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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
H. Peter Anvin50312962009-05-07 16:54:11 -070053Protocol 2.10: (Kernel 2.6.31) Added a protocol for relaxed alignment
H. Peter Anvind2973662009-05-11 16:06:23 -070054 beyond the kernel_alignment added, new init_size and
H. Peter Anvin50312962009-05-07 16:54:11 -070055 pref_address fields. Added extended boot loader IDs.
H. Peter Anvind2973662009-05-11 16:06:23 -070056
Matt Fleming9ca8f722012-07-19 10:23:48 +010057Protocol 2.11: (Kernel 3.6) Added a field for offset of EFI handover
58 protocol entry point.
59
H. Peter Anvin972f7c82013-01-31 20:23:49 -080060Protocol 2.12: (Kernel 3.8) Added the xloadflags field and extension fields
Masanari Iidac9f3f2d2013-07-18 01:29:12 +090061 to struct boot_params for loading bzImage and ramdisk
H. Peter Anvin09c205a2013-01-27 10:43:28 -080062 above 4G in 64bit.
63
Linus Torvalds1da177e2005-04-16 15:20:36 -070064**** MEMORY LAYOUT
65
66The traditional memory map for the kernel loader, used for Image or
67zImage kernels, typically looks like:
68
69 | |
700A0000 +------------------------+
71 | Reserved for BIOS | Do not use. Reserved for BIOS EBDA.
7209A000 +------------------------+
H. Peter Anvindec04cf2007-05-17 15:50:47 -070073 | Command line |
74 | Stack/heap | For use by the kernel real-mode code.
Linus Torvalds1da177e2005-04-16 15:20:36 -070075098000 +------------------------+
76 | Kernel setup | The kernel real-mode code.
77090200 +------------------------+
78 | Kernel boot sector | The kernel legacy boot sector.
79090000 +------------------------+
80 | Protected-mode kernel | The bulk of the kernel image.
81010000 +------------------------+
82 | Boot loader | <- Boot sector entry point 0000:7C00
83001000 +------------------------+
84 | Reserved for MBR/BIOS |
85000800 +------------------------+
86 | Typically used by MBR |
87000600 +------------------------+
88 | BIOS use only |
89000000 +------------------------+
90
91
92When using bzImage, the protected-mode kernel was relocated to
930x100000 ("high memory"), and the kernel real-mode block (boot sector,
94setup, and stack/heap) was made relocatable to any address between
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700950x10000 and end of low memory. Unfortunately, in protocols 2.00 and
962.01 the 0x90000+ memory range is still used internally by the kernel;
97the 2.02 protocol resolves that problem.
Linus Torvalds1da177e2005-04-16 15:20:36 -070098
99It is desirable to keep the "memory ceiling" -- the highest point in
100low memory touched by the boot loader -- as low as possible, since
101some newer BIOSes have begun to allocate some rather large amounts of
102memory, called the Extended BIOS Data Area, near the top of low
103memory. The boot loader should use the "INT 12h" BIOS call to verify
104how much low memory is available.
105
106Unfortunately, if INT 12h reports that the amount of memory is too
107low, there is usually nothing the boot loader can do but to report an
108error to the user. The boot loader should therefore be designed to
109take up as little space in low memory as it reasonably can. For
110zImage or old bzImage kernels, which need data written into the
1110x90000 segment, the boot loader should make sure not to use memory
112above the 0x9A000 point; too many BIOSes will break above that point.
113
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700114For a modern bzImage kernel with boot protocol version >= 2.02, a
115memory layout like the following is suggested:
116
117 ~ ~
118 | Protected-mode kernel |
119100000 +------------------------+
120 | I/O memory hole |
1210A0000 +------------------------+
122 | Reserved for BIOS | Leave as much as possible unused
123 ~ ~
124 | Command line | (Can also be below the X+10000 mark)
125X+10000 +------------------------+
126 | Stack/heap | For use by the kernel real-mode code.
127X+08000 +------------------------+
128 | Kernel setup | The kernel real-mode code.
129 | Kernel boot sector | The kernel legacy boot sector.
130X +------------------------+
131 | Boot loader | <- Boot sector entry point 0000:7C00
132001000 +------------------------+
133 | Reserved for MBR/BIOS |
134000800 +------------------------+
135 | Typically used by MBR |
136000600 +------------------------+
137 | BIOS use only |
138000000 +------------------------+
139
140... where the address X is as low as the design of the boot loader
141permits.
142
Linus Torvalds1da177e2005-04-16 15:20:36 -0700143
144**** THE REAL-MODE KERNEL HEADER
145
146In the following text, and anywhere in the kernel boot sequence, "a
147sector" refers to 512 bytes. It is independent of the actual sector
148size of the underlying medium.
149
150The first step in loading a Linux kernel should be to load the
151real-mode code (boot sector and setup code) and then examine the
152following header at offset 0x01f1. The real-mode code can total up to
15332K, although the boot loader may choose to load only the first two
154sectors (1K) and then examine the bootup sector size.
155
156The header looks like:
157
158Offset Proto Name Meaning
159/Size
160
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -070016101F1/1 ALL(1 setup_sects The size of the setup in sectors
Linus Torvalds1da177e2005-04-16 15:20:36 -070016201F2/2 ALL root_flags If set, the root is mounted readonly
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -070016301F4/4 2.04+(2 syssize The size of the 32-bit code in 16-byte paras
Linus Torvalds1da177e2005-04-16 15:20:36 -070016401F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
16501FA/2 ALL vid_mode Video mode control
16601FC/2 ALL root_dev Default root device number
16701FE/2 ALL boot_flag 0xAA55 magic number
1680200/2 2.00+ jump Jump instruction
1690202/4 2.00+ header Magic signature "HdrS"
1700206/2 2.00+ version Boot protocol version supported
1710208/4 2.00+ realmode_swtch Boot loader hook (see below)
Baodong Chene56d0cf2009-01-08 19:24:29 +0800172020C/2 2.00+ start_sys_seg The load-low segment (0x1000) (obsolete)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700173020E/2 2.00+ kernel_version Pointer to kernel version string
1740210/1 2.00+ type_of_loader Boot loader identifier
1750211/1 2.00+ loadflags Boot protocol option flags
1760212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
1770214/4 2.00+ code32_start Boot loader hook (see below)
1780218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
179021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
1800220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
1810224/2 2.01+ heap_end_ptr Free memory after setup end
H. Peter Anvin50312962009-05-07 16:54:11 -07001820226/1 2.02+(3 ext_loader_ver Extended boot loader version
1830227/1 2.02+(3 ext_loader_type Extended boot loader ID
Linus Torvalds1da177e2005-04-16 15:20:36 -07001840228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
Borislav Petkov3e920b52014-03-12 15:13:02 +0100185022C/4 2.03+ initrd_addr_max Highest legal initrd address
Vivek Goyald263b212006-12-07 02:14:13 +01001860230/4 2.05+ kernel_alignment Physical addr alignment required for kernel
1870234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
H. Peter Anvind2973662009-05-11 16:06:23 -07001880235/1 2.10+ min_alignment Minimum alignment, as a power of two
H. Peter Anvin09c205a2013-01-27 10:43:28 -08001890236/2 2.12+ xloadflags Boot protocol option flags
Bernhard Walle8f9aeca2007-05-02 19:27:10 +02001900238/4 2.06+ cmdline_size Maximum size of the kernel command line
Rusty Russelle5371ac2007-10-21 16:41:33 -0700191023C/4 2.07+ hardware_subarch Hardware subarchitecture
1920240/8 2.07+ hardware_subarch_data Subarchitecture-specific data
Ian Campbell87253d12008-02-19 11:12:30 +00001930248/4 2.08+ payload_offset Offset of kernel payload
194024C/4 2.08+ payload_length Length of kernel payload
Huang, Yingfb884382008-03-28 10:49:50 +08001950250/8 2.09+ setup_data 64-bit physical pointer to linked list
196 of struct setup_data
H. Peter Anvind2973662009-05-11 16:06:23 -07001970258/8 2.10+ pref_address Preferred loading address
1980260/4 2.10+ init_size Linear memory required during initialization
Matt Fleming9ca8f722012-07-19 10:23:48 +01001990264/4 2.11+ handover_offset Offset of handover entry point
Linus Torvalds1da177e2005-04-16 15:20:36 -0700200
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -0700201(1) For backwards compatibility, if the setup_sects field contains 0, the
202 real value is 4.
203
204(2) For boot protocol prior to 2.04, the upper two bytes of the syssize
205 field are unusable, which means the size of a bzImage kernel
206 cannot be determined.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700207
H. Peter Anvin50312962009-05-07 16:54:11 -0700208(3) Ignored, but safe to set, for boot protocols 2.02-2.09.
209
Linus Torvalds1da177e2005-04-16 15:20:36 -0700210If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
211the boot protocol version is "old". Loading an old kernel, the
212following parameters should be assumed:
213
214 Image type = zImage
215 initrd not supported
216 Real-mode kernel must be located at 0x90000.
217
218Otherwise, the "version" field contains the protocol version,
219e.g. protocol version 2.01 will contain 0x0201 in this field. When
220setting fields in the header, you must make sure only to set fields
221supported by the protocol version in use.
222
Linus Torvalds1da177e2005-04-16 15:20:36 -0700223
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700224**** DETAILS OF HEADER FIELDS
Linus Torvalds1da177e2005-04-16 15:20:36 -0700225
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700226For each field, some are information from the kernel to the bootloader
227("read"), some are expected to be filled out by the bootloader
228("write"), and some are expected to be read and modified by the
229bootloader ("modify").
Linus Torvalds1da177e2005-04-16 15:20:36 -0700230
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700231All general purpose boot loaders should write the fields marked
232(obligatory). Boot loaders who want to load the kernel at a
233nonstandard address should fill in the fields marked (reloc); other
234boot loaders can ignore those fields.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700235
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700236The byte order of all fields is littleendian (this is x86, after all.)
237
Rusty Russelle5371ac2007-10-21 16:41:33 -0700238Field name: setup_sects
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700239Type: read
240Offset/size: 0x1f1/1
241Protocol: ALL
242
243 The size of the setup code in 512-byte sectors. If this field is
244 0, the real value is 4. The real-mode code consists of the boot
245 sector (always one 512-byte sector) plus the setup code.
246
247Field name: root_flags
248Type: modify (optional)
249Offset/size: 0x1f2/2
250Protocol: ALL
251
252 If this field is nonzero, the root defaults to readonly. The use of
253 this field is deprecated; use the "ro" or "rw" options on the
254 command line instead.
255
256Field name: syssize
257Type: read
258Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
259Protocol: 2.04+
260
261 The size of the protected-mode code in units of 16-byte paragraphs.
262 For protocol versions older than 2.04 this field is only two bytes
263 wide, and therefore cannot be trusted for the size of a kernel if
264 the LOAD_HIGH flag is set.
265
266Field name: ram_size
267Type: kernel internal
268Offset/size: 0x1f8/2
269Protocol: ALL
270
271 This field is obsolete.
272
273Field name: vid_mode
274Type: modify (obligatory)
275Offset/size: 0x1fa/2
276
277 Please see the section on SPECIAL COMMAND LINE OPTIONS.
278
279Field name: root_dev
280Type: modify (optional)
281Offset/size: 0x1fc/2
282Protocol: ALL
283
284 The default root device device number. The use of this field is
285 deprecated, use the "root=" option on the command line instead.
286
287Field name: boot_flag
288Type: read
289Offset/size: 0x1fe/2
290Protocol: ALL
291
292 Contains 0xAA55. This is the closest thing old Linux kernels have
293 to a magic number.
294
295Field name: jump
296Type: read
297Offset/size: 0x200/2
298Protocol: 2.00+
299
300 Contains an x86 jump instruction, 0xEB followed by a signed offset
301 relative to byte 0x202. This can be used to determine the size of
302 the header.
303
304Field name: header
305Type: read
306Offset/size: 0x202/4
307Protocol: 2.00+
308
309 Contains the magic number "HdrS" (0x53726448).
310
311Field name: version
312Type: read
313Offset/size: 0x206/2
314Protocol: 2.00+
315
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700316 Contains the boot protocol version, in (major << 8)+minor format,
317 e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
318 10.17.
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700319
Baodong Chene56d0cf2009-01-08 19:24:29 +0800320Field name: realmode_swtch
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700321Type: modify (optional)
322Offset/size: 0x208/4
323Protocol: 2.00+
324
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700325 Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700326
Baodong Chene56d0cf2009-01-08 19:24:29 +0800327Field name: start_sys_seg
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700328Type: read
H. Peter Anvina021e512008-07-22 15:33:57 -0400329Offset/size: 0x20c/2
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700330Protocol: 2.00+
331
332 The load low segment (0x1000). Obsolete.
333
334Field name: kernel_version
335Type: read
336Offset/size: 0x20e/2
337Protocol: 2.00+
338
339 If set to a nonzero value, contains a pointer to a NUL-terminated
340 human-readable kernel version number string, less 0x200. This can
341 be used to display the kernel version to the user. This value
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700342 should be less than (0x200*setup_sects).
343
344 For example, if this value is set to 0x1c00, the kernel version
345 number string can be found at offset 0x1e00 in the kernel file.
346 This is a valid value if and only if the "setup_sects" field
347 contains the value 15 or higher, as:
348
349 0x1c00 < 15*0x200 (= 0x1e00) but
350 0x1c00 >= 14*0x200 (= 0x1c00)
351
352 0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700353
354Field name: type_of_loader
355Type: write (obligatory)
356Offset/size: 0x210/1
357Protocol: 2.00+
358
359 If your boot loader has an assigned id (see table below), enter
360 0xTV here, where T is an identifier for the boot loader and V is
361 a version number. Otherwise, enter 0xFF here.
362
H. Peter Anvin50312962009-05-07 16:54:11 -0700363 For boot loader IDs above T = 0xD, write T = 0xE to this field and
364 write the extended ID minus 0x10 to the ext_loader_type field.
365 Similarly, the ext_loader_ver field can be used to provide more than
366 four bits for the bootloader version.
367
368 For example, for T = 0x15, V = 0x234, write:
369
370 type_of_loader <- 0xE4
371 ext_loader_type <- 0x05
372 ext_loader_ver <- 0x23
373
H. Peter Anvinc5e5c422012-07-23 16:16:17 -0700374 Assigned boot loader ids (hexadecimal):
375
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700376 0 LILO (0x00 reserved for pre-2.00 bootloader)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700377 1 Loadlin
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700378 2 bootsect-loader (0x20, all other values reserved)
H. Peter Anvin50312962009-05-07 16:54:11 -0700379 3 Syslinux
H. Peter Anvincd34ab82012-12-12 20:28:39 -0800380 4 Etherboot/gPXE/iPXE
Linus Torvalds1da177e2005-04-16 15:20:36 -0700381 5 ELILO
KOSAKI Motohiro9ee670f2008-11-11 11:49:01 +0900382 7 GRUB
H. Peter Anvin50312962009-05-07 16:54:11 -0700383 8 U-Boot
Jeremy Fitzhardinge354332e2006-09-12 20:35:57 -0700384 9 Xen
H. Peter Anvinc229ec52007-01-26 09:15:47 -0800385 A Gujin
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700386 B Qemu
H. Peter Anvin50312962009-05-07 16:54:11 -0700387 C Arcturus Networks uCbootloader
H. Peter Anvin4bfe24a2012-12-12 20:24:12 -0800388 D kexec-tools
H. Peter Anvin50312962009-05-07 16:54:11 -0700389 E Extended (see ext_loader_type)
390 F Special (0xFF = undefined)
H. Peter Anvinc5e5c422012-07-23 16:16:17 -0700391 10 Reserved
392 11 Minimal Linux Bootloader <http://sebastian-plotz.blogspot.de>
H. Peter Anvin686966d2013-02-08 09:19:33 -0800393 12 OVMF UEFI virtualization stack
Linus Torvalds1da177e2005-04-16 15:20:36 -0700394
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700395 Please contact <hpa@zytor.com> if you need a bootloader ID
396 value assigned.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700397
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700398Field name: loadflags
399Type: modify (obligatory)
400Offset/size: 0x211/1
401Protocol: 2.00+
Linus Torvalds1da177e2005-04-16 15:20:36 -0700402
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700403 This field is a bitmask.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700404
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700405 Bit 0 (read): LOADED_HIGH
406 - If 0, the protected-mode code is loaded at 0x10000.
407 - If 1, the protected-mode code is loaded at 0x100000.
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700408
H. Peter Anvin4039feb2008-05-30 17:16:20 -0700409 Bit 5 (write): QUIET_FLAG
410 - If 0, print early messages.
411 - If 1, suppress early messages.
412 This requests to the kernel (decompressor and early
413 kernel) to not write early messages that require
414 accessing the display hardware directly.
415
Rusty Russelle5371ac2007-10-21 16:41:33 -0700416 Bit 6 (write): KEEP_SEGMENTS
417 Protocol: 2.07+
H. Peter Anvin4039feb2008-05-30 17:16:20 -0700418 - If 0, reload the segment registers in the 32bit entry point.
419 - If 1, do not reload the segment registers in the 32bit entry point.
Rusty Russelle5371ac2007-10-21 16:41:33 -0700420 Assume that %cs %ds %ss %es are all set to flat segments with
421 a base of 0 (or the equivalent for their environment).
422
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700423 Bit 7 (write): CAN_USE_HEAP
424 Set this bit to 1 to indicate that the value entered in the
425 heap_end_ptr is valid. If this field is clear, some setup code
426 functionality will be disabled.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700427
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700428Field name: setup_move_size
429Type: modify (obligatory)
430Offset/size: 0x212/2
431Protocol: 2.00-2.01
Linus Torvalds1da177e2005-04-16 15:20:36 -0700432
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700433 When using protocol 2.00 or 2.01, if the real mode kernel is not
434 loaded at 0x90000, it gets moved there later in the loading
435 sequence. Fill in this field if you want additional data (such as
436 the kernel command line) moved in addition to the real-mode kernel
437 itself.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700438
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700439 The unit is bytes starting with the beginning of the boot sector.
440
441 This field is can be ignored when the protocol is 2.02 or higher, or
442 if the real-mode code is loaded at 0x90000.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700443
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700444Field name: code32_start
445Type: modify (optional, reloc)
446Offset/size: 0x214/4
447Protocol: 2.00+
448
449 The address to jump to in protected mode. This defaults to the load
450 address of the kernel, and can be used by the boot loader to
451 determine the proper load address.
452
453 This field can be modified for two purposes:
454
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700455 1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700456
457 2. if a bootloader which does not install a hook loads a
458 relocatable kernel at a nonstandard address it will have to modify
459 this field to point to the load address.
460
461Field name: ramdisk_image
462Type: write (obligatory)
463Offset/size: 0x218/4
464Protocol: 2.00+
465
466 The 32-bit linear address of the initial ramdisk or ramfs. Leave at
467 zero if there is no initial ramdisk/ramfs.
468
469Field name: ramdisk_size
470Type: write (obligatory)
471Offset/size: 0x21c/4
472Protocol: 2.00+
473
474 Size of the initial ramdisk or ramfs. Leave at zero if there is no
475 initial ramdisk/ramfs.
476
477Field name: bootsect_kludge
478Type: kernel internal
479Offset/size: 0x220/4
480Protocol: 2.00+
481
482 This field is obsolete.
483
484Field name: heap_end_ptr
485Type: write (obligatory)
486Offset/size: 0x224/2
487Protocol: 2.01+
488
489 Set this field to the offset (from the beginning of the real-mode
490 code) of the end of the setup stack/heap, minus 0x0200.
491
H. Peter Anvin50312962009-05-07 16:54:11 -0700492Field name: ext_loader_ver
493Type: write (optional)
494Offset/size: 0x226/1
495Protocol: 2.02+
496
497 This field is used as an extension of the version number in the
498 type_of_loader field. The total version number is considered to be
499 (type_of_loader & 0x0f) + (ext_loader_ver << 4).
500
501 The use of this field is boot loader specific. If not written, it
502 is zero.
503
504 Kernels prior to 2.6.31 did not recognize this field, but it is safe
505 to write for protocol version 2.02 or higher.
506
507Field name: ext_loader_type
508Type: write (obligatory if (type_of_loader & 0xf0) == 0xe0)
509Offset/size: 0x227/1
510Protocol: 2.02+
511
512 This field is used as an extension of the type number in
513 type_of_loader field. If the type in type_of_loader is 0xE, then
514 the actual type is (ext_loader_type + 0x10).
515
516 This field is ignored if the type in type_of_loader is not 0xE.
517
518 Kernels prior to 2.6.31 did not recognize this field, but it is safe
519 to write for protocol version 2.02 or higher.
520
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700521Field name: cmd_line_ptr
522Type: write (obligatory)
523Offset/size: 0x228/4
524Protocol: 2.02+
525
526 Set this field to the linear address of the kernel command line.
527 The kernel command line can be located anywhere between the end of
528 the setup heap and 0xA0000; it does not have to be located in the
529 same 64K segment as the real-mode code itself.
530
531 Fill in this field even if your boot loader does not support a
532 command line, in which case you can point this to an empty string
533 (or better yet, to the string "auto".) If this field is left at
534 zero, the kernel will assume that your boot loader does not support
535 the 2.02+ protocol.
536
Borislav Petkov3e920b52014-03-12 15:13:02 +0100537Field name: initrd_addr_max
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700538Type: read
539Offset/size: 0x22c/4
540Protocol: 2.03+
541
542 The maximum address that may be occupied by the initial
543 ramdisk/ramfs contents. For boot protocols 2.02 or earlier, this
544 field is not present, and the maximum address is 0x37FFFFFF. (This
545 address is defined as the address of the highest safe byte, so if
546 your ramdisk is exactly 131072 bytes long and this field is
547 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
548
549Field name: kernel_alignment
H. Peter Anvind2973662009-05-11 16:06:23 -0700550Type: read/modify (reloc)
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700551Offset/size: 0x230/4
H. Peter Anvind2973662009-05-11 16:06:23 -0700552Protocol: 2.05+ (read), 2.10+ (modify)
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700553
H. Peter Anvind2973662009-05-11 16:06:23 -0700554 Alignment unit required by the kernel (if relocatable_kernel is
555 true.) A relocatable kernel that is loaded at an alignment
556 incompatible with the value in this field will be realigned during
557 kernel initialization.
558
559 Starting with protocol version 2.10, this reflects the kernel
560 alignment preferred for optimal performance; it is possible for the
561 loader to modify this field to permit a lesser alignment. See the
562 min_alignment and pref_address field below.
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700563
564Field name: relocatable_kernel
565Type: read (reloc)
566Offset/size: 0x234/1
567Protocol: 2.05+
568
569 If this field is nonzero, the protected-mode part of the kernel can
570 be loaded at any address that satisfies the kernel_alignment field.
571 After loading, the boot loader must set the code32_start field to
572 point to the loaded code, or to a boot loader hook.
573
H. Peter Anvind2973662009-05-11 16:06:23 -0700574Field name: min_alignment
575Type: read (reloc)
576Offset/size: 0x235/1
577Protocol: 2.10+
578
579 This field, if nonzero, indicates as a power of two the minimum
580 alignment required, as opposed to preferred, by the kernel to boot.
581 If a boot loader makes use of this field, it should update the
582 kernel_alignment field with the alignment unit desired; typically:
583
584 kernel_alignment = 1 << min_alignment
585
586 There may be a considerable performance cost with an excessively
587 misaligned kernel. Therefore, a loader should typically try each
588 power-of-two alignment from kernel_alignment down to this alignment.
589
H. Peter Anvin09c205a2013-01-27 10:43:28 -0800590Field name: xloadflags
591Type: read
592Offset/size: 0x236/2
593Protocol: 2.12+
594
595 This field is a bitmask.
596
597 Bit 0 (read): XLF_KERNEL_64
598 - If 1, this kernel has the legacy 64-bit entry point at 0x200.
599
600 Bit 1 (read): XLF_CAN_BE_LOADED_ABOVE_4G
601 - If 1, kernel/boot_params/cmdline/ramdisk can be above 4G.
602
603 Bit 2 (read): XLF_EFI_HANDOVER_32
604 - If 1, the kernel supports the 32-bit EFI handoff entry point
605 given at handover_offset.
606
607 Bit 3 (read): XLF_EFI_HANDOVER_64
608 - If 1, the kernel supports the 64-bit EFI handoff entry point
609 given at handover_offset + 0x200.
610
Dave Young456a29d2013-12-20 18:02:20 +0800611 Bit 4 (read): XLF_EFI_KEXEC
612 - If 1, the kernel supports kexec EFI boot with EFI runtime support.
613
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700614Field name: cmdline_size
615Type: read
616Offset/size: 0x238/4
617Protocol: 2.06+
618
619 The maximum size of the command line without the terminating
620 zero. This means that the command line can contain at most
621 cmdline_size characters. With protocol version 2.05 and earlier, the
622 maximum size was 255.
Bernhard Walle8f9aeca2007-05-02 19:27:10 +0200623
Rusty Russelle5371ac2007-10-21 16:41:33 -0700624Field name: hardware_subarch
H. Peter Anvin4039feb2008-05-30 17:16:20 -0700625Type: write (optional, defaults to x86/PC)
Rusty Russelle5371ac2007-10-21 16:41:33 -0700626Offset/size: 0x23c/4
627Protocol: 2.07+
628
629 In a paravirtualized environment the hardware low level architectural
630 pieces such as interrupt handling, page table handling, and
631 accessing process control registers needs to be done differently.
632
633 This field allows the bootloader to inform the kernel we are in one
634 one of those environments.
635
636 0x00000000 The default x86/PC environment
637 0x00000001 lguest
638 0x00000002 Xen
Pan, Jacob jun162bc7a2009-08-28 14:52:47 -0700639 0x00000003 Moorestown MID
Thomas Gleixnerc751e172010-11-09 12:08:04 -0800640 0x00000004 CE4100 TV Platform
Rusty Russelle5371ac2007-10-21 16:41:33 -0700641
642Field name: hardware_subarch_data
H. Peter Anvin4039feb2008-05-30 17:16:20 -0700643Type: write (subarch-dependent)
Rusty Russelle5371ac2007-10-21 16:41:33 -0700644Offset/size: 0x240/8
645Protocol: 2.07+
646
647 A pointer to data that is specific to hardware subarch
H. Peter Anvin4039feb2008-05-30 17:16:20 -0700648 This field is currently unused for the default x86/PC environment,
649 do not modify.
Rusty Russelle5371ac2007-10-21 16:41:33 -0700650
Ian Campbell87253d12008-02-19 11:12:30 +0000651Field name: payload_offset
Ian Campbell099e1372008-02-13 20:54:58 +0000652Type: read
653Offset/size: 0x248/4
654Protocol: 2.08+
655
Philipp Kohlbechere1286f22008-12-14 18:50:01 +0100656 If non-zero then this field contains the offset from the beginning
657 of the protected-mode code to the payload.
Ian Campbell87253d12008-02-19 11:12:30 +0000658
659 The payload may be compressed. The format of both the compressed and
660 uncompressed data should be determined using the standard magic
H. Peter Anvinee287582009-01-14 16:07:38 -0800661 numbers. The currently supported compression formats are gzip
Lasse Collin30314802011-01-12 17:01:24 -0800662 (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA
Kyungsik Leef9b493a2013-07-08 16:01:48 -0700663 (magic number 5D 00), XZ (magic number FD 37), and LZ4 (magic number
664 02 21). The uncompressed payload is currently always ELF (magic
665 number 7F 45 4C 46).
666
Ian Campbell87253d12008-02-19 11:12:30 +0000667Field name: payload_length
Ian Campbell099e1372008-02-13 20:54:58 +0000668Type: read
669Offset/size: 0x24c/4
670Protocol: 2.08+
671
Ian Campbell87253d12008-02-19 11:12:30 +0000672 The length of the payload.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700673
H. Peter Anvin4039feb2008-05-30 17:16:20 -0700674Field name: setup_data
675Type: write (special)
676Offset/size: 0x250/8
677Protocol: 2.09+
678
679 The 64-bit physical pointer to NULL terminated single linked list of
680 struct setup_data. This is used to define a more extensible boot
681 parameters passing mechanism. The definition of struct setup_data is
682 as follow:
683
684 struct setup_data {
685 u64 next;
686 u32 type;
687 u32 len;
688 u8 data[0];
689 };
690
691 Where, the next is a 64-bit physical pointer to the next node of
692 linked list, the next field of the last node is 0; the type is used
693 to identify the contents of data; the len is the length of data
694 field; the data holds the real payload.
695
696 This list may be modified at a number of points during the bootup
697 process. Therefore, when modifying this list one should always make
698 sure to consider the case where the linked list already contains
699 entries.
700
H. Peter Anvind2973662009-05-11 16:06:23 -0700701Field name: pref_address
702Type: read (reloc)
703Offset/size: 0x258/8
704Protocol: 2.10+
705
706 This field, if nonzero, represents a preferred load address for the
707 kernel. A relocating bootloader should attempt to load at this
708 address if possible.
709
710 A non-relocatable kernel will unconditionally move itself and to run
711 at this address.
712
713Field name: init_size
714Type: read
Darren Hart11e48fe2011-07-11 20:27:40 -0700715Offset/size: 0x260/4
H. Peter Anvind2973662009-05-11 16:06:23 -0700716
717 This field indicates the amount of linear contiguous memory starting
718 at the kernel runtime start address that the kernel needs before it
719 is capable of examining its memory map. This is not the same thing
720 as the total amount of memory the kernel needs to boot, but it can
721 be used by a relocating boot loader to help select a safe load
722 address for the kernel.
723
724 The kernel runtime start address is determined by the following algorithm:
725
726 if (relocatable_kernel)
727 runtime_start = align_up(load_address, kernel_alignment)
728 else
729 runtime_start = pref_address
730
Matt Fleming9ca8f722012-07-19 10:23:48 +0100731Field name: handover_offset
732Type: read
733Offset/size: 0x264/4
734
735 This field is the offset from the beginning of the kernel image to
736 the EFI handover protocol entry point. Boot loaders using the EFI
737 handover protocol to boot the kernel should jump to this offset.
738
739 See EFI HANDOVER PROTOCOL below for more details.
740
H. Peter Anvin4039feb2008-05-30 17:16:20 -0700741
Ian Campbell7d6e7372008-02-17 20:06:35 +0100742**** THE IMAGE CHECKSUM
743
744From boot protocol version 2.08 onwards the CRC-32 is calculated over
745the entire file using the characteristic polynomial 0x04C11DB7 and an
746initial remainder of 0xffffffff. The checksum is appended to the
747file; therefore the CRC of the file up to the limit specified in the
748syssize field of the header is always 0.
749
H. Peter Anvin4039feb2008-05-30 17:16:20 -0700750
Linus Torvalds1da177e2005-04-16 15:20:36 -0700751**** THE KERNEL COMMAND LINE
752
753The kernel command line has become an important way for the boot
754loader to communicate with the kernel. Some of its options are also
755relevant to the boot loader itself, see "special command line options"
756below.
757
Bernhard Walle8f9aeca2007-05-02 19:27:10 +0200758The kernel command line is a null-terminated string. The maximum
759length can be retrieved from the field cmdline_size. Before protocol
760version 2.06, the maximum was 255 characters. A string that is too
761long will be automatically truncated by the kernel.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700762
763If the boot protocol version is 2.02 or later, the address of the
764kernel command line is given by the header field cmd_line_ptr (see
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -0700765above.) This address can be anywhere between the end of the setup
766heap and 0xA0000.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700767
768If the protocol version is *not* 2.02 or higher, the kernel
769command line is entered using the following protocol:
770
771 At offset 0x0020 (word), "cmd_line_magic", enter the magic
772 number 0xA33F.
773
774 At offset 0x0022 (word), "cmd_line_offset", enter the offset
775 of the kernel command line (relative to the start of the
776 real-mode kernel).
777
778 The kernel command line *must* be within the memory region
779 covered by setup_move_size, so you may need to adjust this
780 field.
781
782
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700783**** MEMORY LAYOUT OF THE REAL-MODE CODE
784
785The real-mode code requires a stack/heap to be set up, as well as
786memory allocated for the kernel command line. This needs to be done
787in the real-mode accessible memory in bottom megabyte.
788
789It should be noted that modern machines often have a sizable Extended
790BIOS Data Area (EBDA). As a result, it is advisable to use as little
791of the low megabyte as possible.
792
793Unfortunately, under the following circumstances the 0x90000 memory
794segment has to be used:
795
796 - When loading a zImage kernel ((loadflags & 0x01) == 0).
797 - When loading a 2.01 or earlier boot protocol kernel.
798
799 -> For the 2.00 and 2.01 boot protocols, the real-mode code
800 can be loaded at another address, but it is internally
801 relocated to 0x90000. For the "old" protocol, the
802 real-mode code must be loaded at 0x90000.
803
804When loading at 0x90000, avoid using memory above 0x9a000.
805
806For boot protocol 2.02 or higher, the command line does not have to be
807located in the same 64K segment as the real-mode setup code; it is
808thus permitted to give the stack/heap the full 64K segment and locate
809the command line above it.
810
811The kernel command line should not be located below the real-mode
812code, nor should it be located in high memory.
813
814
Linus Torvalds1da177e2005-04-16 15:20:36 -0700815**** SAMPLE BOOT CONFIGURATION
816
817As a sample configuration, assume the following layout of the real
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700818mode segment:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700819
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700820 When loading below 0x90000, use the entire segment:
821
822 0x0000-0x7fff Real mode kernel
823 0x8000-0xdfff Stack and heap
824 0xe000-0xffff Kernel command line
825
826 When loading at 0x90000 OR the protocol version is 2.01 or earlier:
827
828 0x0000-0x7fff Real mode kernel
829 0x8000-0x97ff Stack and heap
830 0x9800-0x9fff Kernel command line
Linus Torvalds1da177e2005-04-16 15:20:36 -0700831
832Such a boot loader should enter the following fields in the header:
833
834 unsigned long base_ptr; /* base address for real-mode segment */
835
836 if ( setup_sects == 0 ) {
837 setup_sects = 4;
838 }
839
840 if ( protocol >= 0x0200 ) {
841 type_of_loader = <type code>;
842 if ( loading_initrd ) {
843 ramdisk_image = <initrd_address>;
844 ramdisk_size = <initrd_size>;
845 }
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700846
847 if ( protocol >= 0x0202 && loadflags & 0x01 )
848 heap_end = 0xe000;
849 else
850 heap_end = 0x9800;
851
Linus Torvalds1da177e2005-04-16 15:20:36 -0700852 if ( protocol >= 0x0201 ) {
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700853 heap_end_ptr = heap_end - 0x200;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700854 loadflags |= 0x80; /* CAN_USE_HEAP */
855 }
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700856
Linus Torvalds1da177e2005-04-16 15:20:36 -0700857 if ( protocol >= 0x0202 ) {
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700858 cmd_line_ptr = base_ptr + heap_end;
859 strcpy(cmd_line_ptr, cmdline);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700860 } else {
861 cmd_line_magic = 0xA33F;
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700862 cmd_line_offset = heap_end;
863 setup_move_size = heap_end + strlen(cmdline)+1;
864 strcpy(base_ptr+cmd_line_offset, cmdline);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700865 }
866 } else {
867 /* Very old kernel */
868
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700869 heap_end = 0x9800;
870
Linus Torvalds1da177e2005-04-16 15:20:36 -0700871 cmd_line_magic = 0xA33F;
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700872 cmd_line_offset = heap_end;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700873
874 /* A very old kernel MUST have its real-mode code
875 loaded at 0x90000 */
876
877 if ( base_ptr != 0x90000 ) {
878 /* Copy the real-mode kernel */
879 memcpy(0x90000, base_ptr, (setup_sects+1)*512);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700880 base_ptr = 0x90000; /* Relocated */
881 }
882
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700883 strcpy(0x90000+cmd_line_offset, cmdline);
884
Linus Torvalds1da177e2005-04-16 15:20:36 -0700885 /* It is recommended to clear memory up to the 32K mark */
886 memset(0x90000 + (setup_sects+1)*512, 0,
887 (64-(setup_sects+1))*512);
888 }
889
890
891**** LOADING THE REST OF THE KERNEL
892
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -0700893The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
894in the kernel file (again, if setup_sects == 0 the real value is 4.)
895It should be loaded at address 0x10000 for Image/zImage kernels and
Linus Torvalds1da177e2005-04-16 15:20:36 -07008960x100000 for bzImage kernels.
897
898The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
899bit (LOAD_HIGH) in the loadflags field is set:
900
901 is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
902 load_address = is_bzImage ? 0x100000 : 0x10000;
903
904Note that Image/zImage kernels can be up to 512K in size, and thus use
905the entire 0x10000-0x90000 range of memory. This means it is pretty
906much a requirement for these kernels to load the real-mode part at
9070x90000. bzImage kernels allow much more flexibility.
908
909
910**** SPECIAL COMMAND LINE OPTIONS
911
912If the command line provided by the boot loader is entered by the
913user, the user may expect the following command line options to work.
914They should normally not be deleted from the kernel command line even
915though not all of them are actually meaningful to the kernel. Boot
916loader authors who need additional command line options for the boot
917loader itself should get them registered in
918Documentation/kernel-parameters.txt to make sure they will not
919conflict with actual kernel options now or in the future.
920
921 vga=<mode>
922 <mode> here is either an integer (in C notation, either
923 decimal, octal, or hexadecimal) or one of the strings
924 "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
925 (meaning 0xFFFD). This value should be entered into the
926 vid_mode field, as it is used by the kernel before the command
927 line is parsed.
928
929 mem=<size>
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700930 <size> is an integer in C notation optionally followed by
931 (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
932 << 30, << 40, << 50 or << 60). This specifies the end of
933 memory to the kernel. This affects the possible placement of
934 an initrd, since an initrd should be placed near end of
Linus Torvalds1da177e2005-04-16 15:20:36 -0700935 memory. Note that this is an option to *both* the kernel and
936 the bootloader!
937
938 initrd=<file>
939 An initrd should be loaded. The meaning of <file> is
940 obviously bootloader-dependent, and some boot loaders
941 (e.g. LILO) do not have such a command.
942
943In addition, some boot loaders add the following options to the
944user-specified command line:
945
946 BOOT_IMAGE=<file>
947 The boot image which was loaded. Again, the meaning of <file>
948 is obviously bootloader-dependent.
949
950 auto
951 The kernel was booted without explicit user intervention.
952
953If these options are added by the boot loader, it is highly
954recommended that they are located *first*, before the user-specified
955or configuration-specified command line. Otherwise, "init=/bin/sh"
956gets confused by the "auto" option.
957
958
959**** RUNNING THE KERNEL
960
961The kernel is started by jumping to the kernel entry point, which is
962located at *segment* offset 0x20 from the start of the real mode
963kernel. This means that if you loaded your real-mode kernel code at
9640x90000, the kernel entry point is 9020:0000.
965
966At entry, ds = es = ss should point to the start of the real-mode
967kernel code (0x9000 if the code is loaded at 0x90000), sp should be
968set up properly, normally pointing to the top of the heap, and
969interrupts should be disabled. Furthermore, to guard against bugs in
970the kernel, it is recommended that the boot loader sets fs = gs = ds =
971es = ss.
972
973In our example from above, we would do:
974
975 /* Note: in the case of the "old" kernel protocol, base_ptr must
976 be == 0x90000 at this point; see the previous sample code */
977
978 seg = base_ptr >> 4;
979
980 cli(); /* Enter with interrupts disabled! */
981
982 /* Set up the real-mode kernel stack */
983 _SS = seg;
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700984 _SP = heap_end;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700985
986 _DS = _ES = _FS = _GS = seg;
987 jmp_far(seg+0x20, 0); /* Run the kernel */
988
989If your boot sector accesses a floppy drive, it is recommended to
990switch off the floppy motor before running the kernel, since the
991kernel boot leaves interrupts off and thus the motor will not be
992switched off, especially if the loaded kernel has the floppy driver as
993a demand-loaded module!
994
995
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700996**** ADVANCED BOOT LOADER HOOKS
Linus Torvalds1da177e2005-04-16 15:20:36 -0700997
998If the boot loader runs in a particularly hostile environment (such as
999LOADLIN, which runs under DOS) it may be impossible to follow the
1000standard memory location requirements. Such a boot loader may use the
1001following hooks that, if set, are invoked by the kernel at the
1002appropriate time. The use of these hooks should probably be
1003considered an absolutely last resort!
1004
1005IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
1006%edi across invocation.
1007
1008 realmode_swtch:
1009 A 16-bit real mode far subroutine invoked immediately before
1010 entering protected mode. The default routine disables NMI, so
1011 your routine should probably do so, too.
1012
1013 code32_start:
1014 A 32-bit flat-mode routine *jumped* to immediately after the
1015 transition to protected mode, but before the kernel is
H. Peter Anvinde372ec2007-05-08 20:37:02 -07001016 uncompressed. No segments, except CS, are guaranteed to be
1017 set up (current kernels do, but older ones do not); you should
1018 set them up to BOOT_DS (0x18) yourself.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001019
1020 After completing your hook, you should jump to the address
H. Peter Anvindb2668f2007-05-23 16:59:27 -07001021 that was in this field before your boot loader overwrote it
1022 (relocated, if appropriate.)
Huang, Yingaa694322007-10-24 10:18:49 +08001023
1024
1025**** 32-bit BOOT PROTOCOL
1026
1027For machine with some new BIOS other than legacy BIOS, such as EFI,
1028LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
1029based on legacy BIOS can not be used, so a 32-bit boot protocol needs
1030to be defined.
1031
1032In 32-bit boot protocol, the first step in loading a Linux kernel
1033should be to setup the boot parameters (struct boot_params,
1034traditionally known as "zero page"). The memory for struct boot_params
1035should be allocated and initialized to all zero. Then the setup header
1036from offset 0x01f1 of kernel image on should be loaded into struct
1037boot_params and examined. The end of setup header can be calculated as
1038follow:
1039
1040 0x0202 + byte value at offset 0x0201
1041
1042In addition to read/modify/write the setup header of the struct
1043boot_params as that of 16-bit boot protocol, the boot loader should
1044also fill the additional fields of the struct boot_params as that
1045described in zero-page.txt.
1046
Kees Cookc6039f42012-10-25 20:17:02 -07001047After setting up the struct boot_params, the boot loader can load the
Huang, Yingaa694322007-10-24 10:18:49 +0800104832/64-bit kernel in the same way as that of 16-bit boot protocol.
1049
1050In 32-bit boot protocol, the kernel is started by jumping to the
105132-bit kernel entry point, which is the start address of loaded
105232/64-bit kernel.
1053
1054At entry, the CPU must be in 32-bit protected mode with paging
1055disabled; a GDT must be loaded with the descriptors for selectors
1056__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
Kees Cookc6039f42012-10-25 20:17:02 -07001057segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
Huang, Yingaa694322007-10-24 10:18:49 +08001058must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1059must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
1060address of the struct boot_params; %ebp, %edi and %ebx must be zero.
Matt Fleming9ca8f722012-07-19 10:23:48 +01001061
Yinghai Lu8ee2f2d2013-01-24 12:20:07 -08001062**** 64-bit BOOT PROTOCOL
1063
1064For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader
1065and we need a 64-bit boot protocol.
1066
1067In 64-bit boot protocol, the first step in loading a Linux kernel
1068should be to setup the boot parameters (struct boot_params,
1069traditionally known as "zero page"). The memory for struct boot_params
1070could be allocated anywhere (even above 4G) and initialized to all zero.
1071Then, the setup header at offset 0x01f1 of kernel image on should be
1072loaded into struct boot_params and examined. The end of setup header
1073can be calculated as follows:
1074
1075 0x0202 + byte value at offset 0x0201
1076
1077In addition to read/modify/write the setup header of the struct
1078boot_params as that of 16-bit boot protocol, the boot loader should
1079also fill the additional fields of the struct boot_params as described
1080in zero-page.txt.
1081
1082After setting up the struct boot_params, the boot loader can load
108364-bit kernel in the same way as that of 16-bit boot protocol, but
1084kernel could be loaded above 4G.
1085
1086In 64-bit boot protocol, the kernel is started by jumping to the
108764-bit kernel entry point, which is the start address of loaded
108864-bit kernel plus 0x200.
1089
1090At entry, the CPU must be in 64-bit mode with paging enabled.
1091The range with setup_header.init_size from start address of loaded
1092kernel and zero page and command line buffer get ident mapping;
1093a GDT must be loaded with the descriptors for selectors
1094__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1095segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1096must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1097must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base
1098address of the struct boot_params.
1099
Matt Fleming9ca8f722012-07-19 10:23:48 +01001100**** EFI HANDOVER PROTOCOL
1101
1102This protocol allows boot loaders to defer initialisation to the EFI
1103boot stub. The boot loader is required to load the kernel/initrd(s)
1104from the boot media and jump to the EFI handover protocol entry point
1105which is hdr->handover_offset bytes from the beginning of
1106startup_{32,64}.
1107
1108The function prototype for the handover entry point looks like this,
1109
1110 efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp)
1111
1112'handle' is the EFI image handle passed to the boot loader by the EFI
1113firmware, 'table' is the EFI system table - these are the first two
1114arguments of the "handoff state" as described in section 2.3 of the
1115UEFI specification. 'bp' is the boot loader-allocated boot params.
1116
1117The boot loader *must* fill out the following fields in bp,
1118
1119 o hdr.code32_start
1120 o hdr.cmd_line_ptr
1121 o hdr.cmdline_size
1122 o hdr.ramdisk_image (if applicable)
1123 o hdr.ramdisk_size (if applicable)
1124
1125All other fields should be zero.