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1da177e4 LT |
1 | THE LINUX/I386 BOOT PROTOCOL |
2 | ---------------------------- | |
3 | ||
4 | H. Peter Anvin <hpa@zytor.com> | |
db2668fd | 5 | Last update 2007-05-23 |
1da177e4 LT |
6 | |
7 | On the i386 platform, the Linux kernel uses a rather complicated boot | |
8 | convention. This has evolved partially due to historical aspects, as | |
9 | well as the desire in the early days to have the kernel itself be a | |
10 | bootable image, the complicated PC memory model and due to changed | |
11 | expectations in the PC industry caused by the effective demise of | |
12 | real-mode DOS as a mainstream operating system. | |
13 | ||
de372ecd | 14 | Currently, the following versions of the Linux/i386 boot protocol exist. |
1da177e4 LT |
15 | |
16 | Old kernels: zImage/Image support only. Some very early kernels | |
17 | may not even support a command line. | |
18 | ||
19 | Protocol 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 | ||
25 | Protocol 2.01: (Kernel 1.3.76) Added a heap overrun warning. | |
26 | ||
27 | Protocol 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 | ||
34 | Protocol 2.03: (Kernel 2.4.18-pre1) Explicitly makes the highest possible | |
35 | initrd address available to the bootloader. | |
36 | ||
f8eeaaf4 | 37 | Protocol 2.04: (Kernel 2.6.14) Extend the syssize field to four bytes. |
8f9aeca7 | 38 | |
be274eea VG |
39 | Protocol 2.05: (Kernel 2.6.20) Make protected mode kernel relocatable. |
40 | Introduce relocatable_kernel and kernel_alignment fields. | |
f8eeaaf4 | 41 | |
8f9aeca7 BW |
42 | Protocol 2.06: (Kernel 2.6.22) Added a field that contains the size of |
43 | the boot command line | |
44 | ||
1da177e4 LT |
45 | |
46 | **** MEMORY LAYOUT | |
47 | ||
48 | The traditional memory map for the kernel loader, used for Image or | |
49 | zImage kernels, typically looks like: | |
50 | ||
51 | | | | |
52 | 0A0000 +------------------------+ | |
53 | | Reserved for BIOS | Do not use. Reserved for BIOS EBDA. | |
54 | 09A000 +------------------------+ | |
dec04cff PA |
55 | | Command line | |
56 | | Stack/heap | For use by the kernel real-mode code. | |
1da177e4 LT |
57 | 098000 +------------------------+ |
58 | | Kernel setup | The kernel real-mode code. | |
59 | 090200 +------------------------+ | |
60 | | Kernel boot sector | The kernel legacy boot sector. | |
61 | 090000 +------------------------+ | |
62 | | Protected-mode kernel | The bulk of the kernel image. | |
63 | 010000 +------------------------+ | |
64 | | Boot loader | <- Boot sector entry point 0000:7C00 | |
65 | 001000 +------------------------+ | |
66 | | Reserved for MBR/BIOS | | |
67 | 000800 +------------------------+ | |
68 | | Typically used by MBR | | |
69 | 000600 +------------------------+ | |
70 | | BIOS use only | | |
71 | 000000 +------------------------+ | |
72 | ||
73 | ||
74 | When using bzImage, the protected-mode kernel was relocated to | |
75 | 0x100000 ("high memory"), and the kernel real-mode block (boot sector, | |
76 | setup, and stack/heap) was made relocatable to any address between | |
dec04cff PA |
77 | 0x10000 and end of low memory. Unfortunately, in protocols 2.00 and |
78 | 2.01 the 0x90000+ memory range is still used internally by the kernel; | |
79 | the 2.02 protocol resolves that problem. | |
1da177e4 LT |
80 | |
81 | It is desirable to keep the "memory ceiling" -- the highest point in | |
82 | low memory touched by the boot loader -- as low as possible, since | |
83 | some newer BIOSes have begun to allocate some rather large amounts of | |
84 | memory, called the Extended BIOS Data Area, near the top of low | |
85 | memory. The boot loader should use the "INT 12h" BIOS call to verify | |
86 | how much low memory is available. | |
87 | ||
88 | Unfortunately, if INT 12h reports that the amount of memory is too | |
89 | low, there is usually nothing the boot loader can do but to report an | |
90 | error to the user. The boot loader should therefore be designed to | |
91 | take up as little space in low memory as it reasonably can. For | |
92 | zImage or old bzImage kernels, which need data written into the | |
93 | 0x90000 segment, the boot loader should make sure not to use memory | |
94 | above the 0x9A000 point; too many BIOSes will break above that point. | |
95 | ||
dec04cff PA |
96 | For a modern bzImage kernel with boot protocol version >= 2.02, a |
97 | memory layout like the following is suggested: | |
98 | ||
99 | ~ ~ | |
100 | | Protected-mode kernel | | |
101 | 100000 +------------------------+ | |
102 | | I/O memory hole | | |
103 | 0A0000 +------------------------+ | |
104 | | Reserved for BIOS | Leave as much as possible unused | |
105 | ~ ~ | |
106 | | Command line | (Can also be below the X+10000 mark) | |
107 | X+10000 +------------------------+ | |
108 | | Stack/heap | For use by the kernel real-mode code. | |
109 | X+08000 +------------------------+ | |
110 | | Kernel setup | The kernel real-mode code. | |
111 | | Kernel boot sector | The kernel legacy boot sector. | |
112 | X +------------------------+ | |
113 | | Boot loader | <- Boot sector entry point 0000:7C00 | |
114 | 001000 +------------------------+ | |
115 | | Reserved for MBR/BIOS | | |
116 | 000800 +------------------------+ | |
117 | | Typically used by MBR | | |
118 | 000600 +------------------------+ | |
119 | | BIOS use only | | |
120 | 000000 +------------------------+ | |
121 | ||
122 | ... where the address X is as low as the design of the boot loader | |
123 | permits. | |
124 | ||
1da177e4 LT |
125 | |
126 | **** THE REAL-MODE KERNEL HEADER | |
127 | ||
128 | In the following text, and anywhere in the kernel boot sequence, "a | |
129 | sector" refers to 512 bytes. It is independent of the actual sector | |
130 | size of the underlying medium. | |
131 | ||
132 | The first step in loading a Linux kernel should be to load the | |
133 | real-mode code (boot sector and setup code) and then examine the | |
134 | following header at offset 0x01f1. The real-mode code can total up to | |
135 | 32K, although the boot loader may choose to load only the first two | |
136 | sectors (1K) and then examine the bootup sector size. | |
137 | ||
138 | The header looks like: | |
139 | ||
140 | Offset Proto Name Meaning | |
141 | /Size | |
142 | ||
f8eeaaf4 | 143 | 01F1/1 ALL(1 setup_sects The size of the setup in sectors |
1da177e4 | 144 | 01F2/2 ALL root_flags If set, the root is mounted readonly |
f8eeaaf4 | 145 | 01F4/4 2.04+(2 syssize The size of the 32-bit code in 16-byte paras |
1da177e4 LT |
146 | 01F8/2 ALL ram_size DO NOT USE - for bootsect.S use only |
147 | 01FA/2 ALL vid_mode Video mode control | |
148 | 01FC/2 ALL root_dev Default root device number | |
149 | 01FE/2 ALL boot_flag 0xAA55 magic number | |
150 | 0200/2 2.00+ jump Jump instruction | |
151 | 0202/4 2.00+ header Magic signature "HdrS" | |
152 | 0206/2 2.00+ version Boot protocol version supported | |
153 | 0208/4 2.00+ realmode_swtch Boot loader hook (see below) | |
154 | 020C/2 2.00+ start_sys The load-low segment (0x1000) (obsolete) | |
155 | 020E/2 2.00+ kernel_version Pointer to kernel version string | |
156 | 0210/1 2.00+ type_of_loader Boot loader identifier | |
157 | 0211/1 2.00+ loadflags Boot protocol option flags | |
158 | 0212/2 2.00+ setup_move_size Move to high memory size (used with hooks) | |
159 | 0214/4 2.00+ code32_start Boot loader hook (see below) | |
160 | 0218/4 2.00+ ramdisk_image initrd load address (set by boot loader) | |
161 | 021C/4 2.00+ ramdisk_size initrd size (set by boot loader) | |
162 | 0220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only | |
163 | 0224/2 2.01+ heap_end_ptr Free memory after setup end | |
164 | 0226/2 N/A pad1 Unused | |
165 | 0228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line | |
166 | 022C/4 2.03+ initrd_addr_max Highest legal initrd address | |
d263b213 VG |
167 | 0230/4 2.05+ kernel_alignment Physical addr alignment required for kernel |
168 | 0234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not | |
8f9aeca7 BW |
169 | 0235/3 N/A pad2 Unused |
170 | 0238/4 2.06+ cmdline_size Maximum size of the kernel command line | |
e5371ac5 RR |
171 | 023C/4 2.07+ hardware_subarch Hardware subarchitecture |
172 | 0240/8 2.07+ hardware_subarch_data Subarchitecture-specific data | |
1da177e4 | 173 | |
f8eeaaf4 PA |
174 | (1) For backwards compatibility, if the setup_sects field contains 0, the |
175 | real value is 4. | |
176 | ||
177 | (2) For boot protocol prior to 2.04, the upper two bytes of the syssize | |
178 | field are unusable, which means the size of a bzImage kernel | |
179 | cannot be determined. | |
1da177e4 LT |
180 | |
181 | If the "HdrS" (0x53726448) magic number is not found at offset 0x202, | |
182 | the boot protocol version is "old". Loading an old kernel, the | |
183 | following parameters should be assumed: | |
184 | ||
185 | Image type = zImage | |
186 | initrd not supported | |
187 | Real-mode kernel must be located at 0x90000. | |
188 | ||
189 | Otherwise, the "version" field contains the protocol version, | |
190 | e.g. protocol version 2.01 will contain 0x0201 in this field. When | |
191 | setting fields in the header, you must make sure only to set fields | |
192 | supported by the protocol version in use. | |
193 | ||
dec04cff PA |
194 | |
195 | **** DETAILS OF HEADER FIELDS | |
196 | ||
197 | For each field, some are information from the kernel to the bootloader | |
198 | ("read"), some are expected to be filled out by the bootloader | |
199 | ("write"), and some are expected to be read and modified by the | |
200 | bootloader ("modify"). | |
201 | ||
202 | All general purpose boot loaders should write the fields marked | |
203 | (obligatory). Boot loaders who want to load the kernel at a | |
204 | nonstandard address should fill in the fields marked (reloc); other | |
205 | boot loaders can ignore those fields. | |
206 | ||
db2668fd PA |
207 | The byte order of all fields is littleendian (this is x86, after all.) |
208 | ||
e5371ac5 | 209 | Field name: setup_sects |
dec04cff PA |
210 | Type: read |
211 | Offset/size: 0x1f1/1 | |
212 | Protocol: ALL | |
213 | ||
214 | The size of the setup code in 512-byte sectors. If this field is | |
215 | 0, the real value is 4. The real-mode code consists of the boot | |
216 | sector (always one 512-byte sector) plus the setup code. | |
217 | ||
218 | Field name: root_flags | |
219 | Type: modify (optional) | |
220 | Offset/size: 0x1f2/2 | |
221 | Protocol: ALL | |
222 | ||
223 | If this field is nonzero, the root defaults to readonly. The use of | |
224 | this field is deprecated; use the "ro" or "rw" options on the | |
225 | command line instead. | |
226 | ||
227 | Field name: syssize | |
228 | Type: read | |
229 | Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL) | |
230 | Protocol: 2.04+ | |
231 | ||
232 | The size of the protected-mode code in units of 16-byte paragraphs. | |
233 | For protocol versions older than 2.04 this field is only two bytes | |
234 | wide, and therefore cannot be trusted for the size of a kernel if | |
235 | the LOAD_HIGH flag is set. | |
236 | ||
237 | Field name: ram_size | |
238 | Type: kernel internal | |
239 | Offset/size: 0x1f8/2 | |
240 | Protocol: ALL | |
241 | ||
242 | This field is obsolete. | |
243 | ||
244 | Field name: vid_mode | |
245 | Type: modify (obligatory) | |
246 | Offset/size: 0x1fa/2 | |
247 | ||
248 | Please see the section on SPECIAL COMMAND LINE OPTIONS. | |
249 | ||
250 | Field name: root_dev | |
251 | Type: modify (optional) | |
252 | Offset/size: 0x1fc/2 | |
253 | Protocol: ALL | |
254 | ||
255 | The default root device device number. The use of this field is | |
256 | deprecated, use the "root=" option on the command line instead. | |
257 | ||
258 | Field name: boot_flag | |
259 | Type: read | |
260 | Offset/size: 0x1fe/2 | |
261 | Protocol: ALL | |
262 | ||
263 | Contains 0xAA55. This is the closest thing old Linux kernels have | |
264 | to a magic number. | |
265 | ||
266 | Field name: jump | |
267 | Type: read | |
268 | Offset/size: 0x200/2 | |
269 | Protocol: 2.00+ | |
270 | ||
271 | Contains an x86 jump instruction, 0xEB followed by a signed offset | |
272 | relative to byte 0x202. This can be used to determine the size of | |
273 | the header. | |
274 | ||
275 | Field name: header | |
276 | Type: read | |
277 | Offset/size: 0x202/4 | |
278 | Protocol: 2.00+ | |
279 | ||
280 | Contains the magic number "HdrS" (0x53726448). | |
281 | ||
282 | Field name: version | |
283 | Type: read | |
284 | Offset/size: 0x206/2 | |
285 | Protocol: 2.00+ | |
286 | ||
db2668fd PA |
287 | Contains the boot protocol version, in (major << 8)+minor format, |
288 | e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version | |
289 | 10.17. | |
dec04cff PA |
290 | |
291 | Field name: readmode_swtch | |
292 | Type: modify (optional) | |
293 | Offset/size: 0x208/4 | |
294 | Protocol: 2.00+ | |
295 | ||
db2668fd | 296 | Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.) |
dec04cff PA |
297 | |
298 | Field name: start_sys | |
299 | Type: read | |
300 | Offset/size: 0x20c/4 | |
301 | Protocol: 2.00+ | |
302 | ||
303 | The load low segment (0x1000). Obsolete. | |
304 | ||
305 | Field name: kernel_version | |
306 | Type: read | |
307 | Offset/size: 0x20e/2 | |
308 | Protocol: 2.00+ | |
309 | ||
310 | If set to a nonzero value, contains a pointer to a NUL-terminated | |
311 | human-readable kernel version number string, less 0x200. This can | |
312 | be used to display the kernel version to the user. This value | |
db2668fd PA |
313 | should be less than (0x200*setup_sects). |
314 | ||
315 | For example, if this value is set to 0x1c00, the kernel version | |
316 | number string can be found at offset 0x1e00 in the kernel file. | |
317 | This is a valid value if and only if the "setup_sects" field | |
318 | contains the value 15 or higher, as: | |
319 | ||
320 | 0x1c00 < 15*0x200 (= 0x1e00) but | |
321 | 0x1c00 >= 14*0x200 (= 0x1c00) | |
322 | ||
323 | 0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15. | |
dec04cff PA |
324 | |
325 | Field name: type_of_loader | |
326 | Type: write (obligatory) | |
327 | Offset/size: 0x210/1 | |
328 | Protocol: 2.00+ | |
329 | ||
330 | If your boot loader has an assigned id (see table below), enter | |
331 | 0xTV here, where T is an identifier for the boot loader and V is | |
332 | a version number. Otherwise, enter 0xFF here. | |
333 | ||
334 | Assigned boot loader ids: | |
de372ecd | 335 | 0 LILO (0x00 reserved for pre-2.00 bootloader) |
1da177e4 | 336 | 1 Loadlin |
de372ecd | 337 | 2 bootsect-loader (0x20, all other values reserved) |
1da177e4 LT |
338 | 3 SYSLINUX |
339 | 4 EtherBoot | |
340 | 5 ELILO | |
341 | 7 GRuB | |
342 | 8 U-BOOT | |
354332ee | 343 | 9 Xen |
c229ec5d | 344 | A Gujin |
dec04cff | 345 | B Qemu |
1da177e4 | 346 | |
dec04cff PA |
347 | Please contact <hpa@zytor.com> if you need a bootloader ID |
348 | value assigned. | |
349 | ||
350 | Field name: loadflags | |
351 | Type: modify (obligatory) | |
352 | Offset/size: 0x211/1 | |
353 | Protocol: 2.00+ | |
354 | ||
355 | This field is a bitmask. | |
356 | ||
357 | Bit 0 (read): LOADED_HIGH | |
358 | - If 0, the protected-mode code is loaded at 0x10000. | |
359 | - If 1, the protected-mode code is loaded at 0x100000. | |
360 | ||
e5371ac5 RR |
361 | Bit 6 (write): KEEP_SEGMENTS |
362 | Protocol: 2.07+ | |
363 | - if 0, reload the segment registers in the 32bit entry point. | |
364 | - if 1, do not reload the segment registers in the 32bit entry point. | |
365 | Assume that %cs %ds %ss %es are all set to flat segments with | |
366 | a base of 0 (or the equivalent for their environment). | |
367 | ||
dec04cff PA |
368 | Bit 7 (write): CAN_USE_HEAP |
369 | Set this bit to 1 to indicate that the value entered in the | |
370 | heap_end_ptr is valid. If this field is clear, some setup code | |
371 | functionality will be disabled. | |
372 | ||
373 | Field name: setup_move_size | |
374 | Type: modify (obligatory) | |
375 | Offset/size: 0x212/2 | |
376 | Protocol: 2.00-2.01 | |
377 | ||
378 | When using protocol 2.00 or 2.01, if the real mode kernel is not | |
379 | loaded at 0x90000, it gets moved there later in the loading | |
380 | sequence. Fill in this field if you want additional data (such as | |
381 | the kernel command line) moved in addition to the real-mode kernel | |
382 | itself. | |
383 | ||
384 | The unit is bytes starting with the beginning of the boot sector. | |
385 | ||
386 | This field is can be ignored when the protocol is 2.02 or higher, or | |
387 | if the real-mode code is loaded at 0x90000. | |
388 | ||
389 | Field name: code32_start | |
390 | Type: modify (optional, reloc) | |
391 | Offset/size: 0x214/4 | |
392 | Protocol: 2.00+ | |
393 | ||
394 | The address to jump to in protected mode. This defaults to the load | |
395 | address of the kernel, and can be used by the boot loader to | |
396 | determine the proper load address. | |
397 | ||
398 | This field can be modified for two purposes: | |
399 | ||
db2668fd | 400 | 1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.) |
dec04cff PA |
401 | |
402 | 2. if a bootloader which does not install a hook loads a | |
403 | relocatable kernel at a nonstandard address it will have to modify | |
404 | this field to point to the load address. | |
405 | ||
406 | Field name: ramdisk_image | |
407 | Type: write (obligatory) | |
408 | Offset/size: 0x218/4 | |
409 | Protocol: 2.00+ | |
410 | ||
411 | The 32-bit linear address of the initial ramdisk or ramfs. Leave at | |
412 | zero if there is no initial ramdisk/ramfs. | |
413 | ||
414 | Field name: ramdisk_size | |
415 | Type: write (obligatory) | |
416 | Offset/size: 0x21c/4 | |
417 | Protocol: 2.00+ | |
418 | ||
419 | Size of the initial ramdisk or ramfs. Leave at zero if there is no | |
420 | initial ramdisk/ramfs. | |
421 | ||
422 | Field name: bootsect_kludge | |
423 | Type: kernel internal | |
424 | Offset/size: 0x220/4 | |
425 | Protocol: 2.00+ | |
426 | ||
427 | This field is obsolete. | |
428 | ||
429 | Field name: heap_end_ptr | |
430 | Type: write (obligatory) | |
431 | Offset/size: 0x224/2 | |
432 | Protocol: 2.01+ | |
433 | ||
434 | Set this field to the offset (from the beginning of the real-mode | |
435 | code) of the end of the setup stack/heap, minus 0x0200. | |
436 | ||
437 | Field name: cmd_line_ptr | |
438 | Type: write (obligatory) | |
439 | Offset/size: 0x228/4 | |
440 | Protocol: 2.02+ | |
441 | ||
442 | Set this field to the linear address of the kernel command line. | |
443 | The kernel command line can be located anywhere between the end of | |
444 | the setup heap and 0xA0000; it does not have to be located in the | |
445 | same 64K segment as the real-mode code itself. | |
446 | ||
447 | Fill in this field even if your boot loader does not support a | |
448 | command line, in which case you can point this to an empty string | |
449 | (or better yet, to the string "auto".) If this field is left at | |
450 | zero, the kernel will assume that your boot loader does not support | |
451 | the 2.02+ protocol. | |
452 | ||
453 | Field name: initrd_addr_max | |
454 | Type: read | |
455 | Offset/size: 0x22c/4 | |
456 | Protocol: 2.03+ | |
457 | ||
458 | The maximum address that may be occupied by the initial | |
459 | ramdisk/ramfs contents. For boot protocols 2.02 or earlier, this | |
460 | field is not present, and the maximum address is 0x37FFFFFF. (This | |
461 | address is defined as the address of the highest safe byte, so if | |
462 | your ramdisk is exactly 131072 bytes long and this field is | |
463 | 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.) | |
464 | ||
465 | Field name: kernel_alignment | |
466 | Type: read (reloc) | |
467 | Offset/size: 0x230/4 | |
468 | Protocol: 2.05+ | |
469 | ||
470 | Alignment unit required by the kernel (if relocatable_kernel is true.) | |
471 | ||
472 | Field name: relocatable_kernel | |
473 | Type: read (reloc) | |
474 | Offset/size: 0x234/1 | |
475 | Protocol: 2.05+ | |
476 | ||
477 | If this field is nonzero, the protected-mode part of the kernel can | |
478 | be loaded at any address that satisfies the kernel_alignment field. | |
479 | After loading, the boot loader must set the code32_start field to | |
480 | point to the loaded code, or to a boot loader hook. | |
481 | ||
482 | Field name: cmdline_size | |
483 | Type: read | |
484 | Offset/size: 0x238/4 | |
485 | Protocol: 2.06+ | |
486 | ||
487 | The maximum size of the command line without the terminating | |
488 | zero. This means that the command line can contain at most | |
489 | cmdline_size characters. With protocol version 2.05 and earlier, the | |
490 | maximum size was 255. | |
8f9aeca7 | 491 | |
e5371ac5 RR |
492 | Field name: hardware_subarch |
493 | Type: write | |
494 | Offset/size: 0x23c/4 | |
495 | Protocol: 2.07+ | |
496 | ||
497 | In a paravirtualized environment the hardware low level architectural | |
498 | pieces such as interrupt handling, page table handling, and | |
499 | accessing process control registers needs to be done differently. | |
500 | ||
501 | This field allows the bootloader to inform the kernel we are in one | |
502 | one of those environments. | |
503 | ||
504 | 0x00000000 The default x86/PC environment | |
505 | 0x00000001 lguest | |
506 | 0x00000002 Xen | |
507 | ||
508 | Field name: hardware_subarch_data | |
509 | Type: write | |
510 | Offset/size: 0x240/8 | |
511 | Protocol: 2.07+ | |
512 | ||
513 | A pointer to data that is specific to hardware subarch | |
514 | ||
1da177e4 LT |
515 | |
516 | **** THE KERNEL COMMAND LINE | |
517 | ||
518 | The kernel command line has become an important way for the boot | |
519 | loader to communicate with the kernel. Some of its options are also | |
520 | relevant to the boot loader itself, see "special command line options" | |
521 | below. | |
522 | ||
8f9aeca7 BW |
523 | The kernel command line is a null-terminated string. The maximum |
524 | length can be retrieved from the field cmdline_size. Before protocol | |
525 | version 2.06, the maximum was 255 characters. A string that is too | |
526 | long will be automatically truncated by the kernel. | |
1da177e4 LT |
527 | |
528 | If the boot protocol version is 2.02 or later, the address of the | |
529 | kernel command line is given by the header field cmd_line_ptr (see | |
f8eeaaf4 PA |
530 | above.) This address can be anywhere between the end of the setup |
531 | heap and 0xA0000. | |
1da177e4 LT |
532 | |
533 | If the protocol version is *not* 2.02 or higher, the kernel | |
534 | command line is entered using the following protocol: | |
535 | ||
536 | At offset 0x0020 (word), "cmd_line_magic", enter the magic | |
537 | number 0xA33F. | |
538 | ||
539 | At offset 0x0022 (word), "cmd_line_offset", enter the offset | |
540 | of the kernel command line (relative to the start of the | |
541 | real-mode kernel). | |
542 | ||
543 | The kernel command line *must* be within the memory region | |
544 | covered by setup_move_size, so you may need to adjust this | |
545 | field. | |
546 | ||
547 | ||
de372ecd PA |
548 | **** MEMORY LAYOUT OF THE REAL-MODE CODE |
549 | ||
550 | The real-mode code requires a stack/heap to be set up, as well as | |
551 | memory allocated for the kernel command line. This needs to be done | |
552 | in the real-mode accessible memory in bottom megabyte. | |
553 | ||
554 | It should be noted that modern machines often have a sizable Extended | |
555 | BIOS Data Area (EBDA). As a result, it is advisable to use as little | |
556 | of the low megabyte as possible. | |
557 | ||
558 | Unfortunately, under the following circumstances the 0x90000 memory | |
559 | segment has to be used: | |
560 | ||
561 | - When loading a zImage kernel ((loadflags & 0x01) == 0). | |
562 | - When loading a 2.01 or earlier boot protocol kernel. | |
563 | ||
564 | -> For the 2.00 and 2.01 boot protocols, the real-mode code | |
565 | can be loaded at another address, but it is internally | |
566 | relocated to 0x90000. For the "old" protocol, the | |
567 | real-mode code must be loaded at 0x90000. | |
568 | ||
569 | When loading at 0x90000, avoid using memory above 0x9a000. | |
570 | ||
571 | For boot protocol 2.02 or higher, the command line does not have to be | |
572 | located in the same 64K segment as the real-mode setup code; it is | |
573 | thus permitted to give the stack/heap the full 64K segment and locate | |
574 | the command line above it. | |
575 | ||
576 | The kernel command line should not be located below the real-mode | |
577 | code, nor should it be located in high memory. | |
578 | ||
579 | ||
1da177e4 LT |
580 | **** SAMPLE BOOT CONFIGURATION |
581 | ||
582 | As a sample configuration, assume the following layout of the real | |
de372ecd PA |
583 | mode segment: |
584 | ||
585 | When loading below 0x90000, use the entire segment: | |
586 | ||
587 | 0x0000-0x7fff Real mode kernel | |
588 | 0x8000-0xdfff Stack and heap | |
589 | 0xe000-0xffff Kernel command line | |
1da177e4 | 590 | |
de372ecd PA |
591 | When loading at 0x90000 OR the protocol version is 2.01 or earlier: |
592 | ||
593 | 0x0000-0x7fff Real mode kernel | |
594 | 0x8000-0x97ff Stack and heap | |
595 | 0x9800-0x9fff Kernel command line | |
1da177e4 LT |
596 | |
597 | Such a boot loader should enter the following fields in the header: | |
598 | ||
599 | unsigned long base_ptr; /* base address for real-mode segment */ | |
600 | ||
601 | if ( setup_sects == 0 ) { | |
602 | setup_sects = 4; | |
603 | } | |
604 | ||
605 | if ( protocol >= 0x0200 ) { | |
606 | type_of_loader = <type code>; | |
607 | if ( loading_initrd ) { | |
608 | ramdisk_image = <initrd_address>; | |
609 | ramdisk_size = <initrd_size>; | |
610 | } | |
de372ecd PA |
611 | |
612 | if ( protocol >= 0x0202 && loadflags & 0x01 ) | |
613 | heap_end = 0xe000; | |
614 | else | |
615 | heap_end = 0x9800; | |
616 | ||
1da177e4 | 617 | if ( protocol >= 0x0201 ) { |
de372ecd | 618 | heap_end_ptr = heap_end - 0x200; |
1da177e4 LT |
619 | loadflags |= 0x80; /* CAN_USE_HEAP */ |
620 | } | |
de372ecd | 621 | |
1da177e4 | 622 | if ( protocol >= 0x0202 ) { |
de372ecd PA |
623 | cmd_line_ptr = base_ptr + heap_end; |
624 | strcpy(cmd_line_ptr, cmdline); | |
1da177e4 LT |
625 | } else { |
626 | cmd_line_magic = 0xA33F; | |
de372ecd PA |
627 | cmd_line_offset = heap_end; |
628 | setup_move_size = heap_end + strlen(cmdline)+1; | |
629 | strcpy(base_ptr+cmd_line_offset, cmdline); | |
1da177e4 LT |
630 | } |
631 | } else { | |
632 | /* Very old kernel */ | |
633 | ||
de372ecd PA |
634 | heap_end = 0x9800; |
635 | ||
1da177e4 | 636 | cmd_line_magic = 0xA33F; |
de372ecd | 637 | cmd_line_offset = heap_end; |
1da177e4 LT |
638 | |
639 | /* A very old kernel MUST have its real-mode code | |
640 | loaded at 0x90000 */ | |
641 | ||
642 | if ( base_ptr != 0x90000 ) { | |
643 | /* Copy the real-mode kernel */ | |
644 | memcpy(0x90000, base_ptr, (setup_sects+1)*512); | |
1da177e4 LT |
645 | base_ptr = 0x90000; /* Relocated */ |
646 | } | |
647 | ||
de372ecd PA |
648 | strcpy(0x90000+cmd_line_offset, cmdline); |
649 | ||
1da177e4 LT |
650 | /* It is recommended to clear memory up to the 32K mark */ |
651 | memset(0x90000 + (setup_sects+1)*512, 0, | |
652 | (64-(setup_sects+1))*512); | |
653 | } | |
654 | ||
655 | ||
656 | **** LOADING THE REST OF THE KERNEL | |
657 | ||
f8eeaaf4 PA |
658 | The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512 |
659 | in the kernel file (again, if setup_sects == 0 the real value is 4.) | |
660 | It should be loaded at address 0x10000 for Image/zImage kernels and | |
1da177e4 LT |
661 | 0x100000 for bzImage kernels. |
662 | ||
663 | The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01 | |
664 | bit (LOAD_HIGH) in the loadflags field is set: | |
665 | ||
666 | is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01); | |
667 | load_address = is_bzImage ? 0x100000 : 0x10000; | |
668 | ||
669 | Note that Image/zImage kernels can be up to 512K in size, and thus use | |
670 | the entire 0x10000-0x90000 range of memory. This means it is pretty | |
671 | much a requirement for these kernels to load the real-mode part at | |
672 | 0x90000. bzImage kernels allow much more flexibility. | |
673 | ||
674 | ||
675 | **** SPECIAL COMMAND LINE OPTIONS | |
676 | ||
677 | If the command line provided by the boot loader is entered by the | |
678 | user, the user may expect the following command line options to work. | |
679 | They should normally not be deleted from the kernel command line even | |
680 | though not all of them are actually meaningful to the kernel. Boot | |
681 | loader authors who need additional command line options for the boot | |
682 | loader itself should get them registered in | |
683 | Documentation/kernel-parameters.txt to make sure they will not | |
684 | conflict with actual kernel options now or in the future. | |
685 | ||
686 | vga=<mode> | |
687 | <mode> here is either an integer (in C notation, either | |
688 | decimal, octal, or hexadecimal) or one of the strings | |
689 | "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask" | |
690 | (meaning 0xFFFD). This value should be entered into the | |
691 | vid_mode field, as it is used by the kernel before the command | |
692 | line is parsed. | |
693 | ||
694 | mem=<size> | |
de372ecd PA |
695 | <size> is an integer in C notation optionally followed by |
696 | (case insensitive) K, M, G, T, P or E (meaning << 10, << 20, | |
697 | << 30, << 40, << 50 or << 60). This specifies the end of | |
698 | memory to the kernel. This affects the possible placement of | |
699 | an initrd, since an initrd should be placed near end of | |
1da177e4 LT |
700 | memory. Note that this is an option to *both* the kernel and |
701 | the bootloader! | |
702 | ||
703 | initrd=<file> | |
704 | An initrd should be loaded. The meaning of <file> is | |
705 | obviously bootloader-dependent, and some boot loaders | |
706 | (e.g. LILO) do not have such a command. | |
707 | ||
708 | In addition, some boot loaders add the following options to the | |
709 | user-specified command line: | |
710 | ||
711 | BOOT_IMAGE=<file> | |
712 | The boot image which was loaded. Again, the meaning of <file> | |
713 | is obviously bootloader-dependent. | |
714 | ||
715 | auto | |
716 | The kernel was booted without explicit user intervention. | |
717 | ||
718 | If these options are added by the boot loader, it is highly | |
719 | recommended that they are located *first*, before the user-specified | |
720 | or configuration-specified command line. Otherwise, "init=/bin/sh" | |
721 | gets confused by the "auto" option. | |
722 | ||
723 | ||
724 | **** RUNNING THE KERNEL | |
725 | ||
726 | The kernel is started by jumping to the kernel entry point, which is | |
727 | located at *segment* offset 0x20 from the start of the real mode | |
728 | kernel. This means that if you loaded your real-mode kernel code at | |
729 | 0x90000, the kernel entry point is 9020:0000. | |
730 | ||
731 | At entry, ds = es = ss should point to the start of the real-mode | |
732 | kernel code (0x9000 if the code is loaded at 0x90000), sp should be | |
733 | set up properly, normally pointing to the top of the heap, and | |
734 | interrupts should be disabled. Furthermore, to guard against bugs in | |
735 | the kernel, it is recommended that the boot loader sets fs = gs = ds = | |
736 | es = ss. | |
737 | ||
738 | In our example from above, we would do: | |
739 | ||
740 | /* Note: in the case of the "old" kernel protocol, base_ptr must | |
741 | be == 0x90000 at this point; see the previous sample code */ | |
742 | ||
743 | seg = base_ptr >> 4; | |
744 | ||
745 | cli(); /* Enter with interrupts disabled! */ | |
746 | ||
747 | /* Set up the real-mode kernel stack */ | |
748 | _SS = seg; | |
de372ecd | 749 | _SP = heap_end; |
1da177e4 LT |
750 | |
751 | _DS = _ES = _FS = _GS = seg; | |
752 | jmp_far(seg+0x20, 0); /* Run the kernel */ | |
753 | ||
754 | If your boot sector accesses a floppy drive, it is recommended to | |
755 | switch off the floppy motor before running the kernel, since the | |
756 | kernel boot leaves interrupts off and thus the motor will not be | |
757 | switched off, especially if the loaded kernel has the floppy driver as | |
758 | a demand-loaded module! | |
759 | ||
760 | ||
db2668fd | 761 | **** ADVANCED BOOT LOADER HOOKS |
1da177e4 LT |
762 | |
763 | If the boot loader runs in a particularly hostile environment (such as | |
764 | LOADLIN, which runs under DOS) it may be impossible to follow the | |
765 | standard memory location requirements. Such a boot loader may use the | |
766 | following hooks that, if set, are invoked by the kernel at the | |
767 | appropriate time. The use of these hooks should probably be | |
768 | considered an absolutely last resort! | |
769 | ||
770 | IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and | |
771 | %edi across invocation. | |
772 | ||
773 | realmode_swtch: | |
774 | A 16-bit real mode far subroutine invoked immediately before | |
775 | entering protected mode. The default routine disables NMI, so | |
776 | your routine should probably do so, too. | |
777 | ||
778 | code32_start: | |
779 | A 32-bit flat-mode routine *jumped* to immediately after the | |
780 | transition to protected mode, but before the kernel is | |
de372ecd PA |
781 | uncompressed. No segments, except CS, are guaranteed to be |
782 | set up (current kernels do, but older ones do not); you should | |
783 | set them up to BOOT_DS (0x18) yourself. | |
1da177e4 LT |
784 | |
785 | After completing your hook, you should jump to the address | |
db2668fd PA |
786 | that was in this field before your boot loader overwrote it |
787 | (relocated, if appropriate.) | |
aa69432a HY |
788 | |
789 | ||
790 | **** 32-bit BOOT PROTOCOL | |
791 | ||
792 | For machine with some new BIOS other than legacy BIOS, such as EFI, | |
793 | LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel | |
794 | based on legacy BIOS can not be used, so a 32-bit boot protocol needs | |
795 | to be defined. | |
796 | ||
797 | In 32-bit boot protocol, the first step in loading a Linux kernel | |
798 | should be to setup the boot parameters (struct boot_params, | |
799 | traditionally known as "zero page"). The memory for struct boot_params | |
800 | should be allocated and initialized to all zero. Then the setup header | |
801 | from offset 0x01f1 of kernel image on should be loaded into struct | |
802 | boot_params and examined. The end of setup header can be calculated as | |
803 | follow: | |
804 | ||
805 | 0x0202 + byte value at offset 0x0201 | |
806 | ||
807 | In addition to read/modify/write the setup header of the struct | |
808 | boot_params as that of 16-bit boot protocol, the boot loader should | |
809 | also fill the additional fields of the struct boot_params as that | |
810 | described in zero-page.txt. | |
811 | ||
812 | After setupping the struct boot_params, the boot loader can load the | |
813 | 32/64-bit kernel in the same way as that of 16-bit boot protocol. | |
814 | ||
815 | In 32-bit boot protocol, the kernel is started by jumping to the | |
816 | 32-bit kernel entry point, which is the start address of loaded | |
817 | 32/64-bit kernel. | |
818 | ||
819 | At entry, the CPU must be in 32-bit protected mode with paging | |
820 | disabled; a GDT must be loaded with the descriptors for selectors | |
821 | __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat | |
822 | segment; __BOOS_CS must have execute/read permission, and __BOOT_DS | |
823 | must have read/write permission; CS must be __BOOT_CS and DS, ES, SS | |
824 | must be __BOOT_DS; interrupt must be disabled; %esi must hold the base | |
825 | address of the struct boot_params; %ebp, %edi and %ebx must be zero. |