path: root/Documentation/x86/boot.txt
diff options
Diffstat (limited to 'Documentation/x86/boot.txt')
1 files changed, 114 insertions, 8 deletions
diff --git a/Documentation/x86/boot.txt b/Documentation/x86/boot.txt
index e0203662f9e9..8da3a795083f 100644
--- a/Documentation/x86/boot.txt
+++ b/Documentation/x86/boot.txt
@@ -50,6 +50,10 @@ Protocol 2.08: (Kernel 2.6.26) Added crc32 checksum and ELF format
Protocol 2.09: (Kernel 2.6.26) Added a field of 64-bit physical
pointer to single linked list of struct setup_data.
+Protocol 2.10: (Kernel 2.6.31) Added a protocol for relaxed alignment
+ beyond the kernel_alignment added, new init_size and
+ pref_address fields. Added extended boot loader IDs.
The traditional memory map for the kernel loader, used for Image or
@@ -168,12 +172,13 @@ Offset Proto Name Meaning
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
+0226/1 2.02+(3 ext_loader_ver Extended boot loader version
+0227/1 2.02+(3 ext_loader_type Extended boot loader ID
0228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
022C/4 2.03+ ramdisk_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
-0235/1 N/A pad2 Unused
+0235/1 2.10+ min_alignment Minimum alignment, as a power of two
0236/2 N/A pad3 Unused
0238/4 2.06+ cmdline_size Maximum size of the kernel command line
023C/4 2.07+ hardware_subarch Hardware subarchitecture
@@ -182,6 +187,8 @@ Offset Proto Name Meaning
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
+0258/8 2.10+ pref_address Preferred loading address
+0260/4 2.10+ init_size Linear memory required during initialization
(1) For backwards compatibility, if the setup_sects field contains 0, the
real value is 4.
@@ -190,6 +197,8 @@ Offset Proto Name Meaning
field are unusable, which means the size of a bzImage kernel
cannot be determined.
+(3) Ignored, but safe to set, for boot protocols 2.02-2.09.
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:
@@ -343,18 +352,32 @@ Protocol: 2.00+
0xTV here, where T is an identifier for the boot loader and V is
a version number. Otherwise, enter 0xFF here.
+ For boot loader IDs above T = 0xD, write T = 0xE to this field and
+ write the extended ID minus 0x10 to the ext_loader_type field.
+ Similarly, the ext_loader_ver field can be used to provide more than
+ four bits for the bootloader version.
+ For example, for T = 0x15, V = 0x234, write:
+ type_of_loader <- 0xE4
+ ext_loader_type <- 0x05
+ ext_loader_ver <- 0x23
Assigned boot loader ids:
0 LILO (0x00 reserved for pre-2.00 bootloader)
1 Loadlin
2 bootsect-loader (0x20, all other values reserved)
- 4 EtherBoot
+ 3 Syslinux
+ 4 Etherboot/gPXE
- 8 U-BOOT
+ 8 U-Boot
9 Xen
A Gujin
B Qemu
+ C Arcturus Networks uCbootloader
+ E Extended (see ext_loader_type)
+ F Special (0xFF = undefined)
Please contact <hpa@zytor.com> if you need a bootloader ID
value assigned.
@@ -453,6 +476,35 @@ 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: ext_loader_ver
+Type: write (optional)
+Offset/size: 0x226/1
+Protocol: 2.02+
+ This field is used as an extension of the version number in the
+ type_of_loader field. The total version number is considered to be
+ (type_of_loader & 0x0f) + (ext_loader_ver << 4).
+ The use of this field is boot loader specific. If not written, it
+ is zero.
+ Kernels prior to 2.6.31 did not recognize this field, but it is safe
+ to write for protocol version 2.02 or higher.
+Field name: ext_loader_type
+Type: write (obligatory if (type_of_loader & 0xf0) == 0xe0)
+Offset/size: 0x227/1
+Protocol: 2.02+
+ This field is used as an extension of the type number in
+ type_of_loader field. If the type in type_of_loader is 0xE, then
+ the actual type is (ext_loader_type + 0x10).
+ This field is ignored if the type in type_of_loader is not 0xE.
+ Kernels prior to 2.6.31 did not recognize this field, but it is safe
+ to write for protocol version 2.02 or higher.
Field name: cmd_line_ptr
Type: write (obligatory)
Offset/size: 0x228/4
@@ -482,11 +534,19 @@ Protocol: 2.03+
0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
Field name: kernel_alignment
-Type: read (reloc)
+Type: read/modify (reloc)
Offset/size: 0x230/4
-Protocol: 2.05+
+Protocol: 2.05+ (read), 2.10+ (modify)
+ Alignment unit required by the kernel (if relocatable_kernel is
+ true.) A relocatable kernel that is loaded at an alignment
+ incompatible with the value in this field will be realigned during
+ kernel initialization.
- Alignment unit required by the kernel (if relocatable_kernel is true.)
+ Starting with protocol version 2.10, this reflects the kernel
+ alignment preferred for optimal performance; it is possible for the
+ loader to modify this field to permit a lesser alignment. See the
+ min_alignment and pref_address field below.
Field name: relocatable_kernel
Type: read (reloc)
@@ -498,6 +558,22 @@ Protocol: 2.05+
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: min_alignment
+Type: read (reloc)
+Offset/size: 0x235/1
+Protocol: 2.10+
+ This field, if nonzero, indicates as a power of two the minimum
+ alignment required, as opposed to preferred, by the kernel to boot.
+ If a boot loader makes use of this field, it should update the
+ kernel_alignment field with the alignment unit desired; typically:
+ kernel_alignment = 1 << min_alignment
+ There may be a considerable performance cost with an excessively
+ misaligned kernel. Therefore, a loader should typically try each
+ power-of-two alignment from kernel_alignment down to this alignment.
Field name: cmdline_size
Type: read
Offset/size: 0x238/4
@@ -582,6 +658,36 @@ Protocol: 2.09+
sure to consider the case where the linked list already contains
+Field name: pref_address
+Type: read (reloc)
+Offset/size: 0x258/8
+Protocol: 2.10+
+ This field, if nonzero, represents a preferred load address for the
+ kernel. A relocating bootloader should attempt to load at this
+ address if possible.
+ A non-relocatable kernel will unconditionally move itself and to run
+ at this address.
+Field name: init_size
+Type: read
+Offset/size: 0x25c/4
+ This field indicates the amount of linear contiguous memory starting
+ at the kernel runtime start address that the kernel needs before it
+ is capable of examining its memory map. This is not the same thing
+ as the total amount of memory the kernel needs to boot, but it can
+ be used by a relocating boot loader to help select a safe load
+ address for the kernel.
+ The kernel runtime start address is determined by the following algorithm:
+ if (relocatable_kernel)
+ runtime_start = align_up(load_address, kernel_alignment)
+ else
+ runtime_start = pref_address

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