arch/x86/include/uapi/asm/bootparam.h

struct boot_params

The struct boot_params is also referred to as zeropage.

The x86 boot code expects to be passed a struct boot_param.

This struct is either created by the boot loader or in efi_pe_entry.

`screen_info`	`struct screen_info`	0x000
`apm_bios_info`	`struct apm_bios_info`	0x040
`_pad2[4]`	`__u8`	0x054
`tboot_addr`	`__u64`	0x058
`ist_info`	`struct ist_info`	0x060
`acpi_rsdp_addr`	`__u64`	0x070
`_pad3[8]`	`__u8`	0x078
`hd0_info[16]`	`__u8`	0x080	Obsolete
`hd1_info[16]`	`__u8`	0x090	Obsolete
`sys_desc_table`	`struct sys_desc_table`	0x0a0	Obsolete
`olpc_ofw_header`	`struct olpc_ofw_header`	0x0b0
`ext_ramdisk_image`	`__u32`	0x0c0
`ext_ramdisk_size`	`__u32`	0x0c4
`ext_cmd_line_ptr`	`__u32`	0x0c8	Compare with `hdr->setup_header->cmd_line_ptr` and `CONFIG_CMDLINE`.
`_pad4[112]`	`__u8`	0x0cc
`cc_blob_address`	`__u32`	0x13c
`edid_info`	`struct edid_info`	0x140
`efi_info`	`struct efi_info`	0x1c0
`alt_mem_k`	`__u32`	0x1e0
`scratch`	`__u32`	0x1e4	Scratch field
`e820_entries`	`__u8`	0x1e8
`eddbuf_entries`	`__u8`	0x1e9
`edd_mbr_sig_buf_entries`	`__u8`	0x1ea
`kbd_status`	`__u8`	0x1eb
`secure_boot`	`__u8`	0x1ec
`_pad5[2]`	`__u8`	0x1ed
`sentinel`	`__u8`	0x1ef	Set to a nonzero value (`0xff`) in `arch/x86/boot/header.S`
`_pad6[1]`	`__u8`	0x1f0
`hdr`	`struct setup_header`	0x1f1	setup header, see global symbol `hdr` in `arch/x86/boot/header.S`.
`_pad7[0x290-0x1f1-sizeof(struct setup_header)]`	`__u8`
`edd_mbr_sig_buffer[EDD_MBR_SIG_MAX]`	`__u32`	0x290
`e820_table[E820_MAX_ENTRIES_ZEROPAGE]`	`struct boot_e820_entry`	0x2d0
`_pad8[48]`	`__u8`	0xcd0
`edd_info[EDDMAXNR]`	`struct edd_info`	0xd00
`_pad9[276]`	`__u8`	0xeec

A bootloader is supposed to only take setup_header and put it into a clean boot_params buffer. If it turns out that it is clumsy or too generous with the buffer, it most probably will pick up the sentinel variable too. The fact that this variable then is still 0xff will let kernel know that some variables in boot_params are invalid and kernel should zero out certain portions of boot_params.

Setting the config option DEBUG_BOOT_PARAMS causes the struct boot_param to be exported via debugfs.

struct setup_header

The value of the column rwm indicates if the value is read by the bootloader (r), written by the bootloader (w) or read and written (m = modified) by the bootloader.

		Offset	rwm	Notes
`setup_sects`	`__u8`	497	r	Size of the setup in (512 byte) sectors. The real-mode code consists of the boot sector (512 bytes) plus the setup code. Filled by `arch/x86/boot/tools/build`.
`root_flags`	`__u16`	498	m	If set (!= 0), the root is mounted read only. This value corresponds to `ROOT_RDONLY` (default = `1`). Using `root_flags` is deprecated in favor of the command line options `ro` or `rw`.
`syssize`	`__u32`	500	r	The size of the (protected mode (or 32-bit)) code in 16-byte paragraphs. Filled by `arch/x86/boot/tools/build`.
`ram_size`	`__u16`	504		Obsolete? was used for `bootsect.S` (which seems to be deleted)
`vid_mode`	`__u16`	506	m	Video mode control. Corresponds to value of `SVGA_MODE`
`root_dev`	`__u16`	508	m	Default root device number. Filled by `arch/x86/boot/tools/build`. Using `root_dev` is deprecated in favor of using the `root=` command line option.
`boot_flag`	`__u16`	510	r	`0xAA55` (magic number for the final two bytes of the first sector of an MBR).
`jump`	`__u16`	512	r	Machine code jump instruction: `0xeb` followed by `start_of_setup - 1f` (i. e. a signed offset relative to byte `0x202` = 514). This jump seems to be aligned with the `_start` symbol in the object file? This value can be used to determine the size of the header. This jump instruction limits the maximum size of `setup_header` to 144 bytes (as of Linux version 6.6, the size is 123 bytes).
`header`	`__u32`	514	r	Magic string `HDrS` (`0x53726448`)
`version`	`__u16`	518	r	The header/boot protocol version number (for example `0x020f` for 2.15).
`realmode_swtch`	`__u32`	520	m	Boot loader hook (`0x00000000`?)
`start_sys_seg`	`__u16`	524	r	The had-low segment, corresponding to the value`SYSSEG` (= `0x1000` which is the historical load address `>> 4`). (Obsolete and even meaningless).
`kernel_version`	`__u16`	526	r	Pointer to the kernel version string above the section of header (or so). Value possibly is `kernel_version-512`.
`type_of_loader`	`__u8`	528	w	Identification of boot loader. `0` = ancient bootloader. Newer bootloaders know to change this. See `Documentation/arch/x86/boot/rst`. (7 is GRUB)
`loadflags`	`__u8`	529	m	Boot protocol option flags (bitmap).
`setup_move_size`	`__u16`	530	m	`0x8000` size to move, when setup is not loaded at `0x90000`. We will move setup to `0x90000` then just before jumping into the kernel. However, only the loader knows how much data behind us also needs to be loaded.
`code32_start`	`__u32`	532	m	Boot loader hook: the address to mump to in protected mode. Default for big kernel: `0x100000`. Loaders can put a different start address for 32-bit code here.
`ramdisk_image`	`__u32`	536	w	Linear address of the loaded ramdisk (initrd) image. The loader puts this address.
`ramdisk_size`	`__u32`	540	w	Size of the initial ramdisk (initrd)
`bootsect_kludge`	`__u32`	544		Obsolete. Was used for `bootsect.S` (which seems to be deleted).
`heap_end_ptr`	`__u16`	548	w	Free memory after setup end. `_end+STACK_SIZE-512`
`ext_loader_ver`	`__u8`	550	w	`0`. Extended boot loader version
`ext_loader_type`	`__u8`	551	w	`0`. Extended boot loader type (ID).
`cmd_line_ptr`	`__u32`	552	w	32-bit pointer to kernel command line. Compare with `ext_cmd_line_ptr` (`struct boot_params`) and `CONFIG_CMDLINE`. In header version `0x0202` or later: If nonzero, this value is a pointer to the kernel command line. This command line should be located between the start of setup and the end of low memory (`0xA0000`). Otherwise, it may get overwritten before it is readhj
`initrd_addr_max`	`__u32`	556	r	In header version `0x0203` or later: The highest safe («legal») address for the contents of an initrd. The current kernel allows up to 4 GB, but leave it at 2 GB to avoid possible bootloader bugs.
`kernel_alignment`	`__u32`	560	m	Value of `CONFIG_PHYSICAL_ALIGN`, the phyisical address alignment (if `relocatable_kernel` is set). This is reaquired for protected mode.
`relocatable_kernel`	`__u8`	564	r	`1` if `CONFIG_RELOCTABLE` is defined, `0` otherwise.
`min_alignment`	`__u8`	565	r	Value of `MIN_KERNEL_ALIGN_LG2` (minimum alignment)
`xloadflags`	`__u16`	566	r	Boot protocol option flags
`cmdline_size`	`__u32`	568	r	Maximum length of the command line (`COMMAND_LINE_SIZE - 1`)
`hardware_subarch`	`__u32`	572	w	Hardware subarchitecture. For normal x86 PCs, the default is `0`
`hardware_subarch_data`	`__u64`	580	w	`0`. Subarchitecture specific data.
`payload_offset`	`__u32`	584	r	Offset of kernel payload, corresponds to value of `ZO_input_data` (defined in `arch/x86/boot/zoffset.h`)
`payload_length`	`__u32`	588	r	Length of kernel pyaload, corresponding to value of `ZO_z_input_len` (defined in `arch/x86/boot/zoffset.h`)
`setup_data`	`__u64`	596	w	(Physical) pointer to single linked list of `setup_data`
`pref_address`	`__u64`	604	r	The preferred loading address (value of `LOAD_PHYSICAL_ADDR`)
`init_size`	`__u32`	608	r	Required Kernel initialization size (aka «linear size») = Value of `INIT_SIZE` (which is set to the greater size of the uncompressed or compressed kernel).
`handover_offset`	`__u32`	612	r	Offset of handover entry point. Filled by `arch/x86/boot/tools/build`. With `CONFIG_X86_64` and (the deprecated) `CONFIG_EFI_HANDOVER_PROTOCOL`, the value is `efi64_stub_entry - 0x200`, (yes, this is really how it is defined), otherwise, the value is `efi32_stub_entry`.
`kernel_info_offset`	`__u32`	616	r	Offset of the `kernel_info`. Filled by `arch/x86/boot/tools/build`

struct setup_header corresponds to the «global symbol» hdr in arch/x86/boot/header.S.

arch/x86/include/uapi/asm/bootparam.h

struct boot_params

struct setup_header

See also