2f6de3a199
Signed-off-by: Baodong Chen <[email]chenbdchenbd@gmail.com[email]> Acked-by: Jiri Kosina <jkosina@suse.cz> Signed-off-by: Ingo Molnar <mingo@elte.hu>
901 lines
30 KiB
Plaintext
901 lines
30 KiB
Plaintext
THE LINUX/x86 BOOT PROTOCOL
|
|
---------------------------
|
|
|
|
On the x86 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/x86 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.
|
|
|
|
Protocol 2.07: (Kernel 2.6.24) Added paravirtualised boot protocol.
|
|
Introduced hardware_subarch and hardware_subarch_data
|
|
and KEEP_SEGMENTS flag in load_flags.
|
|
|
|
Protocol 2.08: (Kernel 2.6.26) Added crc32 checksum and ELF format
|
|
payload. Introduced payload_offset and payload_length
|
|
fields to aid in locating the payload.
|
|
|
|
Protocol 2.09: (Kernel 2.6.26) Added a field of 64-bit physical
|
|
pointer to single linked list of struct setup_data.
|
|
|
|
**** 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
|
|
0248/4 2.08+ payload_offset Offset of kernel payload
|
|
024C/4 2.08+ payload_length Length of kernel payload
|
|
0250/8 2.09+ setup_data 64-bit physical pointer to linked list
|
|
of struct setup_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/2
|
|
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 5 (write): QUIET_FLAG
|
|
- If 0, print early messages.
|
|
- If 1, suppress early messages.
|
|
This requests to the kernel (decompressor and early
|
|
kernel) to not write early messages that require
|
|
accessing the display hardware directly.
|
|
|
|
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 (optional, defaults to x86/PC)
|
|
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 (subarch-dependent)
|
|
Offset/size: 0x240/8
|
|
Protocol: 2.07+
|
|
|
|
A pointer to data that is specific to hardware subarch
|
|
This field is currently unused for the default x86/PC environment,
|
|
do not modify.
|
|
|
|
Field name: payload_offset
|
|
Type: read
|
|
Offset/size: 0x248/4
|
|
Protocol: 2.08+
|
|
|
|
If non-zero then this field contains the offset from the beginning
|
|
of the protected-mode code to the payload.
|
|
|
|
The payload may be compressed. The format of both the compressed and
|
|
uncompressed data should be determined using the standard magic
|
|
numbers. Currently only gzip compressed ELF is used.
|
|
|
|
Field name: payload_length
|
|
Type: read
|
|
Offset/size: 0x24c/4
|
|
Protocol: 2.08+
|
|
|
|
The length of the payload.
|
|
|
|
Field name: setup_data
|
|
Type: write (special)
|
|
Offset/size: 0x250/8
|
|
Protocol: 2.09+
|
|
|
|
The 64-bit physical pointer to NULL terminated single linked list of
|
|
struct setup_data. This is used to define a more extensible boot
|
|
parameters passing mechanism. The definition of struct setup_data is
|
|
as follow:
|
|
|
|
struct setup_data {
|
|
u64 next;
|
|
u32 type;
|
|
u32 len;
|
|
u8 data[0];
|
|
};
|
|
|
|
Where, the next is a 64-bit physical pointer to the next node of
|
|
linked list, the next field of the last node is 0; the type is used
|
|
to identify the contents of data; the len is the length of data
|
|
field; the data holds the real payload.
|
|
|
|
This list may be modified at a number of points during the bootup
|
|
process. Therefore, when modifying this list one should always make
|
|
sure to consider the case where the linked list already contains
|
|
entries.
|
|
|
|
|
|
**** THE IMAGE CHECKSUM
|
|
|
|
From boot protocol version 2.08 onwards the CRC-32 is calculated over
|
|
the entire file using the characteristic polynomial 0x04C11DB7 and an
|
|
initial remainder of 0xffffffff. The checksum is appended to the
|
|
file; therefore the CRC of the file up to the limit specified in the
|
|
syssize field of the header is always 0.
|
|
|
|
|
|
**** 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.)
|
|
|
|
|
|
**** 32-bit BOOT PROTOCOL
|
|
|
|
For machine with some new BIOS other than legacy BIOS, such as EFI,
|
|
LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
|
|
based on legacy BIOS can not be used, so a 32-bit boot protocol needs
|
|
to be defined.
|
|
|
|
In 32-bit boot protocol, the first step in loading a Linux kernel
|
|
should be to setup the boot parameters (struct boot_params,
|
|
traditionally known as "zero page"). The memory for struct boot_params
|
|
should be allocated and initialized to all zero. Then the setup header
|
|
from offset 0x01f1 of kernel image on should be loaded into struct
|
|
boot_params and examined. The end of setup header can be calculated as
|
|
follow:
|
|
|
|
0x0202 + byte value at offset 0x0201
|
|
|
|
In addition to read/modify/write the setup header of the struct
|
|
boot_params as that of 16-bit boot protocol, the boot loader should
|
|
also fill the additional fields of the struct boot_params as that
|
|
described in zero-page.txt.
|
|
|
|
After setupping the struct boot_params, the boot loader can load the
|
|
32/64-bit kernel in the same way as that of 16-bit boot protocol.
|
|
|
|
In 32-bit boot protocol, the kernel is started by jumping to the
|
|
32-bit kernel entry point, which is the start address of loaded
|
|
32/64-bit kernel.
|
|
|
|
At entry, the CPU must be in 32-bit protected mode with paging
|
|
disabled; a GDT must be loaded with the descriptors for selectors
|
|
__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
|
|
segment; __BOOS_CS must have execute/read permission, and __BOOT_DS
|
|
must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
|
|
must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
|
|
address of the struct boot_params; %ebp, %edi and %ebx must be zero.
|