1 THE LINUX/I386 BOOT PROTOCOL
2 ----------------------------
4 H. Peter Anvin <hpa@zytor.com>
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.
14 Currently, four versions of the Linux/i386 boot protocol exist.
16 Old kernels: zImage/Image support only. Some very early kernels
17 may not even support a command line.
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
25 Protocol 2.01: (Kernel 1.3.76) Added a heap overrun warning.
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
34 Protocol 2.03: (Kernel 2.4.18-pre1) Explicitly makes the highest possible
35 initrd address available to the bootloader.
40 The traditional memory map for the kernel loader, used for Image or
41 zImage kernels, typically looks like:
44 0A0000 +------------------------+
45 | Reserved for BIOS | Do not use. Reserved for BIOS EBDA.
46 09A000 +------------------------+
47 | Stack/heap/cmdline | For use by the kernel real-mode code.
48 098000 +------------------------+
49 | Kernel setup | The kernel real-mode code.
50 090200 +------------------------+
51 | Kernel boot sector | The kernel legacy boot sector.
52 090000 +------------------------+
53 | Protected-mode kernel | The bulk of the kernel image.
54 010000 +------------------------+
55 | Boot loader | <- Boot sector entry point 0000:7C00
56 001000 +------------------------+
57 | Reserved for MBR/BIOS |
58 000800 +------------------------+
59 | Typically used by MBR |
60 000600 +------------------------+
62 000000 +------------------------+
65 When using bzImage, the protected-mode kernel was relocated to
66 0x100000 ("high memory"), and the kernel real-mode block (boot sector,
67 setup, and stack/heap) was made relocatable to any address between
68 0x10000 and end of low memory. Unfortunately, in protocols 2.00 and
69 2.01 the command line is still required to live in the 0x9XXXX memory
70 range, and that memory range is still overwritten by the early kernel.
71 The 2.02 protocol resolves that problem.
73 It is desirable to keep the "memory ceiling" -- the highest point in
74 low memory touched by the boot loader -- as low as possible, since
75 some newer BIOSes have begun to allocate some rather large amounts of
76 memory, called the Extended BIOS Data Area, near the top of low
77 memory. The boot loader should use the "INT 12h" BIOS call to verify
78 how much low memory is available.
80 Unfortunately, if INT 12h reports that the amount of memory is too
81 low, there is usually nothing the boot loader can do but to report an
82 error to the user. The boot loader should therefore be designed to
83 take up as little space in low memory as it reasonably can. For
84 zImage or old bzImage kernels, which need data written into the
85 0x90000 segment, the boot loader should make sure not to use memory
86 above the 0x9A000 point; too many BIOSes will break above that point.
89 **** THE REAL-MODE KERNEL HEADER
91 In the following text, and anywhere in the kernel boot sequence, "a
92 sector" refers to 512 bytes. It is independent of the actual sector
93 size of the underlying medium.
95 The first step in loading a Linux kernel should be to load the
96 real-mode code (boot sector and setup code) and then examine the
97 following header at offset 0x01f1. The real-mode code can total up to
98 32K, although the boot loader may choose to load only the first two
99 sectors (1K) and then examine the bootup sector size.
101 The header looks like:
103 Offset Proto Name Meaning
106 01F1/1 ALL setup_sects The size of the setup in sectors
107 01F2/2 ALL root_flags If set, the root is mounted readonly
108 01F4/2 ALL syssize DO NOT USE - for bootsect.S use only
109 01F6/2 ALL swap_dev DO NOT USE - obsolete
110 01F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
111 01FA/2 ALL vid_mode Video mode control
112 01FC/2 ALL root_dev Default root device number
113 01FE/2 ALL boot_flag 0xAA55 magic number
114 0200/2 2.00+ jump Jump instruction
115 0202/4 2.00+ header Magic signature "HdrS"
116 0206/2 2.00+ version Boot protocol version supported
117 0208/4 2.00+ realmode_swtch Boot loader hook (see below)
118 020C/2 2.00+ start_sys The load-low segment (0x1000) (obsolete)
119 020E/2 2.00+ kernel_version Pointer to kernel version string
120 0210/1 2.00+ type_of_loader Boot loader identifier
121 0211/1 2.00+ loadflags Boot protocol option flags
122 0212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
123 0214/4 2.00+ code32_start Boot loader hook (see below)
124 0218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
125 021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
126 0220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
127 0224/2 2.01+ heap_end_ptr Free memory after setup end
128 0226/2 N/A pad1 Unused
129 0228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
130 022C/4 2.03+ initrd_addr_max Highest legal initrd address
132 For backwards compatibility, if the setup_sects field contains 0, the
135 If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
136 the boot protocol version is "old". Loading an old kernel, the
137 following parameters should be assumed:
141 Real-mode kernel must be located at 0x90000.
143 Otherwise, the "version" field contains the protocol version,
144 e.g. protocol version 2.01 will contain 0x0201 in this field. When
145 setting fields in the header, you must make sure only to set fields
146 supported by the protocol version in use.
148 The "kernel_version" field, if set to a nonzero value, contains a
149 pointer to a null-terminated human-readable kernel version number
150 string, less 0x200. This can be used to display the kernel version to
151 the user. This value should be less than (0x200*setup_sects). For
152 example, if this value is set to 0x1c00, the kernel version number
153 string can be found at offset 0x1e00 in the kernel file. This is a
154 valid value if and only if the "setup_sects" field contains the value
157 Most boot loaders will simply load the kernel at its target address
158 directly. Such boot loaders do not need to worry about filling in
159 most of the fields in the header. The following fields should be
163 Please see the section on SPECIAL COMMAND LINE OPTIONS.
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.
170 Assigned boot loader ids:
177 Please contact <hpa@zytor.com> if you need a bootloader ID
180 loadflags, heap_end_ptr:
181 If the protocol version is 2.01 or higher, enter the
182 offset limit of the setup heap into heap_end_ptr and set the
183 0x80 bit (CAN_USE_HEAP) of loadflags. heap_end_ptr appears to
184 be relative to the start of setup (offset 0x0200).
187 When using protocol 2.00 or 2.01, if the real mode
188 kernel is not loaded at 0x90000, it gets moved there later in
189 the loading sequence. Fill in this field if you want
190 additional data (such as the kernel command line) moved in
191 addition to the real-mode kernel itself.
193 ramdisk_image, ramdisk_size:
194 If your boot loader has loaded an initial ramdisk (initrd),
195 set ramdisk_image to the 32-bit pointer to the ramdisk data
196 and the ramdisk_size to the size of the ramdisk data.
198 The initrd should typically be located as high in memory as
199 possible, as it may otherwise get overwritten by the early
200 kernel initialization sequence. However, it must never be
201 located above the address specified in the initrd_addr_max
202 field. The initrd should be at least 4K page aligned.
205 If the protocol version is 2.02 or higher, this is a 32-bit
206 pointer to the kernel command line. The kernel command line
207 can be located anywhere between the end of setup and 0xA0000.
208 Fill in this field even if your boot loader does not support a
209 command line, in which case you can point this to an empty
210 string (or better yet, to the string "auto".) If this field
211 is left at zero, the kernel will assume that your boot loader
212 does not support the 2.02+ protocol.
215 The maximum address that may be occupied by the initrd
216 contents. For boot protocols 2.02 or earlier, this field is
217 not present, and the maximum address is 0x37FFFFFF. (This
218 address is defined as the address of the highest safe byte, so
219 if your ramdisk is exactly 131072 bytes long and this field is
220 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
223 **** THE KERNEL COMMAND LINE
225 The kernel command line has become an important way for the boot
226 loader to communicate with the kernel. Some of its options are also
227 relevant to the boot loader itself, see "special command line options"
230 The kernel command line is a null-terminated string up to 255
231 characters long, plus the final null.
233 If the boot protocol version is 2.02 or later, the address of the
234 kernel command line is given by the header field cmd_line_ptr (see
237 If the protocol version is *not* 2.02 or higher, the kernel
238 command line is entered using the following protocol:
240 At offset 0x0020 (word), "cmd_line_magic", enter the magic
243 At offset 0x0022 (word), "cmd_line_offset", enter the offset
244 of the kernel command line (relative to the start of the
247 The kernel command line *must* be within the memory region
248 covered by setup_move_size, so you may need to adjust this
252 **** SAMPLE BOOT CONFIGURATION
254 As a sample configuration, assume the following layout of the real
257 0x0000-0x7FFF Real mode kernel
258 0x8000-0x8FFF Stack and heap
259 0x9000-0x90FF Kernel command line
261 Such a boot loader should enter the following fields in the header:
263 unsigned long base_ptr; /* base address for real-mode segment */
265 if ( setup_sects == 0 ) {
269 if ( protocol >= 0x0200 ) {
270 type_of_loader = <type code>;
271 if ( loading_initrd ) {
272 ramdisk_image = <initrd_address>;
273 ramdisk_size = <initrd_size>;
275 if ( protocol >= 0x0201 ) {
276 heap_end_ptr = 0x9000 - 0x200;
277 loadflags |= 0x80; /* CAN_USE_HEAP */
279 if ( protocol >= 0x0202 ) {
280 cmd_line_ptr = base_ptr + 0x9000;
282 cmd_line_magic = 0xA33F;
283 cmd_line_offset = 0x9000;
284 setup_move_size = 0x9100;
287 /* Very old kernel */
289 cmd_line_magic = 0xA33F;
290 cmd_line_offset = 0x9000;
292 /* A very old kernel MUST have its real-mode code
295 if ( base_ptr != 0x90000 ) {
296 /* Copy the real-mode kernel */
297 memcpy(0x90000, base_ptr, (setup_sects+1)*512);
298 /* Copy the command line */
299 memcpy(0x99000, base_ptr+0x9000, 256);
301 base_ptr = 0x90000; /* Relocated */
304 /* It is recommended to clear memory up to the 32K mark */
305 memset(0x90000 + (setup_sects+1)*512, 0,
306 (64-(setup_sects+1))*512);
310 **** LOADING THE REST OF THE KERNEL
312 The non-real-mode kernel starts at offset (setup_sects+1)*512 in the
313 kernel file (again, if setup_sects == 0 the real value is 4.) It
314 should be loaded at address 0x10000 for Image/zImage kernels and
315 0x100000 for bzImage kernels.
317 The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
318 bit (LOAD_HIGH) in the loadflags field is set:
320 is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
321 load_address = is_bzImage ? 0x100000 : 0x10000;
323 Note that Image/zImage kernels can be up to 512K in size, and thus use
324 the entire 0x10000-0x90000 range of memory. This means it is pretty
325 much a requirement for these kernels to load the real-mode part at
326 0x90000. bzImage kernels allow much more flexibility.
329 **** SPECIAL COMMAND LINE OPTIONS
331 If the command line provided by the boot loader is entered by the
332 user, the user may expect the following command line options to work.
333 They should normally not be deleted from the kernel command line even
334 though not all of them are actually meaningful to the kernel. Boot
335 loader authors who need additional command line options for the boot
336 loader itself should get them registered in
337 linux/Documentation/kernel-parameters.txt to make sure they will not
338 conflict with actual kernel options now or in the future.
341 <mode> here is either an integer (in C notation, either
342 decimal, octal, or hexadecimal) or one of the strings
343 "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
344 (meaning 0xFFFD). This value should be entered into the
345 vid_mode field, as it is used by the kernel before the command
349 <size> is an integer in C notation optionally followed by K, M
350 or G (meaning << 10, << 20 or << 30). This specifies the end
351 of memory to the kernel. This affects the possible placement
352 of an initrd, since an initrd should be placed near end of
353 memory. Note that this is an option to *both* the kernel and
357 An initrd should be loaded. The meaning of <file> is
358 obviously bootloader-dependent, and some boot loaders
359 (e.g. LILO) do not have such a command.
361 In addition, some boot loaders add the following options to the
362 user-specified command line:
365 The boot image which was loaded. Again, the meaning of <file>
366 is obviously bootloader-dependent.
369 The kernel was booted without explicit user intervention.
371 If these options are added by the boot loader, it is highly
372 recommended that they are located *first*, before the user-specified
373 or configuration-specified command line. Otherwise, "init=/bin/sh"
374 gets confused by the "auto" option.
377 **** RUNNING THE KERNEL
379 The kernel is started by jumping to the kernel entry point, which is
380 located at *segment* offset 0x20 from the start of the real mode
381 kernel. This means that if you loaded your real-mode kernel code at
382 0x90000, the kernel entry point is 9020:0000.
384 At entry, ds = es = ss should point to the start of the real-mode
385 kernel code (0x9000 if the code is loaded at 0x90000), sp should be
386 set up properly, normally pointing to the top of the heap, and
387 interrupts should be disabled. Furthermore, to guard against bugs in
388 the kernel, it is recommended that the boot loader sets fs = gs = ds =
391 In our example from above, we would do:
393 /* Note: in the case of the "old" kernel protocol, base_ptr must
394 be == 0x90000 at this point; see the previous sample code */
398 cli(); /* Enter with interrupts disabled! */
400 /* Set up the real-mode kernel stack */
402 _SP = 0x9000; /* Load SP immediately after loading SS! */
404 _DS = _ES = _FS = _GS = seg;
405 jmp_far(seg+0x20, 0); /* Run the kernel */
407 If your boot sector accesses a floppy drive, it is recommended to
408 switch off the floppy motor before running the kernel, since the
409 kernel boot leaves interrupts off and thus the motor will not be
410 switched off, especially if the loaded kernel has the floppy driver as
411 a demand-loaded module!
414 **** ADVANCED BOOT TIME HOOKS
416 If the boot loader runs in a particularly hostile environment (such as
417 LOADLIN, which runs under DOS) it may be impossible to follow the
418 standard memory location requirements. Such a boot loader may use the
419 following hooks that, if set, are invoked by the kernel at the
420 appropriate time. The use of these hooks should probably be
421 considered an absolutely last resort!
423 IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
424 %edi across invocation.
427 A 16-bit real mode far subroutine invoked immediately before
428 entering protected mode. The default routine disables NMI, so
429 your routine should probably do so, too.
432 A 32-bit flat-mode routine *jumped* to immediately after the
433 transition to protected mode, but before the kernel is
434 uncompressed. No segments, except CS, are set up; you should
435 set them up to KERNEL_DS (0x18) yourself.
437 After completing your hook, you should jump to the address
438 that was in this field before your boot loader overwrote it.