| blob | 7b4596ac41208f49d83b3ed09b8ebb7774c7e010 |
1 THE LINUX/x86 BOOT PROTOCOL
2 ---------------------------
4 On the x86 platform, the Linux kernel uses a rather complicated boot
5 convention. This has evolved partially due to historical aspects, as
6 well as the desire in the early days to have the kernel itself be a
7 bootable image, the complicated PC memory model and due to changed
8 expectations in the PC industry caused by the effective demise of
9 real-mode DOS as a mainstream operating system.
11 Currently, the following versions of the Linux/x86 boot protocol exist.
13 Old kernels: zImage/Image support only. Some very early kernels
14 may not even support a command line.
16 Protocol 2.00: (Kernel 1.3.73) Added bzImage and initrd support, as
17 well as a formalized way to communicate between the
18 boot loader and the kernel. setup.S made relocatable,
19 although the traditional setup area still assumed
20 writable.
22 Protocol 2.01: (Kernel 1.3.76) Added a heap overrun warning.
24 Protocol 2.02: (Kernel 2.4.0-test3-pre3) New command line protocol.
25 Lower the conventional memory ceiling. No overwrite
26 of the traditional setup area, thus making booting
27 safe for systems which use the EBDA from SMM or 32-bit
28 BIOS entry points. zImage deprecated but still
29 supported.
31 Protocol 2.03: (Kernel 2.4.18-pre1) Explicitly makes the highest possible
32 initrd address available to the bootloader.
34 Protocol 2.04: (Kernel 2.6.14) Extend the syssize field to four bytes.
36 Protocol 2.05: (Kernel 2.6.20) Make protected mode kernel relocatable.
37 Introduce relocatable_kernel and kernel_alignment fields.
39 Protocol 2.06: (Kernel 2.6.22) Added a field that contains the size of
40 the boot command line.
42 Protocol 2.07: (Kernel 2.6.24) Added paravirtualised boot protocol.
43 Introduced hardware_subarch and hardware_subarch_data
44 and KEEP_SEGMENTS flag in load_flags.
46 Protocol 2.08: (Kernel 2.6.26) Added crc32 checksum and ELF format
47 payload. Introduced payload_offset and payload_length
48 fields to aid in locating the payload.
50 Protocol 2.09: (Kernel 2.6.26) Added a field of 64-bit physical
51 pointer to single linked list of struct setup_data.
53 **** MEMORY LAYOUT
55 The traditional memory map for the kernel loader, used for Image or
56 zImage kernels, typically looks like:
58 | |
59 0A0000 +------------------------+
60 | Reserved for BIOS | Do not use. Reserved for BIOS EBDA.
61 09A000 +------------------------+
62 | Command line |
63 | Stack/heap | For use by the kernel real-mode code.
64 098000 +------------------------+
65 | Kernel setup | The kernel real-mode code.
66 090200 +------------------------+
67 | Kernel boot sector | The kernel legacy boot sector.
68 090000 +------------------------+
69 | Protected-mode kernel | The bulk of the kernel image.
70 010000 +------------------------+
71 | Boot loader | <- Boot sector entry point 0000:7C00
72 001000 +------------------------+
73 | Reserved for MBR/BIOS |
74 000800 +------------------------+
75 | Typically used by MBR |
76 000600 +------------------------+
77 | BIOS use only |
78 000000 +------------------------+
81 When using bzImage, the protected-mode kernel was relocated to
82 0x100000 ("high memory"), and the kernel real-mode block (boot sector,
83 setup, and stack/heap) was made relocatable to any address between
84 0x10000 and end of low memory. Unfortunately, in protocols 2.00 and
85 2.01 the 0x90000+ memory range is still used internally by the kernel;
86 the 2.02 protocol resolves that problem.
88 It is desirable to keep the "memory ceiling" -- the highest point in
89 low memory touched by the boot loader -- as low as possible, since
90 some newer BIOSes have begun to allocate some rather large amounts of
91 memory, called the Extended BIOS Data Area, near the top of low
92 memory. The boot loader should use the "INT 12h" BIOS call to verify
93 how much low memory is available.
95 Unfortunately, if INT 12h reports that the amount of memory is too
96 low, there is usually nothing the boot loader can do but to report an
97 error to the user. The boot loader should therefore be designed to
98 take up as little space in low memory as it reasonably can. For
99 zImage or old bzImage kernels, which need data written into the
100 0x90000 segment, the boot loader should make sure not to use memory
101 above the 0x9A000 point; too many BIOSes will break above that point.
103 For a modern bzImage kernel with boot protocol version >= 2.02, a
104 memory layout like the following is suggested:
106 ~ ~
107 | Protected-mode kernel |
108 100000 +------------------------+
109 | I/O memory hole |
110 0A0000 +------------------------+
111 | Reserved for BIOS | Leave as much as possible unused
112 ~ ~
113 | Command line | (Can also be below the X+10000 mark)
114 X+10000 +------------------------+
115 | Stack/heap | For use by the kernel real-mode code.
116 X+08000 +------------------------+
117 | Kernel setup | The kernel real-mode code.
118 | Kernel boot sector | The kernel legacy boot sector.
119 X +------------------------+
120 | Boot loader | <- Boot sector entry point 0000:7C00
121 001000 +------------------------+
122 | Reserved for MBR/BIOS |
123 000800 +------------------------+
124 | Typically used by MBR |
125 000600 +------------------------+
126 | BIOS use only |
127 000000 +------------------------+
129 ... where the address X is as low as the design of the boot loader
130 permits.
133 **** THE REAL-MODE KERNEL HEADER
135 In the following text, and anywhere in the kernel boot sequence, "a
136 sector" refers to 512 bytes. It is independent of the actual sector
137 size of the underlying medium.
139 The first step in loading a Linux kernel should be to load the
140 real-mode code (boot sector and setup code) and then examine the
141 following header at offset 0x01f1. The real-mode code can total up to
142 32K, although the boot loader may choose to load only the first two
143 sectors (1K) and then examine the bootup sector size.
145 The header looks like:
147 Offset Proto Name Meaning
148 /Size
150 01F1/1 ALL(1 setup_sects The size of the setup in sectors
151 01F2/2 ALL root_flags If set, the root is mounted readonly
152 01F4/4 2.04+(2 syssize The size of the 32-bit code in 16-byte paras
153 01F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
154 01FA/2 ALL vid_mode Video mode control
155 01FC/2 ALL root_dev Default root device number
156 01FE/2 ALL boot_flag 0xAA55 magic number
157 0200/2 2.00+ jump Jump instruction
158 0202/4 2.00+ header Magic signature "HdrS"
159 0206/2 2.00+ version Boot protocol version supported
160 0208/4 2.00+ realmode_swtch Boot loader hook (see below)
161 020C/2 2.00+ start_sys The load-low segment (0x1000) (obsolete)
162 020E/2 2.00+ kernel_version Pointer to kernel version string
163 0210/1 2.00+ type_of_loader Boot loader identifier
164 0211/1 2.00+ loadflags Boot protocol option flags
165 0212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
166 0214/4 2.00+ code32_start Boot loader hook (see below)
167 0218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
168 021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
169 0220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
170 0224/2 2.01+ heap_end_ptr Free memory after setup end
171 0226/2 N/A pad1 Unused
172 0228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
173 022C/4 2.03+ initrd_addr_max Highest legal initrd address
174 0230/4 2.05+ kernel_alignment Physical addr alignment required for kernel
175 0234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
176 0235/3 N/A pad2 Unused
177 0238/4 2.06+ cmdline_size Maximum size of the kernel command line
178 023C/4 2.07+ hardware_subarch Hardware subarchitecture
179 0240/8 2.07+ hardware_subarch_data Subarchitecture-specific data
180 0248/4 2.08+ payload_offset Offset of kernel payload
181 024C/4 2.08+ payload_length Length of kernel payload
182 0250/8 2.09+ setup_data 64-bit physical pointer to linked list
183 of struct setup_data
185 (1) For backwards compatibility, if the setup_sects field contains 0, the
186 real value is 4.
188 (2) For boot protocol prior to 2.04, the upper two bytes of the syssize
189 field are unusable, which means the size of a bzImage kernel
190 cannot be determined.
192 If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
193 the boot protocol version is "old". Loading an old kernel, the
194 following parameters should be assumed:
196 Image type = zImage
197 initrd not supported
198 Real-mode kernel must be located at 0x90000.
200 Otherwise, the "version" field contains the protocol version,
201 e.g. protocol version 2.01 will contain 0x0201 in this field. When
202 setting fields in the header, you must make sure only to set fields
203 supported by the protocol version in use.
206 **** DETAILS OF HEADER FIELDS
208 For each field, some are information from the kernel to the bootloader
209 ("read"), some are expected to be filled out by the bootloader
210 ("write"), and some are expected to be read and modified by the
211 bootloader ("modify").
213 All general purpose boot loaders should write the fields marked
214 (obligatory). Boot loaders who want to load the kernel at a
215 nonstandard address should fill in the fields marked (reloc); other
216 boot loaders can ignore those fields.
218 The byte order of all fields is littleendian (this is x86, after all.)
220 Field name: setup_sects
221 Type: read
222 Offset/size: 0x1f1/1
223 Protocol: ALL
225 The size of the setup code in 512-byte sectors. If this field is
226 0, the real value is 4. The real-mode code consists of the boot
227 sector (always one 512-byte sector) plus the setup code.
229 Field name: root_flags
230 Type: modify (optional)
231 Offset/size: 0x1f2/2
232 Protocol: ALL
234 If this field is nonzero, the root defaults to readonly. The use of
235 this field is deprecated; use the "ro" or "rw" options on the
236 command line instead.
238 Field name: syssize
239 Type: read
240 Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
241 Protocol: 2.04+
243 The size of the protected-mode code in units of 16-byte paragraphs.
244 For protocol versions older than 2.04 this field is only two bytes
245 wide, and therefore cannot be trusted for the size of a kernel if
246 the LOAD_HIGH flag is set.
248 Field name: ram_size
249 Type: kernel internal
250 Offset/size: 0x1f8/2
251 Protocol: ALL
253 This field is obsolete.
255 Field name: vid_mode
256 Type: modify (obligatory)
257 Offset/size: 0x1fa/2
259 Please see the section on SPECIAL COMMAND LINE OPTIONS.
261 Field name: root_dev
262 Type: modify (optional)
263 Offset/size: 0x1fc/2
264 Protocol: ALL
266 The default root device device number. The use of this field is
267 deprecated, use the "root=" option on the command line instead.
269 Field name: boot_flag
270 Type: read
271 Offset/size: 0x1fe/2
272 Protocol: ALL
274 Contains 0xAA55. This is the closest thing old Linux kernels have
275 to a magic number.
277 Field name: jump
278 Type: read
279 Offset/size: 0x200/2
280 Protocol: 2.00+
282 Contains an x86 jump instruction, 0xEB followed by a signed offset
283 relative to byte 0x202. This can be used to determine the size of
284 the header.
286 Field name: header
287 Type: read
288 Offset/size: 0x202/4
289 Protocol: 2.00+
291 Contains the magic number "HdrS" (0x53726448).
293 Field name: version
294 Type: read
295 Offset/size: 0x206/2
296 Protocol: 2.00+
298 Contains the boot protocol version, in (major << 8)+minor format,
299 e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
300 10.17.
302 Field name: readmode_swtch
303 Type: modify (optional)
304 Offset/size: 0x208/4
305 Protocol: 2.00+
307 Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
309 Field name: start_sys
310 Type: read
311 Offset/size: 0x20c/2
312 Protocol: 2.00+
314 The load low segment (0x1000). Obsolete.
316 Field name: kernel_version
317 Type: read
318 Offset/size: 0x20e/2
319 Protocol: 2.00+
321 If set to a nonzero value, contains a pointer to a NUL-terminated
322 human-readable kernel version number string, less 0x200. This can
323 be used to display the kernel version to the user. This value
324 should be less than (0x200*setup_sects).
326 For example, if this value is set to 0x1c00, the kernel version
327 number string can be found at offset 0x1e00 in the kernel file.
328 This is a valid value if and only if the "setup_sects" field
329 contains the value 15 or higher, as:
331 0x1c00 < 15*0x200 (= 0x1e00) but
332 0x1c00 >= 14*0x200 (= 0x1c00)
334 0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
336 Field name: type_of_loader
337 Type: write (obligatory)
338 Offset/size: 0x210/1
339 Protocol: 2.00+
341 If your boot loader has an assigned id (see table below), enter
342 0xTV here, where T is an identifier for the boot loader and V is
343 a version number. Otherwise, enter 0xFF here.
345 Assigned boot loader ids:
346 0 LILO (0x00 reserved for pre-2.00 bootloader)
347 1 Loadlin
348 2 bootsect-loader (0x20, all other values reserved)
349 3 SYSLINUX
350 4 EtherBoot
351 5 ELILO
352 7 GRUB
353 8 U-BOOT
354 9 Xen
355 A Gujin
356 B Qemu
358 Please contact <hpa@zytor.com> if you need a bootloader ID
359 value assigned.
361 Field name: loadflags
362 Type: modify (obligatory)
363 Offset/size: 0x211/1
364 Protocol: 2.00+
366 This field is a bitmask.
368 Bit 0 (read): LOADED_HIGH
369 - If 0, the protected-mode code is loaded at 0x10000.
370 - If 1, the protected-mode code is loaded at 0x100000.
372 Bit 5 (write): QUIET_FLAG
373 - If 0, print early messages.
374 - If 1, suppress early messages.
375 This requests to the kernel (decompressor and early
376 kernel) to not write early messages that require
377 accessing the display hardware directly.
379 Bit 6 (write): KEEP_SEGMENTS
380 Protocol: 2.07+
381 - If 0, reload the segment registers in the 32bit entry point.
382 - If 1, do not reload the segment registers in the 32bit entry point.
383 Assume that %cs %ds %ss %es are all set to flat segments with
384 a base of 0 (or the equivalent for their environment).
386 Bit 7 (write): CAN_USE_HEAP
387 Set this bit to 1 to indicate that the value entered in the
388 heap_end_ptr is valid. If this field is clear, some setup code
389 functionality will be disabled.
391 Field name: setup_move_size
392 Type: modify (obligatory)
393 Offset/size: 0x212/2
394 Protocol: 2.00-2.01
396 When using protocol 2.00 or 2.01, if the real mode kernel is not
397 loaded at 0x90000, it gets moved there later in the loading
398 sequence. Fill in this field if you want additional data (such as
399 the kernel command line) moved in addition to the real-mode kernel
400 itself.
402 The unit is bytes starting with the beginning of the boot sector.
404 This field is can be ignored when the protocol is 2.02 or higher, or
405 if the real-mode code is loaded at 0x90000.
407 Field name: code32_start
408 Type: modify (optional, reloc)
409 Offset/size: 0x214/4
410 Protocol: 2.00+
412 The address to jump to in protected mode. This defaults to the load
413 address of the kernel, and can be used by the boot loader to
414 determine the proper load address.
416 This field can be modified for two purposes:
418 1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
420 2. if a bootloader which does not install a hook loads a
421 relocatable kernel at a nonstandard address it will have to modify
422 this field to point to the load address.
424 Field name: ramdisk_image
425 Type: write (obligatory)
426 Offset/size: 0x218/4
427 Protocol: 2.00+
429 The 32-bit linear address of the initial ramdisk or ramfs. Leave at
430 zero if there is no initial ramdisk/ramfs.
432 Field name: ramdisk_size
433 Type: write (obligatory)
434 Offset/size: 0x21c/4
435 Protocol: 2.00+
437 Size of the initial ramdisk or ramfs. Leave at zero if there is no
438 initial ramdisk/ramfs.
440 Field name: bootsect_kludge
441 Type: kernel internal
442 Offset/size: 0x220/4
443 Protocol: 2.00+
445 This field is obsolete.
447 Field name: heap_end_ptr
448 Type: write (obligatory)
449 Offset/size: 0x224/2
450 Protocol: 2.01+
452 Set this field to the offset (from the beginning of the real-mode
453 code) of the end of the setup stack/heap, minus 0x0200.
455 Field name: cmd_line_ptr
456 Type: write (obligatory)
457 Offset/size: 0x228/4
458 Protocol: 2.02+
460 Set this field to the linear address of the kernel command line.
461 The kernel command line can be located anywhere between the end of
462 the setup heap and 0xA0000; it does not have to be located in the
463 same 64K segment as the real-mode code itself.
465 Fill in this field even if your boot loader does not support a
466 command line, in which case you can point this to an empty string
467 (or better yet, to the string "auto".) If this field is left at
468 zero, the kernel will assume that your boot loader does not support
469 the 2.02+ protocol.
471 Field name: initrd_addr_max
472 Type: read
473 Offset/size: 0x22c/4
474 Protocol: 2.03+
476 The maximum address that may be occupied by the initial
477 ramdisk/ramfs contents. For boot protocols 2.02 or earlier, this
478 field is not present, and the maximum address is 0x37FFFFFF. (This
479 address is defined as the address of the highest safe byte, so if
480 your ramdisk is exactly 131072 bytes long and this field is
481 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
483 Field name: kernel_alignment
484 Type: read (reloc)
485 Offset/size: 0x230/4
486 Protocol: 2.05+
488 Alignment unit required by the kernel (if relocatable_kernel is true.)
490 Field name: relocatable_kernel
491 Type: read (reloc)
492 Offset/size: 0x234/1
493 Protocol: 2.05+
495 If this field is nonzero, the protected-mode part of the kernel can
496 be loaded at any address that satisfies the kernel_alignment field.
497 After loading, the boot loader must set the code32_start field to
498 point to the loaded code, or to a boot loader hook.
500 Field name: cmdline_size
501 Type: read
502 Offset/size: 0x238/4
503 Protocol: 2.06+
505 The maximum size of the command line without the terminating
506 zero. This means that the command line can contain at most
507 cmdline_size characters. With protocol version 2.05 and earlier, the
508 maximum size was 255.
510 Field name: hardware_subarch
511 Type: write (optional, defaults to x86/PC)
512 Offset/size: 0x23c/4
513 Protocol: 2.07+
515 In a paravirtualized environment the hardware low level architectural
516 pieces such as interrupt handling, page table handling, and
517 accessing process control registers needs to be done differently.
519 This field allows the bootloader to inform the kernel we are in one
520 one of those environments.
522 0x00000000 The default x86/PC environment
523 0x00000001 lguest
524 0x00000002 Xen
526 Field name: hardware_subarch_data
527 Type: write (subarch-dependent)
528 Offset/size: 0x240/8
529 Protocol: 2.07+
531 A pointer to data that is specific to hardware subarch
532 This field is currently unused for the default x86/PC environment,
533 do not modify.
535 Field name: payload_offset
536 Type: read
537 Offset/size: 0x248/4
538 Protocol: 2.08+
540 If non-zero then this field contains the offset from the beginning
541 of the protected-mode code to the payload.
543 The payload may be compressed. The format of both the compressed and
544 uncompressed data should be determined using the standard magic
545 numbers. Currently only gzip compressed ELF is used.
547 Field name: payload_length
548 Type: read
549 Offset/size: 0x24c/4
550 Protocol: 2.08+
552 The length of the payload.
554 Field name: setup_data
555 Type: write (special)
556 Offset/size: 0x250/8
557 Protocol: 2.09+
559 The 64-bit physical pointer to NULL terminated single linked list of
560 struct setup_data. This is used to define a more extensible boot
561 parameters passing mechanism. The definition of struct setup_data is
562 as follow:
564 struct setup_data {
565 u64 next;
566 u32 type;
567 u32 len;
568 u8 data[0];
569 };
571 Where, the next is a 64-bit physical pointer to the next node of
572 linked list, the next field of the last node is 0; the type is used
573 to identify the contents of data; the len is the length of data
574 field; the data holds the real payload.
576 This list may be modified at a number of points during the bootup
577 process. Therefore, when modifying this list one should always make
578 sure to consider the case where the linked list already contains
579 entries.
582 **** THE IMAGE CHECKSUM
584 From boot protocol version 2.08 onwards the CRC-32 is calculated over
585 the entire file using the characteristic polynomial 0x04C11DB7 and an
586 initial remainder of 0xffffffff. The checksum is appended to the
587 file; therefore the CRC of the file up to the limit specified in the
588 syssize field of the header is always 0.
591 **** THE KERNEL COMMAND LINE
593 The kernel command line has become an important way for the boot
594 loader to communicate with the kernel. Some of its options are also
595 relevant to the boot loader itself, see "special command line options"
596 below.
598 The kernel command line is a null-terminated string. The maximum
599 length can be retrieved from the field cmdline_size. Before protocol
600 version 2.06, the maximum was 255 characters. A string that is too
601 long will be automatically truncated by the kernel.
603 If the boot protocol version is 2.02 or later, the address of the
604 kernel command line is given by the header field cmd_line_ptr (see
605 above.) This address can be anywhere between the end of the setup
606 heap and 0xA0000.
608 If the protocol version is *not* 2.02 or higher, the kernel
609 command line is entered using the following protocol:
611 At offset 0x0020 (word), "cmd_line_magic", enter the magic
612 number 0xA33F.
614 At offset 0x0022 (word), "cmd_line_offset", enter the offset
615 of the kernel command line (relative to the start of the
616 real-mode kernel).
618 The kernel command line *must* be within the memory region
619 covered by setup_move_size, so you may need to adjust this
620 field.
623 **** MEMORY LAYOUT OF THE REAL-MODE CODE
625 The real-mode code requires a stack/heap to be set up, as well as
626 memory allocated for the kernel command line. This needs to be done
627 in the real-mode accessible memory in bottom megabyte.
629 It should be noted that modern machines often have a sizable Extended
630 BIOS Data Area (EBDA). As a result, it is advisable to use as little
631 of the low megabyte as possible.
633 Unfortunately, under the following circumstances the 0x90000 memory
634 segment has to be used:
636 - When loading a zImage kernel ((loadflags & 0x01) == 0).
637 - When loading a 2.01 or earlier boot protocol kernel.
639 -> For the 2.00 and 2.01 boot protocols, the real-mode code
640 can be loaded at another address, but it is internally
641 relocated to 0x90000. For the "old" protocol, the
642 real-mode code must be loaded at 0x90000.
644 When loading at 0x90000, avoid using memory above 0x9a000.
646 For boot protocol 2.02 or higher, the command line does not have to be
647 located in the same 64K segment as the real-mode setup code; it is
648 thus permitted to give the stack/heap the full 64K segment and locate
649 the command line above it.
651 The kernel command line should not be located below the real-mode
652 code, nor should it be located in high memory.
655 **** SAMPLE BOOT CONFIGURATION
657 As a sample configuration, assume the following layout of the real
658 mode segment:
660 When loading below 0x90000, use the entire segment:
662 0x0000-0x7fff Real mode kernel
663 0x8000-0xdfff Stack and heap
664 0xe000-0xffff Kernel command line
666 When loading at 0x90000 OR the protocol version is 2.01 or earlier:
668 0x0000-0x7fff Real mode kernel
669 0x8000-0x97ff Stack and heap
670 0x9800-0x9fff Kernel command line
672 Such a boot loader should enter the following fields in the header:
674 unsigned long base_ptr; /* base address for real-mode segment */
676 if ( setup_sects == 0 ) {
677 setup_sects = 4;
678 }
680 if ( protocol >= 0x0200 ) {
681 type_of_loader = <type code>;
682 if ( loading_initrd ) {
683 ramdisk_image = <initrd_address>;
684 ramdisk_size = <initrd_size>;
685 }
687 if ( protocol >= 0x0202 && loadflags & 0x01 )
688 heap_end = 0xe000;
689 else
690 heap_end = 0x9800;
692 if ( protocol >= 0x0201 ) {
693 heap_end_ptr = heap_end - 0x200;
694 loadflags |= 0x80; /* CAN_USE_HEAP */
695 }
697 if ( protocol >= 0x0202 ) {
698 cmd_line_ptr = base_ptr + heap_end;
699 strcpy(cmd_line_ptr, cmdline);
700 } else {
701 cmd_line_magic = 0xA33F;
702 cmd_line_offset = heap_end;
703 setup_move_size = heap_end + strlen(cmdline)+1;
704 strcpy(base_ptr+cmd_line_offset, cmdline);
705 }
706 } else {
707 /* Very old kernel */
709 heap_end = 0x9800;
711 cmd_line_magic = 0xA33F;
712 cmd_line_offset = heap_end;
714 /* A very old kernel MUST have its real-mode code
715 loaded at 0x90000 */
717 if ( base_ptr != 0x90000 ) {
718 /* Copy the real-mode kernel */
719 memcpy(0x90000, base_ptr, (setup_sects+1)*512);
720 base_ptr = 0x90000; /* Relocated */
721 }
723 strcpy(0x90000+cmd_line_offset, cmdline);
725 /* It is recommended to clear memory up to the 32K mark */
726 memset(0x90000 + (setup_sects+1)*512, 0,
727 (64-(setup_sects+1))*512);
728 }
731 **** LOADING THE REST OF THE KERNEL
733 The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
734 in the kernel file (again, if setup_sects == 0 the real value is 4.)
735 It should be loaded at address 0x10000 for Image/zImage kernels and
736 0x100000 for bzImage kernels.
738 The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
739 bit (LOAD_HIGH) in the loadflags field is set:
741 is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
742 load_address = is_bzImage ? 0x100000 : 0x10000;
744 Note that Image/zImage kernels can be up to 512K in size, and thus use
745 the entire 0x10000-0x90000 range of memory. This means it is pretty
746 much a requirement for these kernels to load the real-mode part at
747 0x90000. bzImage kernels allow much more flexibility.
750 **** SPECIAL COMMAND LINE OPTIONS
752 If the command line provided by the boot loader is entered by the
753 user, the user may expect the following command line options to work.
754 They should normally not be deleted from the kernel command line even
755 though not all of them are actually meaningful to the kernel. Boot
756 loader authors who need additional command line options for the boot
757 loader itself should get them registered in
758 Documentation/kernel-parameters.txt to make sure they will not
759 conflict with actual kernel options now or in the future.
761 vga=<mode>
762 <mode> here is either an integer (in C notation, either
763 decimal, octal, or hexadecimal) or one of the strings
764 "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
765 (meaning 0xFFFD). This value should be entered into the
766 vid_mode field, as it is used by the kernel before the command
767 line is parsed.
769 mem=<size>
770 <size> is an integer in C notation optionally followed by
771 (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
772 << 30, << 40, << 50 or << 60). This specifies the end of
773 memory to the kernel. This affects the possible placement of
774 an initrd, since an initrd should be placed near end of
775 memory. Note that this is an option to *both* the kernel and
776 the bootloader!
778 initrd=<file>
779 An initrd should be loaded. The meaning of <file> is
780 obviously bootloader-dependent, and some boot loaders
781 (e.g. LILO) do not have such a command.
783 In addition, some boot loaders add the following options to the
784 user-specified command line:
786 BOOT_IMAGE=<file>
787 The boot image which was loaded. Again, the meaning of <file>
788 is obviously bootloader-dependent.
790 auto
791 The kernel was booted without explicit user intervention.
793 If these options are added by the boot loader, it is highly
794 recommended that they are located *first*, before the user-specified
795 or configuration-specified command line. Otherwise, "init=/bin/sh"
796 gets confused by the "auto" option.
799 **** RUNNING THE KERNEL
801 The kernel is started by jumping to the kernel entry point, which is
802 located at *segment* offset 0x20 from the start of the real mode
803 kernel. This means that if you loaded your real-mode kernel code at
804 0x90000, the kernel entry point is 9020:0000.
806 At entry, ds = es = ss should point to the start of the real-mode
807 kernel code (0x9000 if the code is loaded at 0x90000), sp should be
808 set up properly, normally pointing to the top of the heap, and
809 interrupts should be disabled. Furthermore, to guard against bugs in
810 the kernel, it is recommended that the boot loader sets fs = gs = ds =
811 es = ss.
813 In our example from above, we would do:
815 /* Note: in the case of the "old" kernel protocol, base_ptr must
816 be == 0x90000 at this point; see the previous sample code */
818 seg = base_ptr >> 4;
820 cli(); /* Enter with interrupts disabled! */
822 /* Set up the real-mode kernel stack */
823 _SS = seg;
824 _SP = heap_end;
826 _DS = _ES = _FS = _GS = seg;
827 jmp_far(seg+0x20, 0); /* Run the kernel */
829 If your boot sector accesses a floppy drive, it is recommended to
830 switch off the floppy motor before running the kernel, since the
831 kernel boot leaves interrupts off and thus the motor will not be
832 switched off, especially if the loaded kernel has the floppy driver as
833 a demand-loaded module!
836 **** ADVANCED BOOT LOADER HOOKS
838 If the boot loader runs in a particularly hostile environment (such as
839 LOADLIN, which runs under DOS) it may be impossible to follow the
840 standard memory location requirements. Such a boot loader may use the
841 following hooks that, if set, are invoked by the kernel at the
842 appropriate time. The use of these hooks should probably be
843 considered an absolutely last resort!
845 IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
846 %edi across invocation.
848 realmode_swtch:
849 A 16-bit real mode far subroutine invoked immediately before
850 entering protected mode. The default routine disables NMI, so
851 your routine should probably do so, too.
853 code32_start:
854 A 32-bit flat-mode routine *jumped* to immediately after the
855 transition to protected mode, but before the kernel is
856 uncompressed. No segments, except CS, are guaranteed to be
857 set up (current kernels do, but older ones do not); you should
858 set them up to BOOT_DS (0x18) yourself.
860 After completing your hook, you should jump to the address
861 that was in this field before your boot loader overwrote it
862 (relocated, if appropriate.)
865 **** 32-bit BOOT PROTOCOL
867 For machine with some new BIOS other than legacy BIOS, such as EFI,
868 LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
869 based on legacy BIOS can not be used, so a 32-bit boot protocol needs
870 to be defined.
872 In 32-bit boot protocol, the first step in loading a Linux kernel
873 should be to setup the boot parameters (struct boot_params,
874 traditionally known as "zero page"). The memory for struct boot_params
875 should be allocated and initialized to all zero. Then the setup header
876 from offset 0x01f1 of kernel image on should be loaded into struct
877 boot_params and examined. The end of setup header can be calculated as
878 follow:
880 0x0202 + byte value at offset 0x0201
882 In addition to read/modify/write the setup header of the struct
883 boot_params as that of 16-bit boot protocol, the boot loader should
884 also fill the additional fields of the struct boot_params as that
885 described in zero-page.txt.
887 After setupping the struct boot_params, the boot loader can load the
888 32/64-bit kernel in the same way as that of 16-bit boot protocol.
890 In 32-bit boot protocol, the kernel is started by jumping to the
891 32-bit kernel entry point, which is the start address of loaded
892 32/64-bit kernel.
894 At entry, the CPU must be in 32-bit protected mode with paging
895 disabled; a GDT must be loaded with the descriptors for selectors
896 __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
897 segment; __BOOS_CS must have execute/read permission, and __BOOT_DS
898 must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
899 must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
900 address of the struct boot_params; %ebp, %edi and %ebx must be zero.