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Linus Torvalds1da177e2005-04-16 15:20:36 -07001 THE LINUX/I386 BOOT PROTOCOL
2 ----------------------------
3
4 H. Peter Anvin <hpa@zytor.com>
H. Peter Anvindb2668f2007-05-23 16:59:27 -07005 Last update 2007-05-23
Linus Torvalds1da177e2005-04-16 15:20:36 -07006
7On the i386 platform, the Linux kernel uses a rather complicated boot
8convention. This has evolved partially due to historical aspects, as
9well as the desire in the early days to have the kernel itself be a
10bootable image, the complicated PC memory model and due to changed
11expectations in the PC industry caused by the effective demise of
12real-mode DOS as a mainstream operating system.
13
H. Peter Anvinde372ec2007-05-08 20:37:02 -070014Currently, the following versions of the Linux/i386 boot protocol exist.
Linus Torvalds1da177e2005-04-16 15:20:36 -070015
16Old kernels: zImage/Image support only. Some very early kernels
17 may not even support a command line.
18
19Protocol 2.00: (Kernel 1.3.73) Added bzImage and initrd support, as
20 well as a formalized way to communicate between the
21 boot loader and the kernel. setup.S made relocatable,
22 although the traditional setup area still assumed
23 writable.
24
25Protocol 2.01: (Kernel 1.3.76) Added a heap overrun warning.
26
27Protocol 2.02: (Kernel 2.4.0-test3-pre3) New command line protocol.
28 Lower the conventional memory ceiling. No overwrite
29 of the traditional setup area, thus making booting
30 safe for systems which use the EBDA from SMM or 32-bit
31 BIOS entry points. zImage deprecated but still
32 supported.
33
34Protocol 2.03: (Kernel 2.4.18-pre1) Explicitly makes the highest possible
35 initrd address available to the bootloader.
36
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -070037Protocol 2.04: (Kernel 2.6.14) Extend the syssize field to four bytes.
Bernhard Walle8f9aeca2007-05-02 19:27:10 +020038
Vivek Goyalbe274ee2006-12-07 02:14:04 +010039Protocol 2.05: (Kernel 2.6.20) Make protected mode kernel relocatable.
40 Introduce relocatable_kernel and kernel_alignment fields.
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -070041
Bernhard Walle8f9aeca2007-05-02 19:27:10 +020042Protocol 2.06: (Kernel 2.6.22) Added a field that contains the size of
Ian Campbell4c0587e2008-04-27 12:21:11 +010043 the boot command line.
Bernhard Walle8f9aeca2007-05-02 19:27:10 +020044
Ian Campbell4c0587e2008-04-27 12:21:11 +010045Protocol 2.07: (Kernel 2.6.24) Added paravirtualised boot protocol.
46 Introduced hardware_subarch and hardware_subarch_data
47 and KEEP_SEGMENTS flag in load_flags.
48
49Protocol 2.08: (Kernel 2.6.26) Added crc32 checksum and ELF format
50 payload. Introduced payload_offset and payload length
51 fields to aid in locating the payload.
52
53Protocol 2.09: (Kernel 2.6.26) Added a field of 64-bit physical
Huang, Yingfb884382008-03-28 10:49:50 +080054 pointer to single linked list of struct setup_data.
Linus Torvalds1da177e2005-04-16 15:20:36 -070055
56**** MEMORY LAYOUT
57
58The traditional memory map for the kernel loader, used for Image or
59zImage kernels, typically looks like:
60
61 | |
620A0000 +------------------------+
63 | Reserved for BIOS | Do not use. Reserved for BIOS EBDA.
6409A000 +------------------------+
H. Peter Anvindec04cf2007-05-17 15:50:47 -070065 | Command line |
66 | Stack/heap | For use by the kernel real-mode code.
Linus Torvalds1da177e2005-04-16 15:20:36 -070067098000 +------------------------+
68 | Kernel setup | The kernel real-mode code.
69090200 +------------------------+
70 | Kernel boot sector | The kernel legacy boot sector.
71090000 +------------------------+
72 | Protected-mode kernel | The bulk of the kernel image.
73010000 +------------------------+
74 | Boot loader | <- Boot sector entry point 0000:7C00
75001000 +------------------------+
76 | Reserved for MBR/BIOS |
77000800 +------------------------+
78 | Typically used by MBR |
79000600 +------------------------+
80 | BIOS use only |
81000000 +------------------------+
82
83
84When using bzImage, the protected-mode kernel was relocated to
850x100000 ("high memory"), and the kernel real-mode block (boot sector,
86setup, and stack/heap) was made relocatable to any address between
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700870x10000 and end of low memory. Unfortunately, in protocols 2.00 and
882.01 the 0x90000+ memory range is still used internally by the kernel;
89the 2.02 protocol resolves that problem.
Linus Torvalds1da177e2005-04-16 15:20:36 -070090
91It is desirable to keep the "memory ceiling" -- the highest point in
92low memory touched by the boot loader -- as low as possible, since
93some newer BIOSes have begun to allocate some rather large amounts of
94memory, called the Extended BIOS Data Area, near the top of low
95memory. The boot loader should use the "INT 12h" BIOS call to verify
96how much low memory is available.
97
98Unfortunately, if INT 12h reports that the amount of memory is too
99low, there is usually nothing the boot loader can do but to report an
100error to the user. The boot loader should therefore be designed to
101take up as little space in low memory as it reasonably can. For
102zImage or old bzImage kernels, which need data written into the
1030x90000 segment, the boot loader should make sure not to use memory
104above the 0x9A000 point; too many BIOSes will break above that point.
105
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700106For a modern bzImage kernel with boot protocol version >= 2.02, a
107memory layout like the following is suggested:
108
109 ~ ~
110 | Protected-mode kernel |
111100000 +------------------------+
112 | I/O memory hole |
1130A0000 +------------------------+
114 | Reserved for BIOS | Leave as much as possible unused
115 ~ ~
116 | Command line | (Can also be below the X+10000 mark)
117X+10000 +------------------------+
118 | Stack/heap | For use by the kernel real-mode code.
119X+08000 +------------------------+
120 | Kernel setup | The kernel real-mode code.
121 | Kernel boot sector | The kernel legacy boot sector.
122X +------------------------+
123 | Boot loader | <- Boot sector entry point 0000:7C00
124001000 +------------------------+
125 | Reserved for MBR/BIOS |
126000800 +------------------------+
127 | Typically used by MBR |
128000600 +------------------------+
129 | BIOS use only |
130000000 +------------------------+
131
132... where the address X is as low as the design of the boot loader
133permits.
134
Linus Torvalds1da177e2005-04-16 15:20:36 -0700135
136**** THE REAL-MODE KERNEL HEADER
137
138In the following text, and anywhere in the kernel boot sequence, "a
139sector" refers to 512 bytes. It is independent of the actual sector
140size of the underlying medium.
141
142The first step in loading a Linux kernel should be to load the
143real-mode code (boot sector and setup code) and then examine the
144following header at offset 0x01f1. The real-mode code can total up to
14532K, although the boot loader may choose to load only the first two
146sectors (1K) and then examine the bootup sector size.
147
148The header looks like:
149
150Offset Proto Name Meaning
151/Size
152
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -070015301F1/1 ALL(1 setup_sects The size of the setup in sectors
Linus Torvalds1da177e2005-04-16 15:20:36 -070015401F2/2 ALL root_flags If set, the root is mounted readonly
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -070015501F4/4 2.04+(2 syssize The size of the 32-bit code in 16-byte paras
Linus Torvalds1da177e2005-04-16 15:20:36 -070015601F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
15701FA/2 ALL vid_mode Video mode control
15801FC/2 ALL root_dev Default root device number
15901FE/2 ALL boot_flag 0xAA55 magic number
1600200/2 2.00+ jump Jump instruction
1610202/4 2.00+ header Magic signature "HdrS"
1620206/2 2.00+ version Boot protocol version supported
1630208/4 2.00+ realmode_swtch Boot loader hook (see below)
164020C/2 2.00+ start_sys The load-low segment (0x1000) (obsolete)
165020E/2 2.00+ kernel_version Pointer to kernel version string
1660210/1 2.00+ type_of_loader Boot loader identifier
1670211/1 2.00+ loadflags Boot protocol option flags
1680212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
1690214/4 2.00+ code32_start Boot loader hook (see below)
1700218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
171021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
1720220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
1730224/2 2.01+ heap_end_ptr Free memory after setup end
1740226/2 N/A pad1 Unused
1750228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
176022C/4 2.03+ initrd_addr_max Highest legal initrd address
Vivek Goyald263b212006-12-07 02:14:13 +01001770230/4 2.05+ kernel_alignment Physical addr alignment required for kernel
1780234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
Bernhard Walle8f9aeca2007-05-02 19:27:10 +02001790235/3 N/A pad2 Unused
1800238/4 2.06+ cmdline_size Maximum size of the kernel command line
Rusty Russelle5371ac2007-10-21 16:41:33 -0700181023C/4 2.07+ hardware_subarch Hardware subarchitecture
1820240/8 2.07+ hardware_subarch_data Subarchitecture-specific data
Ian Campbell87253d12008-02-19 11:12:30 +00001830248/4 2.08+ payload_offset Offset of kernel payload
184024C/4 2.08+ payload_length Length of kernel payload
Huang, Yingfb884382008-03-28 10:49:50 +08001850250/8 2.09+ setup_data 64-bit physical pointer to linked list
186 of struct setup_data
Linus Torvalds1da177e2005-04-16 15:20:36 -0700187
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -0700188(1) For backwards compatibility, if the setup_sects field contains 0, the
189 real value is 4.
190
191(2) For boot protocol prior to 2.04, the upper two bytes of the syssize
192 field are unusable, which means the size of a bzImage kernel
193 cannot be determined.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700194
195If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
196the boot protocol version is "old". Loading an old kernel, the
197following parameters should be assumed:
198
199 Image type = zImage
200 initrd not supported
201 Real-mode kernel must be located at 0x90000.
202
203Otherwise, the "version" field contains the protocol version,
204e.g. protocol version 2.01 will contain 0x0201 in this field. When
205setting fields in the header, you must make sure only to set fields
206supported by the protocol version in use.
207
Linus Torvalds1da177e2005-04-16 15:20:36 -0700208
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700209**** DETAILS OF HEADER FIELDS
Linus Torvalds1da177e2005-04-16 15:20:36 -0700210
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700211For each field, some are information from the kernel to the bootloader
212("read"), some are expected to be filled out by the bootloader
213("write"), and some are expected to be read and modified by the
214bootloader ("modify").
Linus Torvalds1da177e2005-04-16 15:20:36 -0700215
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700216All general purpose boot loaders should write the fields marked
217(obligatory). Boot loaders who want to load the kernel at a
218nonstandard address should fill in the fields marked (reloc); other
219boot loaders can ignore those fields.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700220
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700221The byte order of all fields is littleendian (this is x86, after all.)
222
Rusty Russelle5371ac2007-10-21 16:41:33 -0700223Field name: setup_sects
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700224Type: read
225Offset/size: 0x1f1/1
226Protocol: ALL
227
228 The size of the setup code in 512-byte sectors. If this field is
229 0, the real value is 4. The real-mode code consists of the boot
230 sector (always one 512-byte sector) plus the setup code.
231
232Field name: root_flags
233Type: modify (optional)
234Offset/size: 0x1f2/2
235Protocol: ALL
236
237 If this field is nonzero, the root defaults to readonly. The use of
238 this field is deprecated; use the "ro" or "rw" options on the
239 command line instead.
240
241Field name: syssize
242Type: read
243Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
244Protocol: 2.04+
245
246 The size of the protected-mode code in units of 16-byte paragraphs.
247 For protocol versions older than 2.04 this field is only two bytes
248 wide, and therefore cannot be trusted for the size of a kernel if
249 the LOAD_HIGH flag is set.
250
251Field name: ram_size
252Type: kernel internal
253Offset/size: 0x1f8/2
254Protocol: ALL
255
256 This field is obsolete.
257
258Field name: vid_mode
259Type: modify (obligatory)
260Offset/size: 0x1fa/2
261
262 Please see the section on SPECIAL COMMAND LINE OPTIONS.
263
264Field name: root_dev
265Type: modify (optional)
266Offset/size: 0x1fc/2
267Protocol: ALL
268
269 The default root device device number. The use of this field is
270 deprecated, use the "root=" option on the command line instead.
271
272Field name: boot_flag
273Type: read
274Offset/size: 0x1fe/2
275Protocol: ALL
276
277 Contains 0xAA55. This is the closest thing old Linux kernels have
278 to a magic number.
279
280Field name: jump
281Type: read
282Offset/size: 0x200/2
283Protocol: 2.00+
284
285 Contains an x86 jump instruction, 0xEB followed by a signed offset
286 relative to byte 0x202. This can be used to determine the size of
287 the header.
288
289Field name: header
290Type: read
291Offset/size: 0x202/4
292Protocol: 2.00+
293
294 Contains the magic number "HdrS" (0x53726448).
295
296Field name: version
297Type: read
298Offset/size: 0x206/2
299Protocol: 2.00+
300
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700301 Contains the boot protocol version, in (major << 8)+minor format,
302 e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
303 10.17.
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700304
305Field name: readmode_swtch
306Type: modify (optional)
307Offset/size: 0x208/4
308Protocol: 2.00+
309
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700310 Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700311
312Field name: start_sys
313Type: read
314Offset/size: 0x20c/4
315Protocol: 2.00+
316
317 The load low segment (0x1000). Obsolete.
318
319Field name: kernel_version
320Type: read
321Offset/size: 0x20e/2
322Protocol: 2.00+
323
324 If set to a nonzero value, contains a pointer to a NUL-terminated
325 human-readable kernel version number string, less 0x200. This can
326 be used to display the kernel version to the user. This value
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700327 should be less than (0x200*setup_sects).
328
329 For example, if this value is set to 0x1c00, the kernel version
330 number string can be found at offset 0x1e00 in the kernel file.
331 This is a valid value if and only if the "setup_sects" field
332 contains the value 15 or higher, as:
333
334 0x1c00 < 15*0x200 (= 0x1e00) but
335 0x1c00 >= 14*0x200 (= 0x1c00)
336
337 0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700338
339Field name: type_of_loader
340Type: write (obligatory)
341Offset/size: 0x210/1
342Protocol: 2.00+
343
344 If your boot loader has an assigned id (see table below), enter
345 0xTV here, where T is an identifier for the boot loader and V is
346 a version number. Otherwise, enter 0xFF here.
347
348 Assigned boot loader ids:
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700349 0 LILO (0x00 reserved for pre-2.00 bootloader)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700350 1 Loadlin
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700351 2 bootsect-loader (0x20, all other values reserved)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700352 3 SYSLINUX
353 4 EtherBoot
354 5 ELILO
355 7 GRuB
356 8 U-BOOT
Jeremy Fitzhardinge354332e2006-09-12 20:35:57 -0700357 9 Xen
H. Peter Anvinc229ec52007-01-26 09:15:47 -0800358 A Gujin
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700359 B Qemu
Linus Torvalds1da177e2005-04-16 15:20:36 -0700360
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700361 Please contact <hpa@zytor.com> if you need a bootloader ID
362 value assigned.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700363
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700364Field name: loadflags
365Type: modify (obligatory)
366Offset/size: 0x211/1
367Protocol: 2.00+
Linus Torvalds1da177e2005-04-16 15:20:36 -0700368
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700369 This field is a bitmask.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700370
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700371 Bit 0 (read): LOADED_HIGH
372 - If 0, the protected-mode code is loaded at 0x10000.
373 - If 1, the protected-mode code is loaded at 0x100000.
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700374
Rusty Russelle5371ac2007-10-21 16:41:33 -0700375 Bit 6 (write): KEEP_SEGMENTS
376 Protocol: 2.07+
377 - if 0, reload the segment registers in the 32bit entry point.
378 - if 1, do not reload the segment registers in the 32bit entry point.
379 Assume that %cs %ds %ss %es are all set to flat segments with
380 a base of 0 (or the equivalent for their environment).
381
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700382 Bit 7 (write): CAN_USE_HEAP
383 Set this bit to 1 to indicate that the value entered in the
384 heap_end_ptr is valid. If this field is clear, some setup code
385 functionality will be disabled.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700386
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700387Field name: setup_move_size
388Type: modify (obligatory)
389Offset/size: 0x212/2
390Protocol: 2.00-2.01
Linus Torvalds1da177e2005-04-16 15:20:36 -0700391
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700392 When using protocol 2.00 or 2.01, if the real mode kernel is not
393 loaded at 0x90000, it gets moved there later in the loading
394 sequence. Fill in this field if you want additional data (such as
395 the kernel command line) moved in addition to the real-mode kernel
396 itself.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700397
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700398 The unit is bytes starting with the beginning of the boot sector.
399
400 This field is can be ignored when the protocol is 2.02 or higher, or
401 if the real-mode code is loaded at 0x90000.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700402
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700403Field name: code32_start
404Type: modify (optional, reloc)
405Offset/size: 0x214/4
406Protocol: 2.00+
407
408 The address to jump to in protected mode. This defaults to the load
409 address of the kernel, and can be used by the boot loader to
410 determine the proper load address.
411
412 This field can be modified for two purposes:
413
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700414 1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700415
416 2. if a bootloader which does not install a hook loads a
417 relocatable kernel at a nonstandard address it will have to modify
418 this field to point to the load address.
419
420Field name: ramdisk_image
421Type: write (obligatory)
422Offset/size: 0x218/4
423Protocol: 2.00+
424
425 The 32-bit linear address of the initial ramdisk or ramfs. Leave at
426 zero if there is no initial ramdisk/ramfs.
427
428Field name: ramdisk_size
429Type: write (obligatory)
430Offset/size: 0x21c/4
431Protocol: 2.00+
432
433 Size of the initial ramdisk or ramfs. Leave at zero if there is no
434 initial ramdisk/ramfs.
435
436Field name: bootsect_kludge
437Type: kernel internal
438Offset/size: 0x220/4
439Protocol: 2.00+
440
441 This field is obsolete.
442
443Field name: heap_end_ptr
444Type: write (obligatory)
445Offset/size: 0x224/2
446Protocol: 2.01+
447
448 Set this field to the offset (from the beginning of the real-mode
449 code) of the end of the setup stack/heap, minus 0x0200.
450
451Field name: cmd_line_ptr
452Type: write (obligatory)
453Offset/size: 0x228/4
454Protocol: 2.02+
455
456 Set this field to the linear address of the kernel command line.
457 The kernel command line can be located anywhere between the end of
458 the setup heap and 0xA0000; it does not have to be located in the
459 same 64K segment as the real-mode code itself.
460
461 Fill in this field even if your boot loader does not support a
462 command line, in which case you can point this to an empty string
463 (or better yet, to the string "auto".) If this field is left at
464 zero, the kernel will assume that your boot loader does not support
465 the 2.02+ protocol.
466
467Field name: initrd_addr_max
468Type: read
469Offset/size: 0x22c/4
470Protocol: 2.03+
471
472 The maximum address that may be occupied by the initial
473 ramdisk/ramfs contents. For boot protocols 2.02 or earlier, this
474 field is not present, and the maximum address is 0x37FFFFFF. (This
475 address is defined as the address of the highest safe byte, so if
476 your ramdisk is exactly 131072 bytes long and this field is
477 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
478
479Field name: kernel_alignment
480Type: read (reloc)
481Offset/size: 0x230/4
482Protocol: 2.05+
483
484 Alignment unit required by the kernel (if relocatable_kernel is true.)
485
486Field name: relocatable_kernel
487Type: read (reloc)
488Offset/size: 0x234/1
489Protocol: 2.05+
490
491 If this field is nonzero, the protected-mode part of the kernel can
492 be loaded at any address that satisfies the kernel_alignment field.
493 After loading, the boot loader must set the code32_start field to
494 point to the loaded code, or to a boot loader hook.
495
496Field name: cmdline_size
497Type: read
498Offset/size: 0x238/4
499Protocol: 2.06+
500
501 The maximum size of the command line without the terminating
502 zero. This means that the command line can contain at most
503 cmdline_size characters. With protocol version 2.05 and earlier, the
504 maximum size was 255.
Bernhard Walle8f9aeca2007-05-02 19:27:10 +0200505
Rusty Russelle5371ac2007-10-21 16:41:33 -0700506Field name: hardware_subarch
507Type: write
508Offset/size: 0x23c/4
509Protocol: 2.07+
510
511 In a paravirtualized environment the hardware low level architectural
512 pieces such as interrupt handling, page table handling, and
513 accessing process control registers needs to be done differently.
514
515 This field allows the bootloader to inform the kernel we are in one
516 one of those environments.
517
518 0x00000000 The default x86/PC environment
519 0x00000001 lguest
520 0x00000002 Xen
521
522Field name: hardware_subarch_data
523Type: write
524Offset/size: 0x240/8
525Protocol: 2.07+
526
527 A pointer to data that is specific to hardware subarch
528
Ian Campbell87253d12008-02-19 11:12:30 +0000529Field name: payload_offset
Ian Campbell099e1372008-02-13 20:54:58 +0000530Type: read
531Offset/size: 0x248/4
532Protocol: 2.08+
533
534 If non-zero then this field contains the offset from the end of the
Ian Campbell87253d12008-02-19 11:12:30 +0000535 real-mode code to the payload.
536
537 The payload may be compressed. The format of both the compressed and
538 uncompressed data should be determined using the standard magic
539 numbers. Currently only gzip compressed ELF is used.
Ian Campbell099e1372008-02-13 20:54:58 +0000540
Ian Campbell87253d12008-02-19 11:12:30 +0000541Field name: payload_length
Ian Campbell099e1372008-02-13 20:54:58 +0000542Type: read
543Offset/size: 0x24c/4
544Protocol: 2.08+
545
Ian Campbell87253d12008-02-19 11:12:30 +0000546 The length of the payload.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700547
Ian Campbell7d6e7372008-02-17 20:06:35 +0100548**** THE IMAGE CHECKSUM
549
550From boot protocol version 2.08 onwards the CRC-32 is calculated over
551the entire file using the characteristic polynomial 0x04C11DB7 and an
552initial remainder of 0xffffffff. The checksum is appended to the
553file; therefore the CRC of the file up to the limit specified in the
554syssize field of the header is always 0.
555
Linus Torvalds1da177e2005-04-16 15:20:36 -0700556**** THE KERNEL COMMAND LINE
557
558The kernel command line has become an important way for the boot
559loader to communicate with the kernel. Some of its options are also
560relevant to the boot loader itself, see "special command line options"
561below.
562
Bernhard Walle8f9aeca2007-05-02 19:27:10 +0200563The kernel command line is a null-terminated string. The maximum
564length can be retrieved from the field cmdline_size. Before protocol
565version 2.06, the maximum was 255 characters. A string that is too
566long will be automatically truncated by the kernel.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700567
568If the boot protocol version is 2.02 or later, the address of the
569kernel command line is given by the header field cmd_line_ptr (see
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -0700570above.) This address can be anywhere between the end of the setup
571heap and 0xA0000.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700572
573If the protocol version is *not* 2.02 or higher, the kernel
574command line is entered using the following protocol:
575
576 At offset 0x0020 (word), "cmd_line_magic", enter the magic
577 number 0xA33F.
578
579 At offset 0x0022 (word), "cmd_line_offset", enter the offset
580 of the kernel command line (relative to the start of the
581 real-mode kernel).
582
583 The kernel command line *must* be within the memory region
584 covered by setup_move_size, so you may need to adjust this
585 field.
586
Huang, Yingfb884382008-03-28 10:49:50 +0800587Field name: setup_data
588Type: write (obligatory)
589Offset/size: 0x250/8
590Protocol: 2.09+
591
592 The 64-bit physical pointer to NULL terminated single linked list of
593 struct setup_data. This is used to define a more extensible boot
594 parameters passing mechanism. The definition of struct setup_data is
595 as follow:
596
597 struct setup_data {
598 u64 next;
599 u32 type;
600 u32 len;
601 u8 data[0];
602 };
603
604 Where, the next is a 64-bit physical pointer to the next node of
605 linked list, the next field of the last node is 0; the type is used
606 to identify the contents of data; the len is the length of data
607 field; the data holds the real payload.
608
Linus Torvalds1da177e2005-04-16 15:20:36 -0700609
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700610**** MEMORY LAYOUT OF THE REAL-MODE CODE
611
612The real-mode code requires a stack/heap to be set up, as well as
613memory allocated for the kernel command line. This needs to be done
614in the real-mode accessible memory in bottom megabyte.
615
616It should be noted that modern machines often have a sizable Extended
617BIOS Data Area (EBDA). As a result, it is advisable to use as little
618of the low megabyte as possible.
619
620Unfortunately, under the following circumstances the 0x90000 memory
621segment has to be used:
622
623 - When loading a zImage kernel ((loadflags & 0x01) == 0).
624 - When loading a 2.01 or earlier boot protocol kernel.
625
626 -> For the 2.00 and 2.01 boot protocols, the real-mode code
627 can be loaded at another address, but it is internally
628 relocated to 0x90000. For the "old" protocol, the
629 real-mode code must be loaded at 0x90000.
630
631When loading at 0x90000, avoid using memory above 0x9a000.
632
633For boot protocol 2.02 or higher, the command line does not have to be
634located in the same 64K segment as the real-mode setup code; it is
635thus permitted to give the stack/heap the full 64K segment and locate
636the command line above it.
637
638The kernel command line should not be located below the real-mode
639code, nor should it be located in high memory.
640
641
Linus Torvalds1da177e2005-04-16 15:20:36 -0700642**** SAMPLE BOOT CONFIGURATION
643
644As a sample configuration, assume the following layout of the real
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700645mode segment:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700646
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700647 When loading below 0x90000, use the entire segment:
648
649 0x0000-0x7fff Real mode kernel
650 0x8000-0xdfff Stack and heap
651 0xe000-0xffff Kernel command line
652
653 When loading at 0x90000 OR the protocol version is 2.01 or earlier:
654
655 0x0000-0x7fff Real mode kernel
656 0x8000-0x97ff Stack and heap
657 0x9800-0x9fff Kernel command line
Linus Torvalds1da177e2005-04-16 15:20:36 -0700658
659Such a boot loader should enter the following fields in the header:
660
661 unsigned long base_ptr; /* base address for real-mode segment */
662
663 if ( setup_sects == 0 ) {
664 setup_sects = 4;
665 }
666
667 if ( protocol >= 0x0200 ) {
668 type_of_loader = <type code>;
669 if ( loading_initrd ) {
670 ramdisk_image = <initrd_address>;
671 ramdisk_size = <initrd_size>;
672 }
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700673
674 if ( protocol >= 0x0202 && loadflags & 0x01 )
675 heap_end = 0xe000;
676 else
677 heap_end = 0x9800;
678
Linus Torvalds1da177e2005-04-16 15:20:36 -0700679 if ( protocol >= 0x0201 ) {
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700680 heap_end_ptr = heap_end - 0x200;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700681 loadflags |= 0x80; /* CAN_USE_HEAP */
682 }
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700683
Linus Torvalds1da177e2005-04-16 15:20:36 -0700684 if ( protocol >= 0x0202 ) {
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700685 cmd_line_ptr = base_ptr + heap_end;
686 strcpy(cmd_line_ptr, cmdline);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700687 } else {
688 cmd_line_magic = 0xA33F;
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700689 cmd_line_offset = heap_end;
690 setup_move_size = heap_end + strlen(cmdline)+1;
691 strcpy(base_ptr+cmd_line_offset, cmdline);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700692 }
693 } else {
694 /* Very old kernel */
695
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700696 heap_end = 0x9800;
697
Linus Torvalds1da177e2005-04-16 15:20:36 -0700698 cmd_line_magic = 0xA33F;
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700699 cmd_line_offset = heap_end;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700700
701 /* A very old kernel MUST have its real-mode code
702 loaded at 0x90000 */
703
704 if ( base_ptr != 0x90000 ) {
705 /* Copy the real-mode kernel */
706 memcpy(0x90000, base_ptr, (setup_sects+1)*512);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700707 base_ptr = 0x90000; /* Relocated */
708 }
709
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700710 strcpy(0x90000+cmd_line_offset, cmdline);
711
Linus Torvalds1da177e2005-04-16 15:20:36 -0700712 /* It is recommended to clear memory up to the 32K mark */
713 memset(0x90000 + (setup_sects+1)*512, 0,
714 (64-(setup_sects+1))*512);
715 }
716
717
718**** LOADING THE REST OF THE KERNEL
719
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -0700720The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
721in the kernel file (again, if setup_sects == 0 the real value is 4.)
722It should be loaded at address 0x10000 for Image/zImage kernels and
Linus Torvalds1da177e2005-04-16 15:20:36 -07007230x100000 for bzImage kernels.
724
725The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
726bit (LOAD_HIGH) in the loadflags field is set:
727
728 is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
729 load_address = is_bzImage ? 0x100000 : 0x10000;
730
731Note that Image/zImage kernels can be up to 512K in size, and thus use
732the entire 0x10000-0x90000 range of memory. This means it is pretty
733much a requirement for these kernels to load the real-mode part at
7340x90000. bzImage kernels allow much more flexibility.
735
736
737**** SPECIAL COMMAND LINE OPTIONS
738
739If the command line provided by the boot loader is entered by the
740user, the user may expect the following command line options to work.
741They should normally not be deleted from the kernel command line even
742though not all of them are actually meaningful to the kernel. Boot
743loader authors who need additional command line options for the boot
744loader itself should get them registered in
745Documentation/kernel-parameters.txt to make sure they will not
746conflict with actual kernel options now or in the future.
747
748 vga=<mode>
749 <mode> here is either an integer (in C notation, either
750 decimal, octal, or hexadecimal) or one of the strings
751 "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
752 (meaning 0xFFFD). This value should be entered into the
753 vid_mode field, as it is used by the kernel before the command
754 line is parsed.
755
756 mem=<size>
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700757 <size> is an integer in C notation optionally followed by
758 (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
759 << 30, << 40, << 50 or << 60). This specifies the end of
760 memory to the kernel. This affects the possible placement of
761 an initrd, since an initrd should be placed near end of
Linus Torvalds1da177e2005-04-16 15:20:36 -0700762 memory. Note that this is an option to *both* the kernel and
763 the bootloader!
764
765 initrd=<file>
766 An initrd should be loaded. The meaning of <file> is
767 obviously bootloader-dependent, and some boot loaders
768 (e.g. LILO) do not have such a command.
769
770In addition, some boot loaders add the following options to the
771user-specified command line:
772
773 BOOT_IMAGE=<file>
774 The boot image which was loaded. Again, the meaning of <file>
775 is obviously bootloader-dependent.
776
777 auto
778 The kernel was booted without explicit user intervention.
779
780If these options are added by the boot loader, it is highly
781recommended that they are located *first*, before the user-specified
782or configuration-specified command line. Otherwise, "init=/bin/sh"
783gets confused by the "auto" option.
784
785
786**** RUNNING THE KERNEL
787
788The kernel is started by jumping to the kernel entry point, which is
789located at *segment* offset 0x20 from the start of the real mode
790kernel. This means that if you loaded your real-mode kernel code at
7910x90000, the kernel entry point is 9020:0000.
792
793At entry, ds = es = ss should point to the start of the real-mode
794kernel code (0x9000 if the code is loaded at 0x90000), sp should be
795set up properly, normally pointing to the top of the heap, and
796interrupts should be disabled. Furthermore, to guard against bugs in
797the kernel, it is recommended that the boot loader sets fs = gs = ds =
798es = ss.
799
800In our example from above, we would do:
801
802 /* Note: in the case of the "old" kernel protocol, base_ptr must
803 be == 0x90000 at this point; see the previous sample code */
804
805 seg = base_ptr >> 4;
806
807 cli(); /* Enter with interrupts disabled! */
808
809 /* Set up the real-mode kernel stack */
810 _SS = seg;
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700811 _SP = heap_end;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700812
813 _DS = _ES = _FS = _GS = seg;
814 jmp_far(seg+0x20, 0); /* Run the kernel */
815
816If your boot sector accesses a floppy drive, it is recommended to
817switch off the floppy motor before running the kernel, since the
818kernel boot leaves interrupts off and thus the motor will not be
819switched off, especially if the loaded kernel has the floppy driver as
820a demand-loaded module!
821
822
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700823**** ADVANCED BOOT LOADER HOOKS
Linus Torvalds1da177e2005-04-16 15:20:36 -0700824
825If the boot loader runs in a particularly hostile environment (such as
826LOADLIN, which runs under DOS) it may be impossible to follow the
827standard memory location requirements. Such a boot loader may use the
828following hooks that, if set, are invoked by the kernel at the
829appropriate time. The use of these hooks should probably be
830considered an absolutely last resort!
831
832IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
833%edi across invocation.
834
835 realmode_swtch:
836 A 16-bit real mode far subroutine invoked immediately before
837 entering protected mode. The default routine disables NMI, so
838 your routine should probably do so, too.
839
840 code32_start:
841 A 32-bit flat-mode routine *jumped* to immediately after the
842 transition to protected mode, but before the kernel is
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700843 uncompressed. No segments, except CS, are guaranteed to be
844 set up (current kernels do, but older ones do not); you should
845 set them up to BOOT_DS (0x18) yourself.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700846
847 After completing your hook, you should jump to the address
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700848 that was in this field before your boot loader overwrote it
849 (relocated, if appropriate.)
Huang, Yingaa694322007-10-24 10:18:49 +0800850
851
852**** 32-bit BOOT PROTOCOL
853
854For machine with some new BIOS other than legacy BIOS, such as EFI,
855LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
856based on legacy BIOS can not be used, so a 32-bit boot protocol needs
857to be defined.
858
859In 32-bit boot protocol, the first step in loading a Linux kernel
860should be to setup the boot parameters (struct boot_params,
861traditionally known as "zero page"). The memory for struct boot_params
862should be allocated and initialized to all zero. Then the setup header
863from offset 0x01f1 of kernel image on should be loaded into struct
864boot_params and examined. The end of setup header can be calculated as
865follow:
866
867 0x0202 + byte value at offset 0x0201
868
869In addition to read/modify/write the setup header of the struct
870boot_params as that of 16-bit boot protocol, the boot loader should
871also fill the additional fields of the struct boot_params as that
872described in zero-page.txt.
873
874After setupping the struct boot_params, the boot loader can load the
87532/64-bit kernel in the same way as that of 16-bit boot protocol.
876
877In 32-bit boot protocol, the kernel is started by jumping to the
87832-bit kernel entry point, which is the start address of loaded
87932/64-bit kernel.
880
881At entry, the CPU must be in 32-bit protected mode with paging
882disabled; a GDT must be loaded with the descriptors for selectors
883__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
884segment; __BOOS_CS must have execute/read permission, and __BOOT_DS
885must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
886must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
887address of the struct boot_params; %ebp, %edi and %ebx must be zero.