15:20:36 -0700692 if ( protocol >= 0x0201 ) {
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700693 heap_end_ptr = heap_end - 0x200;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700694 loadflags |= 0x80; /* CAN_USE_HEAP */
695 }
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700696
Linus Torvalds1da177e2005-04-16 15:20:36 -0700697 if ( protocol >= 0x0202 ) {
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700698 cmd_line_ptr = base_ptr + heap_end;
699 strcpy(cmd_line_ptr, cmdline);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700700 } else {
701 cmd_line_magic = 0xA33F;
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700702 cmd_line_offset = heap_end;
703 setup_move_size = heap_end + strlen(cmdline)+1;
704 strcpy(base_ptr+cmd_line_offset, cmdline);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700705 }
706 } else {
707 /* Very old kernel */
708
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700709 heap_end = 0x9800;
710
Linus Torvalds1da177e2005-04-16 15:20:36 -0700711 cmd_line_magic = 0xA33F;
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700712 cmd_line_offset = heap_end;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700713
714 /* A very old kernel MUST have its real-mode code
715 loaded at 0x90000 */
716
717 if ( base_ptr != 0x90000 ) {
718 /* Copy the real-mode kernel */
719 memcpy(0x90000, base_ptr, (setup_sects+1)*512);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700720 base_ptr = 0x90000; /* Relocated */
721 }
722
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700723 strcpy(0x90000+cmd_line_offset, cmdline);
724
Linus Torvalds1da177e2005-04-16 15:20:36 -0700725 /* It is recommended to clear memory up to the 32K mark */
726 memset(0x90000 + (setup_sects+1)*512, 0,
727 (64-(setup_sects+1))*512);
728 }
729
730
731**** LOADING THE REST OF THE KERNEL
732
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -0700733The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
734in the kernel file (again, if setup_sects == 0 the real value is 4.)
735It should be loaded at address 0x10000 for Image/zImage kernels and
Linus Torvalds1da177e2005-04-16 15:20:36 -07007360x100000 for bzImage kernels.
737
738The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
739bit (LOAD_HIGH) in the loadflags field is set:
740
741 is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
742 load_address = is_bzImage ? 0x100000 : 0x10000;
743
744Note that Image/zImage kernels can be up to 512K in size, and thus use
745the entire 0x10000-0x90000 range of memory. This means it is pretty
746much a requirement for these kernels to load the real-mode part at
7470x90000. bzImage kernels allow much more flexibility.
748
749
750**** SPECIAL COMMAND LINE OPTIONS
751
752If the command line provided by the boot loader is entered by the
753user, the user may expect the following command line options to work.
754They should normally not be deleted from the kernel command line even
755though not all of them are actually meaningful to the kernel. Boot
756loader authors who need additional command line options for the boot
757loader itself should get them registered in
758Documentation/kernel-parameters.txt to make sure they will not
759conflict with actual kernel options now or in the future.
760
761 vga=<mode>
762 <mode> here is either an integer (in C notation, either
763 decimal, octal, or hexadecimal) or one of the strings
764 "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
765 (meaning 0xFFFD). This value should be entered into the
766 vid_mode field, as it is used by the kernel before the command
767 line is parsed.
768
769 mem=<size>
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700770 <size> is an integer in C notation optionally followed by
771 (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
772 << 30, << 40, << 50 or << 60). This specifies the end of
773 memory to the kernel. This affects the possible placement of
774 an initrd, since an initrd should be placed near end of
Linus Torvalds1da177e2005-04-16 15:20:36 -0700775 memory. Note that this is an option to *both* the kernel and
776 the bootloader!
777
778 initrd=<file>
779 An initrd should be loaded. The meaning of <file> is
780 obviously bootloader-dependent, and some boot loaders
781 (e.g. LILO) do not have such a command.
782
783In addition, some boot loaders add the following options to the
784user-specified command line:
785
786 BOOT_IMAGE=<file>
787 The boot image which was loaded. Again, the meaning of <file>
788 is obviously bootloader-dependent.
789
790 auto
791 The kernel was booted without explicit user intervention.
792
793If these options are added by the boot loader, it is highly
794recommended that they are located *first*, before the user-specified
795or configuration-specified command line. Otherwise, "init=/bin/sh"
796gets confused by the "auto" option.
797
798
799**** RUNNING THE KERNEL
800
801The kernel is started by jumping to the kernel entry point, which is
802located at *segment* offset 0x20 from the start of the real mode
803kernel. This means that if you loaded your real-mode kernel code at
8040x90000, the kernel entry point is 9020:0000.
805
806At entry, ds = es = ss should point to the start of the real-mode
807kernel code (0x9000 if the code is loaded at 0x90000), sp should be
808set up properly, normally pointing to the top of the heap, and
809interrupts should be disabled. Furthermore, to guard against bugs in
810the kernel, it is recommended that the boot loader sets fs = gs = ds =
811es = ss.
812
813In our example from above, we would do:
814
815 /* Note: in the case of the "old" kernel protocol, base_ptr must
816 be == 0x90000 at this point; see the previous sample code */
817
818 seg = base_ptr >> 4;
819
820 cli(); /* Enter with interrupts disabled! */
821
822 /* Set up the real-mode kernel stack */
823 _SS = seg;
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700824 _SP = heap_end;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700825
826 _DS = _ES = _FS = _GS = seg;
827 jmp_far(seg+0x20, 0); /* Run the kernel */
828
829If your boot sector accesses a floppy drive, it is recommended to
830switch off the floppy motor before running the kernel, since the
831kernel boot leaves interrupts off and thus the motor will not be
832switched off, especially if the loaded kernel has the floppy driver as
833a demand-loaded module!
834
835
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700836**** ADVANCED BOOT LOADER HOOKS
Linus Torvalds1da177e2005-04-16 15:20:36 -0700837
838If the boot loader runs in a particularly hostile environment (such as
839LOADLIN, which runs under DOS) it may be impossible to follow the
840standard memory location requirements. Such a boot loader may use the
841following hooks that, if set, are invoked by the kernel at the
842appropriate time. The use of these hooks should probably be
843considered an absolutely last resort!
844
845IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
846%edi across invocation.
847
848 realmode_swtch:
849 A 16-bit real mode far subroutine invoked immediately before
850 entering protected mode. The default routine disables NMI, so
851 your routine should probably do so, too.
852
853 code32_start:
854 A 32-bit flat-mode routine *jumped* to immediately after the
855 transition to protected mode, but before the kernel is
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700856 uncompressed. No segments, except CS, are guaranteed to be
857 set up (current kernels do, but older ones do not); you should
858 set them up to BOOT_DS (0x18) yourself.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700859
860 After completing your hook, you should jump to the address
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700861 that was in this field before your boot loader overwrote it
862 (relocated, if appropriate.)
Huang, Yingaa694322007-10-24 10:18:49 +0800863
864
865**** 32-bit BOOT PROTOCOL
866
867For machine with some new BIOS other than legacy BIOS, such as EFI,
868LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
869based on legacy BIOS can not be used, so a 32-bit boot protocol needs
870to be defined.
871
872In 32-bit boot protocol, the first step in loading a Linux kernel
873should be to setup the boot parameters (struct boot_params,
874traditionally known as "zero page"). The memory for struct boot_params
875should be allocated and initialized to all zero. Then the setup header
876from offset 0x01f1 of kernel image on should be loaded into struct
877boot_params and examined. The end of setup header can be calculated as
878follow:
879
880 0x0202 + byte value at offset 0x0201
881
882In addition to read/modify/write the setup header of the struct
883boot_params as that of 16-bit boot protocol, the boot loader should
884also fill the additional fields of the struct boot_params as that
885described in zero-page.txt.
886
887After setupping the struct boot_params, the boot loader can load the
88832/64-bit kernel in the same way as that of 16-bit boot protocol.
889
890In 32-bit boot protocol, the kernel is started by jumping to the
89132-bit kernel entry point, which is the start address of loaded
89232/64-bit kernel.
893
894At entry, the CPU must be in 32-bit protected mode with paging
895disabled; a GDT must be loaded with the descriptors for selectors
896__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
897segment; __BOOS_CS must have execute/read permission, and __BOOT_DS
898must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
899must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
900address of the struct boot_params; %ebp, %edi and %ebx must be zero.