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+ ----------------------------
+ H. Peter Anvin <>
+ Last update 2002-01-01
+On the i386 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, four versions of the Linux/i386 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.
+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 +------------------------+
+ | Stack/heap/cmdline | 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 command line is still required to live in the 0x9XXXX memory
+range, and that memory range is still overwritten by the early kernel.
+The 2.02 protocol resolves that problem.
+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.
+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
+01F1/1 ALL setup_sects The size of the setup in sectors
+01F2/2 ALL root_flags If set, the root is mounted readonly
+01F4/2 ALL syssize DO NOT USE - for bootsect.S use only
+01F6/2 ALL swap_dev DO NOT USE - obsolete
+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 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
+0226/2 N/A pad1 Unused
+0228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
+022C/4 2.03+ initrd_addr_max Highest legal initrd address
+For backwards compatibility, if the setup_sects field contains 0, the
+real value is 4.
+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.
+The "kernel_version" field, if set to a nonzero value, contains a
+pointer to a null-terminated human-readable kernel version number
+string, less 0x200. This can be used to display the kernel version to
+the user. This value should be less than (0x200*setup_sects). For
+example, if this value is set to 0x1c00, the kernel version number
+string can be found at offset 0x1e00 in the kernel file. This is a
+valid value if and only if the "setup_sects" field contains the value
+14 or higher.
+Most boot loaders will simply load the kernel at its target address
+directly. Such boot loaders do not need to worry about filling in
+most of the fields in the header. The following fields should be
+filled out, however:
+ vid_mode:
+ Please see the section on SPECIAL COMMAND LINE OPTIONS.
+ type_of_loader:
+ If your boot loader has an assigned id (see table below), enter
+ 0xTV here, where T is an identifier for the boot loader and V is
+ a version number. Otherwise, enter 0xFF here.
+ Assigned boot loader ids:
+ 0 LILO
+ 1 Loadlin
+ 2 bootsect-loader
+ 4 EtherBoot
+ 7 GRuB
+ 8 U-BOOT
+ Please contact <> if you need a bootloader ID
+ value assigned.
+ loadflags, heap_end_ptr:
+ If the protocol version is 2.01 or higher, enter the
+ offset limit of the setup heap into heap_end_ptr and set the
+ 0x80 bit (CAN_USE_HEAP) of loadflags. heap_end_ptr appears to
+ be relative to the start of setup (offset 0x0200).
+ setup_move_size:
+ When using protocol 2.00 or 2.01, if the real mode
+ kernel is not loaded at 0x90000, it gets moved there later in
+ the loading sequence. Fill in this field if you want
+ additional data (such as the kernel command line) moved in
+ addition to the real-mode kernel itself.
+ ramdisk_image, ramdisk_size:
+ If your boot loader has loaded an initial ramdisk (initrd),
+ set ramdisk_image to the 32-bit pointer to the ramdisk data
+ and the ramdisk_size to the size of the ramdisk data.
+ The initrd should typically be located as high in memory as
+ possible, as it may otherwise get overwritten by the early
+ kernel initialization sequence. However, it must never be
+ located above the address specified in the initrd_addr_max
+ field. The initrd should be at least 4K page aligned.
+ cmd_line_ptr:
+ If the protocol version is 2.02 or higher, this is a 32-bit
+ pointer to the kernel command line. The kernel command line
+ can be located anywhere between the end of setup and 0xA0000.
+ Fill in this field even if your boot loader does not support a
+ command line, in which case you can point this to an empty
+ string (or better yet, to the string "auto".) If this field
+ is left at zero, the kernel will assume that your boot loader
+ does not support the 2.02+ protocol.
+ ramdisk_max:
+ The maximum address that may be occupied by the initrd
+ contents. For boot protocols 2.02 or earlier, this field is
+ not present, and the maximum address is 0x37FFFFFF. (This
+ address is defined as the address of the highest safe byte, so
+ if your ramdisk is exactly 131072 bytes long and this field is
+ 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
+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"
+The kernel command line is a null-terminated string up to 255
+characters long, plus the final null.
+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
+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.
+As a sample configuration, assume the following layout of the real
+mode segment:
+ 0x0000-0x7FFF Real mode kernel
+ 0x8000-0x8FFF Stack and heap
+ 0x9000-0x90FF 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 >= 0x0201 ) {
+ heap_end_ptr = 0x9000 - 0x200;
+ loadflags |= 0x80; /* CAN_USE_HEAP */
+ }
+ if ( protocol >= 0x0202 ) {
+ cmd_line_ptr = base_ptr + 0x9000;
+ } else {
+ cmd_line_magic = 0xA33F;
+ cmd_line_offset = 0x9000;
+ setup_move_size = 0x9100;
+ }
+ } else {
+ /* Very old kernel */
+ cmd_line_magic = 0xA33F;
+ cmd_line_offset = 0x9000;
+ /* 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);
+ /* Copy the command line */
+ memcpy(0x99000, base_ptr+0x9000, 256);
+ base_ptr = 0x90000; /* Relocated */
+ }
+ /* It is recommended to clear memory up to the 32K mark */
+ memset(0x90000 + (setup_sects+1)*512, 0,
+ (64-(setup_sects+1))*512);
+ }
+The 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.
+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 K, M
+ or G (meaning << 10, << 20 or << 30). 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.
+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 = 0x9000; /* Load SP immediately after loading SS! */
+ _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!
+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 set up; you should
+ set them up to KERNEL_DS (0x18) yourself.
+ After completing your hook, you should jump to the address
+ that was in this field before your boot loader overwrote it.