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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
Linus Torvalds1da177e2005-04-16 15:20:36 -0700171
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -0700172(1) For backwards compatibility, if the setup_sects field contains 0, the
173 real value is 4.
174
175(2) For boot protocol prior to 2.04, the upper two bytes of the syssize
176 field are unusable, which means the size of a bzImage kernel
177 cannot be determined.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700178
179If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
180the boot protocol version is "old". Loading an old kernel, the
181following parameters should be assumed:
182
183 Image type = zImage
184 initrd not supported
185 Real-mode kernel must be located at 0x90000.
186
187Otherwise, the "version" field contains the protocol version,
188e.g. protocol version 2.01 will contain 0x0201 in this field. When
189setting fields in the header, you must make sure only to set fields
190supported by the protocol version in use.
191
Linus Torvalds1da177e2005-04-16 15:20:36 -0700192
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700193**** DETAILS OF HEADER FIELDS
Linus Torvalds1da177e2005-04-16 15:20:36 -0700194
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700195For each field, some are information from the kernel to the bootloader
196("read"), some are expected to be filled out by the bootloader
197("write"), and some are expected to be read and modified by the
198bootloader ("modify").
Linus Torvalds1da177e2005-04-16 15:20:36 -0700199
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700200All general purpose boot loaders should write the fields marked
201(obligatory). Boot loaders who want to load the kernel at a
202nonstandard address should fill in the fields marked (reloc); other
203boot loaders can ignore those fields.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700204
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700205The byte order of all fields is littleendian (this is x86, after all.)
206
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700207Field name: setup_secs
208Type: read
209Offset/size: 0x1f1/1
210Protocol: ALL
211
212 The size of the setup code in 512-byte sectors. If this field is
213 0, the real value is 4. The real-mode code consists of the boot
214 sector (always one 512-byte sector) plus the setup code.
215
216Field name: root_flags
217Type: modify (optional)
218Offset/size: 0x1f2/2
219Protocol: ALL
220
221 If this field is nonzero, the root defaults to readonly. The use of
222 this field is deprecated; use the "ro" or "rw" options on the
223 command line instead.
224
225Field name: syssize
226Type: read
227Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
228Protocol: 2.04+
229
230 The size of the protected-mode code in units of 16-byte paragraphs.
231 For protocol versions older than 2.04 this field is only two bytes
232 wide, and therefore cannot be trusted for the size of a kernel if
233 the LOAD_HIGH flag is set.
234
235Field name: ram_size
236Type: kernel internal
237Offset/size: 0x1f8/2
238Protocol: ALL
239
240 This field is obsolete.
241
242Field name: vid_mode
243Type: modify (obligatory)
244Offset/size: 0x1fa/2
245
246 Please see the section on SPECIAL COMMAND LINE OPTIONS.
247
248Field name: root_dev
249Type: modify (optional)
250Offset/size: 0x1fc/2
251Protocol: ALL
252
253 The default root device device number. The use of this field is
254 deprecated, use the "root=" option on the command line instead.
255
256Field name: boot_flag
257Type: read
258Offset/size: 0x1fe/2
259Protocol: ALL
260
261 Contains 0xAA55. This is the closest thing old Linux kernels have
262 to a magic number.
263
264Field name: jump
265Type: read
266Offset/size: 0x200/2
267Protocol: 2.00+
268
269 Contains an x86 jump instruction, 0xEB followed by a signed offset
270 relative to byte 0x202. This can be used to determine the size of
271 the header.
272
273Field name: header
274Type: read
275Offset/size: 0x202/4
276Protocol: 2.00+
277
278 Contains the magic number "HdrS" (0x53726448).
279
280Field name: version
281Type: read
282Offset/size: 0x206/2
283Protocol: 2.00+
284
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700285 Contains the boot protocol version, in (major << 8)+minor format,
286 e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
287 10.17.
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700288
289Field name: readmode_swtch
290Type: modify (optional)
291Offset/size: 0x208/4
292Protocol: 2.00+
293
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700294 Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700295
296Field name: start_sys
297Type: read
298Offset/size: 0x20c/4
299Protocol: 2.00+
300
301 The load low segment (0x1000). Obsolete.
302
303Field name: kernel_version
304Type: read
305Offset/size: 0x20e/2
306Protocol: 2.00+
307
308 If set to a nonzero value, contains a pointer to a NUL-terminated
309 human-readable kernel version number string, less 0x200. This can
310 be used to display the kernel version to the user. This value
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700311 should be less than (0x200*setup_sects).
312
313 For example, if this value is set to 0x1c00, the kernel version
314 number string can be found at offset 0x1e00 in the kernel file.
315 This is a valid value if and only if the "setup_sects" field
316 contains the value 15 or higher, as:
317
318 0x1c00 < 15*0x200 (= 0x1e00) but
319 0x1c00 >= 14*0x200 (= 0x1c00)
320
321 0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700322
323Field name: type_of_loader
324Type: write (obligatory)
325Offset/size: 0x210/1
326Protocol: 2.00+
327
328 If your boot loader has an assigned id (see table below), enter
329 0xTV here, where T is an identifier for the boot loader and V is
330 a version number. Otherwise, enter 0xFF here.
331
332 Assigned boot loader ids:
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700333 0 LILO (0x00 reserved for pre-2.00 bootloader)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700334 1 Loadlin
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700335 2 bootsect-loader (0x20, all other values reserved)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700336 3 SYSLINUX
337 4 EtherBoot
338 5 ELILO
339 7 GRuB
340 8 U-BOOT
Jeremy Fitzhardinge354332e2006-09-12 20:35:57 -0700341 9 Xen
H. Peter Anvinc229ec52007-01-26 09:15:47 -0800342 A Gujin
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700343 B Qemu
Linus Torvalds1da177e2005-04-16 15:20:36 -0700344
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700345 Please contact <hpa@zytor.com> if you need a bootloader ID
346 value assigned.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700347
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700348Field name: loadflags
349Type: modify (obligatory)
350Offset/size: 0x211/1
351Protocol: 2.00+
Linus Torvalds1da177e2005-04-16 15:20:36 -0700352
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700353 This field is a bitmask.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700354
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700355 Bit 0 (read): LOADED_HIGH
356 - If 0, the protected-mode code is loaded at 0x10000.
357 - If 1, the protected-mode code is loaded at 0x100000.
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700358
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700359 Bit 7 (write): CAN_USE_HEAP
360 Set this bit to 1 to indicate that the value entered in the
361 heap_end_ptr is valid. If this field is clear, some setup code
362 functionality will be disabled.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700363
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700364Field name: setup_move_size
365Type: modify (obligatory)
366Offset/size: 0x212/2
367Protocol: 2.00-2.01
Linus Torvalds1da177e2005-04-16 15:20:36 -0700368
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700369 When using protocol 2.00 or 2.01, if the real mode kernel is not
370 loaded at 0x90000, it gets moved there later in the loading
371 sequence. Fill in this field if you want additional data (such as
372 the kernel command line) moved in addition to the real-mode kernel
373 itself.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700374
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700375 The unit is bytes starting with the beginning of the boot sector.
376
377 This field is can be ignored when the protocol is 2.02 or higher, or
378 if the real-mode code is loaded at 0x90000.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700379
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700380Field name: code32_start
381Type: modify (optional, reloc)
382Offset/size: 0x214/4
383Protocol: 2.00+
384
385 The address to jump to in protected mode. This defaults to the load
386 address of the kernel, and can be used by the boot loader to
387 determine the proper load address.
388
389 This field can be modified for two purposes:
390
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700391 1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
H. Peter Anvindec04cf2007-05-17 15:50:47 -0700392
393 2. if a bootloader which does not install a hook loads a
394 relocatable kernel at a nonstandard address it will have to modify
395 this field to point to the load address.
396
397Field name: ramdisk_image
398Type: write (obligatory)
399Offset/size: 0x218/4
400Protocol: 2.00+
401
402 The 32-bit linear address of the initial ramdisk or ramfs. Leave at
403 zero if there is no initial ramdisk/ramfs.
404
405Field name: ramdisk_size
406Type: write (obligatory)
407Offset/size: 0x21c/4
408Protocol: 2.00+
409
410 Size of the initial ramdisk or ramfs. Leave at zero if there is no
411 initial ramdisk/ramfs.
412
413Field name: bootsect_kludge
414Type: kernel internal
415Offset/size: 0x220/4
416Protocol: 2.00+
417
418 This field is obsolete.
419
420Field name: heap_end_ptr
421Type: write (obligatory)
422Offset/size: 0x224/2
423Protocol: 2.01+
424
425 Set this field to the offset (from the beginning of the real-mode
426 code) of the end of the setup stack/heap, minus 0x0200.
427
428Field name: cmd_line_ptr
429Type: write (obligatory)
430Offset/size: 0x228/4
431Protocol: 2.02+
432
433 Set this field to the linear address of the kernel command line.
434 The kernel command line can be located anywhere between the end of
435 the setup heap and 0xA0000; it does not have to be located in the
436 same 64K segment as the real-mode code itself.
437
438 Fill in this field even if your boot loader does not support a
439 command line, in which case you can point this to an empty string
440 (or better yet, to the string "auto".) If this field is left at
441 zero, the kernel will assume that your boot loader does not support
442 the 2.02+ protocol.
443
444Field name: initrd_addr_max
445Type: read
446Offset/size: 0x22c/4
447Protocol: 2.03+
448
449 The maximum address that may be occupied by the initial
450 ramdisk/ramfs contents. For boot protocols 2.02 or earlier, this
451 field is not present, and the maximum address is 0x37FFFFFF. (This
452 address is defined as the address of the highest safe byte, so if
453 your ramdisk is exactly 131072 bytes long and this field is
454 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
455
456Field name: kernel_alignment
457Type: read (reloc)
458Offset/size: 0x230/4
459Protocol: 2.05+
460
461 Alignment unit required by the kernel (if relocatable_kernel is true.)
462
463Field name: relocatable_kernel
464Type: read (reloc)
465Offset/size: 0x234/1
466Protocol: 2.05+
467
468 If this field is nonzero, the protected-mode part of the kernel can
469 be loaded at any address that satisfies the kernel_alignment field.
470 After loading, the boot loader must set the code32_start field to
471 point to the loaded code, or to a boot loader hook.
472
473Field name: cmdline_size
474Type: read
475Offset/size: 0x238/4
476Protocol: 2.06+
477
478 The maximum size of the command line without the terminating
479 zero. This means that the command line can contain at most
480 cmdline_size characters. With protocol version 2.05 and earlier, the
481 maximum size was 255.
Bernhard Walle8f9aeca2007-05-02 19:27:10 +0200482
Linus Torvalds1da177e2005-04-16 15:20:36 -0700483
484**** THE KERNEL COMMAND LINE
485
486The kernel command line has become an important way for the boot
487loader to communicate with the kernel. Some of its options are also
488relevant to the boot loader itself, see "special command line options"
489below.
490
Bernhard Walle8f9aeca2007-05-02 19:27:10 +0200491The kernel command line is a null-terminated string. The maximum
492length can be retrieved from the field cmdline_size. Before protocol
493version 2.06, the maximum was 255 characters. A string that is too
494long will be automatically truncated by the kernel.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700495
496If the boot protocol version is 2.02 or later, the address of the
497kernel command line is given by the header field cmd_line_ptr (see
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -0700498above.) This address can be anywhere between the end of the setup
499heap and 0xA0000.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700500
501If the protocol version is *not* 2.02 or higher, the kernel
502command line is entered using the following protocol:
503
504 At offset 0x0020 (word), "cmd_line_magic", enter the magic
505 number 0xA33F.
506
507 At offset 0x0022 (word), "cmd_line_offset", enter the offset
508 of the kernel command line (relative to the start of the
509 real-mode kernel).
510
511 The kernel command line *must* be within the memory region
512 covered by setup_move_size, so you may need to adjust this
513 field.
514
515
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700516**** MEMORY LAYOUT OF THE REAL-MODE CODE
517
518The real-mode code requires a stack/heap to be set up, as well as
519memory allocated for the kernel command line. This needs to be done
520in the real-mode accessible memory in bottom megabyte.
521
522It should be noted that modern machines often have a sizable Extended
523BIOS Data Area (EBDA). As a result, it is advisable to use as little
524of the low megabyte as possible.
525
526Unfortunately, under the following circumstances the 0x90000 memory
527segment has to be used:
528
529 - When loading a zImage kernel ((loadflags & 0x01) == 0).
530 - When loading a 2.01 or earlier boot protocol kernel.
531
532 -> For the 2.00 and 2.01 boot protocols, the real-mode code
533 can be loaded at another address, but it is internally
534 relocated to 0x90000. For the "old" protocol, the
535 real-mode code must be loaded at 0x90000.
536
537When loading at 0x90000, avoid using memory above 0x9a000.
538
539For boot protocol 2.02 or higher, the command line does not have to be
540located in the same 64K segment as the real-mode setup code; it is
541thus permitted to give the stack/heap the full 64K segment and locate
542the command line above it.
543
544The kernel command line should not be located below the real-mode
545code, nor should it be located in high memory.
546
547
Linus Torvalds1da177e2005-04-16 15:20:36 -0700548**** SAMPLE BOOT CONFIGURATION
549
550As a sample configuration, assume the following layout of the real
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700551mode segment:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700552
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700553 When loading below 0x90000, use the entire segment:
554
555 0x0000-0x7fff Real mode kernel
556 0x8000-0xdfff Stack and heap
557 0xe000-0xffff Kernel command line
558
559 When loading at 0x90000 OR the protocol version is 2.01 or earlier:
560
561 0x0000-0x7fff Real mode kernel
562 0x8000-0x97ff Stack and heap
563 0x9800-0x9fff Kernel command line
Linus Torvalds1da177e2005-04-16 15:20:36 -0700564
565Such a boot loader should enter the following fields in the header:
566
567 unsigned long base_ptr; /* base address for real-mode segment */
568
569 if ( setup_sects == 0 ) {
570 setup_sects = 4;
571 }
572
573 if ( protocol >= 0x0200 ) {
574 type_of_loader = <type code>;
575 if ( loading_initrd ) {
576 ramdisk_image = <initrd_address>;
577 ramdisk_size = <initrd_size>;
578 }
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700579
580 if ( protocol >= 0x0202 && loadflags & 0x01 )
581 heap_end = 0xe000;
582 else
583 heap_end = 0x9800;
584
Linus Torvalds1da177e2005-04-16 15:20:36 -0700585 if ( protocol >= 0x0201 ) {
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700586 heap_end_ptr = heap_end - 0x200;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700587 loadflags |= 0x80; /* CAN_USE_HEAP */
588 }
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700589
Linus Torvalds1da177e2005-04-16 15:20:36 -0700590 if ( protocol >= 0x0202 ) {
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700591 cmd_line_ptr = base_ptr + heap_end;
592 strcpy(cmd_line_ptr, cmdline);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700593 } else {
594 cmd_line_magic = 0xA33F;
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700595 cmd_line_offset = heap_end;
596 setup_move_size = heap_end + strlen(cmdline)+1;
597 strcpy(base_ptr+cmd_line_offset, cmdline);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700598 }
599 } else {
600 /* Very old kernel */
601
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700602 heap_end = 0x9800;
603
Linus Torvalds1da177e2005-04-16 15:20:36 -0700604 cmd_line_magic = 0xA33F;
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700605 cmd_line_offset = heap_end;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700606
607 /* A very old kernel MUST have its real-mode code
608 loaded at 0x90000 */
609
610 if ( base_ptr != 0x90000 ) {
611 /* Copy the real-mode kernel */
612 memcpy(0x90000, base_ptr, (setup_sects+1)*512);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700613 base_ptr = 0x90000; /* Relocated */
614 }
615
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700616 strcpy(0x90000+cmd_line_offset, cmdline);
617
Linus Torvalds1da177e2005-04-16 15:20:36 -0700618 /* It is recommended to clear memory up to the 32K mark */
619 memset(0x90000 + (setup_sects+1)*512, 0,
620 (64-(setup_sects+1))*512);
621 }
622
623
624**** LOADING THE REST OF THE KERNEL
625
H. Peter Anvinf8eeaaf2005-09-06 15:17:24 -0700626The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
627in the kernel file (again, if setup_sects == 0 the real value is 4.)
628It should be loaded at address 0x10000 for Image/zImage kernels and
Linus Torvalds1da177e2005-04-16 15:20:36 -07006290x100000 for bzImage kernels.
630
631The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
632bit (LOAD_HIGH) in the loadflags field is set:
633
634 is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
635 load_address = is_bzImage ? 0x100000 : 0x10000;
636
637Note that Image/zImage kernels can be up to 512K in size, and thus use
638the entire 0x10000-0x90000 range of memory. This means it is pretty
639much a requirement for these kernels to load the real-mode part at
6400x90000. bzImage kernels allow much more flexibility.
641
642
643**** SPECIAL COMMAND LINE OPTIONS
644
645If the command line provided by the boot loader is entered by the
646user, the user may expect the following command line options to work.
647They should normally not be deleted from the kernel command line even
648though not all of them are actually meaningful to the kernel. Boot
649loader authors who need additional command line options for the boot
650loader itself should get them registered in
651Documentation/kernel-parameters.txt to make sure they will not
652conflict with actual kernel options now or in the future.
653
654 vga=<mode>
655 <mode> here is either an integer (in C notation, either
656 decimal, octal, or hexadecimal) or one of the strings
657 "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
658 (meaning 0xFFFD). This value should be entered into the
659 vid_mode field, as it is used by the kernel before the command
660 line is parsed.
661
662 mem=<size>
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700663 <size> is an integer in C notation optionally followed by
664 (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
665 << 30, << 40, << 50 or << 60). This specifies the end of
666 memory to the kernel. This affects the possible placement of
667 an initrd, since an initrd should be placed near end of
Linus Torvalds1da177e2005-04-16 15:20:36 -0700668 memory. Note that this is an option to *both* the kernel and
669 the bootloader!
670
671 initrd=<file>
672 An initrd should be loaded. The meaning of <file> is
673 obviously bootloader-dependent, and some boot loaders
674 (e.g. LILO) do not have such a command.
675
676In addition, some boot loaders add the following options to the
677user-specified command line:
678
679 BOOT_IMAGE=<file>
680 The boot image which was loaded. Again, the meaning of <file>
681 is obviously bootloader-dependent.
682
683 auto
684 The kernel was booted without explicit user intervention.
685
686If these options are added by the boot loader, it is highly
687recommended that they are located *first*, before the user-specified
688or configuration-specified command line. Otherwise, "init=/bin/sh"
689gets confused by the "auto" option.
690
691
692**** RUNNING THE KERNEL
693
694The kernel is started by jumping to the kernel entry point, which is
695located at *segment* offset 0x20 from the start of the real mode
696kernel. This means that if you loaded your real-mode kernel code at
6970x90000, the kernel entry point is 9020:0000.
698
699At entry, ds = es = ss should point to the start of the real-mode
700kernel code (0x9000 if the code is loaded at 0x90000), sp should be
701set up properly, normally pointing to the top of the heap, and
702interrupts should be disabled. Furthermore, to guard against bugs in
703the kernel, it is recommended that the boot loader sets fs = gs = ds =
704es = ss.
705
706In our example from above, we would do:
707
708 /* Note: in the case of the "old" kernel protocol, base_ptr must
709 be == 0x90000 at this point; see the previous sample code */
710
711 seg = base_ptr >> 4;
712
713 cli(); /* Enter with interrupts disabled! */
714
715 /* Set up the real-mode kernel stack */
716 _SS = seg;
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700717 _SP = heap_end;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700718
719 _DS = _ES = _FS = _GS = seg;
720 jmp_far(seg+0x20, 0); /* Run the kernel */
721
722If your boot sector accesses a floppy drive, it is recommended to
723switch off the floppy motor before running the kernel, since the
724kernel boot leaves interrupts off and thus the motor will not be
725switched off, especially if the loaded kernel has the floppy driver as
726a demand-loaded module!
727
728
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700729**** ADVANCED BOOT LOADER HOOKS
Linus Torvalds1da177e2005-04-16 15:20:36 -0700730
731If the boot loader runs in a particularly hostile environment (such as
732LOADLIN, which runs under DOS) it may be impossible to follow the
733standard memory location requirements. Such a boot loader may use the
734following hooks that, if set, are invoked by the kernel at the
735appropriate time. The use of these hooks should probably be
736considered an absolutely last resort!
737
738IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
739%edi across invocation.
740
741 realmode_swtch:
742 A 16-bit real mode far subroutine invoked immediately before
743 entering protected mode. The default routine disables NMI, so
744 your routine should probably do so, too.
745
746 code32_start:
747 A 32-bit flat-mode routine *jumped* to immediately after the
748 transition to protected mode, but before the kernel is
H. Peter Anvinde372ec2007-05-08 20:37:02 -0700749 uncompressed. No segments, except CS, are guaranteed to be
750 set up (current kernels do, but older ones do not); you should
751 set them up to BOOT_DS (0x18) yourself.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700752
753 After completing your hook, you should jump to the address
H. Peter Anvindb2668f2007-05-23 16:59:27 -0700754 that was in this field before your boot loader overwrote it
755 (relocated, if appropriate.)