THE LINUX/I386 BOOT PROTOCOL
----------------------------
H. Peter Anvin <hpa@zytor.com>
Last update 2007-05-23
On the i386 platform, the Linux kernel uses a rather complicated boot
convention. This has evolved partially due to historical aspects, as
well as the desire in the early days to have the kernel itself be a
bootable image, the complicated PC memory model and due to changed
expectations in the PC industry caused by the effective demise of
real-mode DOS as a mainstream operating system.
Currently, the following versions of the Linux/i386 boot protocol exist.
Old kernels: zImage/Image support only. Some very early kernels
may not even support a command line.
Protocol 2.00: (Kernel 1.3.73) Added bzImage and initrd support, as
well as a formalized way to communicate between the
boot loader and the kernel. setup.S made relocatable,
although the traditional setup area still assumed
writable.
Protocol 2.01: (Kernel 1.3.76) Added a heap overrun warning.
Protocol 2.02: (Kernel 2.4.0-test3-pre3) New command line protocol.
Lower the conventional memory ceiling. No overwrite
of the traditional setup area, thus making booting
safe for systems which use the EBDA from SMM or 32-bit
BIOS entry points. zImage deprecated but still
supported.
Protocol 2.03: (Kernel 2.4.18-pre1) Explicitly makes the highest possible
initrd address available to the bootloader.
Protocol 2.04: (Kernel 2.6.14) Extend the syssize field to four bytes.
Protocol 2.05: (Kernel 2.6.20) Make protected mode kernel relocatable.
Introduce relocatable_kernel and kernel_alignment fields.
Protocol 2.06: (Kernel 2.6.22) Added a field that contains the size of
the boot command line
**** MEMORY LAYOUT
The traditional memory map for the kernel loader, used for Image or
zImage kernels, typically looks like:
| |
0A0000 +------------------------+
| Reserved for BIOS | Do not use. Reserved for BIOS EBDA.
09A000 +------------------------+
| Command line |
| Stack/heap | For use by the kernel real-mode code.
098000 +------------------------+
| Kernel setup | The kernel real-mode code.
090200 +------------------------+
| Kernel boot sector | The kernel legacy boot sector.
090000 +------------------------+
| Protected-mode kernel | The bulk of the kernel image.
010000 +------------------------+
| Boot loader | <- Boot sector entry point 0000:7C00
001000 +------------------------+
| Reserved for MBR/BIOS |
000800 +------------------------+
| Typically used by MBR |
000600 +------------------------+
| BIOS use only |
000000 +------------------------+
When using bzImage, the protected-mode kernel was relocated to
0x100000 ("high memory"), and the kernel real-mode block (boot sector,
setup, and stack/heap) was made relocatable to any address between
0x10000 and end of low memory. Unfortunately, in protocols 2.00 and
2.01 the 0x90000+ memory range is still used internally by the kernel;
the 2.02 protocol resolves that problem.
It is desirable to keep the "memory ceiling" -- the highest point in
low memory touched by the boot loader -- as low as possible, since
some newer BIOSes have begun to allocate some rather large amounts of
memory, called the Extended BIOS Data Area, near the top of low
memory. The boot loader should use the "INT 12h" BIOS call to verify
how much low memory is available.
Unfortunately, if INT 12h reports that the amount of memory is too
low, there is usually nothing the boot loader can do but to report an
error to the user. The boot loader should therefore be designed to
take up as little space in low memory as it reasonably can. For
zImage or old bzImage kernels, which need data written into the
0x90000 segment, the boot loader should make sure not to use memory
above the 0x9A000 point; too many BIOSes will break above that point.
For a modern bzImage kernel with boot protocol version >= 2.02, a
memory layout like the following is suggested:
~ ~
| Protected-mode kernel |
100000 +------------------------+
| I/O memory hole |
0A0000 +------------------------+
| Reserved for BIOS | Leave as much as possible unused
~ ~
| Command line | (Can also be below the X+10000 mark)
X+10000 +------------------------+
| Stack/heap | For use by the kernel real-mode code.
X+08000 +------------------------+
| Kernel setup | The kernel real-mode code.
| Kernel boot sector | The kernel legacy boot sector.
X +------------------------+
| Boot loader | <- Boot sector entry point 0000:7C00
001000 +------------------------+
| Reserved for MBR/BIOS |
000800 +------------------------+
| Typically used by MBR |
000600 +------------------------+
| BIOS use only |
000000 +------------------------+
... where the address X is as low as the design of the boot loader
permits.
**** THE REAL-MODE KERNEL HEADER
In the following text, and anywhere in the kernel boot sequence, "a
sector" refers to 512 bytes. It is independent of the actual sector
size of the underlying medium.
The first step in loading a Linux kernel should be to load the
real-mode code (boot sector and setup code) and then examine the
following header at offset 0x01f1. The real-mode code can total up to
32K, although the boot loader may choose to load only the first two
sectors (1K) and then examine the bootup sector size.
The header looks like:
Offset Proto Name Meaning
/Size
01F1/1 ALL(1 setup_sects The size of the setup in sectors
01F2/2 ALL root_flags If set, the root is mounted readonly
01F4/4 2.04+(2 syssize The size of the 32-bit code in 16-byte paras
01F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
01FA/2 ALL vid_mode Video mode control
01FC/2 ALL root_dev Default root device number
01FE/2 ALL boot_flag 0xAA55 magic number
0200/2 2.00+ jump Jump instruction
0202/4 2.00+ header Magic signature "HdrS"
0206/2 2.00+ version Boot protocol version supported
0208/4 2.00+ realmode_swtch Boot loader hook (see below)
020C/2 2.00+ start_sys The load-low segment (0x1000) (obsolete)
020E/2 2.00+ kernel_version Pointer to kernel version string
0210/1 2.00+ type_of_loader Boot loader identifier
0211/1 2.00+ loadflags Boot protocol option flags
0212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
0214/4 2.00+ code32_start Boot loader hook (see below)
0218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
0220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
0224/2 2.01+ heap_end_ptr Free memory after setup end
0226/2 N/A pad1 Unused
0228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
022C/4 2.03+ initrd_addr_max Highest legal initrd address
0230/4 2.05+ kernel_alignment Physical addr alignment required for kernel
0234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
0235/3 N/A pad2 Unused
0238/4 2.06+ cmdline_size Maximum size of the kernel command line
023C/4 2.07+ hardware_subarch Hardware subarchitecture
0240/8 2.07+ hardware_subarch_data Subarchitecture-specific data
(1) For backwards compatibility, if the setup_sects field contains 0, the
real value is 4.
(2) For boot protocol prior to 2.04, the upper two bytes of the syssize
field are unusable, which means the size of a bzImage kernel
cannot be determined.
If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
the boot protocol version is "old". Loading an old kernel, the
following parameters should be assumed:
Image type = zImage
initrd not supported
Real-mode kernel must be located at 0x90000.
Otherwise, the "version" field contains the protocol version,
e.g. protocol version 2.01 will contain 0x0201 in this field. When
setting fields in the header, you must make sure only to set fields
supported by the protocol version in use.
**** DETAILS OF HEADER FIELDS
For each field, some are information from the kernel to the bootloader
("read"), some are expected to be filled out by the bootloader
("write"), and some are expected to be read and modified by the
bootloader ("modify").
All general purpose boot loaders should write the fields marked
(obligatory). Boot loaders who want to load the kernel at a
nonstandard address should fill in the fields marked (reloc); other
boot loaders can ignore those fields.
The byte order of all fields is littleendian (this is x86, after all.)
Field name: setup_sects
Type: read
Offset/size: 0x1f1/1
Protocol: ALL
The size of the setup code in 512-byte sectors. If this field is
0, the real value is 4. The real-mode code consists of the boot
sector (always one 512-byte sector) plus the setup code.
Field name: root_flags
Type: modify (optional)
Offset/size: 0x1f2/2
Protocol: ALL
If this field is nonzero, the root defaults to readonly. The use of
this field is deprecated; use the "ro" or "rw" options on the
command line instead.
Field name: syssize
Type: read
Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
Protocol: 2.04+
The size of the protected-mode code in units of 16-byte paragraphs.
For protocol versions older than 2.04 this field is only two bytes
wide, and therefore cannot be trusted for the size of a kernel if
the LOAD_HIGH flag is set.
Field name: ram_size
Type: kernel internal
Offset/size: 0x1f8/2
Protocol: ALL
This field is obsolete.
Field name: vid_mode
Type: modify (obligatory)
Offset/size: 0x1fa/2
Please see the section on SPECIAL COMMAND LINE OPTIONS.
Field name: root_dev
Type: modify (optional)
Offset/size: 0x1fc/2
Protocol: ALL
The default root device device number. The use of this field is
deprecated, use the "root=" option on the command line instead.
Field name: boot_flag
Type: read
Offset/size: 0x1fe/2
Protocol: ALL
Contains 0xAA55. This is the closest thing old Linux kernels have
to a magic number.
Field name: jump
Type: read
Offset/size: 0x200/2
Protocol: 2.00+
Contains an x86 jump instruction, 0xEB followed by a signed offset
relative to byte 0x202. This can be used to determine the size of
the header.
Field name: header
Type: read
Offset/size: 0x202/4
Protocol: 2.00+
Contains the magic number "HdrS" (0x53726448).
Field name: version
Type: read
Offset/size: 0x206/2
Protocol: 2.00+
Contains the boot protocol version, in (major << 8)+minor format,
e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
10.17.
Field name: readmode_swtch
Type: modify (optional)
Offset/size: 0x208/4
Protocol: 2.00+
Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
Field name: start_sys
Type: read
Offset/size: 0x20c/4
Protocol: 2.00+
The load low segment (0x1000). Obsolete.
Field name: kernel_version
Type: read
Offset/size: 0x20e/2
Protocol: 2.00+
If set to a nonzero value, contains a pointer to a NUL-terminated
human-readable kernel version number string, less 0x200. This can
be used to display the kernel version to the user. This value
should be less than (0x200*setup_sects).
For example, if this value is set to 0x1c00, the kernel version
number string can be found at offset 0x1e00 in the kernel file.
This is a valid value if and only if the "setup_sects" field
contains the value 15 or higher, as:
0x1c00 < 15*0x200 (= 0x1e00) but
0x1c00 >= 14*0x200 (= 0x1c00)
0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
Field name: type_of_loader
Type: write (obligatory)
Offset/size: 0x210/1
Protocol: 2.00+
If your boot loader has an assigned id (see table below), enter
0xTV here, where T is an identifier for the boot loader and V is
a version number. Otherwise, enter 0xFF here.
Assigned boot loader ids:
0 LILO (0x00 reserved for pre-2.00 bootloader)
1 Loadlin
2 bootsect-loader (0x20, all other values reserved)
3 SYSLINUX
4 EtherBoot
5 ELILO
7 GRuB
8 U-BOOT
9 Xen
A Gujin
B Qemu
Please contact <hpa@zytor.com> if you need a bootloader ID
value assigned.
Field name: loadflags
Type: modify (obligatory)
Offset/size: 0x211/1
Protocol: 2.00+
This field is a bitmask.
Bit 0 (read): LOADED_HIGH
- If 0, the protected-mode code is loaded at 0x10000.
- If 1, the protected-mode code is loaded at 0x100000.
Bit 6 (write): KEEP_SEGMENTS
Protocol: 2.07+
- if 0, reload the segment registers in the 32bit entry point.
- if 1, do not reload the segment registers in the 32bit entry point.
Assume that %cs %ds %ss %es are all set to flat segments with
a base of 0 (or the equivalent for their environment).
Bit 7 (write): CAN_USE_HEAP
Set this bit to 1 to indicate that the value entered in the
heap_end_ptr is valid. If this field is clear, some setup code
functionality will be disabled.
Field name: setup_move_size
Type: modify (obligatory)
Offset/size: 0x212/2
Protocol: 2.00-2.01
When using protocol 2.00 or 2.01, if the real mode kernel is not
loaded at 0x90000, it gets moved there later in the loading
sequence. Fill in this field if you want additional data (such as
the kernel command line) moved in addition to the real-mode kernel
itself.
The unit is bytes starting with the beginning of the boot sector.
This field is can be ignored when the protocol is 2.02 or higher, or
if the real-mode code is loaded at 0x90000.
Field name: code32_start
Type: modify (optional, reloc)
Offset/size: 0x214/4
Protocol: 2.00+
The address to jump to in protected mode. This defaults to the load
address of the kernel, and can be used by the boot loader to
determine the proper load address.
This field can be modified for two purposes:
1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
2. if a bootloader which does not install a hook loads a
relocatable kernel at a nonstandard address it will have to modify
this field to point to the load address.
Field name: ramdisk_image
Type: write (obligatory)
Offset/size: 0x218/4
Protocol: 2.00+
The 32-bit linear address of the initial ramdisk or ramfs. Leave at
zero if there is no initial ramdisk/ramfs.
Field name: ramdisk_size
Type: write (obligatory)
Offset/size: 0x21c/4
Protocol: 2.00+
Size of the initial ramdisk or ramfs. Leave at zero if there is no
initial ramdisk/ramfs.
Field name: bootsect_kludge
Type: kernel internal
Offset/size: 0x220/4
Protocol: 2.00+
This field is obsolete.
Field name: heap_end_ptr
Type: write (obligatory)
Offset/size: 0x224/2
Protocol: 2.01+
Set this field to the offset (from the beginning of the real-mode
code) of the end of the setup stack/heap, minus 0x0200.
Field name: cmd_line_ptr
Type: write (obligatory)
Offset/size: 0x228/4
Protocol: 2.02+
Set this field to the linear address of the kernel command line.
The kernel command line can be located anywhere between the end of
the setup heap and 0xA0000; it does not have to be located in the
same 64K segment as the real-mode code itself.
Fill in this field even if your boot loader does not support a
command line, in which case you can point this to an empty string
(or better yet, to the string "auto".) If this field is left at
zero, the kernel will assume that your boot loader does not support
the 2.02+ protocol.
Field name: initrd_addr_max
Type: read
Offset/size: 0x22c/4
Protocol: 2.03+
The maximum address that may be occupied by the initial
ramdisk/ramfs contents. For boot protocols 2.02 or earlier, this
field is not present, and the maximum address is 0x37FFFFFF. (This
address is defined as the address of the highest safe byte, so if
your ramdisk is exactly 131072 bytes long and this field is
0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
Field name: kernel_alignment
Type: read (reloc)
Offset/size: 0x230/4
Protocol: 2.05+
Alignment unit required by the kernel (if relocatable_kernel is true.)
Field name: relocatable_kernel
Type: read (reloc)
Offset/size: 0x234/1
Protocol: 2.05+
If this field is nonzero, the protected-mode part of the kernel can
be loaded at any address that satisfies the kernel_alignment field.
After loading, the boot loader must set the code32_start field to
point to the loaded code, or to a boot loader hook.
Field name: cmdline_size
Type: read
Offset/size: 0x238/4
Protocol: 2.06+
The maximum size of the command line without the terminating
zero. This means that the command line can contain at most
cmdline_size characters. With protocol version 2.05 and earlier, the
maximum size was 255.
Field name: hardware_subarch
Type: write
Offset/size: 0x23c/4
Protocol: 2.07+
In a paravirtualized environment the hardware low level architectural
pieces such as interrupt handling, page table handling, and
accessing process control registers needs to be done differently.
This field allows the bootloader to inform the kernel we are in one
one of those environments.
0x00000000 The default x86/PC environment
0x00000001 lguest
0x00000002 Xen
Field name: hardware_subarch_data
Type: write
Offset/size: 0x240/8
Protocol: 2.07+
A pointer to data that is specific to hardware subarch
**** THE KERNEL COMMAND LINE
The kernel command line has become an important way for the boot
loader to communicate with the kernel. Some of its options are also
relevant to the boot loader itself, see "special command line options"
below.
The kernel command line is a null-terminated string. The maximum
length can be retrieved from the field cmdline_size. Before protocol
version 2.06, the maximum was 255 characters. A string that is too
long will be automatically truncated by the kernel.
If the boot protocol version is 2.02 or later, the address of the
kernel command line is given by the header field cmd_line_ptr (see
above.) This address can be anywhere between the end of the setup
heap and 0xA0000.
If the protocol version is *not* 2.02 or higher, the kernel
command line is entered using the following protocol:
At offset 0x0020 (word), "cmd_line_magic", enter the magic
number 0xA33F.
At offset 0x0022 (word), "cmd_line_offset", enter the offset
of the kernel command line (relative to the start of the
real-mode kernel).
The kernel command line *must* be within the memory region
covered by setup_move_size, so you may need to adjust this
field.
**** MEMORY LAYOUT OF THE REAL-MODE CODE
The real-mode code requires a stack/heap to be set up, as well as
memory allocated for the kernel command line. This needs to be done
in the real-mode accessible memory in bottom megabyte.
It should be noted that modern machines often have a sizable Extended
BIOS Data Area (EBDA). As a result, it is advisable to use as little
of the low megabyte as possible.
Unfortunately, under the following circumstances the 0x90000 memory
segment has to be used:
- When loading a zImage kernel ((loadflags & 0x01) == 0).
- When loading a 2.01 or earlier boot protocol kernel.
-> For the 2.00 and 2.01 boot protocols, the real-mode code
can be loaded at another address, but it is internally
relocated to 0x90000. For the "old" protocol, the
real-mode code must be loaded at 0x90000.
When loading at 0x90000, avoid using memory above 0x9a000.
For boot protocol 2.02 or higher, the command line does not have to be
located in the same 64K segment as the real-mode setup code; it is
thus permitted to give the stack/heap the full 64K segment and locate
the command line above it.
The kernel command line should not be located below the real-mode
code, nor should it be located in high memory.
**** SAMPLE BOOT CONFIGURATION
As a sample configuration, assume the following layout of the real
mode segment:
When loading below 0x90000, use the entire segment:
0x0000-0x7fff Real mode kernel
0x8000-0xdfff Stack and heap
0xe000-0xffff Kernel command line
When loading at 0x90000 OR the protocol version is 2.01 or earlier:
0x0000-0x7fff Real mode kernel
0x8000-0x97ff Stack and heap
0x9800-0x9fff Kernel command line
Such a boot loader should enter the following fields in the header:
unsigned long base_ptr; /* base address for real-mode segment */
if ( setup_sects == 0 ) {
setup_sects = 4;
}
if ( protocol >= 0x0200 ) {
type_of_loader = <type code>;
if ( loading_initrd ) {
ramdisk_image = <initrd_address>;
ramdisk_size = <initrd_size>;
}
if ( protocol >= 0x0202 && loadflags & 0x01 )
heap_end = 0xe000;
else
heap_end = 0x9800;
if ( protocol >= 0x0201 ) {
heap_end_ptr = heap_end - 0x200;
loadflags |= 0x80; /* CAN_USE_HEAP */
}
if ( protocol >= 0x0202 ) {
cmd_line_ptr = base_ptr + heap_end;
strcpy(cmd_line_ptr, cmdline);
} else {
cmd_line_magic = 0xA33F;
cmd_line_offset = heap_end;
setup_move_size = heap_end + strlen(cmdline)+1;
strcpy(base_ptr+cmd_line_offset, cmdline);
}
} else {
/* Very old kernel */
heap_end = 0x9800;
cmd_line_magic = 0xA33F;
cmd_line_offset = heap_end;
/* A very old kernel MUST have its real-mode code
loaded at 0x90000 */
if ( base_ptr != 0x90000 ) {
/* Copy the real-mode kernel */
memcpy(0x90000, base_ptr, (setup_sects+1)*512);
base_ptr = 0x90000; /* Relocated */
}
strcpy(0x90000+cmd_line_offset, cmdline);
/* It is recommended to clear memory up to the 32K mark */
memset(0x90000 + (setup_sects+1)*512, 0,
(64-(setup_sects+1))*512);
}
**** LOADING THE REST OF THE KERNEL
The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
in the kernel file (again, if setup_sects == 0 the real value is 4.)
It should be loaded at address 0x10000 for Image/zImage kernels and
0x100000 for bzImage kernels.
The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
bit (LOAD_HIGH) in the loadflags field is set:
is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
load_address = is_bzImage ? 0x100000 : 0x10000;
Note that Image/zImage kernels can be up to 512K in size, and thus use
the entire 0x10000-0x90000 range of memory. This means it is pretty
much a requirement for these kernels to load the real-mode part at
0x90000. bzImage kernels allow much more flexibility.
**** SPECIAL COMMAND LINE OPTIONS
If the command line provided by the boot loader is entered by the
user, the user may expect the following command line options to work.
They should normally not be deleted from the kernel command line even
though not all of them are actually meaningful to the kernel. Boot
loader authors who need additional command line options for the boot
loader itself should get them registered in
Documentation/kernel-parameters.txt to make sure they will not
conflict with actual kernel options now or in the future.
vga=<mode>
<mode> here is either an integer (in C notation, either
decimal, octal, or hexadecimal) or one of the strings
"normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
(meaning 0xFFFD). This value should be entered into the
vid_mode field, as it is used by the kernel before the command
line is parsed.
mem=<size>
<size> is an integer in C notation optionally followed by
(case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
<< 30, << 40, << 50 or << 60). This specifies the end of
memory to the kernel. This affects the possible placement of
an initrd, since an initrd should be placed near end of
memory. Note that this is an option to *both* the kernel and
the bootloader!
initrd=<file>
An initrd should be loaded. The meaning of <file> is
obviously bootloader-dependent, and some boot loaders
(e.g. LILO) do not have such a command.
In addition, some boot loaders add the following options to the
user-specified command line:
BOOT_IMAGE=<file>
The boot image which was loaded. Again, the meaning of <file>
is obviously bootloader-dependent.
auto
The kernel was booted without explicit user intervention.
If these options are added by the boot loader, it is highly
recommended that they are located *first*, before the user-specified
or configuration-specified command line. Otherwise, "init=/bin/sh"
gets confused by the "auto" option.
**** RUNNING THE KERNEL
The kernel is started by jumping to the kernel entry point, which is
located at *segment* offset 0x20 from the start of the real mode
kernel. This means that if you loaded your real-mode kernel code at
0x90000, the kernel entry point is 9020:0000.
At entry, ds = es = ss should point to the start of the real-mode
kernel code (0x9000 if the code is loaded at 0x90000), sp should be
set up properly, normally pointing to the top of the heap, and
interrupts should be disabled. Furthermore, to guard against bugs in
the kernel, it is recommended that the boot loader sets fs = gs = ds =
es = ss.
In our example from above, we would do:
/* Note: in the case of the "old" kernel protocol, base_ptr must
be == 0x90000 at this point; see the previous sample code */
seg = base_ptr >> 4;
cli(); /* Enter with interrupts disabled! */
/* Set up the real-mode kernel stack */
_SS = seg;
_SP = heap_end;
_DS = _ES = _FS = _GS = seg;
jmp_far(seg+0x20, 0); /* Run the kernel */
If your boot sector accesses a floppy drive, it is recommended to
switch off the floppy motor before running the kernel, since the
kernel boot leaves interrupts off and thus the motor will not be
switched off, especially if the loaded kernel has the floppy driver as
a demand-loaded module!
**** ADVANCED BOOT LOADER HOOKS
If the boot loader runs in a particularly hostile environment (such as
LOADLIN, which runs under DOS) it may be impossible to follow the
standard memory location requirements. Such a boot loader may use the
following hooks that, if set, are invoked by the kernel at the
appropriate time. The use of these hooks should probably be
considered an absolutely last resort!
IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
%edi across invocation.
realmode_swtch:
A 16-bit real mode far subroutine invoked immediately before
entering protected mode. The default routine disables NMI, so
your routine should probably do so, too.
code32_start:
A 32-bit flat-mode routine *jumped* to immediately after the
transition to protected mode, but before the kernel is
uncompressed. No segments, except CS, are guaranteed to be
set up (current kernels do, but older ones do not); you should
set them up to BOOT_DS (0x18) yourself.
After completing your hook, you should jump to the address
that was in this field before your boot loader overwrote it
(relocated, if appropriate.)
