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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>
5 Last update 2002-01-01
6
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
14Currently, four versions of the Linux/i386 boot protocol exist.
15
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
37
38**** MEMORY LAYOUT
39
40The traditional memory map for the kernel loader, used for Image or
41zImage kernels, typically looks like:
42
43 | |
440A0000 +------------------------+
45 | Reserved for BIOS | Do not use. Reserved for BIOS EBDA.
4609A000 +------------------------+
47 | Stack/heap/cmdline | For use by the kernel real-mode code.
48098000 +------------------------+
49 | Kernel setup | The kernel real-mode code.
50090200 +------------------------+
51 | Kernel boot sector | The kernel legacy boot sector.
52090000 +------------------------+
53 | Protected-mode kernel | The bulk of the kernel image.
54010000 +------------------------+
55 | Boot loader | <- Boot sector entry point 0000:7C00
56001000 +------------------------+
57 | Reserved for MBR/BIOS |
58000800 +------------------------+
59 | Typically used by MBR |
60000600 +------------------------+
61 | BIOS use only |
62000000 +------------------------+
63
64
65When using bzImage, the protected-mode kernel was relocated to
660x100000 ("high memory"), and the kernel real-mode block (boot sector,
67setup, and stack/heap) was made relocatable to any address between
680x10000 and end of low memory. Unfortunately, in protocols 2.00 and
692.01 the command line is still required to live in the 0x9XXXX memory
70range, and that memory range is still overwritten by the early kernel.
71The 2.02 protocol resolves that problem.
72
73It is desirable to keep the "memory ceiling" -- the highest point in
74low memory touched by the boot loader -- as low as possible, since
75some newer BIOSes have begun to allocate some rather large amounts of
76memory, called the Extended BIOS Data Area, near the top of low
77memory. The boot loader should use the "INT 12h" BIOS call to verify
78how much low memory is available.
79
80Unfortunately, if INT 12h reports that the amount of memory is too
81low, there is usually nothing the boot loader can do but to report an
82error to the user. The boot loader should therefore be designed to
83take up as little space in low memory as it reasonably can. For
84zImage or old bzImage kernels, which need data written into the
850x90000 segment, the boot loader should make sure not to use memory
86above the 0x9A000 point; too many BIOSes will break above that point.
87
88
89**** THE REAL-MODE KERNEL HEADER
90
91In the following text, and anywhere in the kernel boot sequence, "a
92sector" refers to 512 bytes. It is independent of the actual sector
93size of the underlying medium.
94
95The first step in loading a Linux kernel should be to load the
96real-mode code (boot sector and setup code) and then examine the
97following header at offset 0x01f1. The real-mode code can total up to
9832K, although the boot loader may choose to load only the first two
99sectors (1K) and then examine the bootup sector size.
100
101The header looks like:
102
103Offset Proto Name Meaning
104/Size
105
10601F1/1 ALL setup_sects The size of the setup in sectors
10701F2/2 ALL root_flags If set, the root is mounted readonly
10801F4/2 ALL syssize DO NOT USE - for bootsect.S use only
10901F6/2 ALL swap_dev DO NOT USE - obsolete
11001F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
11101FA/2 ALL vid_mode Video mode control
11201FC/2 ALL root_dev Default root device number
11301FE/2 ALL boot_flag 0xAA55 magic number
1140200/2 2.00+ jump Jump instruction
1150202/4 2.00+ header Magic signature "HdrS"
1160206/2 2.00+ version Boot protocol version supported
1170208/4 2.00+ realmode_swtch Boot loader hook (see below)
118020C/2 2.00+ start_sys The load-low segment (0x1000) (obsolete)
119020E/2 2.00+ kernel_version Pointer to kernel version string
1200210/1 2.00+ type_of_loader Boot loader identifier
1210211/1 2.00+ loadflags Boot protocol option flags
1220212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
1230214/4 2.00+ code32_start Boot loader hook (see below)
1240218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
125021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
1260220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
1270224/2 2.01+ heap_end_ptr Free memory after setup end
1280226/2 N/A pad1 Unused
1290228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
130022C/4 2.03+ initrd_addr_max Highest legal initrd address
131
132For backwards compatibility, if the setup_sects field contains 0, the
133real value is 4.
134
135If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
136the boot protocol version is "old". Loading an old kernel, the
137following parameters should be assumed:
138
139 Image type = zImage
140 initrd not supported
141 Real-mode kernel must be located at 0x90000.
142
143Otherwise, the "version" field contains the protocol version,
144e.g. protocol version 2.01 will contain 0x0201 in this field. When
145setting fields in the header, you must make sure only to set fields
146supported by the protocol version in use.
147
148The "kernel_version" field, if set to a nonzero value, contains a
149pointer to a null-terminated human-readable kernel version number
150string, less 0x200. This can be used to display the kernel version to
151the user. This value should be less than (0x200*setup_sects). For
152example, if this value is set to 0x1c00, the kernel version number
153string can be found at offset 0x1e00 in the kernel file. This is a
154valid value if and only if the "setup_sects" field contains the value
15514 or higher.
156
157Most boot loaders will simply load the kernel at its target address
158directly. Such boot loaders do not need to worry about filling in
159most of the fields in the header. The following fields should be
160filled out, however:
161
162 vid_mode:
163 Please see the section on SPECIAL COMMAND LINE OPTIONS.
164
165 type_of_loader:
166 If your boot loader has an assigned id (see table below), enter
167 0xTV here, where T is an identifier for the boot loader and V is
168 a version number. Otherwise, enter 0xFF here.
169
170 Assigned boot loader ids:
171 0 LILO
172 1 Loadlin
173 2 bootsect-loader
174 3 SYSLINUX
175 4 EtherBoot
176 5 ELILO
177 7 GRuB
178 8 U-BOOT
179
180 Please contact <hpa@zytor.com> if you need a bootloader ID
181 value assigned.
182
183 loadflags, heap_end_ptr:
184 If the protocol version is 2.01 or higher, enter the
185 offset limit of the setup heap into heap_end_ptr and set the
186 0x80 bit (CAN_USE_HEAP) of loadflags. heap_end_ptr appears to
187 be relative to the start of setup (offset 0x0200).
188
189 setup_move_size:
190 When using protocol 2.00 or 2.01, if the real mode
191 kernel is not loaded at 0x90000, it gets moved there later in
192 the loading sequence. Fill in this field if you want
193 additional data (such as the kernel command line) moved in
194 addition to the real-mode kernel itself.
195
196 ramdisk_image, ramdisk_size:
197 If your boot loader has loaded an initial ramdisk (initrd),
198 set ramdisk_image to the 32-bit pointer to the ramdisk data
199 and the ramdisk_size to the size of the ramdisk data.
200
201 The initrd should typically be located as high in memory as
202 possible, as it may otherwise get overwritten by the early
203 kernel initialization sequence. However, it must never be
204 located above the address specified in the initrd_addr_max
205 field. The initrd should be at least 4K page aligned.
206
207 cmd_line_ptr:
208 If the protocol version is 2.02 or higher, this is a 32-bit
209 pointer to the kernel command line. The kernel command line
210 can be located anywhere between the end of setup and 0xA0000.
211 Fill in this field even if your boot loader does not support a
212 command line, in which case you can point this to an empty
213 string (or better yet, to the string "auto".) If this field
214 is left at zero, the kernel will assume that your boot loader
215 does not support the 2.02+ protocol.
216
217 ramdisk_max:
218 The maximum address that may be occupied by the initrd
219 contents. For boot protocols 2.02 or earlier, this field is
220 not present, and the maximum address is 0x37FFFFFF. (This
221 address is defined as the address of the highest safe byte, so
222 if your ramdisk is exactly 131072 bytes long and this field is
223 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
224
225
226**** THE KERNEL COMMAND LINE
227
228The kernel command line has become an important way for the boot
229loader to communicate with the kernel. Some of its options are also
230relevant to the boot loader itself, see "special command line options"
231below.
232
233The kernel command line is a null-terminated string up to 255
234characters long, plus the final null.
235
236If the boot protocol version is 2.02 or later, the address of the
237kernel command line is given by the header field cmd_line_ptr (see
238above.)
239
240If the protocol version is *not* 2.02 or higher, the kernel
241command line is entered using the following protocol:
242
243 At offset 0x0020 (word), "cmd_line_magic", enter the magic
244 number 0xA33F.
245
246 At offset 0x0022 (word), "cmd_line_offset", enter the offset
247 of the kernel command line (relative to the start of the
248 real-mode kernel).
249
250 The kernel command line *must* be within the memory region
251 covered by setup_move_size, so you may need to adjust this
252 field.
253
254
255**** SAMPLE BOOT CONFIGURATION
256
257As a sample configuration, assume the following layout of the real
258mode segment:
259
260 0x0000-0x7FFF Real mode kernel
261 0x8000-0x8FFF Stack and heap
262 0x9000-0x90FF Kernel command line
263
264Such a boot loader should enter the following fields in the header:
265
266 unsigned long base_ptr; /* base address for real-mode segment */
267
268 if ( setup_sects == 0 ) {
269 setup_sects = 4;
270 }
271
272 if ( protocol >= 0x0200 ) {
273 type_of_loader = <type code>;
274 if ( loading_initrd ) {
275 ramdisk_image = <initrd_address>;
276 ramdisk_size = <initrd_size>;
277 }
278 if ( protocol >= 0x0201 ) {
279 heap_end_ptr = 0x9000 - 0x200;
280 loadflags |= 0x80; /* CAN_USE_HEAP */
281 }
282 if ( protocol >= 0x0202 ) {
283 cmd_line_ptr = base_ptr + 0x9000;
284 } else {
285 cmd_line_magic = 0xA33F;
286 cmd_line_offset = 0x9000;
287 setup_move_size = 0x9100;
288 }
289 } else {
290 /* Very old kernel */
291
292 cmd_line_magic = 0xA33F;
293 cmd_line_offset = 0x9000;
294
295 /* A very old kernel MUST have its real-mode code
296 loaded at 0x90000 */
297
298 if ( base_ptr != 0x90000 ) {
299 /* Copy the real-mode kernel */
300 memcpy(0x90000, base_ptr, (setup_sects+1)*512);
301 /* Copy the command line */
302 memcpy(0x99000, base_ptr+0x9000, 256);
303
304 base_ptr = 0x90000; /* Relocated */
305 }
306
307 /* It is recommended to clear memory up to the 32K mark */
308 memset(0x90000 + (setup_sects+1)*512, 0,
309 (64-(setup_sects+1))*512);
310 }
311
312
313**** LOADING THE REST OF THE KERNEL
314
315The non-real-mode kernel starts at offset (setup_sects+1)*512 in the
316kernel file (again, if setup_sects == 0 the real value is 4.) It
317should be loaded at address 0x10000 for Image/zImage kernels and
3180x100000 for bzImage kernels.
319
320The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
321bit (LOAD_HIGH) in the loadflags field is set:
322
323 is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
324 load_address = is_bzImage ? 0x100000 : 0x10000;
325
326Note that Image/zImage kernels can be up to 512K in size, and thus use
327the entire 0x10000-0x90000 range of memory. This means it is pretty
328much a requirement for these kernels to load the real-mode part at
3290x90000. bzImage kernels allow much more flexibility.
330
331
332**** SPECIAL COMMAND LINE OPTIONS
333
334If the command line provided by the boot loader is entered by the
335user, the user may expect the following command line options to work.
336They should normally not be deleted from the kernel command line even
337though not all of them are actually meaningful to the kernel. Boot
338loader authors who need additional command line options for the boot
339loader itself should get them registered in
340Documentation/kernel-parameters.txt to make sure they will not
341conflict with actual kernel options now or in the future.
342
343 vga=<mode>
344 <mode> here is either an integer (in C notation, either
345 decimal, octal, or hexadecimal) or one of the strings
346 "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
347 (meaning 0xFFFD). This value should be entered into the
348 vid_mode field, as it is used by the kernel before the command
349 line is parsed.
350
351 mem=<size>
352 <size> is an integer in C notation optionally followed by K, M
353 or G (meaning << 10, << 20 or << 30). This specifies the end
354 of memory to the kernel. This affects the possible placement
355 of an initrd, since an initrd should be placed near end of
356 memory. Note that this is an option to *both* the kernel and
357 the bootloader!
358
359 initrd=<file>
360 An initrd should be loaded. The meaning of <file> is
361 obviously bootloader-dependent, and some boot loaders
362 (e.g. LILO) do not have such a command.
363
364In addition, some boot loaders add the following options to the
365user-specified command line:
366
367 BOOT_IMAGE=<file>
368 The boot image which was loaded. Again, the meaning of <file>
369 is obviously bootloader-dependent.
370
371 auto
372 The kernel was booted without explicit user intervention.
373
374If these options are added by the boot loader, it is highly
375recommended that they are located *first*, before the user-specified
376or configuration-specified command line. Otherwise, "init=/bin/sh"
377gets confused by the "auto" option.
378
379
380**** RUNNING THE KERNEL
381
382The kernel is started by jumping to the kernel entry point, which is
383located at *segment* offset 0x20 from the start of the real mode
384kernel. This means that if you loaded your real-mode kernel code at
3850x90000, the kernel entry point is 9020:0000.
386
387At entry, ds = es = ss should point to the start of the real-mode
388kernel code (0x9000 if the code is loaded at 0x90000), sp should be
389set up properly, normally pointing to the top of the heap, and
390interrupts should be disabled. Furthermore, to guard against bugs in
391the kernel, it is recommended that the boot loader sets fs = gs = ds =
392es = ss.
393
394In our example from above, we would do:
395
396 /* Note: in the case of the "old" kernel protocol, base_ptr must
397 be == 0x90000 at this point; see the previous sample code */
398
399 seg = base_ptr >> 4;
400
401 cli(); /* Enter with interrupts disabled! */
402
403 /* Set up the real-mode kernel stack */
404 _SS = seg;
405 _SP = 0x9000; /* Load SP immediately after loading SS! */
406
407 _DS = _ES = _FS = _GS = seg;
408 jmp_far(seg+0x20, 0); /* Run the kernel */
409
410If your boot sector accesses a floppy drive, it is recommended to
411switch off the floppy motor before running the kernel, since the
412kernel boot leaves interrupts off and thus the motor will not be
413switched off, especially if the loaded kernel has the floppy driver as
414a demand-loaded module!
415
416
417**** ADVANCED BOOT TIME HOOKS
418
419If the boot loader runs in a particularly hostile environment (such as
420LOADLIN, which runs under DOS) it may be impossible to follow the
421standard memory location requirements. Such a boot loader may use the
422following hooks that, if set, are invoked by the kernel at the
423appropriate time. The use of these hooks should probably be
424considered an absolutely last resort!
425
426IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
427%edi across invocation.
428
429 realmode_swtch:
430 A 16-bit real mode far subroutine invoked immediately before
431 entering protected mode. The default routine disables NMI, so
432 your routine should probably do so, too.
433
434 code32_start:
435 A 32-bit flat-mode routine *jumped* to immediately after the
436 transition to protected mode, but before the kernel is
437 uncompressed. No segments, except CS, are set up; you should
438 set them up to KERNEL_DS (0x18) yourself.
439
440 After completing your hook, you should jump to the address
441 that was in this field before your boot loader overwrote it.