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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
43 the boot command line
44
Linus Torvalds1da177e2005-04-16 15:20:36 -070045
46**** MEMORY LAYOUT
47
48The traditional memory map for the kernel loader, used for Image or
49zImage kernels, typically looks like:
50
51 | |
520A0000 +------------------------+
53 | Reserved for BIOS | Do not use. Reserved for BIOS EBDA.
5409A000 +------------------------+
H. Peter Anvindec04cf2007-05-17 15:50:47 -070055 | Command line |
56 | Stack/heap | For use by the kernel real-mode code.
Linus Torvalds1da177e2005-04-16 15:20:36 -070057098000 +------------------------+
58 | Kernel setup | The kernel real-mode code.
59090200 +------------------------+
60 | Kernel boot sector | The kernel legacy boot sector.
61090000 +------------------------+
62 | Protected-mode kernel | The bulk of the kernel image.
63010000 +------------------------+
64 | Boot loader | <- Boot sector entry point 0000:7C00
65001000 +------------------------+
66 | Reserved for MBR/BIOS |
67000800 +------------------------+
68 | Typically used by MBR |
69000600 +------------------------+
70 | BIOS use only |
71000000 +------------------------+
72
73
74When using bzImage, the protected-mode kernel was relocated to
750x100000 ("high memory"), and the kernel real-mode block (boot sector,
76setup, and stack/heap) was made relocatable to any address between
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700770x10000 and end of low memory. Unfortunately, in protocols 2.00 and
782.01 the 0x90000+ memory range is still used internally by the kernel;
79the 2.02 protocol resolves that problem.
Linus Torvalds1da177e2005-04-16 15:20:36 -070080
81It is desirable to keep the "memory ceiling" -- the highest point in
82low memory touched by the boot loader -- as low as possible, since
83some newer BIOSes have begun to allocate some rather large amounts of
84memory, called the Extended BIOS Data Area, near the top of low
85memory. The boot loader should use the "INT 12h" BIOS call to verify
86how much low memory is available.
87
88Unfortunately, if INT 12h reports that the amount of memory is too
89low, there is usually nothing the boot loader can do but to report an
90error to the user. The boot loader should therefore be designed to
91take up as little space in low memory as it reasonably can. For
92zImage or old bzImage kernels, which need data written into the
930x90000 segment, the boot loader should make sure not to use memory
94above the 0x9A000 point; too many BIOSes will break above that point.
95
H. Peter Anvindec04cf2007-05-17 15:50:47 -070096For a modern bzImage kernel with boot protocol version >= 2.02, a
97memory layout like the following is suggested:
98
99 ~ ~
100 | Protected-mode kernel |
101100000 +------------------------+
102 | I/O memory hole |
1030A0000 +------------------------+
104 | Reserved for BIOS | Leave as much as possible unused
105 ~ ~
106 | Command line | (Can also be below the X+10000 mark)
107X+10000 +------------------------+
108 | Stack/heap | For use by the kernel real-mode code.
109X+08000 +------------------------+
110 | Kernel setup | The kernel real-mode code.
111 | Kernel boot sector | The kernel legacy boot sector.
112X +------------------------+
113 | Boot loader | <- Boot sector entry point 0000:7C00
114001000 +------------------------+
115 | Reserved for MBR/BIOS |
116000800 +------------------------+
117 | Typically used by MBR |
118000600 +------------------------+
119 | BIOS use only |
120000000 +------------------------+
121
122... where the address X is as low as the design of the boot loader
123permits.
124
Linus Torvalds1da177e2005-04-16 15:20:36 -0700125
126**** THE REAL-MODE KERNEL HEADER
127
128In the following text, and anywhere in the kernel boot sequence, "a
129sector" refers to 512 bytes. It is independent of the actual sector
130size of the underlying medium.
131
132The first step in loading a Linux kernel should be to load the
133real-mode code (boot sector and setup code) and then examine the
134following header at offset 0x01f1. The real-mode code can total up to
13532K, although the boot loader may choose to load only the first two
136sectors (1K) and then examine the bootup sector size.
137
138The header looks like:
139
140Offset Proto Name Meaning
141/Size
142
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -070014301F1/1 ALL(1 setup_sects The size of the setup in sectors
Linus Torvalds1da177e2005-04-16 15:20:36 -070014401F2/2 ALL root_flags If set, the root is mounted readonly
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -070014501F4/4 2.04+(2 syssize The size of the 32-bit code in 16-byte paras
Linus Torvalds1da177e2005-04-16 15:20:36 -070014601F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
14701FA/2 ALL vid_mode Video mode control
14801FC/2 ALL root_dev Default root device number
14901FE/2 ALL boot_flag 0xAA55 magic number
1500200/2 2.00+ jump Jump instruction
1510202/4 2.00+ header Magic signature "HdrS"
1520206/2 2.00+ version Boot protocol version supported
1530208/4 2.00+ realmode_swtch Boot loader hook (see below)
154020C/2 2.00+ start_sys The load-low segment (0x1000) (obsolete)
155020E/2 2.00+ kernel_version Pointer to kernel version string
1560210/1 2.00+ type_of_loader Boot loader identifier
1570211/1 2.00+ loadflags Boot protocol option flags
1580212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
1590214/4 2.00+ code32_start Boot loader hook (see below)
1600218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
161021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
1620220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
1630224/2 2.01+ heap_end_ptr Free memory after setup end
1640226/2 N/A pad1 Unused
1650228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
166022C/4 2.03+ initrd_addr_max Highest legal initrd address
Vivek Goyald263b212006-12-07 02:14:13 +01001670230/4 2.05+ kernel_alignment Physical addr alignment required for kernel
1680234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
Bernhard Walle8f9aeca2007-05-02 19:27:10 +02001690235/3 N/A pad2 Unused
1700238/4 2.06+ cmdline_size Maximum size of the kernel command line
Rusty Russelle5371ac2007-10-21 16:41:33 -0700171023C/4 2.07+ hardware_subarch Hardware subarchitecture
1720240/8 2.07+ hardware_subarch_data Subarchitecture-specific data
Ian Campbell099e1372008-02-13 20:54:58 +00001730248/4 2.08+ compressed_payload_offset
174024C/4 2.08+ compressed_payload_length
Linus Torvalds1da177e2005-04-16 15:20:36 -0700175
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -0700176(1) For backwards compatibility, if the setup_sects field contains 0, the
177 real value is 4.
178
179(2) For boot protocol prior to 2.04, the upper two bytes of the syssize
180 field are unusable, which means the size of a bzImage kernel
181 cannot be determined.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700182
183If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
184the boot protocol version is "old". Loading an old kernel, the
185following parameters should be assumed:
186
187 Image type = zImage
188 initrd not supported
189 Real-mode kernel must be located at 0x90000.
190
191Otherwise, the "version" field contains the protocol version,
192e.g. protocol version 2.01 will contain 0x0201 in this field. When
193setting fields in the header, you must make sure only to set fields
194supported by the protocol version in use.
195
Linus Torvalds1da177e2005-04-16 15:20:36 -0700196
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700197**** DETAILS OF HEADER FIELDS
Linus Torvalds1da177e2005-04-16 15:20:36 -0700198
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700199For each field, some are information from the kernel to the bootloader
200("read"), some are expected to be filled out by the bootloader
201("write"), and some are expected to be read and modified by the
202bootloader ("modify").
Linus Torvalds1da177e2005-04-16 15:20:36 -0700203
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700204All general purpose boot loaders should write the fields marked
205(obligatory). Boot loaders who want to load the kernel at a
206nonstandard address should fill in the fields marked (reloc); other
207boot loaders can ignore those fields.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700208
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700209The byte order of all fields is littleendian (this is x86, after all.)
210
Rusty Russelle5371ac2007-10-21 16:41:33 -0700211Field name: setup_sects
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700212Type: read
213Offset/size: 0x1f1/1
214Protocol: ALL
215
216 The size of the setup code in 512-byte sectors. If this field is
217 0, the real value is 4. The real-mode code consists of the boot
218 sector (always one 512-byte sector) plus the setup code.
219
220Field name: root_flags
221Type: modify (optional)
222Offset/size: 0x1f2/2
223Protocol: ALL
224
225 If this field is nonzero, the root defaults to readonly. The use of
226 this field is deprecated; use the "ro" or "rw" options on the
227 command line instead.
228
229Field name: syssize
230Type: read
231Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
232Protocol: 2.04+
233
234 The size of the protected-mode code in units of 16-byte paragraphs.
235 For protocol versions older than 2.04 this field is only two bytes
236 wide, and therefore cannot be trusted for the size of a kernel if
237 the LOAD_HIGH flag is set.
238
239Field name: ram_size
240Type: kernel internal
241Offset/size: 0x1f8/2
242Protocol: ALL
243
244 This field is obsolete.
245
246Field name: vid_mode
247Type: modify (obligatory)
248Offset/size: 0x1fa/2
249
250 Please see the section on SPECIAL COMMAND LINE OPTIONS.
251
252Field name: root_dev
253Type: modify (optional)
254Offset/size: 0x1fc/2
255Protocol: ALL
256
257 The default root device device number. The use of this field is
258 deprecated, use the "root=" option on the command line instead.
259
260Field name: boot_flag
261Type: read
262Offset/size: 0x1fe/2
263Protocol: ALL
264
265 Contains 0xAA55. This is the closest thing old Linux kernels have
266 to a magic number.
267
268Field name: jump
269Type: read
270Offset/size: 0x200/2
271Protocol: 2.00+
272
273 Contains an x86 jump instruction, 0xEB followed by a signed offset
274 relative to byte 0x202. This can be used to determine the size of
275 the header.
276
277Field name: header
278Type: read
279Offset/size: 0x202/4
280Protocol: 2.00+
281
282 Contains the magic number "HdrS" (0x53726448).
283
284Field name: version
285Type: read
286Offset/size: 0x206/2
287Protocol: 2.00+
288
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700289 Contains the boot protocol version, in (major << 8)+minor format,
290 e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
291 10.17.
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700292
293Field name: readmode_swtch
294Type: modify (optional)
295Offset/size: 0x208/4
296Protocol: 2.00+
297
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700298 Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700299
300Field name: start_sys
301Type: read
302Offset/size: 0x20c/4
303Protocol: 2.00+
304
305 The load low segment (0x1000). Obsolete.
306
307Field name: kernel_version
308Type: read
309Offset/size: 0x20e/2
310Protocol: 2.00+
311
312 If set to a nonzero value, contains a pointer to a NUL-terminated
313 human-readable kernel version number string, less 0x200. This can
314 be used to display the kernel version to the user. This value
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700315 should be less than (0x200*setup_sects).
316
317 For example, if this value is set to 0x1c00, the kernel version
318 number string can be found at offset 0x1e00 in the kernel file.
319 This is a valid value if and only if the "setup_sects" field
320 contains the value 15 or higher, as:
321
322 0x1c00 < 15*0x200 (= 0x1e00) but
323 0x1c00 >= 14*0x200 (= 0x1c00)
324
325 0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700326
327Field name: type_of_loader
328Type: write (obligatory)
329Offset/size: 0x210/1
330Protocol: 2.00+
331
332 If your boot loader has an assigned id (see table below), enter
333 0xTV here, where T is an identifier for the boot loader and V is
334 a version number. Otherwise, enter 0xFF here.
335
336 Assigned boot loader ids:
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700337 0 LILO (0x00 reserved for pre-2.00 bootloader)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700338 1 Loadlin
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700339 2 bootsect-loader (0x20, all other values reserved)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700340 3 SYSLINUX
341 4 EtherBoot
342 5 ELILO
343 7 GRuB
344 8 U-BOOT
Jeremy Fitzhardinge354332e2006-09-12 20:35:57 -0700345 9 Xen
H. Peter Anvinc229ec52007-01-26 09:15:47 -0800346 A Gujin
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700347 B Qemu
Linus Torvalds1da177e2005-04-16 15:20:36 -0700348
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700349 Please contact <hpa@zytor.com> if you need a bootloader ID
350 value assigned.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700351
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700352Field name: loadflags
353Type: modify (obligatory)
354Offset/size: 0x211/1
355Protocol: 2.00+
Linus Torvalds1da177e2005-04-16 15:20:36 -0700356
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700357 This field is a bitmask.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700358
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700359 Bit 0 (read): LOADED_HIGH
360 - If 0, the protected-mode code is loaded at 0x10000.
361 - If 1, the protected-mode code is loaded at 0x100000.
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700362
Rusty Russelle5371ac2007-10-21 16:41:33 -0700363 Bit 6 (write): KEEP_SEGMENTS
364 Protocol: 2.07+
365 - if 0, reload the segment registers in the 32bit entry point.
366 - if 1, do not reload the segment registers in the 32bit entry point.
367 Assume that %cs %ds %ss %es are all set to flat segments with
368 a base of 0 (or the equivalent for their environment).
369
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700370 Bit 7 (write): CAN_USE_HEAP
371 Set this bit to 1 to indicate that the value entered in the
372 heap_end_ptr is valid. If this field is clear, some setup code
373 functionality will be disabled.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700374
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700375Field name: setup_move_size
376Type: modify (obligatory)
377Offset/size: 0x212/2
378Protocol: 2.00-2.01
Linus Torvalds1da177e2005-04-16 15:20:36 -0700379
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700380 When using protocol 2.00 or 2.01, if the real mode kernel is not
381 loaded at 0x90000, it gets moved there later in the loading
382 sequence. Fill in this field if you want additional data (such as
383 the kernel command line) moved in addition to the real-mode kernel
384 itself.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700385
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700386 The unit is bytes starting with the beginning of the boot sector.
387
388 This field is can be ignored when the protocol is 2.02 or higher, or
389 if the real-mode code is loaded at 0x90000.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700390
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700391Field name: code32_start
392Type: modify (optional, reloc)
393Offset/size: 0x214/4
394Protocol: 2.00+
395
396 The address to jump to in protected mode. This defaults to the load
397 address of the kernel, and can be used by the boot loader to
398 determine the proper load address.
399
400 This field can be modified for two purposes:
401
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700402 1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700403
404 2. if a bootloader which does not install a hook loads a
405 relocatable kernel at a nonstandard address it will have to modify
406 this field to point to the load address.
407
408Field name: ramdisk_image
409Type: write (obligatory)
410Offset/size: 0x218/4
411Protocol: 2.00+
412
413 The 32-bit linear address of the initial ramdisk or ramfs. Leave at
414 zero if there is no initial ramdisk/ramfs.
415
416Field name: ramdisk_size
417Type: write (obligatory)
418Offset/size: 0x21c/4
419Protocol: 2.00+
420
421 Size of the initial ramdisk or ramfs. Leave at zero if there is no
422 initial ramdisk/ramfs.
423
424Field name: bootsect_kludge
425Type: kernel internal
426Offset/size: 0x220/4
427Protocol: 2.00+
428
429 This field is obsolete.
430
431Field name: heap_end_ptr
432Type: write (obligatory)
433Offset/size: 0x224/2
434Protocol: 2.01+
435
436 Set this field to the offset (from the beginning of the real-mode
437 code) of the end of the setup stack/heap, minus 0x0200.
438
439Field name: cmd_line_ptr
440Type: write (obligatory)
441Offset/size: 0x228/4
442Protocol: 2.02+
443
444 Set this field to the linear address of the kernel command line.
445 The kernel command line can be located anywhere between the end of
446 the setup heap and 0xA0000; it does not have to be located in the
447 same 64K segment as the real-mode code itself.
448
449 Fill in this field even if your boot loader does not support a
450 command line, in which case you can point this to an empty string
451 (or better yet, to the string "auto".) If this field is left at
452 zero, the kernel will assume that your boot loader does not support
453 the 2.02+ protocol.
454
455Field name: initrd_addr_max
456Type: read
457Offset/size: 0x22c/4
458Protocol: 2.03+
459
460 The maximum address that may be occupied by the initial
461 ramdisk/ramfs contents. For boot protocols 2.02 or earlier, this
462 field is not present, and the maximum address is 0x37FFFFFF. (This
463 address is defined as the address of the highest safe byte, so if
464 your ramdisk is exactly 131072 bytes long and this field is
465 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
466
467Field name: kernel_alignment
468Type: read (reloc)
469Offset/size: 0x230/4
470Protocol: 2.05+
471
472 Alignment unit required by the kernel (if relocatable_kernel is true.)
473
474Field name: relocatable_kernel
475Type: read (reloc)
476Offset/size: 0x234/1
477Protocol: 2.05+
478
479 If this field is nonzero, the protected-mode part of the kernel can
480 be loaded at any address that satisfies the kernel_alignment field.
481 After loading, the boot loader must set the code32_start field to
482 point to the loaded code, or to a boot loader hook.
483
484Field name: cmdline_size
485Type: read
486Offset/size: 0x238/4
487Protocol: 2.06+
488
489 The maximum size of the command line without the terminating
490 zero. This means that the command line can contain at most
491 cmdline_size characters. With protocol version 2.05 and earlier, the
492 maximum size was 255.
Bernhard Walle8f9aeca2007-05-02 19:27:10 +0200493
Rusty Russelle5371ac2007-10-21 16:41:33 -0700494Field name: hardware_subarch
495Type: write
496Offset/size: 0x23c/4
497Protocol: 2.07+
498
499 In a paravirtualized environment the hardware low level architectural
500 pieces such as interrupt handling, page table handling, and
501 accessing process control registers needs to be done differently.
502
503 This field allows the bootloader to inform the kernel we are in one
504 one of those environments.
505
506 0x00000000 The default x86/PC environment
507 0x00000001 lguest
508 0x00000002 Xen
509
510Field name: hardware_subarch_data
511Type: write
512Offset/size: 0x240/8
513Protocol: 2.07+
514
515 A pointer to data that is specific to hardware subarch
516
Ian Campbell099e1372008-02-13 20:54:58 +0000517Field name: compressed_payload_offset
518Type: read
519Offset/size: 0x248/4
520Protocol: 2.08+
521
522 If non-zero then this field contains the offset from the end of the
523 real-mode code to the compressed payload. The compression format
524 should be determined using the standard magic number, currently only
525 gzip is used.
526
527Field name: compressed_payload_length
528Type: read
529Offset/size: 0x24c/4
530Protocol: 2.08+
531
532 The length of the compressed payload.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700533
Ian Campbell7d6e7372008-02-17 20:06:35 +0100534**** THE IMAGE CHECKSUM
535
536From boot protocol version 2.08 onwards the CRC-32 is calculated over
537the entire file using the characteristic polynomial 0x04C11DB7 and an
538initial remainder of 0xffffffff. The checksum is appended to the
539file; therefore the CRC of the file up to the limit specified in the
540syssize field of the header is always 0.
541
Linus Torvalds1da177e2005-04-16 15:20:36 -0700542**** THE KERNEL COMMAND LINE
543
544The kernel command line has become an important way for the boot
545loader to communicate with the kernel. Some of its options are also
546relevant to the boot loader itself, see "special command line options"
547below.
548
Bernhard Walle8f9aeca2007-05-02 19:27:10 +0200549The kernel command line is a null-terminated string. The maximum
550length can be retrieved from the field cmdline_size. Before protocol
551version 2.06, the maximum was 255 characters. A string that is too
552long will be automatically truncated by the kernel.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700553
554If the boot protocol version is 2.02 or later, the address of the
555kernel command line is given by the header field cmd_line_ptr (see
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -0700556above.) This address can be anywhere between the end of the setup
557heap and 0xA0000.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700558
559If the protocol version is *not* 2.02 or higher, the kernel
560command line is entered using the following protocol:
561
562 At offset 0x0020 (word), "cmd_line_magic", enter the magic
563 number 0xA33F.
564
565 At offset 0x0022 (word), "cmd_line_offset", enter the offset
566 of the kernel command line (relative to the start of the
567 real-mode kernel).
568
569 The kernel command line *must* be within the memory region
570 covered by setup_move_size, so you may need to adjust this
571 field.
572
573
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700574**** MEMORY LAYOUT OF THE REAL-MODE CODE
575
576The real-mode code requires a stack/heap to be set up, as well as
577memory allocated for the kernel command line. This needs to be done
578in the real-mode accessible memory in bottom megabyte.
579
580It should be noted that modern machines often have a sizable Extended
581BIOS Data Area (EBDA). As a result, it is advisable to use as little
582of the low megabyte as possible.
583
584Unfortunately, under the following circumstances the 0x90000 memory
585segment has to be used:
586
587 - When loading a zImage kernel ((loadflags & 0x01) == 0).
588 - When loading a 2.01 or earlier boot protocol kernel.
589
590 -> For the 2.00 and 2.01 boot protocols, the real-mode code
591 can be loaded at another address, but it is internally
592 relocated to 0x90000. For the "old" protocol, the
593 real-mode code must be loaded at 0x90000.
594
595When loading at 0x90000, avoid using memory above 0x9a000.
596
597For boot protocol 2.02 or higher, the command line does not have to be
598located in the same 64K segment as the real-mode setup code; it is
599thus permitted to give the stack/heap the full 64K segment and locate
600the command line above it.
601
602The kernel command line should not be located below the real-mode
603code, nor should it be located in high memory.
604
605
Linus Torvalds1da177e2005-04-16 15:20:36 -0700606**** SAMPLE BOOT CONFIGURATION
607
608As a sample configuration, assume the following layout of the real
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700609mode segment:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700610
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700611 When loading below 0x90000, use the entire segment:
612
613 0x0000-0x7fff Real mode kernel
614 0x8000-0xdfff Stack and heap
615 0xe000-0xffff Kernel command line
616
617 When loading at 0x90000 OR the protocol version is 2.01 or earlier:
618
619 0x0000-0x7fff Real mode kernel
620 0x8000-0x97ff Stack and heap
621 0x9800-0x9fff Kernel command line
Linus Torvalds1da177e2005-04-16 15:20:36 -0700622
623Such a boot loader should enter the following fields in the header:
624
625 unsigned long base_ptr; /* base address for real-mode segment */
626
627 if ( setup_sects == 0 ) {
628 setup_sects = 4;
629 }
630
631 if ( protocol >= 0x0200 ) {
632 type_of_loader = <type code>;
633 if ( loading_initrd ) {
634 ramdisk_image = <initrd_address>;
635 ramdisk_size = <initrd_size>;
636 }
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700637
638 if ( protocol >= 0x0202 && loadflags & 0x01 )
639 heap_end = 0xe000;
640 else
641 heap_end = 0x9800;
642
Linus Torvalds1da177e2005-04-16 15:20:36 -0700643 if ( protocol >= 0x0201 ) {
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700644 heap_end_ptr = heap_end - 0x200;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700645 loadflags |= 0x80; /* CAN_USE_HEAP */
646 }
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700647
Linus Torvalds1da177e2005-04-16 15:20:36 -0700648 if ( protocol >= 0x0202 ) {
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700649 cmd_line_ptr = base_ptr + heap_end;
650 strcpy(cmd_line_ptr, cmdline);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700651 } else {
652 cmd_line_magic = 0xA33F;
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700653 cmd_line_offset = heap_end;
654 setup_move_size = heap_end + strlen(cmdline)+1;
655 strcpy(base_ptr+cmd_line_offset, cmdline);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700656 }
657 } else {
658 /* Very old kernel */
659
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700660 heap_end = 0x9800;
661
Linus Torvalds1da177e2005-04-16 15:20:36 -0700662 cmd_line_magic = 0xA33F;
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700663 cmd_line_offset = heap_end;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700664
665 /* A very old kernel MUST have its real-mode code
666 loaded at 0x90000 */
667
668 if ( base_ptr != 0x90000 ) {
669 /* Copy the real-mode kernel */
670 memcpy(0x90000, base_ptr, (setup_sects+1)*512);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700671 base_ptr = 0x90000; /* Relocated */
672 }
673
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700674 strcpy(0x90000+cmd_line_offset, cmdline);
675
Linus Torvalds1da177e2005-04-16 15:20:36 -0700676 /* It is recommended to clear memory up to the 32K mark */
677 memset(0x90000 + (setup_sects+1)*512, 0,
678 (64-(setup_sects+1))*512);
679 }
680
681
682**** LOADING THE REST OF THE KERNEL
683
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -0700684The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
685in the kernel file (again, if setup_sects == 0 the real value is 4.)
686It should be loaded at address 0x10000 for Image/zImage kernels and
Linus Torvalds1da177e2005-04-16 15:20:36 -07006870x100000 for bzImage kernels.
688
689The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
690bit (LOAD_HIGH) in the loadflags field is set:
691
692 is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
693 load_address = is_bzImage ? 0x100000 : 0x10000;
694
695Note that Image/zImage kernels can be up to 512K in size, and thus use
696the entire 0x10000-0x90000 range of memory. This means it is pretty
697much a requirement for these kernels to load the real-mode part at
6980x90000. bzImage kernels allow much more flexibility.
699
700
701**** SPECIAL COMMAND LINE OPTIONS
702
703If the command line provided by the boot loader is entered by the
704user, the user may expect the following command line options to work.
705They should normally not be deleted from the kernel command line even
706though not all of them are actually meaningful to the kernel. Boot
707loader authors who need additional command line options for the boot
708loader itself should get them registered in
709Documentation/kernel-parameters.txt to make sure they will not
710conflict with actual kernel options now or in the future.
711
712 vga=<mode>
713 <mode> here is either an integer (in C notation, either
714 decimal, octal, or hexadecimal) or one of the strings
715 "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
716 (meaning 0xFFFD). This value should be entered into the
717 vid_mode field, as it is used by the kernel before the command
718 line is parsed.
719
720 mem=<size>
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700721 <size> is an integer in C notation optionally followed by
722 (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
723 << 30, << 40, << 50 or << 60). This specifies the end of
724 memory to the kernel. This affects the possible placement of
725 an initrd, since an initrd should be placed near end of
Linus Torvalds1da177e2005-04-16 15:20:36 -0700726 memory. Note that this is an option to *both* the kernel and
727 the bootloader!
728
729 initrd=<file>
730 An initrd should be loaded. The meaning of <file> is
731 obviously bootloader-dependent, and some boot loaders
732 (e.g. LILO) do not have such a command.
733
734In addition, some boot loaders add the following options to the
735user-specified command line:
736
737 BOOT_IMAGE=<file>
738 The boot image which was loaded. Again, the meaning of <file>
739 is obviously bootloader-dependent.
740
741 auto
742 The kernel was booted without explicit user intervention.
743
744If these options are added by the boot loader, it is highly
745recommended that they are located *first*, before the user-specified
746or configuration-specified command line. Otherwise, "init=/bin/sh"
747gets confused by the "auto" option.
748
749
750**** RUNNING THE KERNEL
751
752The kernel is started by jumping to the kernel entry point, which is
753located at *segment* offset 0x20 from the start of the real mode
754kernel. This means that if you loaded your real-mode kernel code at
7550x90000, the kernel entry point is 9020:0000.
756
757At entry, ds = es = ss should point to the start of the real-mode
758kernel code (0x9000 if the code is loaded at 0x90000), sp should be
759set up properly, normally pointing to the top of the heap, and
760interrupts should be disabled. Furthermore, to guard against bugs in
761the kernel, it is recommended that the boot loader sets fs = gs = ds =
762es = ss.
763
764In our example from above, we would do:
765
766 /* Note: in the case of the "old" kernel protocol, base_ptr must
767 be == 0x90000 at this point; see the previous sample code */
768
769 seg = base_ptr >> 4;
770
771 cli(); /* Enter with interrupts disabled! */
772
773 /* Set up the real-mode kernel stack */
774 _SS = seg;
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700775 _SP = heap_end;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700776
777 _DS = _ES = _FS = _GS = seg;
778 jmp_far(seg+0x20, 0); /* Run the kernel */
779
780If your boot sector accesses a floppy drive, it is recommended to
781switch off the floppy motor before running the kernel, since the
782kernel boot leaves interrupts off and thus the motor will not be
783switched off, especially if the loaded kernel has the floppy driver as
784a demand-loaded module!
785
786
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700787**** ADVANCED BOOT LOADER HOOKS
Linus Torvalds1da177e2005-04-16 15:20:36 -0700788
789If the boot loader runs in a particularly hostile environment (such as
790LOADLIN, which runs under DOS) it may be impossible to follow the
791standard memory location requirements. Such a boot loader may use the
792following hooks that, if set, are invoked by the kernel at the
793appropriate time. The use of these hooks should probably be
794considered an absolutely last resort!
795
796IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
797%edi across invocation.
798
799 realmode_swtch:
800 A 16-bit real mode far subroutine invoked immediately before
801 entering protected mode. The default routine disables NMI, so
802 your routine should probably do so, too.
803
804 code32_start:
805 A 32-bit flat-mode routine *jumped* to immediately after the
806 transition to protected mode, but before the kernel is
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700807 uncompressed. No segments, except CS, are guaranteed to be
808 set up (current kernels do, but older ones do not); you should
809 set them up to BOOT_DS (0x18) yourself.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700810
811 After completing your hook, you should jump to the address
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700812 that was in this field before your boot loader overwrote it
813 (relocated, if appropriate.)
Huang, Yingaa694322007-10-24 10:18:49 +0800814
815
816**** 32-bit BOOT PROTOCOL
817
818For machine with some new BIOS other than legacy BIOS, such as EFI,
819LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
820based on legacy BIOS can not be used, so a 32-bit boot protocol needs
821to be defined.
822
823In 32-bit boot protocol, the first step in loading a Linux kernel
824should be to setup the boot parameters (struct boot_params,
825traditionally known as "zero page"). The memory for struct boot_params
826should be allocated and initialized to all zero. Then the setup header
827from offset 0x01f1 of kernel image on should be loaded into struct
828boot_params and examined. The end of setup header can be calculated as
829follow:
830
831 0x0202 + byte value at offset 0x0201
832
833In addition to read/modify/write the setup header of the struct
834boot_params as that of 16-bit boot protocol, the boot loader should
835also fill the additional fields of the struct boot_params as that
836described in zero-page.txt.
837
838After setupping the struct boot_params, the boot loader can load the
83932/64-bit kernel in the same way as that of 16-bit boot protocol.
840
841In 32-bit boot protocol, the kernel is started by jumping to the
84232-bit kernel entry point, which is the start address of loaded
84332/64-bit kernel.
844
845At entry, the CPU must be in 32-bit protected mode with paging
846disabled; a GDT must be loaded with the descriptors for selectors
847__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
848segment; __BOOS_CS must have execute/read permission, and __BOOT_DS
849must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
850must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
851address of the struct boot_params; %ebp, %edi and %ebx must be zero.