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                     THE LINUX/I386 BOOT PROTOCOL

                     ----------------------------

                    H. Peter Anvin 

                        Last update 2002-01-01

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, four 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.

**** 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  +------------------------+

        |  Stack/heap/cmdline    |      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 command line is still required to live in the 0x9XXXX memory

range, and that memory range is still overwritten by the early 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.

**** 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     setup_sects     The size of the setup in sectors

01F2/2  ALL     root_flags      If set, the root is mounted readonly

01F4/2  ALL     syssize         DO NOT USE - for bootsect.S use only

01F6/2  ALL     swap_dev        DO NOT USE - obsolete

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

For backwards compatibility, if the setup_sects field contains 0, the

real value is 4.

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.

The "kernel_version" field, if set to a nonzero value, contains a

pointer to a null-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

14 or higher.

Most boot loaders will simply load the kernel at its target address

directly.  Such boot loaders do not need to worry about filling in

most of the fields in the header.  The following fields should be

filled out, however:

  vid_mode:

        Please see the section on SPECIAL COMMAND LINE OPTIONS.

  type_of_loader:

        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:

        1  Loadlin

        2  bootsect-loader

        3  SYSLINUX

        4  EtherBoot

        Please contact  if you need a bootloader ID

        value assigned.

  loadflags, heap_end_ptr:

        If the protocol version is 2.01 or higher, enter the

        offset limit of the setup heap into heap_end_ptr and set the

        0x80 bit (CAN_USE_HEAP) of loadflags.  heap_end_ptr appears to

        be relative to the start of setup (offset 0x0200).

  setup_move_size:

        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.

  ramdisk_image, ramdisk_size:

        If your boot loader has loaded an initial ramdisk (initrd),

        set ramdisk_image to the 32-bit pointer to the ramdisk data

        and the ramdisk_size to the size of the ramdisk data.

        The initrd should typically be located as high in memory as

        possible, as it may otherwise get overwritten by the early

        kernel initialization sequence.  However, it must never be

        located above the address specified in the initrd_addr_max

        field.  The initrd should be at least 4K page aligned.

  cmd_line_ptr:

        If the protocol version is 2.02 or higher, this is a 32-bit

        pointer to the kernel command line.  The kernel command line

        can be located anywhere between the end of setup and 0xA0000.

        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.

  ramdisk_max:

        The maximum address that may be occupied by the initrd

        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.)

**** 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 up to 255

characters long, plus the final null.

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.)

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.

**** SAMPLE BOOT CONFIGURATION

As a sample configuration, assume the following layout of the real

mode segment:

        0x0000-0x7FFF   Real mode kernel

        0x8000-0x8FFF   Stack and heap

        0x9000-0x90FF   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 = ;

                if ( loading_initrd ) {

                        ramdisk_image = ;

                        ramdisk_size = ;

                }

                if ( protocol >= 0x0201 ) {

                        heap_end_ptr = 0x9000 - 0x200;

                        loadflags |= 0x80; /* CAN_USE_HEAP */

                }

                if ( protocol >= 0x0202 ) {

                        cmd_line_ptr = base_ptr + 0x9000;

                } else {

                        cmd_line_magic  = 0xA33F;

                        cmd_line_offset = 0x9000;

                        setup_move_size = 0x9100;

                }

        } else {

                /* Very old kernel */

                cmd_line_magic  = 0xA33F;

                cmd_line_offset = 0x9000;

                /* 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);

                        /* Copy the command line */

                        memcpy(0x99000, base_ptr+0x9000, 256);

                        base_ptr = 0x90000;              /* Relocated */

                }

                /* 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 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

linux/Documentation/kernel-parameters.txt to make sure they will not

conflict with actual kernel options now or in the future.

  vga=

         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=

         is an integer in C notation optionally followed by K, M

        or G (meaning << 10, << 20 or << 30).  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=

        An initrd should be loaded.  The meaning of  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=

        The boot image which was loaded.  Again, the meaning of 

        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 = 0x9000;   /* Load SP immediately after loading SS! */

        _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 TIME 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 set up; you should

        set them up to KERNEL_DS (0x18) yourself.

        After completing your hook, you should jump to the address

        that was in this field before your boot loader overwrote it.