| THE LINUX/x86 BOOT PROTOCOL |
| --------------------------- |
| |
| On the x86 platform, the Linux kernel uses a rather complicated boot |
| convention. This has evolved partially due to historical aspects, as |
| well as the desire in the early days to have the kernel itself be a |
| bootable image, the complicated PC memory model and due to changed |
| expectations in the PC industry caused by the effective demise of |
| real-mode DOS as a mainstream operating system. |
| |
| Currently, the following versions of the Linux/x86 boot protocol exist. |
| |
| Old kernels: zImage/Image support only. Some very early kernels |
| may not even support a command line. |
| |
| Protocol 2.00: (Kernel 1.3.73) Added bzImage and initrd support, as |
| well as a formalized way to communicate between the |
| boot loader and the kernel. setup.S made relocatable, |
| although the traditional setup area still assumed |
| writable. |
| |
| Protocol 2.01: (Kernel 1.3.76) Added a heap overrun warning. |
| |
| Protocol 2.02: (Kernel 2.4.0-test3-pre3) New command line protocol. |
| Lower the conventional memory ceiling. No overwrite |
| of the traditional setup area, thus making booting |
| safe for systems which use the EBDA from SMM or 32-bit |
| BIOS entry points. zImage deprecated but still |
| supported. |
| |
| Protocol 2.03: (Kernel 2.4.18-pre1) Explicitly makes the highest possible |
| initrd address available to the bootloader. |
| |
| Protocol 2.04: (Kernel 2.6.14) Extend the syssize field to four bytes. |
| |
| Protocol 2.05: (Kernel 2.6.20) Make protected mode kernel relocatable. |
| Introduce relocatable_kernel and kernel_alignment fields. |
| |
| Protocol 2.06: (Kernel 2.6.22) Added a field that contains the size of |
| the boot command line. |
| |
| Protocol 2.07: (Kernel 2.6.24) Added paravirtualised boot protocol. |
| Introduced hardware_subarch and hardware_subarch_data |
| and KEEP_SEGMENTS flag in load_flags. |
| |
| Protocol 2.08: (Kernel 2.6.26) Added crc32 checksum and ELF format |
| payload. Introduced payload_offset and payload_length |
| fields to aid in locating the payload. |
| |
| 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. |
| |
| Protocol 2.11: (Kernel 3.6) Added a field for offset of EFI handover |
| protocol entry point. |
| |
| Protocol 2.12: (Kernel 3.8) Added the xloadflags field and extension fields |
| to struct boot_params for for loading bzImage and ramdisk |
| above 4G in 64bit. |
| |
| **** MEMORY LAYOUT |
| |
| The traditional memory map for the kernel loader, used for Image or |
| zImage 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 0000:7C00 |
| 001000 +------------------------+ |
| | Reserved for MBR/BIOS | |
| 000800 +------------------------+ |
| | Typically used by MBR | |
| 000600 +------------------------+ |
| | BIOS use only | |
| 000000 +------------------------+ |
| |
| |
| 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 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 |
| some newer BIOSes have begun to allocate some rather large amounts of |
| memory, called the Extended BIOS Data Area, near the top of low |
| memory. The boot loader should use the "INT 12h" BIOS call to verify |
| how much low memory is available. |
| |
| Unfortunately, if INT 12h reports that the amount of memory is too |
| low, there is usually nothing the boot loader can do but to report an |
| error to the user. The boot loader should therefore be designed to |
| take up as little space in low memory as it reasonably can. For |
| 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 |
| |
| In the following text, and anywhere in the kernel boot sequence, "a |
| sector" refers to 512 bytes. It is independent of the actual sector |
| size of the underlying medium. |
| |
| The first step in loading a Linux kernel should be to load the |
| real-mode code (boot sector and setup code) and then examine the |
| following header at offset 0x01f1. The real-mode code can total up to |
| 32K, although the boot loader may choose to load only the first two |
| sectors (1K) and then examine the bootup sector size. |
| |
| The header looks like: |
| |
| Offset Proto Name Meaning |
| /Size |
| |
| 01F1/1 ALL(1 setup_sects The size of the setup in sectors |
| 01F2/2 ALL root_flags If set, the root is mounted readonly |
| 01F4/4 2.04+(2 syssize The size of the 32-bit code in 16-byte paras |
| 01F8/2 ALL ram_size DO NOT USE - for bootsect.S use only |
| 01FA/2 ALL vid_mode Video mode control |
| 01FC/2 ALL root_dev Default root device number |
| 01FE/2 ALL boot_flag 0xAA55 magic number |
| 0200/2 2.00+ jump Jump instruction |
| 0202/4 2.00+ header Magic signature "HdrS" |
| 0206/2 2.00+ version Boot protocol version supported |
| 0208/4 2.00+ realmode_swtch Boot loader hook (see below) |
| 020C/2 2.00+ start_sys_seg The load-low segment (0x1000) (obsolete) |
| 020E/2 2.00+ kernel_version Pointer to kernel version string |
| 0210/1 2.00+ type_of_loader Boot loader identifier |
| 0211/1 2.00+ loadflags Boot protocol option flags |
| 0212/2 2.00+ setup_move_size Move to high memory size (used with hooks) |
| 0214/4 2.00+ code32_start Boot loader hook (see below) |
| 0218/4 2.00+ ramdisk_image initrd load address (set by boot loader) |
| 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 |
| 0226/1 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 2.10+ min_alignment Minimum alignment, as a power of two |
| 0236/2 2.12+ xloadflags Boot protocol option flags |
| 0238/4 2.06+ cmdline_size Maximum size of the kernel command line |
| 023C/4 2.07+ hardware_subarch Hardware subarchitecture |
| 0240/8 2.07+ hardware_subarch_data Subarchitecture-specific data |
| 0248/4 2.08+ payload_offset Offset of kernel payload |
| 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 |
| 0264/4 2.11+ handover_offset Offset of handover entry point |
| |
| (1) For backwards compatibility, if the setup_sects field contains 0, the |
| real value is 4. |
| |
| (2) For boot protocol prior to 2.04, the upper two bytes of the syssize |
| 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: |
| |
| Image type = zImage |
| initrd not supported |
| Real-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. When |
| setting fields in the header, you must make sure only to set fields |
| supported by the protocol version in use. |
| |
| |
| **** 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. |
| |
| The byte order of all fields is littleendian (this is x86, after all.) |
| |
| Field name: setup_sects |
| 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, in (major << 8)+minor format, |
| e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version |
| 10.17. |
| |
| Field name: realmode_swtch |
| Type: modify (optional) |
| Offset/size: 0x208/4 |
| Protocol: 2.00+ |
| |
| Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.) |
| |
| Field name: start_sys_seg |
| Type: read |
| Offset/size: 0x20c/2 |
| 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 NUL-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 15 or higher, as: |
| |
| 0x1c00 < 15*0x200 (= 0x1e00) but |
| 0x1c00 >= 14*0x200 (= 0x1c00) |
| |
| 0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15. |
| |
| 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. |
| |
| 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 (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. |
| |
| 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 5 (write): QUIET_FLAG |
| - If 0, print early messages. |
| - If 1, suppress early messages. |
| This requests to the kernel (decompressor and early |
| kernel) to not write early messages that require |
| accessing the display hardware directly. |
| |
| Bit 6 (write): KEEP_SEGMENTS |
| Protocol: 2.07+ |
| - If 0, reload the segment registers in the 32bit entry point. |
| - If 1, do not reload the segment registers in the 32bit entry point. |
| Assume that %cs %ds %ss %es are all set to flat segments with |
| a base of 0 (or the equivalent for their environment). |
| |
| 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 ADVANCED BOOT LOADER HOOKS below.) |
| |
| 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: 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 |
| 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: ramdisk_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/modify (reloc) |
| Offset/size: 0x230/4 |
| 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. |
| |
| 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) |
| 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: 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: xloadflags |
| Type: read |
| Offset/size: 0x236/2 |
| Protocol: 2.12+ |
| |
| 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 point |
| given at handover_offset. |
| |
| Bit 3 (read): XLF_EFI_HANDOVER_64 |
| - If 1, the kernel supports the 64-bit EFI handoff entry point |
| given at handover_offset + 0x200. |
| |
| 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. |
| |
| Field name: hardware_subarch |
| Type: write (optional, defaults to x86/PC) |
| Offset/size: 0x23c/4 |
| Protocol: 2.07+ |
| |
| In a paravirtualized environment the hardware low level architectural |
| pieces such as interrupt handling, page table handling, and |
| accessing process control registers needs to be done differently. |
| |
| This field allows the bootloader to inform the kernel we are in one |
| one of those environments. |
| |
| 0x00000000 The default x86/PC environment |
| 0x00000001 lguest |
| 0x00000002 Xen |
| 0x00000003 Moorestown MID |
| 0x00000004 CE4100 TV Platform |
| |
| Field name: hardware_subarch_data |
| Type: write (subarch-dependent) |
| Offset/size: 0x240/8 |
| Protocol: 2.07+ |
| |
| A pointer to data that is specific to hardware subarch |
| This field is currently unused for the default x86/PC environment, |
| do not modify. |
| |
| Field name: payload_offset |
| Type: read |
| Offset/size: 0x248/4 |
| Protocol: 2.08+ |
| |
| If non-zero then this field contains the offset from the beginning |
| of the protected-mode code to the payload. |
| |
| The payload may be compressed. The format of both the compressed and |
| uncompressed data should be determined using the standard magic |
| numbers. The currently supported compression formats are gzip |
| (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA |
| (magic number 5D 00), and XZ (magic number FD 37). The uncompressed |
| payload is currently always ELF (magic number 7F 45 4C 46). |
| |
| Field name: payload_length |
| Type: read |
| Offset/size: 0x24c/4 |
| Protocol: 2.08+ |
| |
| The length of the payload. |
| |
| Field name: setup_data |
| Type: write (special) |
| Offset/size: 0x250/8 |
| Protocol: 2.09+ |
| |
| The 64-bit physical pointer to NULL terminated single linked list of |
| struct setup_data. This is used to define a more extensible boot |
| parameters passing mechanism. The definition of struct setup_data is |
| as 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 of |
| linked list, the next field of the last node is 0; the type is used |
| to identify the contents of data; the len is the length of data |
| field; the data holds the real payload. |
| |
| This list may be modified at a number of points during the bootup |
| process. Therefore, when modifying this list one should always make |
| sure to consider the case where the linked list already contains |
| entries. |
| |
| 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: 0x260/4 |
| |
| 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 |
| |
| Field name: handover_offset |
| Type: read |
| Offset/size: 0x264/4 |
| |
| This field is the offset from the beginning of the kernel image to |
| the EFI handover protocol entry point. Boot loaders using the EFI |
| handover protocol to boot the kernel should jump to this offset. |
| |
| See EFI HANDOVER PROTOCOL below for more details. |
| |
| |
| **** THE IMAGE CHECKSUM |
| |
| From boot protocol version 2.08 onwards the CRC-32 is calculated over |
| the entire file using the characteristic polynomial 0x04C11DB7 and an |
| initial remainder of 0xffffffff. The checksum is appended to the |
| file; therefore the CRC of the file up to the limit specified in the |
| syssize field of the header is always 0. |
| |
| |
| **** THE KERNEL COMMAND LINE |
| |
| The kernel command line has become an important way for the boot |
| loader to communicate with the kernel. Some of its options are also |
| relevant to the boot loader itself, see "special command line options" |
| below. |
| |
| The kernel command line is a null-terminated string. The maximum |
| length can be retrieved from the field cmdline_size. Before protocol |
| version 2.06, the maximum was 255 characters. A string that is too |
| long will be automatically truncated by the kernel. |
| |
| If the boot protocol version is 2.02 or later, the address of the |
| kernel command line is given by the header field cmd_line_ptr (see |
| above.) This address can be anywhere between the end of the setup |
| heap and 0xA0000. |
| |
| If the protocol version is *not* 2.02 or higher, the kernel |
| command line is entered using the following protocol: |
| |
| At offset 0x0020 (word), "cmd_line_magic", enter the magic |
| number 0xA33F. |
| |
| At offset 0x0022 (word), "cmd_line_offset", enter the offset |
| of the kernel command line (relative to the start of the |
| real-mode kernel). |
| |
| The kernel command line *must* be within the memory region |
| covered by setup_move_size, so you may need to adjust this |
| field. |
| |
| |
| **** MEMORY LAYOUT OF THE REAL-MODE CODE |
| |
| The real-mode code requires a stack/heap to be set up, as well as |
| memory allocated for the kernel command line. This needs to be done |
| in the real-mode accessible memory in bottom megabyte. |
| |
| It should be noted that modern machines often have a sizable Extended |
| BIOS Data Area (EBDA). As a result, it is advisable to use as little |
| of the low megabyte as possible. |
| |
| Unfortunately, under the following circumstances the 0x90000 memory |
| segment has to be used: |
| |
| - When loading a zImage kernel ((loadflags & 0x01) == 0). |
| - When loading a 2.01 or earlier boot protocol kernel. |
| |
| -> For the 2.00 and 2.01 boot protocols, the real-mode code |
| can be loaded at another address, but it is internally |
| relocated to 0x90000. For the "old" protocol, the |
| real-mode code must be loaded at 0x90000. |
| |
| When loading at 0x90000, avoid using memory above 0x9a000. |
| |
| For boot protocol 2.02 or higher, the command line does not have to be |
| located in the same 64K segment as the real-mode setup code; it is |
| thus permitted to give the stack/heap the full 64K segment and locate |
| the command line above it. |
| |
| The kernel command line should not be located below the real-mode |
| code, nor should it be located in high memory. |
| |
| |
| **** SAMPLE BOOT CONFIGURATION |
| |
| As a sample configuration, assume the following layout of the real |
| mode 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); |
| } |
| |
| |
| **** LOADING THE REST OF THE KERNEL |
| |
| The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512 |
| in the kernel file (again, if setup_sects == 0 the real value is 4.) |
| It should be loaded at address 0x10000 for Image/zImage kernels and |
| 0x100000 for bzImage kernels. |
| |
| The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01 |
| bit (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 use |
| the entire 0x10000-0x90000 range of memory. This means it is pretty |
| much a requirement for these kernels to load the real-mode part at |
| 0x90000. bzImage kernels allow much more flexibility. |
| |
| |
| **** SPECIAL COMMAND LINE OPTIONS |
| |
| If the command line provided by the boot loader is entered by the |
| user, the user may expect the following command line options to work. |
| They should normally not be deleted from the kernel command line even |
| though not all of them are actually meaningful to the kernel. Boot |
| loader authors who need additional command line options for the boot |
| loader itself should get them registered in |
| Documentation/kernel-parameters.txt to make sure they will not |
| conflict with actual kernel options now or in the future. |
| |
| vga=<mode> |
| <mode> here is either an integer (in C notation, either |
| decimal, octal, or hexadecimal) or one of the strings |
| "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask" |
| (meaning 0xFFFD). This value should be entered into the |
| vid_mode field, as it is used by the kernel before the command |
| line 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 of |
| memory to the kernel. This affects the possible placement of |
| an initrd, since an initrd should be placed near end of |
| memory. Note that this is an option to *both* the kernel and |
| the bootloader! |
| |
| initrd=<file> |
| An initrd should be loaded. The meaning of <file> is |
| obviously 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 the |
| user-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 highly |
| recommended that they are located *first*, before the user-specified |
| or configuration-specified command line. Otherwise, "init=/bin/sh" |
| gets confused by the "auto" option. |
| |
| |
| **** RUNNING THE KERNEL |
| |
| The kernel is started by jumping to the kernel entry point, which is |
| located at *segment* offset 0x20 from the start of the real mode |
| kernel. This means that if you loaded your real-mode kernel code at |
| 0x90000, the kernel entry point is 9020:0000. |
| |
| At entry, ds = es = ss should point to the start of the real-mode |
| kernel code (0x9000 if the code is loaded at 0x90000), sp should be |
| set up properly, normally pointing to the top of the heap, and |
| interrupts should be disabled. Furthermore, to guard against bugs in |
| the 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 to |
| switch off the floppy motor before running the kernel, since the |
| kernel boot leaves interrupts off and thus the motor will not be |
| switched off, especially if the loaded kernel has the floppy driver as |
| a demand-loaded module! |
| |
| |
| **** ADVANCED BOOT LOADER HOOKS |
| |
| If the boot loader runs in a particularly hostile environment (such as |
| LOADLIN, which runs under DOS) it may be impossible to follow the |
| standard memory location requirements. Such a boot loader may use the |
| following hooks that, if set, are invoked by the kernel at the |
| appropriate time. The use of these hooks should probably be |
| considered 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 before |
| entering protected mode. The default routine disables NMI, so |
| your routine should probably do so, too. |
| |
| code32_start: |
| A 32-bit flat-mode routine *jumped* to immediately after the |
| transition to protected mode, but before the kernel is |
| uncompressed. No segments, except CS, are guaranteed to be |
| set up (current kernels do, but older ones do not); you should |
| set them up to BOOT_DS (0x18) yourself. |
| |
| After completing your hook, you should jump to the address |
| that was in this field before your boot loader overwrote it |
| (relocated, if appropriate.) |
| |
| |
| **** 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 kernel |
| based on legacy BIOS can not be used, so a 32-bit boot protocol needs |
| to be defined. |
| |
| In 32-bit boot protocol, the first step in loading a Linux kernel |
| should be to setup the boot parameters (struct boot_params, |
| traditionally known as "zero page"). The memory for struct boot_params |
| should be allocated and initialized to all zero. Then the setup header |
| from offset 0x01f1 of kernel image on should be loaded into struct |
| boot_params and examined. The end of setup header can be calculated as |
| follow: |
| |
| 0x0202 + byte value at offset 0x0201 |
| |
| In addition to read/modify/write the setup header of the struct |
| boot_params as that of 16-bit boot protocol, the boot loader should |
| also fill the additional fields of the struct boot_params as that |
| described in zero-page.txt. |
| |
| After setting up the struct boot_params, the boot loader can load the |
| 32/64-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 the |
| 32-bit kernel entry point, which is the start address of loaded |
| 32/64-bit kernel. |
| |
| At entry, the CPU must be in 32-bit protected mode with paging |
| disabled; a GDT must be loaded with the descriptors for selectors |
| __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat |
| segment; __BOOT_CS must have execute/read permission, and __BOOT_DS |
| must have read/write permission; CS must be __BOOT_CS and DS, ES, SS |
| must be __BOOT_DS; interrupt must be disabled; %esi must hold the base |
| address of the struct boot_params; %ebp, %edi and %ebx must be zero. |
| |
| **** EFI HANDOVER PROTOCOL |
| |
| This protocol allows boot loaders to defer initialisation to the EFI |
| boot stub. The boot loader is required to load the kernel/initrd(s) |
| from the boot media and jump to the EFI handover protocol entry point |
| which is hdr->handover_offset bytes from the beginning of |
| startup_{32,64}. |
| |
| 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 EFI |
| firmware, 'table' is the EFI system table - these are the first two |
| arguments of the "handoff state" as described in section 2.3 of the |
| UEFI specification. 'bp' is the boot loader-allocated boot params. |
| |
| The boot loader *must* fill out the following fields in bp, |
| |
| o hdr.code32_start |
| o hdr.cmd_line_ptr |
| o hdr.cmdline_size |
| o hdr.ramdisk_image (if applicable) |
| o hdr.ramdisk_size (if applicable) |
| |
| All other fields should be zero. |