multiboot.texi

i386的bootloader源码grub

        +-------------------+
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 |
        +-------------------+
@end group
@end example

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 @samp{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 @samp{flags} word is set, then the @samp{mem_*} fields
are valid. @samp{mem_lower} and @samp{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 @samp{flags} word is set, then the @samp{boot_device}
field is valid, and indicates which @sc{bios} disk device the boot
loader loaded the OS image from. If the OS image was not loaded from a
@sc{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 @dfn{root} device, but is not required to. The
@samp{boot_device} field is laid out in four one-byte subfields as
follows:

@example
@group
+-------+-------+-------+-------+
| drive | part1 | part2 | part3 |
+-------+-------+-------+-------+
@end group
@end example

The first byte contains the @sc{bios} drive number as understood by the
@sc{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. @samp{part1}
specifies the @dfn{top-level} partition number, @samp{part2} specifies a
@dfn{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 @samp{part1} contains the DOS partition
number, and @samp{part2} and @samp{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 @dfn{disklabel} strategy, then @samp{part1} contains the DOS
partition number, @samp{part2} contains the BSD sub-partition within
that DOS partition, and @samp{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
@samp{part1} will be 5, and @samp{part2} and @samp{part3} will both be
0xFF.

If bit 2 of the @samp{flags} longword is set, the @samp{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 @samp{flags} is set, then the @samp{mods} fields
indicate to the kernel what boot modules were loaded along with the
kernel image, and where they can be found. @samp{mods_count} contains
the number of modules loaded; @samp{mods_addr} contains the physical
address of the first module structure. @samp{mods_count} may be zero,
indicating no boot modules were loaded, even if bit 1 of @samp{flags} is
set. Each module structure is formatted as follows:

@example
@group
        +-------------------+
0       | mod_start         |
4       | mod_end           |
        +-------------------+
8       | string            |
        +-------------------+
12      | reserved (0)      |
        +-------------------+
@end group
@end example

The first two fields contain the start and end addresses of the boot
module itself. The @samp{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 @samp{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 @samp{reserved} field must be
set to 0 by the boot loader and ignored by the operating system.

@strong{Caution:} Bits 4 & 5 are mutually exclusive.

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

@example
@group
        +-------------------+
28      | tabsize           |
32      | strsize           |
36      | addr              |
40      | reserved (0)      |
        +-------------------+
@end group
@end example

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

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

@example
@group
        +-------------------+
28      | num               |
32      | size              |
36      | addr              |
40      | shndx             |
        +-------------------+
@end group
@end example

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 @samp{shdr_*} entries
(@samp{shdr_num}, etc.) in the Executable and Linkable Format (@sc{elf})
specification in the program header. All sections are loaded, and the
physical address fields of the @sc{elf} section header then refer to where
the sections are in memory (refer to the i386 @sc{elf} documentation for
details as to how to read the section header(s)). Note that
@samp{shdr_num} may be 0, indicating no symbols, even if bit 5 in the
@samp{flags} word is set.

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

@example
@group
        +-------------------+
-4      | size              |
        +-------------------+
0       | base_addr_low     |
4       | base_addr_high    |
8       | length_low        |
12      | length_high       |
16      | type              |
        +-------------------+
@end group
@end example

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

The map provided is guaranteed to list all standard @sc{ram} that should
be available for normal use.

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

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

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

@table @samp
@item 0
CHS mode (traditional cylinder/head/sector addressing mode).

@item 1
LBA mode (Logical Block Addressing mode).
@end table

The three fields, @samp{drive_cylinders}, @samp{drive_heads} and
@samp{drive_sectors}, indicate the geometry of the drive detected by the
@sc{bios}. @samp{drive_cylinders} contains the number of the
cylinders. @samp{drive_heads} contains the number of the
heads. @samp{drive_sectors} contains the number of the sectors per
track.

The @samp{drive_ports} field contains the array of the I/O ports used
for the drive in the @sc{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 @sc{dma} controller's ports).

If bit 8 in the @samp{flags} is set, then the @samp{config_table} field
is valid, and indicates the address of the @sc{rom} configuration table
returned by the @dfn{GET CONFIGURATION} @sc{bios} call. If the @sc{bios}
call fails, then the size of the table must be @emph{zero}.

If bit 9 in the @samp{flags} is set, the @samp{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 @samp{flags} is set, the @samp{apm_table} field is
valid, and contains the physical address of an @sc{apm} table defined as
below:

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

The fields @samp{version}, @samp{cseg}, @samp{offset}, @samp{cseg_16},
@samp{dseg}, @samp{flags}, @samp{cseg_len}, @samp{cseg_16_len},
@samp{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 @samp{offset} is 4
bytes, and the others are 2 bytes. See
@uref{http://www.microsoft.com/hwdev/busbios/amp_12.htm, Advanced Power