Copyright © 1995, 96 Bryan Ford <baford@cs.utah.edu> Copyright © 1995, 96 Erich Stefan Boleyn <erich@uruk.org> Copyright © 1999, 2000 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.

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Multiboot Specification
***********************

This file documents Multiboot Specification, the proposal for the boot sequence standard. This edition documents version 0.6.93.

Introduction to Multiboot Specification
***************************************

This chapter describes some rough information on the Multiboot Specification. Note that this is not a part of the specification itself.

The background of Multiboot Specification
=========================================

Every operating system ever created tends to have its own boot loader. Installing a new operating system on a machine generally involves installing a whole new set of boot mechanisms, each with completely different install-time and boot-time user interfaces. Getting multiple operating systems to coexist reliably on one machine through typical chaining mechanisms can be a nightmare. There is little or no choice of boot loaders for a particular operating system --- if the one that comes with the operating system doesn't do exactly what you want, or doesn't work on your machine, you're screwed.

While we may not be able to fix this problem in existing commercial operating systems, it shouldn't be too difficult for a few people in the free operating system communities to put their heads together and solve this problem for the popular free operating systems. That's what this specification aims for. Basically, it specifies an interface between a boot loader and a operating system, such that any complying boot loader should be able to load any complying operating system. This specification does not specify how boot loaders should work --- only how they must interface with the operating system being loaded.

The target architecture
=======================

This specification is primarily targeted at PC, since they are the most common and have the largest variety of operating systems and boot loaders. However, to the extent that certain other architectures may need a boot specification and do not have one already, a variation of this specification, stripped of the x86-specific details, could be adopted for them as well.

The target operating systems
============================

This specification is targeted toward free 32-bit operating systems that can be fairly easily modified to support the specification without going through lots of bureaucratic rigmarole. The particular free operating systems that this specification is being primarily designed for are Linux, FreeBSD, NetBSD, Mach, and VSTa. It is hoped that other emerging free operating systems will adopt it from the start, and thus immediately be able to take advantage of existing boot loaders. It would be nice if commercial operating system vendors eventually adopted this specification as well, but that's probably a pipe dream.

Boot sources
============

It should be possible to write compliant boot loaders that load the OS image from a variety of sources, including floppy disk, hard disk, and across a network.

Disk-based boot loaders may use a variety of techniques to find the relevant OS image and boot module data on disk, such as by interpretation of specific file systems (e.g. the BSD/Mach boot loader), using precalculated block lists (e.g. LILO), loading from a special boot partition (e.g. OS/2), or even loading from within another operating system (e.g. the VSTa boot code, which loads from DOS). Similarly, network-based boot loaders could use a variety of network hardware and protocols.

It is hoped that boot loaders will be created that support multiple loading mechanisms, increasing their portability, robustness, and user-friendliness.

Configure an operating system at boot-time
==========================================

It is often necessary for one reason or another for the user to be able to provide some configuration information to an operating system dynamically at boot time. While this specification should not dictate how this configuration information is obtained by the boot loader, it should provide a standard means for the boot loader to pass such information to the operating system.

How to make OS development easier
=================================

OS images should be easy to generate. Ideally, an OS image should simply be an ordinary 32-bit executable file in whatever file format the operating system normally uses. It should be possible to nm or disassemble OS images just like normal executables. Specialized tools should not be required to create OS images in a special file format. If this means shifting some work from the operating system to a boot loader, that is probably appropriate, because all the memory consumed by the boot loader will typically be made available again after the boot process is created, whereas every bit of code in the OS image typically has to remain in memory forever. The operating system should not have to worry about getting into 32-bit mode initially, because mode switching code generally needs to be in the boot loader anyway in order to load operating system data above the 1MB boundary, and forcing the operating system to do this makes creation of OS images much more difficult.

Unfortunately, there is a horrendous variety of executable file formats even among free Unix-like PC-based operating systems --- generally a different format for each operating system. Most of the relevant free operating systems use some variant of a.out format, but some are moving to ELF. It is highly desirable for boot loaders not to have to be able to interpret all the different types of executable file formats in existence in order to load the OS image --- otherwise the boot loader effectively becomes operating system specific again.

This specification adopts a compromise solution to this problem. Multiboot-compliant OS images always contain a magic Multiboot header (see OS image format), which allows the boot loader to load the image without having to understand numerous a.out variants or other executable formats. This magic header does not need to be at the very beginning of the executable file, so kernel images can still conform to the local a.out format variant in addition to being Multiboot-compliant.

Boot modules
============

Many modern operating system kernels, such as those of VSTa and Mach, do not by themselves contain enough mechanism to get the system fully operational: they require the presence of additional software modules at boot time in order to access devices, mount file systems, etc. While these additional modules could be embedded in the main OS image along with the kernel itself, and the resulting image be split apart manually by the operating system when it receives control, it is often more flexible, more space-efficient, and more convenient to the operating system and user if the boot loader can load these additional modules independently in the first place.

Thus, this specification should provide a standard method for a boot loader to indicate to the operating system what auxiliary boot modules were loaded, and where they can be found. Boot loaders don't have to support multiple boot modules, but they are strongly encouraged to, because some operating systems will be unable to boot without them.

The definitions of terms used through the specification
*******************************************************

must
     We use the term must, when any boot loader or OS image needs to follow a rule --- otherwise, the boot loader or OS image is not Multiboot-compliant.

should
     We use the term should, when any boot loader or OS image is recommended to follow a rule, but it doesn't need to follow the rule.

may
     We use the term may, when any boot loader or OS image is allowed to follow a rule.

boot loader
     Whatever program or set of programs loads the image of the final operating system to be run on the machine. The boot loader may itself consist of several stages, but that is an implementation detail not relevant to this specification. Only the final stage of the boot loader --- the stage that eventually transfers control to an operating system --- must follow the rules specified in this document in order to be Multiboot-compliant; earlier boot loader stages may be designed in whatever way is most convenient.

OS image
     The initial binary image that a boot loader loads into memory and transfers control to start an operating system. The OS image is typically an executable containing the operating system kernel.

boot module
     Other auxiliary files that a boot loader loads into memory along with an OS image, but does not interpret in any way other than passing their locations to the operating system when it is invoked.

Multiboot-compliant
     A boot loader or an OS image which follows the rules defined as must is Multiboot-compliant. When this specification specifies a rule as should or may, a Multiboot-complaint boot loader/OS image doesn't need to follow the rule.

u8
     The type of unsigned 8-bit data.

u16
     The type of unsigned 16-bit data. Because the target architecture is little-endian, WORD is coded in little-endian.

u32
     The type of unsigned 32-bit data. Because the target architecture is little-endian, DWORD is coded in little-endian.

u64
     The type of unsigned 64-bit data. Because the target architecture is little-endian, QWORD is coded in little-endian.

The exact definitions of Multiboot Specification
************************************************

There are three main aspects of a boot loader/OS image interface:

 1. The format of an OS image as seen by a boot loader.

 2. The state of a machine when a boot loader starts an operating system.

 3. The format of information passed by a boot loader to an operating system.

OS image format
===============

An OS image may be an ordinary 32-bit executable file in the standard format for that particular operating system, except that it may be linked at a non-default load address to avoid loading on top of the PC's I/O region or other reserved areas, and of course it should not use shared libraries or other fancy features.

An OS image must contain an additional header called Multiboot header, besides the headers of the format used by the OS image. The Multiboot header must be contained completely within the first 8192 bytes of the OS image, and must be longword (32-bit) aligned. In general, it should come as early as possible, and may be embedded in the beginning of the text segment after the real executable header.

The layout of Multiboot header
------------------------------

The layout of the Multiboot header must be as follows:

Offset   Type    Field Name      Note
0        u32   magic           required
4        u32   flags           required
8        u32   checksum        required
12       u32   header_addr     if flags[16] is set
16       u32   load_addr       if flags[16] is set
20       u32   load_end_addr   if flags[16] is set
24       u32   bss_end_addr    if flags[16] is set
28       u32   entry_addr      if flags[16] is set
32       u32   mode_type       if flags[2] is set
36       u32   width           if flags[2] is set
40       u32   height          if flags[2] is set
44       u32   depth           if flags[2] is set

The fields magic, flags and checksum are defined in Header magic fields, the fields header_addr, load_addr, load_end_addr, bss_end_addr and entry_addr are defined in Header address fields, and the fields mode_type, width, height and depth are defind in Header graphics fields.

The magic fields of Multiboot header
------------------------------------

magic
     The field magic is the magic number identifying the header, which must be the hexadecimal value 0x1BADB002.

flags
     The field flags specifies features that the OS image requests or requires of an boot loader. Bits 0-15 indicate requirements; if the boot loader sees any of these bits set but doesn't understand the flag or can't fulfill the requirements it indicates for some reason, it must notify the user and fail to load the OS image. Bits 16-31 indicate optional features; if any bits in this range are set but the boot loader doesn't understand them, it may simply ignore them and proceed as usual. Naturally, all as-yet-undefined bits in the flags word must be set to zero in OS images. This way, the flags fields serves for version control as well as simple feature selection.

     If bit 0 in the flags word is set, then all boot modules loaded along with the operating system must be aligned on page (4KB) boundaries. Some operating systems expect to be able to map the pages containing boot modules directly into a paged address space during startup, and thus need the boot modules to be page-aligned.

     If bit 1 in the flags word is set, then information on available memory via at least the mem_* fields of the Multiboot information structure (see Boot information format) must be included. If the boot loader is capable of passing a memory map (the mmap_* fields) and one exists, then it may be included as well.

     If bit 2 in the flags word is set, information about the video mode table (see Boot information format) must be available to the kernel.

     If bit 16 in the flags word is set, then the fields at offsets 8-24 in the Multiboot header are valid, and the boot loader should use them instead of the fields in the actual executable header to calculate where to load the OS image. This information does not need to be provided if the kernel image is in ELF format, but it must be provided if the images is in a.out format or in some other format. Compliant boot loaders must be able to load images that either are in ELF format or contain the load address information embedded in the Multiboot header; they may also directly support other executable formats, such as particular a.out variants, but are not required to.

checksum
     The field checksum is a 32-bit unsigned value which, when added to the other magic fields (i.e. magic and flags), must have a 32-bit unsigned sum of zero.

The address fields of Multiboot header
--------------------------------------

All of the address fields enabled by flag bit 16 are physical addresses. The meaning of each is as follows:

header_addr
     Contains the address corresponding to the beginning of the Multiboot header --- the physical memory location at which the magic value is supposed to be loaded. This field serves to synchronize the mapping between OS image offsets and physical memory addresses.

load_addr
     Contains the physical address of the beginning of the text segment. The offset in the OS image file at which to start loading is defined by the offset at which the header was found, minus (header_addr - load_addr). load_addr must be less than or equal to header_addr.

load_end_addr
     Contains the physical address of the end of the data segment. (load_end_addr - load_addr) specifies how much data to load. This implies that the text and data segments must be consecutive in the OS image; this is true for existing a.out executable formats.

bss_end_addr
     Contains the physical address of the end of the bss segment. The boot loader initializes this area to zero, and reserves the memory it occupies to avoid placing boot modules and other data relevant to the operating system in that area.

entry_addr
     The physical address to which the boot loader should jump in order to start running the operating system.

The graphics fields of Multiboot header
---------------------------------------

Also, information about a preferred default mode can be specified in the mode_type, width, height and depth fields. This is only a recommended mode by the kernel. If the mode exists, the boot loader should set it if the user hasn't specified a mode. If not, it should fall back to a similar mode, if available.

Valid numbers for mode_type is 0 for linear graphics mode and 1 for EGA-standard text mode. Everything else is reserved for future expansion. Please note that even if you set this field to indicate that you want a graphics mode, you might get a text mode.

width and height is specified in pixels, if graphics mode, or characters in EGA text mode. depth is given in bits per pixel for graphics, or zero for EGA text modes.

Any, some or all of those three fields may be set to zero, indicating to the boot loader that no preference is given. This makes it possible for a kernel to just say it wants a 32-bit resolution, for example.

Machine state
=============

FIXME: Use a table like the chapter ``Header layout''.

When the boot loader invokes the 32-bit operating system, the machine must have the following state:

  * CS must be a 32-bit read/execute code segment with an offset of 0 and a limit of 0xFFFFFFFF.

  * DS, ES, FS, GS, and SS must be a 32-bit read/write data segment with an offset of 0 and a limit of 0xFFFFFFFF.

  * The address 20 line must be usable for standard linear 32-bit addressing of memory (in standard PC hardware, it is wired to 0 at bootup, forcing addresses in the 1-2 MB range to be mapped to the 0-1 MB range, 3-4 is mapped to 2-3, etc.).

  * Paging must be turned off.

  * The processor interrupt flag must be turned off.

  * EAX must contain the magic value 0x2BADB002; the presence of this value indicates to the operating system that it was loaded by a Multiboot-compliant boot loader (e.g. as opposed to another type of boot loader that the operating system can also be loaded from).

  * EBX must contain the 32-bit physical address of the Multiboot information structure provided by the boot loader (see Boot information format).

All other processor registers and flag bits are undefined. This includes, in particular:

  * ESP: the 32-bit operating system must create its own stack as soon as it needs one.

  * GDTR: Even though the segment registers are set up as described above, the GDTR may be invalid, so the operating system must not load any segment registers (even just reloading the same values!) until it sets up its own GDT.

  * IDTR: The operating system must leave interrupts disabled until it sets up its own IDT.

However, other machine state should be left by the boot loader in normal working order, i.e. as initialized by the BIOS (or DOS, if that's what the boot loader runs from). In other words, the operating system should be able to make BIOS calls and such after being loaded, as long as it does not overwrite the BIOS data structures before doing so. Also, the boot loader must leave the PIC programmed with the normal BIOS/DOS values, even if it changed them during the switch to 32-bit mode.

Boot information format
=======================

FIXME: Split this chapter like the chapter ``OS image format''.

Upon entry to the operating system, the EBX register contains the physical address of a Multiboot information data structure, through which the boot loader communicates vital information to the operating system. The operating system can use or ignore any parts of the structure as it chooses; all information passed by the boot loader is advisory only.

The Multiboot information structure and its related substructures may be placed anywhere in memory by the boot loader (with the exception of the memory reserved for the kernel and boot modules, of course). It is the operating system's responsibility to avoid overwriting this memory until it is done using it.

The format of the Multiboot information structure (as defined so far) follows:

             +-------------------+
     0       | flags             |    (required)
             +-------------------+
     4       | mem_lower         |    (present if flags[0] is set)
     8       | mem_upper         |    (present if flags[0] is set)
             +-------------------+
     12      | boot_device       |    (present if flags[1] is set)
             +-------------------+
     16      | cmdline           |    (present if flags[2] is set)
             +-------------------+
     20      | mods_count        |    (present if flags[3] is set)
     24      | mods_addr         |    (present if flags[3] is set)
             +-------------------+
     28 - 40 | syms              |    (present if flags[4] or
             |                   |                flags[5] is set)
             +-------------------+
     44      | mmap_length       |    (present if flags[6] is set)
     48      | mmap_addr         |    (present if flags[6] is set)
             +-------------------+
     52      | drives_length     |    (present if flags[7] is set)
     56      | drives_addr       |    (present if flags[7] is set)
             +-------------------+
     60      | config_table      |    (present if flags[8] is set)
             +-------------------+
     64      | boot_loader_name  |    (present if flags[9] is set)
             +-------------------+
     68      | apm_table         |    (present if flags[10] is set)
             +-------------------+
     72      | vbe_control_info  |    (present if flags[11] is set)
     76      | vbe_mode_info     |
     80      | vbe_mode          |
     82      | vbe_interface_seg |
     84      | vbe_interface_off |
     86      | vbe_interface_len |
             +-------------------+

The first longword indicates the presence and validity of other fields in the Multiboot information structure. All as-yet-undefined bits must be set to zero by the boot loader. Any set bits that the operating system does not understand should be ignored. Thus, the flags field also functions as a version indicator, allowing the Multiboot information structure to be expanded in the future without breaking anything.

If bit 0 in the flags word is set, then the mem_* fields are valid. mem_lower and mem_upper indicate the amount of lower and upper memory, respectively, in kilobytes. Lower memory starts at address 0, and upper memory starts at address 1 megabyte. The maximum possible value for lower memory is 640 kilobytes. The value returned for upper memory is maximally the address of the first upper memory hole minus 1 megabyte. It is not guaranteed to be this value.

If bit 1 in the flags word is set, then the boot_device field is valid, and indicates which BIOS disk device the boot loader loaded the OS image from. If the OS image was not loaded from a BIOS disk, then this field must not be present (bit 3 must be clear). The operating system may use this field as a hint for determining its own root device, but is not required to. The boot_device field is laid out in four one-byte subfields as follows:

     +-------+-------+-------+-------+
     | drive | part1 | part2 | part3 |
     +-------+-------+-------+-------+

The first byte contains the BIOS drive number as understood by the BIOS INT 0x13 low-level disk interface: e.g. 0x00 for the first floppy disk or 0x80 for the first hard disk.

The three remaining bytes specify the boot partition. part1 specifies the top-level partition number, part2 specifies a sub-partition in the top-level partition, etc. Partition numbers always start from zero. Unused partition bytes must be set to 0xFF. For example, if the disk is partitioned using a simple one-level DOS partitioning scheme, then part1 contains the DOS partition number, and part2 and part3 are both 0xFF. As another example, if a disk is partitioned first into DOS partitions, and then one of those DOS partitions is subdivided into several BSD partitions using BSD's disklabel strategy, then part1 contains the DOS partition number, part2 contains the BSD sub-partition within that DOS partition, and part3 is 0xFF.

DOS extended partitions are indicated as partition numbers starting from 4 and increasing, rather than as nested sub-partitions, even though the underlying disk layout of extended partitions is hierarchical in nature. For example, if the boot loader boots from the second extended partition on a disk partitioned in conventional DOS style, then part1 will be 5, and part2 and part3 will both be 0xFF.

If bit 2 of the flags longword is set, the cmdline field is valid, and contains the physical address of the command line to be passed to the kernel. The command line is a normal C-style zero-terminated string.

If bit 3 of the flags is set, then the mods fields indicate to the kernel what boot modules were loaded along with the kernel image, and where they can be found. mods_count contains the number of modules loaded; mods_addr contains the physical address of the first module structure. mods_count may be zero, indicating no boot modules were loaded, even if bit 1 of flags is set. Each module structure is formatted as follows:

             +-------------------+
     0       | mod_start         |
     4       | mod_end           |
             +-------------------+
     8       | string            |
             +-------------------+
     12      | reserved (0)      |
             +-------------------+

The first two fields contain the start and end addresses of the boot module itself. The string field provides an arbitrary string to be associated with that particular boot module; it is a zero-terminated ASCII string, just like the kernel command line. The string field may be 0 if there is no string associated with the module. Typically the string might be a command line (e.g. if the operating system treats boot modules as executable programs), or a pathname (e.g. if the operating system treats boot modules as files in a file system), but its exact use is specific to the operating system. The reserved field must be set to 0 by the boot loader and ignored by the operating system.

Caution: Bits 4 & 5 are mutually exclusive.

If bit 4 in the flags word is set, then the following fields in the Multiboot information structure starting at byte 28 are valid:

             +-------------------+
     28      | tabsize           |
     32      | strsize           |
     36      | addr              |
     40      | reserved (0)      |
             +-------------------+

These indicate where the symbol table from an a.out kernel image can be found. addr is the physical address of the size (4-byte unsigned long) of an array of a.out format nlist structures, followed immediately by the array itself, then the size (4-byte unsigned long) of a set of zero-terminated ASCII strings (plus sizeof(unsigned long) in this case), and finally the set of strings itself. tabsize is equal to its size parameter (found at the beginning of the symbol section), and strsize is equal to its size parameter (found at the beginning of the string section) of the following string table to which the symbol table refers. Note that tabsize may be 0, indicating no symbols, even if bit 4 in the flags word is set.

If bit 5 in the flags word is set, then the following fields in the Multiboot information structure starting at byte 28 are valid:

             +-------------------+
     28      | num               |
     32      | size              |
     36      | addr              |
     40      | shndx             |
             +-------------------+

These indicate where the section header table from an ELF kernel is, the size of each entry, number of entries, and the string table used as the index of names. They correspond to the shdr_* entries (shdr_num, etc.) in the Executable and Linkable Format (ELF) specification in the program header. All sections are loaded, and the physical address fields of the ELF section header then refer to where the sections are in memory (refer to the i386 ELF documentation for details as to how to read the section header(s)). Note that shdr_num may be 0, indicating no symbols, even if bit 5 in the flags word is set.

If bit 6 in the flags word is set, then the mmap_* fields are valid, and indicate the address and length of a buffer containing a memory map of the machine provided by the BIOS. mmap_addr is the address, and mmap_length is the total size of the buffer. The buffer consists of one or more of the following size/structure pairs (size is really used for skipping to the next pair):

             +-------------------+
     -4      | size              |
             +-------------------+
     0       | base_addr_low     |
     4       | base_addr_high    |
     8       | length_low        |
     12      | length_high       |
     16      | type              |
             +-------------------+

where size is the size of the associated structure in bytes, which can be greater than the minimum of 20 bytes. base_addr_low is the lower 32 bits of the starting address, and base_addr_high is the upper 32 bits, for a total of a 64-bit starting address. length_low is the lower 32 bits of the size of the memory region in bytes, and length_high is the upper 32 bits, for a total of a 64-bit length. type is the variety of address range represented, where a value of 1 indicates available RAM, and all other values currently indicated a reserved area.

The map provided is guaranteed to list all standard RAM that should be available for normal use.

If bit 7 in the flags is set, then the drives_* fields are valid, and indicate the address of the physical address of the first drive structure and the size of drive structures. drives_addr is the address, and drives_length is the total size of drive structures. Note that drives_length may be zero. Each drive structure is formatted as follows:

             +-------------------+
     0       | size              |
             +-------------------+
     4       | drive_number      |
             +-------------------+
     5       | drive_mode        |
             +-------------------+
     6       | drive_cylinders   |
     8       | drive_heads       |
     9       | drive_sectors     |
             +-------------------+
     10 - xx | drive_ports       |
             +-------------------+

The size field specifies the size of this structure. The size varies, depending on the number of ports. Note that the size may not be equal to (10 + 2 * the number of ports), because of an alignment.

The drive_number field contains the BIOS drive number. The drive_mode field represents the access mode used by the boot loader. Currently, the following modes are defined:

0
     CHS mode (traditional cylinder/head/sector addressing mode).

1
     LBA mode (Logical Block Addressing mode).

The three fields, drive_cylinders, drive_heads and drive_sectors, indicate the geometry of the drive detected by the BIOS. drive_cylinders contains the number of the cylinders. drive_heads contains the number of the heads. drive_sectors contains the number of the sectors per track.

The drive_ports field contains the array of the I/O ports used for the drive in the BIOS code. The array consists of zero or more unsigned two-bytes integers, and is terminated with zero. Note that the array may contain any number of I/O ports that are not related to the drive actually (such as DMA controller's ports).

If bit 8 in the flags is set, then the config_table field is valid, and indicates the address of the ROM configuration table returned by the GET CONFIGURATION BIOS call. If the BIOS call fails, then the size of the table must be zero.

If bit 9 in the flags is set, the boot_loader_name field is valid, and contains the physical address of the name of a boot loader booting the kernel. The name is a normal C-style zero-terminated string.

If bit 10 in the flags is set, the apm_table field is valid, and contains the physical address of an APM table defined as below:

             +----------------------+
     0       | version              |
     2       | cseg                 |
     4       | offset               |
     8       | cseg_16              |
     10      | dseg                 |
     12      | flags                |
     14      | cseg_len             |
     16      | cseg_16_len          |
     18      | dseg_len             |
             +----------------------+

The fields version, cseg, offset, cseg_16, dseg, flags, cseg_len, cseg_16_len, dseg_len indicate the version number, the protected mode 32-bit code segment, the offset of the entry point, the protected mode 16-bit code segment, the protected mode 16-bit data segment, the flags, the length of the protected mode 32-bit code segment, the length of the protected mode 16-bit code segment, and the length of the protected mode 16-bit data segment, respectively. Only the field offset is 4 bytes, and the others are 2 bytes. See Advanced Power Management (APM) BIOS Interface Specification (http://www.microsoft.com/hwdev/busbios/amp_12.htm), for more information.

If bit 11 in the flags is set, the graphics table is available. This must only be done if the kernel has indicated in the Multiboot Header that it accepts a graphics mode.

The fields vbe_control_info and vbe_mode_info contain the physical addresses of VBE control information returned by the VBE Function 00h and VBE mode information returned by the VBE Function 01h, respectively.

The field vbe_mode indicates current video mode in the format specified in VBE 3.0.

The rest fields vbe_interface_seg, vbe_interface_off, and vbe_interface_len contain the table of a protected mode interface defined in VBE 2.0+. If this information is not available, those fields contain zero. Note that VBE 3.0 defines another protected mode interface which is incompatible with the old one. If you want to use the new protected mode interface, you will have to find the table yourself.

The fields for the graphics table are designed for VBE, but Multiboot boot loaders may simulate VBE on non-VBE modes, as if they were VBE modes.

Examples
********

Caution: The following items are not part of the specification document, but are included for prospective operating system and boot loader writers.

Notes on PC
===========

In reference to bit 0 of the flags parameter in the Multiboot information structure, if the bootloader in question uses older BIOS interfaces, or the newest ones are not available (see description about bit 6), then a maximum of either 15 or 63 megabytes of memory may be reported. It is highly recommended that boot loaders perform a thorough memory probe.

In reference to bit 1 of the flags parameter in the Multiboot information structure, it is recognized that determination of which BIOS drive maps to which device driver in an operating system is non-trivial, at best. Many kludges have been made to various operating systems instead of solving this problem, most of them breaking under many conditions. To encourage the use of general-purpose solutions to this problem, there are 2 BIOS device mapping techniques (see BIOS device mapping techniques).

In reference to bit 6 of the flags parameter in the Multiboot information structure, it is important to note that the data structure used there (starting with BaseAddrLow) is the data returned by the INT 15h, AX=E820h --- Query System Address Map call. See Query System Address Map (The GRUB Manual), for more information. The interface here is meant to allow a boot loader to work unmodified with any reasonable extensions of the BIOS interface, passing along any extra data to be interpreted by the operating system as desired.

BIOS device mapping techniques
==============================

Both of these techniques should be usable from any PC operating system, and neither require any special support in the drivers themselves. This section will be flushed out into detailed explanations, particularly for the I/O restriction technique.

The general rule is that the data comparison technique is the quick and dirty solution. It works most of the time, but doesn't cover all the bases, and is relatively simple.

The I/O restriction technique is much more complex, but it has potential to solve the problem under all conditions, plus allow access of the remaining BIOS devices when not all of them have operating system drivers.

Data comparison technique
-------------------------

Before activating any of the device drivers, gather enough data from similar sectors on each of the disks such that each one can be uniquely identified.

After activating the device drivers, compare data from the drives using the operating system drivers. This should hopefully be sufficient to provide such a mapping.

Problems:

 1. The data on some BIOS devices might be identical (so the part reading the drives from the BIOS should have some mechanism to give up).

 2. There might be extra drives not accessible from the BIOS which are identical to some drive used by the BIOS (so it should be capable of giving up there as well).

I/O restriction technique
-------------------------

This first step may be unnecessary, but first create copy-on-write mappings for the device drivers writing into PC RAM. Keep the original copies for the clean BIOS virtual machine to be created later.

For each device driver brought online, determine which BIOS devices become inaccessible by:

 1. Create a clean BIOS virtual machine.

 2. Set the I/O permission map for the I/O area claimed by the device driver to no permissions (neither read nor write).

 3. Access each device.

 4. Record which devices succeed, and those which try to access the restricted I/O areas (hopefully, this will be an xor situation).

For each device driver, given how many of the BIOS devices were subsumed by it (there should be no gaps in this list), it should be easy to determine which devices on the controller these are.

In general, you have at most 2 disks from each controller given BIOS numbers, but they pretty much always count from the lowest logically numbered devices on the controller.

Example OS code
===============

In this distribution, the example Multiboot kernel kernel is included. The kernel just prints out the Multiboot information structure on the screen, so you can make use of the kernel to test a Multiboot-compliant boot loader and for reference to how to implement a Multiboot kernel. The source files can be found under the directory docs in the GRUB distribution.

The kernel kernel consists of only three files: boot.S, kernel.c and multiboot.h. The assembly source boot.S is written in GAS (see GNU assembler (The GNU assembler)), and contains the Multiboot information structure to comply with the specification. When a Multiboot-compliant boot loader loads and execute it, it initialize the stack pointer and EFLAGS, and then call the function cmain defined in kernel.c. If cmain returns to the callee, then it shows a message to inform the user of the halt state and stops forever until you push the reset key. The file kernel.c contains the function cmain, which checks if the magic number passed by the boot loader is valid and so on, and some functions to print messages on the screen. The file multiboot.h defines some macros, such as the magic number for the Multiboot header, the Multiboot header structure and the Multiboot information structure.

multiboot.h
-----------

This is the source code in the file multiboot.h:

          [file multiboot.h.texi not included]

boot.S
------

In the file boot.S:

          [file boot.S.texi not included]

kernel.c
--------

And, in the file kernel.c:

          [file kernel.c.texi not included]

Other Multiboot kernels
-----------------------

Other useful information should be available in Multiboot kernels, such as GNU Mach and Fiasco http://os.inf.tu-dresden.de/fiasco/. And, it is worth mentioning the OSKit http://www.cs.utah.edu/projects/flux/oskit/, which provides a library supporting the specification.

Example boot loader code
========================

The GNU GRUB (see GRUB (The GRUB manual)) project is a full Multiboot-compliant boot loader, supporting all required and optional features present in this specification. A public release has not been made, but the test release is available from:

ftp://alpha.gnu.org/gnu/hurd/src/

See the webpage http://www.gnu.org/software/grub/grub.html, for more information.

The change log of this specification
************************************

0.7

       * Multiboot Standard is renamed to Multiboot Specification.

       * Graphics fields are added to Multiboot header.

       * BIOS drive information, BIOS configuration table, the name of a boot loader, APM information, and graphics information are added to Multiboot information.

       * Rewritten in Texinfo format.

       * Rewritten, using more strict words.

       * The maintainer changes to the GNU GRUB maintainer team bug-grub@gnu.org, from Bryan Ford and Erich Stefan Boleyn.

0.6

       * A few wording changes.

       * Header checksum.

       * Clasification of machine state passed to an operating system.

0.5

       * Name change.

0.4

       * Major changes plus HTMLification.