Note

The protocol version number should be changed only if the setup headeris changed. There is no need to update the version number if boot_paramsor kernel_info are changed. Additionally, it is recommended to usexloadflags (in this case the protocol version number should not beupdated either) or kernel_info to communicate supported Linux kernelfeatures to the boot loader. Due to very limited space available inthe original setup header every update to it should be consideredwith great care. Starting from the protocol 2.15 the primary way tocommunicate things to the boot loader is the kernel_info.

1.1. Memory Layout¶

The traditional memory map for the kernel loader, used for Image orzImage 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 0000:7C00001000  +------------------------+        |  Reserved for MBR/BIOS |000800  +------------------------+        |  Typically used by MBR |000600  +------------------------+        |  BIOS use only         |000000  +------------------------+

When using bzImage, the protected-mode kernel was relocated to0x100000 (“high memory”), and the kernel real-mode block (boot sector,setup, and stack/heap) was made relocatable to any address between0x10000 and end of low memory. Unfortunately, in protocols 2.00 and2.01 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 inlow memory touched by the boot loader – as low as possible, sincesome newer BIOSes have begun to allocate some rather large amounts ofmemory, called the Extended BIOS Data Area, near the top of lowmemory. The boot loader should use the “INT 12h” BIOS call to verifyhow much low memory is available.

Unfortunately, if INT 12h reports that the amount of memory is toolow, there is usually nothing the boot loader can do but to report anerror to the user. The boot loader should therefore be designed totake up as little space in low memory as it reasonably can. ForzImage or old bzImage kernels, which need data written into the0x90000 segment, the boot loader should make sure not to use memoryabove the 0x9A000 point; too many BIOSes will break above that point.

For a modern bzImage kernel with boot protocol version >= 2.02, amemory 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.

1.2. The Real-Mode Kernel Header¶

In the following text, and anywhere in the kernel boot sequence, “asector” refers to 512 bytes. It is independent of the actual sectorsize of the underlying medium.

The first step in loading a Linux kernel should be to load thereal-mode code (boot sector and setup code) and then examine thefollowing header at offset 0x01f1. The real-mode code can total up to32K, although the boot loader may choose to load only the first twosectors (1K) and then examine the bootup sector size.

The header looks like:

If the “HdrS” (0x53726448) magic number is not found at offset 0x202,the boot protocol version is “old”. Loading an old kernel, thefollowing parameters should be assumed:

Image type = zImageinitrd not supportedReal-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. Whensetting fields in the header, you must make sure only to set fieldssupported by the protocol version in use.

1.3. 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 thebootloader (“modify”).

All general purpose boot loaders should write the fields marked(obligatory). Boot loaders who want to load the kernel at anonstandard address should fill in the fields marked (reloc); otherboot loaders can ignore those fields.

The byte order of all fields is littleendian (this is x86, after all.)

The size of the setup code in 512-byte sectors. If this field is0, the real value is 4. The real-mode code consists of the bootsector (always one 512-byte sector) plus the setup code.

If this field is nonzero, the root defaults to readonly. The use ofthis field is deprecated; use the “ro” or “rw” options on thecommand line instead.

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 byteswide, and therefore cannot be trusted for the size of a kernel ifthe LOAD_HIGH flag is set.

This field is obsolete.

Please see the section on SPECIAL COMMAND LINE OPTIONS.

The default root device device number. The use of this field isdeprecated, use the “root=” option on the command line instead.

Contains 0xAA55. This is the closest thing old Linux kernels haveto a magic number.

Contains an x86 jump instruction, 0xEB followed by a signed offsetrelative to byte 0x202. This can be used to determine the size ofthe header.

Contains the magic number “HdrS” (0x53726448).

Contains the boot protocol version, in (major << 8)+minor format,e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version10.17.

Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)

The load low segment (0x1000). Obsolete.

If set to a nonzero value, contains a pointer to a NUL-terminatedhuman-readable kernel version number string, less 0x200. This canbe used to display the kernel version to the user. This valueshould be less than (0x200*setup_sects).

For example, if this value is set to 0x1c00, the kernel versionnumber string can be found at offset 0x1e00 in the kernel file.This is a valid value if and only if the “setup_sects” fieldcontains the value 15 or higher, as:

0x1c00  < 15*0x200 (= 0x1e00) but0x1c00 >= 14*0x200 (= 0x1c00)0x1c00 >> 9 = 14, So the minimum value for setup_secs is 15.

If your boot loader has an assigned id (see table below), enter0xTV here, where T is an identifier for the boot loader and V isa version number. Otherwise, enter 0xFF here.

For boot loader IDs above T = 0xD, write T = 0xE to this field andwrite the extended ID minus 0x10 to the ext_loader_type field.Similarly, the ext_loader_ver field can be used to provide more thanfour bits for the bootloader version.

For example, for T = 0x15, V = 0x234, write:

type_of_loader  <- 0xE4ext_loader_type <- 0x05ext_loader_ver  <- 0x23

Assigned boot loader ids (hexadecimal):

0	LILO(0x00 reserved for pre-2.00 bootloader)
1	Loadlin
2	bootsect-loader(0x20, all other values reserved)
3	Syslinux
4	Etherboot/gPXE/iPXE
5	ELILO
7	GRUB
8	U-Boot
9	Xen
A	Gujin
B	Qemu
C	Arcturus Networks uCbootloader
D	kexec-tools
E	Extended (see ext_loader_type)
F	Special (0xFF = undefined)
10	Reserved
11	Minimal Linux Bootloader<http://sebastian-plotz.blogspot.de>
12	OVMF UEFI virtualization stack

Please contact <hpa@zytor.com> if you need a bootloader ID value assigned.

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 1 (kernel internal): KASLR_FLAG

• Used internally by the compressed kernel to communicateKASLR status to kernel proper.

  • If 1, KASLR enabled.
  • If 0, KASLR disabled.

Bit 5 (write): QUIET_FLAG

• If 0, print early messages.

• If 1, suppress early messages.

  This requests to the kernel (decompressor and earlykernel) to not write early messages that requireaccessing the display hardware directly.

Bit 6 (obsolete): KEEP_SEGMENTS

Protocol: 2.07+

• This flag is obsolete.

Bit 7 (write): CAN_USE_HEAP

Set this bit to 1 to indicate that the value entered in theheap_end_ptr is valid. If this field is clear, some setup codefunctionality will be disabled.

When using protocol 2.00 or 2.01, if the real mode kernel is notloaded at 0x90000, it gets moved there later in the loadingsequence. Fill in this field if you want additional data (such asthe kernel command line) moved in addition to the real-mode kernelitself.

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, orif the real-mode code is loaded at 0x90000.

The address to jump to in protected mode. This defaults to the loadaddress of the kernel, and can be used by the boot loader todetermine 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 arelocatable kernel at a nonstandard address it will have to modifythis field to point to the load address.

The 32-bit linear address of the initial ramdisk or ramfs. Leave atzero if there is no initial ramdisk/ramfs.

Size of the initial ramdisk or ramfs. Leave at zero if there is noinitial ramdisk/ramfs.

This field is obsolete.

Set this field to the offset (from the beginning of the real-modecode) of the end of the setup stack/heap, minus 0x0200.

This field is used as an extension of the version number in thetype_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, itis zero.

Kernels prior to 2.6.31 did not recognize this field, but it is safeto write for protocol version 2.02 or higher.

This field is used as an extension of the type number intype_of_loader field. If the type in type_of_loader is 0xE, thenthe 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 safeto write for protocol version 2.02 or higher.

Set this field to the linear address of the kernel command line.The kernel command line can be located anywhere between the end ofthe setup heap and 0xA0000; it does not have to be located in thesame 64K segment as the real-mode code itself.

Fill in this field even if your boot loader does not support acommand line, in which case you can point this to an empty string(or better yet, to the string “auto”.) If this field is left atzero, the kernel will assume that your boot loader does not supportthe 2.02+ protocol.

The maximum address that may be occupied by the initialramdisk/ramfs contents. For boot protocols 2.02 or earlier, thisfield is not present, and the maximum address is 0x37FFFFFF. (Thisaddress is defined as the address of the highest safe byte, so ifyour ramdisk is exactly 131072 bytes long and this field is0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)

Alignment unit required by the kernel (if relocatable_kernel istrue.) A relocatable kernel that is loaded at an alignmentincompatible with the value in this field will be realigned duringkernel initialization.

Starting with protocol version 2.10, this reflects the kernelalignment preferred for optimal performance; it is possible for theloader to modify this field to permit a lesser alignment. See themin_alignment and pref_address field below.

If this field is nonzero, the protected-mode part of the kernel canbe loaded at any address that satisfies the kernel_alignment field.After loading, the boot loader must set the code32_start field topoint to the loaded code, or to a boot loader hook.

This field, if nonzero, indicates as a power of two the minimumalignment required, as opposed to preferred, by the kernel to boot.If a boot loader makes use of this field, it should update thekernel_alignment field with the alignment unit desired; typically:

kernel_alignment = 1 << min_alignment

There may be a considerable performance cost with an excessivelymisaligned kernel. Therefore, a loader should typically try eachpower-of-two alignment from kernel_alignment down to this alignment.

This field is a bitmask.

Bit 0 (read): XLF_KERNEL_64

• If 1, this kernel has the legacy 64-bit entry point at 0x200.

Bit 1 (read): XLF_CAN_BE_LOADED_ABOVE_4G

• If 1, kernel/boot_params/cmdline/ramdisk can be above 4G.

Bit 2 (read): XLF_EFI_HANDOVER_32

• If 1, the kernel supports the 32-bit EFI handoff entry pointgiven at handover_offset.

Bit 3 (read): XLF_EFI_HANDOVER_64

• If 1, the kernel supports the 64-bit EFI handoff entry pointgiven at handover_offset + 0x200.

Bit 4 (read): XLF_EFI_KEXEC

• If 1, the kernel supports kexec EFI boot with EFI runtime support.

The maximum size of the command line without the terminatingzero. This means that the command line can contain at mostcmdline_size characters. With protocol version 2.05 and earlier, themaximum size was 255.

In a paravirtualized environment the hardware low level architecturalpieces such as interrupt handling, page table handling, andaccessing process control registers needs to be done differently.

This field allows the bootloader to inform the kernel we are in oneone of those environments.

0x00000000	The default x86/PC environment
0x00000001	lguest
0x00000002	Xen
0x00000003	Moorestown MID
0x00000004	CE4100 TV Platform

A pointer to data that is specific to hardware subarchThis field is currently unused for the default x86/PC environment,do not modify.

If non-zero then this field contains the offset from the beginningof the protected-mode code to the payload.

The payload may be compressed. The format of both the compressed anduncompressed data should be determined using the standard magicnumbers. The currently supported compression formats are gzip(magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA(magic number 5D 00), XZ (magic number FD 37), and LZ4 (magic number02 21). The uncompressed payload is currently always ELF (magicnumber 7F 45 4C 46).

The length of the payload.

The 64-bit physical pointer to NULL terminated single linked list ofstruct setup_data. This is used to define a more extensible bootparameters passing mechanism. The definition of struct setup_data isas 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 oflinked list, the next field of the last node is 0; the type is usedto identify the contents of data; the len is the length of datafield; the data holds the real payload.

This list may be modified at a number of points during the bootupprocess. Therefore, when modifying this list one should always makesure to consider the case where the linked list already containsentries.

The setup_data is a bit awkward to use for extremely large data objects,both because the setup_data header has to be adjacent to the data objectand because it has a 32-bit length field. However, it is important thatintermediate stages of the boot process have a way to identify whichchunks of memory are occupied by kernel data.

Thus setup_indirect struct and SETUP_INDIRECT type were introduced inprotocol 2.15:

struct setup_indirect {  __u32 type;  __u32 reserved;  /* Reserved, must be set to zero. */  __u64 len;  __u64 addr;};

The type member is a SETUP_INDIRECT | SETUP_* type. However, it cannot beSETUP_INDIRECT itself since making the setup_indirect a tree structurecould require a lot of stack space in something that needs to parse itand stack space can be limited in boot contexts.

Let’s give an example how to point to SETUP_E820_EXT data using setup_indirect.In this case setup_data and setup_indirect will look like this:

struct setup_data {  __u64 next = 0 or <addr_of_next_setup_data_struct>;  __u32 type = SETUP_INDIRECT;  __u32 len = sizeof(setup_data);  __u8 data[sizeof(setup_indirect)] = struct setup_indirect {    __u32 type = SETUP_INDIRECT | SETUP_E820_EXT;    __u32 reserved = 0;    __u64 len = <len_of_SETUP_E820_EXT_data>;    __u64 addr = <addr_of_SETUP_E820_EXT_data>;  }}

This field, if nonzero, represents a preferred load address for thekernel. A relocating bootloader should attempt to load at thisaddress if possible.

A non-relocatable kernel will unconditionally move itself and to runat this address.

This field indicates the amount of linear contiguous memory startingat the kernel runtime start address that the kernel needs before itis capable of examining its memory map. This is not the same thingas the total amount of memory the kernel needs to boot, but it canbe used by a relocating boot loader to help select a safe loadaddress 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)elseruntime_start = pref_address

This field is the offset from the beginning of the kernel image tothe EFI handover protocol entry point. Boot loaders using the EFIhandover protocol to boot the kernel should jump to this offset.

See EFI HANDOVER PROTOCOL below for more details.

This field is the offset from the beginning of the kernel image to thekernel_info. The kernel_info structure is embedded in the Linux imagein the uncompressed protected mode region.

1.4. The kernel_info¶

The relationships between the headers are analogous to the various datasections:

setup_header = .databoot_params/setup_data = .bss

What is missing from the above list? That’s right:

kernel_info = .rodata

We have been (ab)using .data for things that could go into .rodata or .bss fora long time, for lack of alternatives and – especially early on – inertia.Also, the BIOS stub is responsible for creating boot_params, so it isn’tavailable to a BIOS-based loader (setup_data is, though).

setup_header is permanently limited to 144 bytes due to the reach of the2-byte jump field, which doubles as a length field for the structure, combinedwith the size of the “hole” in struct boot_params that a protected-mode loaderor the BIOS stub has to copy it into. It is currently 119 bytes long, whichleaves us with 25 very precious bytes. This isn’t something that can be fixedwithout revising the boot protocol entirely, breaking backwards compatibility.

boot_params proper is limited to 4096 bytes, but can be arbitrarily extendedby adding setup_data entries. It cannot be used to communicate properties ofthe kernel image, because it is .bss and has no image-provided content.

kernel_info solves this by providing an extensible place for information aboutthe kernel image. It is readonly, because the kernel cannot rely on abootloader copying its contents anywhere, but that is OK; if it becomesnecessary it can still contain data items that an enabled bootloader would beexpected to copy into a setup_data chunk.

All kernel_info data should be part of this structure. Fixed size data have tobe put before kernel_info_var_len_data label. Variable size data have to be putafter kernel_info_var_len_data label. Each chunk of variable size data has tobe prefixed with header/magic and its size, e.g.:

kernel_info:        .ascii  "LToP"          /* Header, Linux top (structure). */        .long   kernel_info_var_len_data - kernel_info        .long   kernel_info_end - kernel_info        .long   0x01234567      /* Some fixed size data for the bootloaders. */kernel_info_var_len_data:example_struct:                 /* Some variable size data for the bootloaders. */        .ascii  "0123"          /* Header/Magic. */        .long   example_struct_end - example_struct        .ascii  "Struct"        .long   0x89012345example_struct_end:example_strings:                /* Some variable size data for the bootloaders. */        .ascii  "ABCD"          /* Header/Magic. */        .long   example_strings_end - example_strings        .asciz  "String_0"        .asciz  "String_1"example_strings_end:kernel_info_end:

This way the kernel_info is self-contained blob.

1.5. Details of the kernel_info Fields¶

Contains the magic number “LToP” (0x506f544c).

This field contains the size of the kernel_info including kernel_info.header.It does not count kernel_info.kernel_info_var_len_data size. This field should beused by the bootloaders to detect supported fixed size fields in the kernel_infoand beginning of kernel_info.kernel_info_var_len_data.

This field contains the size of the kernel_info including kernel_info.headerand kernel_info.kernel_info_var_len_data.

This field contains maximal allowed type for setup_data and setup_indirect structs.

1.6. The Image Checksum¶

From boot protocol version 2.08 onwards the CRC-32 is calculated overthe entire file using the characteristic polynomial 0x04C11DB7 and aninitial remainder of 0xffffffff. The checksum is appended to thefile; therefore the CRC of the file up to the limit specified in thesyssize field of the header is always 0.

1.7. The Kernel Command Line¶

The kernel command line has become an important way for the bootloader to communicate with the kernel. Some of its options are alsorelevant to the boot loader itself, see “special command line options”below.

The kernel command line is a null-terminated string. The maximumlength can be retrieved from the field cmdline_size. Before protocolversion 2.06, the maximum was 255 characters. A string that is toolong will be automatically truncated by the kernel.

If the boot protocol version is 2.02 or later, the address of thekernel command line is given by the header field cmd_line_ptr (seeabove.) This address can be anywhere between the end of the setupheap and 0xA0000.

If the protocol version isnot 2.02 or higher, the kernelcommand line is entered using the following protocol:

• At offset 0x0020 (word), “cmd_line_magic”, enter the magicnumber 0xA33F.
• At offset 0x0022 (word), “cmd_line_offset”, enter the offsetof the kernel command line (relative to the start of thereal-mode kernel).
• The kernel command linemust be within the memory regioncovered by setup_move_size, so you may need to adjust thisfield.

1.8. Memory Layout of The Real-Mode Code¶

The real-mode code requires a stack/heap to be set up, as well asmemory allocated for the kernel command line. This needs to be donein the real-mode accessible memory in bottom megabyte.

It should be noted that modern machines often have a sizable ExtendedBIOS Data Area (EBDA). As a result, it is advisable to use as littleof the low megabyte as possible.

Unfortunately, under the following circumstances the 0x90000 memorysegment has to be used:

• When loading a zImage kernel ((loadflags & 0x01) == 0).
• When loading a 2.01 or earlier boot protocol kernel.

When loading at 0x90000, avoid using memory above 0x9a000.

For boot protocol 2.02 or higher, the command line does not have to belocated in the same 64K segment as the real-mode setup code; it isthus permitted to give the stack/heap the full 64K segment and locatethe command line above it.

The kernel command line should not be located below the real-modecode, nor should it be located in high memory.

1.9. Sample Boot Configuartion¶

As a sample configuration, assume the following layout of the realmode 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);}

1.10. Loading The Rest of The Kernel¶

The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512in the kernel file (again, if setup_sects == 0 the real value is 4.)It should be loaded at address 0x10000 for Image/zImage kernels and0x100000 for bzImage kernels.

The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01bit (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 usethe entire 0x10000-0x90000 range of memory. This means it is prettymuch a requirement for these kernels to load the real-mode part at0x90000. bzImage kernels allow much more flexibility.

1.11. Special Command Line Options¶

If the command line provided by the boot loader is entered by theuser, the user may expect the following command line options to work.They should normally not be deleted from the kernel command line eventhough not all of them are actually meaningful to the kernel. Bootloader authors who need additional command line options for the bootloader itself should get them registered inDocumentation/admin-guide/kernel-parameters.rst to make sure they will notconflict with actual kernel options now or in the future.

vga=<mode>

<mode> here is either an integer (in C notation, eitherdecimal, octal, or hexadecimal) or one of the strings“normal” (meaning 0xFFFF), “ext” (meaning 0xFFFE) or “ask”(meaning 0xFFFD). This value should be entered into thevid_mode field, as it is used by the kernel before the commandline 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 ofmemory to the kernel. This affects the possible placement ofan initrd, since an initrd should be placed near end ofmemory. Note that this is an option toboth the kernel andthe bootloader!

initrd=<file>

An initrd should be loaded. The meaning of <file> isobviously 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 theuser-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 highlyrecommended that they are locatedfirst, before the user-specifiedor configuration-specified command line. Otherwise, “init=/bin/sh”gets confused by the “auto” option.

1.12. Running the Kernel¶

The kernel is started by jumping to the kernel entry point, which islocated atsegment offset 0x20 from the start of the real modekernel. This means that if you loaded your real-mode kernel code at0x90000, the kernel entry point is 9020:0000.

At entry, ds = es = ss should point to the start of the real-modekernel code (0x9000 if the code is loaded at 0x90000), sp should beset up properly, normally pointing to the top of the heap, andinterrupts should be disabled. Furthermore, to guard against bugs inthe 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 toswitch off the floppy motor before running the kernel, since thekernel boot leaves interrupts off and thus the motor will not beswitched off, especially if the loaded kernel has the floppy driver asa demand-loaded module!

1.13. Advanced Boot Loader Hooks¶

If the boot loader runs in a particularly hostile environment (such asLOADLIN, which runs under DOS) it may be impossible to follow thestandard memory location requirements. Such a boot loader may use thefollowing hooks that, if set, are invoked by the kernel at theappropriate time. The use of these hooks should probably beconsidered 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 beforeentering protected mode. The default routine disables NMI, soyour routine should probably do so, too.

code32_start:

A 32-bit flat-mode routinejumped to immediately after thetransition to protected mode, but before the kernel isuncompressed. No segments, except CS, are guaranteed to beset up (current kernels do, but older ones do not); you shouldset them up to BOOT_DS (0x18) yourself.

After completing your hook, you should jump to the addressthat was in this field before your boot loader overwrote it(relocated, if appropriate.)

1.14. 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 kernelbased on legacy BIOS can not be used, so a 32-bit boot protocol needsto be defined.

In 32-bit boot protocol, the first step in loading a Linux kernelshould be to setup the boot parameters (struct boot_params,traditionally known as “zero page”). The memory for struct boot_paramsshould be allocated and initialized to all zero. Then the setup headerfrom offset 0x01f1 of kernel image on should be loaded into structboot_params and examined. The end of setup header can be calculated asfollow:

0x0202 + byte value at offset 0x0201

In addition to read/modify/write the setup header of the structboot_params as that of 16-bit boot protocol, the boot loader shouldalso fill the additional fields of the struct boot_params as thatdescribed in zero-page.txt.

After setting up the struct boot_params, the boot loader can load the32/64-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 the32-bit kernel entry point, which is the start address of loaded32/64-bit kernel.

At entry, the CPU must be in 32-bit protected mode with pagingdisabled; a GDT must be loaded with the descriptors for selectors__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flatsegment; __BOOT_CS must have execute/read permission, and __BOOT_DSmust have read/write permission; CS must be __BOOT_CS and DS, ES, SSmust be __BOOT_DS; interrupt must be disabled; %esi must hold the baseaddress of the struct boot_params; %ebp, %edi and %ebx must be zero.

1.15. 64-bit Boot Protocol¶

For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloaderand we need a 64-bit boot protocol.

In 64-bit boot protocol, the first step in loading a Linux kernelshould be to setup the boot parameters (struct boot_params,traditionally known as “zero page”). The memory for struct boot_paramscould be allocated anywhere (even above 4G) and initialized to all zero.Then, the setup header at offset 0x01f1 of kernel image on should beloaded into struct boot_params and examined. The end of setup headercan be calculated as follows:

0x0202 + byte value at offset 0x0201

In addition to read/modify/write the setup header of the structboot_params as that of 16-bit boot protocol, the boot loader shouldalso fill the additional fields of the struct boot_params as describedin zero-page.txt.

After setting up the struct boot_params, the boot loader can load64-bit kernel in the same way as that of 16-bit boot protocol, butkernel could be loaded above 4G.

In 64-bit boot protocol, the kernel is started by jumping to the64-bit kernel entry point, which is the start address of loaded64-bit kernel plus 0x200.

At entry, the CPU must be in 64-bit mode with paging enabled.The range with setup_header.init_size from start address of loadedkernel and zero page and command line buffer get ident mapping;a GDT must be loaded with the descriptors for selectors__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flatsegment; __BOOT_CS must have execute/read permission, and __BOOT_DSmust have read/write permission; CS must be __BOOT_CS and DS, ES, SSmust be __BOOT_DS; interrupt must be disabled; %rsi must hold the baseaddress of the struct boot_params.

1.16. EFI Handover Protocol (deprecated)¶

This protocol allows boot loaders to defer initialisation to the EFIboot stub. The boot loader is required to load the kernel/initrd(s)from the boot media and jump to the EFI handover protocol entry pointwhich is hdr->handover_offset bytes from the beginning ofstartup_{32,64}.

The boot loader MUST respect the kernel’s PE/COFF metadata when it comesto section alignment, the memory footprint of the executable image beyondthe size of the file itself, and any other aspect of the PE/COFF headerthat may affect correct operation of the image as a PE/COFF binary in theexecution context provided by the EFI firmware.

The function prototype for the handover entry point looks like this:

efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp)

‘handle’ is the EFI image handle passed to the boot loader by the EFIfirmware, ‘table’ is the EFI system table - these are the first twoarguments of the “handoff state” as described in section 2.3 of theUEFI specification. ‘bp’ is the boot loader-allocated boot params.

The boot loadermust fill out the following fields in bp:

- hdr.cmd_line_ptr- hdr.ramdisk_image (if applicable)- hdr.ramdisk_size  (if applicable)

All other fields should be zero.

NOTE: The EFI Handover Protocol is deprecated in favour of the ordinary PE/COFF

entry point, combined with the LINUX_EFI_INITRD_MEDIA_GUID based initrdloading protocol (refer to [0] for an example of the bootloader side ofthis), which removes the need for any knowledge on the part of the EFIbootloader regarding the internal representation of boot_params or anyrequirements/limitations regarding the placement of the command lineand ramdisk in memory, or the placement of the kernel image itself.

[0]https://github.com/u-boot/u-boot/commit/ec80b4735a593961fe701cc3a5d717d4739b0fd0