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Further update of the i386 boot documentation
A number of items in the i386 boot documentation have been either vague, outdated or hard to read. This text revision adds several more examples, including a memory map for a modern kernel load. It also adds a field-by-field detailed description of the setup header, and a bootloader ID for Qemu. Signed-off-by: H. Peter Anvin <hpa@zytor.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
H. Peter Anvin
Linus Torvalds
parent
66123549db
commit
dec04cff50
@ -2,7 +2,7 @@
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----------------------------
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H. Peter Anvin <hpa@zytor.com>
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Last update 2007-05-07
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Last update 2007-05-16
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On the i386 platform, the Linux kernel uses a rather complicated boot
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convention. This has evolved partially due to historical aspects, as
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@ -52,7 +52,8 @@ zImage kernels, typically looks like:
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0A0000 +------------------------+
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| Reserved for BIOS | Do not use. Reserved for BIOS EBDA.
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09A000 +------------------------+
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| Stack/heap/cmdline | For use by the kernel real-mode code.
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| Command line |
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| Stack/heap | For use by the kernel real-mode code.
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098000 +------------------------+
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| Kernel setup | The kernel real-mode code.
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090200 +------------------------+
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@ -73,10 +74,9 @@ zImage kernels, typically looks like:
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When using bzImage, the protected-mode kernel was relocated to
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0x100000 ("high memory"), and the kernel real-mode block (boot sector,
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setup, and stack/heap) was made relocatable to any address between
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0x10000 and end of low memory. Unfortunately, in protocols 2.00 and
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2.01 the command line is still required to live in the 0x9XXXX memory
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range, and that memory range is still overwritten by the early kernel.
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The 2.02 protocol resolves that problem.
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0x10000 and end of low memory. Unfortunately, in protocols 2.00 and
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2.01 the 0x90000+ memory range is still used internally by the kernel;
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the 2.02 protocol resolves that problem.
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It is desirable to keep the "memory ceiling" -- the highest point in
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low memory touched by the boot loader -- as low as possible, since
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@ -93,6 +93,35 @@ zImage or old bzImage kernels, which need data written into the
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0x90000 segment, the boot loader should make sure not to use memory
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above the 0x9A000 point; too many BIOSes will break above that point.
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For a modern bzImage kernel with boot protocol version >= 2.02, a
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memory layout like the following is suggested:
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~ ~
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| Protected-mode kernel |
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100000 +------------------------+
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| I/O memory hole |
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0A0000 +------------------------+
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| Reserved for BIOS | Leave as much as possible unused
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~ ~
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| Command line | (Can also be below the X+10000 mark)
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X+10000 +------------------------+
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| Stack/heap | For use by the kernel real-mode code.
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X+08000 +------------------------+
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| Kernel setup | The kernel real-mode code.
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| Kernel boot sector | The kernel legacy boot sector.
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X +------------------------+
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| Boot loader | <- Boot sector entry point 0000:7C00
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001000 +------------------------+
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| Reserved for MBR/BIOS |
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000800 +------------------------+
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| Typically used by MBR |
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000600 +------------------------+
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| BIOS use only |
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000000 +------------------------+
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... where the address X is as low as the design of the boot loader
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permits.
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**** THE REAL-MODE KERNEL HEADER
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@ -160,29 +189,136 @@ e.g. protocol version 2.01 will contain 0x0201 in this field. When
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setting fields in the header, you must make sure only to set fields
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supported by the protocol version in use.
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The "kernel_version" field, if set to a nonzero value, contains a
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pointer to a null-terminated human-readable kernel version number
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string, less 0x200. This can be used to display the kernel version to
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the user. This value should be less than (0x200*setup_sects). For
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example, if this value is set to 0x1c00, the kernel version number
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string can be found at offset 0x1e00 in the kernel file. This is a
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valid value if and only if the "setup_sects" field contains the value
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14 or higher.
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Most boot loaders will simply load the kernel at its target address
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directly. Such boot loaders do not need to worry about filling in
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most of the fields in the header. The following fields should be
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filled out, however:
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**** DETAILS OF HEADER FIELDS
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vid_mode:
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Please see the section on SPECIAL COMMAND LINE OPTIONS.
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For each field, some are information from the kernel to the bootloader
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("read"), some are expected to be filled out by the bootloader
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("write"), and some are expected to be read and modified by the
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bootloader ("modify").
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type_of_loader:
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If your boot loader has an assigned id (see table below), enter
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0xTV here, where T is an identifier for the boot loader and V is
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a version number. Otherwise, enter 0xFF here.
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All general purpose boot loaders should write the fields marked
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(obligatory). Boot loaders who want to load the kernel at a
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nonstandard address should fill in the fields marked (reloc); other
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boot loaders can ignore those fields.
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Assigned boot loader ids:
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Field name: setup_secs
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Type: read
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Offset/size: 0x1f1/1
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Protocol: ALL
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The size of the setup code in 512-byte sectors. If this field is
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0, the real value is 4. The real-mode code consists of the boot
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sector (always one 512-byte sector) plus the setup code.
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Field name: root_flags
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Type: modify (optional)
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Offset/size: 0x1f2/2
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Protocol: ALL
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If this field is nonzero, the root defaults to readonly. The use of
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this field is deprecated; use the "ro" or "rw" options on the
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command line instead.
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Field name: syssize
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Type: read
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Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
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Protocol: 2.04+
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The size of the protected-mode code in units of 16-byte paragraphs.
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For protocol versions older than 2.04 this field is only two bytes
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wide, and therefore cannot be trusted for the size of a kernel if
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the LOAD_HIGH flag is set.
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Field name: ram_size
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Type: kernel internal
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Offset/size: 0x1f8/2
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Protocol: ALL
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This field is obsolete.
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Field name: vid_mode
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Type: modify (obligatory)
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Offset/size: 0x1fa/2
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Please see the section on SPECIAL COMMAND LINE OPTIONS.
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Field name: root_dev
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Type: modify (optional)
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Offset/size: 0x1fc/2
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Protocol: ALL
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The default root device device number. The use of this field is
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deprecated, use the "root=" option on the command line instead.
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Field name: boot_flag
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Type: read
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Offset/size: 0x1fe/2
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Protocol: ALL
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Contains 0xAA55. This is the closest thing old Linux kernels have
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to a magic number.
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Field name: jump
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Type: read
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Offset/size: 0x200/2
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Protocol: 2.00+
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Contains an x86 jump instruction, 0xEB followed by a signed offset
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relative to byte 0x202. This can be used to determine the size of
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the header.
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Field name: header
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Type: read
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Offset/size: 0x202/4
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Protocol: 2.00+
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Contains the magic number "HdrS" (0x53726448).
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Field name: version
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Type: read
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Offset/size: 0x206/2
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Protocol: 2.00+
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Contains the boot protocol version, e.g. 0x0204 for version 2.04.
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Field name: readmode_swtch
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Type: modify (optional)
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Offset/size: 0x208/4
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Protocol: 2.00+
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Boot loader hook (see separate chapter.)
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Field name: start_sys
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Type: read
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Offset/size: 0x20c/4
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Protocol: 2.00+
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The load low segment (0x1000). Obsolete.
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Field name: kernel_version
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Type: read
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Offset/size: 0x20e/2
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Protocol: 2.00+
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If set to a nonzero value, contains a pointer to a NUL-terminated
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human-readable kernel version number string, less 0x200. This can
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be used to display the kernel version to the user. This value
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should be less than (0x200*setup_sects). For example, if this value
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is set to 0x1c00, the kernel version number string can be found at
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offset 0x1e00 in the kernel file. This is a valid value if and only
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if the "setup_sects" field contains the value 14 or higher.
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Field name: type_of_loader
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Type: write (obligatory)
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Offset/size: 0x210/1
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Protocol: 2.00+
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If your boot loader has an assigned id (see table below), enter
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0xTV here, where T is an identifier for the boot loader and V is
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a version number. Otherwise, enter 0xFF here.
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Assigned boot loader ids:
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0 LILO (0x00 reserved for pre-2.00 bootloader)
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1 Loadlin
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2 bootsect-loader (0x20, all other values reserved)
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@ -193,60 +329,145 @@ filled out, however:
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8 U-BOOT
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9 Xen
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A Gujin
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B Qemu
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Please contact <hpa@zytor.com> if you need a bootloader ID
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value assigned.
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Please contact <hpa@zytor.com> if you need a bootloader ID
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value assigned.
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loadflags, heap_end_ptr:
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If the protocol version is 2.01 or higher, enter the
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offset limit of the setup heap into heap_end_ptr and set the
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0x80 bit (CAN_USE_HEAP) of loadflags. heap_end_ptr appears to
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be relative to the start of setup (offset 0x0200).
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Field name: loadflags
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Type: modify (obligatory)
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Offset/size: 0x211/1
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Protocol: 2.00+
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setup_move_size:
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When using protocol 2.00 or 2.01, if the real mode
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kernel is not loaded at 0x90000, it gets moved there later in
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the loading sequence. Fill in this field if you want
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additional data (such as the kernel command line) moved in
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addition to the real-mode kernel itself.
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This field is a bitmask.
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The unit is bytes starting with the beginning of the boot
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sector.
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Bit 0 (read): LOADED_HIGH
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- If 0, the protected-mode code is loaded at 0x10000.
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- If 1, the protected-mode code is loaded at 0x100000.
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ramdisk_image, ramdisk_size:
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If your boot loader has loaded an initial ramdisk (initrd),
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set ramdisk_image to the 32-bit pointer to the ramdisk data
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and the ramdisk_size to the size of the ramdisk data.
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Bit 7 (write): CAN_USE_HEAP
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Set this bit to 1 to indicate that the value entered in the
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heap_end_ptr is valid. If this field is clear, some setup code
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functionality will be disabled.
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The initrd should typically be located as high in memory as
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possible, as it may otherwise get overwritten by the early
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kernel initialization sequence. However, it must never be
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located above the address specified in the initrd_addr_max
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field. The initrd should be at least 4K page aligned.
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Field name: setup_move_size
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Type: modify (obligatory)
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Offset/size: 0x212/2
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Protocol: 2.00-2.01
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cmd_line_ptr:
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If the protocol version is 2.02 or higher, this is a 32-bit
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pointer to the kernel command line. The kernel command line
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can be located anywhere between the end of setup and 0xA0000.
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Fill in this field even if your boot loader does not support a
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command line, in which case you can point this to an empty
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string (or better yet, to the string "auto".) If this field
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is left at zero, the kernel will assume that your boot loader
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does not support the 2.02+ protocol.
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When using protocol 2.00 or 2.01, if the real mode kernel is not
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loaded at 0x90000, it gets moved there later in the loading
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sequence. Fill in this field if you want additional data (such as
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the kernel command line) moved in addition to the real-mode kernel
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itself.
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ramdisk_max:
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The maximum address that may be occupied by the initrd
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contents. For boot protocols 2.02 or earlier, this field is
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not present, and the maximum address is 0x37FFFFFF. (This
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address is defined as the address of the highest safe byte, so
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if your ramdisk is exactly 131072 bytes long and this field is
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0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
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The unit is bytes starting with the beginning of the boot sector.
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This field is can be ignored when the protocol is 2.02 or higher, or
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if the real-mode code is loaded at 0x90000.
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cmdline_size:
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The maximum size of the command line without the terminating
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zero. This means that the command line can contain at most
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cmdline_size characters. With protocol version 2.05 and
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earlier, the maximum size was 255.
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Field name: code32_start
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Type: modify (optional, reloc)
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Offset/size: 0x214/4
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Protocol: 2.00+
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The address to jump to in protected mode. This defaults to the load
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address of the kernel, and can be used by the boot loader to
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determine the proper load address.
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This field can be modified for two purposes:
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1. as a boot loader hook (see separate chapter.)
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2. if a bootloader which does not install a hook loads a
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relocatable kernel at a nonstandard address it will have to modify
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this field to point to the load address.
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Field name: ramdisk_image
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Type: write (obligatory)
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Offset/size: 0x218/4
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Protocol: 2.00+
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The 32-bit linear address of the initial ramdisk or ramfs. Leave at
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zero if there is no initial ramdisk/ramfs.
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Field name: ramdisk_size
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Type: write (obligatory)
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Offset/size: 0x21c/4
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Protocol: 2.00+
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Size of the initial ramdisk or ramfs. Leave at zero if there is no
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initial ramdisk/ramfs.
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Field name: bootsect_kludge
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Type: kernel internal
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Offset/size: 0x220/4
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Protocol: 2.00+
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This field is obsolete.
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Field name: heap_end_ptr
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Type: write (obligatory)
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Offset/size: 0x224/2
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Protocol: 2.01+
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Set this field to the offset (from the beginning of the real-mode
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code) of the end of the setup stack/heap, minus 0x0200.
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Field name: cmd_line_ptr
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Type: write (obligatory)
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Offset/size: 0x228/4
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Protocol: 2.02+
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Set this field to the linear address of the kernel command line.
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The kernel command line can be located anywhere between the end of
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the setup heap and 0xA0000; it does not have to be located in the
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same 64K segment as the real-mode code itself.
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Fill in this field even if your boot loader does not support a
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command line, in which case you can point this to an empty string
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(or better yet, to the string "auto".) If this field is left at
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zero, the kernel will assume that your boot loader does not support
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the 2.02+ protocol.
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Field name: initrd_addr_max
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Type: read
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Offset/size: 0x22c/4
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Protocol: 2.03+
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The maximum address that may be occupied by the initial
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ramdisk/ramfs contents. For boot protocols 2.02 or earlier, this
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field is not present, and the maximum address is 0x37FFFFFF. (This
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address is defined as the address of the highest safe byte, so if
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your ramdisk is exactly 131072 bytes long and this field is
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0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
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Field name: kernel_alignment
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Type: read (reloc)
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Offset/size: 0x230/4
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Protocol: 2.05+
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Alignment unit required by the kernel (if relocatable_kernel is true.)
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Field name: relocatable_kernel
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Type: read (reloc)
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Offset/size: 0x234/1
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Protocol: 2.05+
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If this field is nonzero, the protected-mode part of the kernel can
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be loaded at any address that satisfies the kernel_alignment field.
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After loading, the boot loader must set the code32_start field to
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point to the loaded code, or to a boot loader hook.
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Field name: cmdline_size
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Type: read
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Offset/size: 0x238/4
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Protocol: 2.06+
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The maximum size of the command line without the terminating
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zero. This means that the command line can contain at most
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cmdline_size characters. With protocol version 2.05 and earlier, the
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maximum size was 255.
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**** THE KERNEL COMMAND LINE
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Block a user