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vague, outdated or hard to read.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 <[email protected]>
diff --git a/Documentation/i386/boot.txt b/Documentation/i386/boot.txt
index d01b7a2..5150e76 100644
--- a/Documentation/i386/boot.txt
+++ b/Documentation/i386/boot.txt
@@ -2,7 +2,7 @@
----------------------------
H. Peter Anvin <[email protected]>
- Last update 2007-05-07
+ Last update 2007-05-16
On the i386 platform, the Linux kernel uses a rather complicated boot
convention. This has evolved partially due to historical aspects, as
@@ -52,7 +52,8 @@ zImage kernels, typically looks like:
0A0000 +------------------------+
| Reserved for BIOS | Do not use. Reserved for BIOS EBDA.
09A000 +------------------------+
- | Stack/heap/cmdline | For use by the kernel real-mode code.
+ | Command line |
+ | Stack/heap | For use by the kernel real-mode code.
098000 +------------------------+
| Kernel setup | The kernel real-mode code.
090200 +------------------------+
@@ -73,10 +74,9 @@ zImage kernels, typically looks like:
When using bzImage, the protected-mode kernel was relocated to
0x100000 ("high memory"), and the kernel real-mode block (boot sector,
setup, and stack/heap) was made relocatable to any address between
-0x10000 and end of low memory. Unfortunately, in protocols 2.00 and
-2.01 the command line is still required to live in the 0x9XXXX memory
-range, and that memory range is still overwritten by the early kernel.
-The 2.02 protocol resolves that problem.
+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
@@ -93,6 +93,35 @@ 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
@@ -160,29 +189,136 @@ e.g. protocol version 2.01 will contain 0x0201 in this field. When
setting fields in the header, you must make sure only to set fields
supported by the protocol version in use.
-The "kernel_version" field, if set to a nonzero value, contains a
-pointer to a null-terminated human-readable kernel version number
-string, less 0x200. This can be used to display the kernel version to
-the user. This value should be less than (0x200*setup_sects). For
-example, if this value is set to 0x1c00, the kernel version number
-string can be found at offset 0x1e00 in the kernel file. This is a
-valid value if and only if the "setup_sects" field contains the value
-14 or higher.
-
-Most boot loaders will simply load the kernel at its target address
-directly. Such boot loaders do not need to worry about filling in
-most of the fields in the header. The following fields should be
-filled out, however:
-
- vid_mode:
- Please see the section on SPECIAL COMMAND LINE OPTIONS.
-
- type_of_loader:
- If your boot loader has an assigned id (see table below), enter
- 0xTV here, where T is an identifier for the boot loader and V is
- a version number. Otherwise, enter 0xFF here.
-
- Assigned boot loader ids:
+
+**** 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.
+
+Field name: setup_secs
+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, e.g. 0x0204 for version 2.04.
+
+Field name: readmode_swtch
+Type: modify (optional)
+Offset/size: 0x208/4
+Protocol: 2.00+
+
+ Boot loader hook (see separate chapter.)
+
+Field name: start_sys
+Type: read
+Offset/size: 0x20c/4
+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 null-terminated
+ human-readable kernel version number string, less 0x200. This can
+ be used to display the kernel version to the user. This value
+ should be less than (0x200*setup_sects). For example, if this value
+ is set to 0x1c00, the kernel version number string can be found at
+ offset 0x1e00 in the kernel file. This is a valid value if and only
+ if the "setup_sects" field contains the value 14 or higher.
+
+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)
@@ -193,60 +329,145 @@ filled out, however:
8 U-BOOT
9 Xen
A Gujin
-
- Please contact <[email protected]> if you need a bootloader ID
- value assigned.
-
- loadflags, heap_end_ptr:
- If the protocol version is 2.01 or higher, enter the
- offset limit of the setup heap into heap_end_ptr and set the
- 0x80 bit (CAN_USE_HEAP) of loadflags. heap_end_ptr appears to
- be relative to the start of setup (offset 0x0200).
-
- setup_move_size:
- When using protocol 2.00 or 2.01, if the real mode
- kernel is not loaded at 0x90000, it gets moved there later in
- the loading sequence. Fill in this field if you want
- additional data (such as the kernel command line) moved in
- addition to the real-mode kernel itself.
-
- The unit is bytes starting with the beginning of the boot
- sector.
-
- ramdisk_image, ramdisk_size:
- If your boot loader has loaded an initial ramdisk (initrd),
- set ramdisk_image to the 32-bit pointer to the ramdisk data
- and the ramdisk_size to the size of the ramdisk data.
-
- The initrd should typically be located as high in memory as
- possible, as it may otherwise get overwritten by the early
- kernel initialization sequence. However, it must never be
- located above the address specified in the initrd_addr_max
- field. The initrd should be at least 4K page aligned.
-
- cmd_line_ptr:
- If the protocol version is 2.02 or higher, this is a 32-bit
- pointer to the kernel command line. The kernel command line
- can be located anywhere between the end of setup and 0xA0000.
- Fill in this field even if your boot loader does not support a
- command line, in which case you can point this to an empty
- string (or better yet, to the string "auto".) If this field
- is left at zero, the kernel will assume that your boot loader
- does not support the 2.02+ protocol.
-
- ramdisk_max:
- The maximum address that may be occupied by the initrd
- contents. For boot protocols 2.02 or earlier, this field is
- not present, and the maximum address is 0x37FFFFFF. (This
- address is defined as the address of the highest safe byte, so
- if your ramdisk is exactly 131072 bytes long and this field is
- 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
-
- cmdline_size:
- 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.
+ B Qemu
+
+ Please contact <[email protected]> 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 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 separate chapter.)
+
+ 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.
**** THE KERNEL COMMAND LINE
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