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Diffstat (limited to 'Documentation/i386/boot.txt')
| -rw-r--r-- | Documentation/i386/boot.txt | 401 |
1 files changed, 317 insertions, 84 deletions
diff --git a/Documentation/i386/boot.txt b/Documentation/i386/boot.txt index d01b7a2a0f2..35985b34d5a 100644 --- a/Documentation/i386/boot.txt +++ b/Documentation/i386/boot.txt @@ -2,7 +2,7 @@ ---------------------------- H. Peter Anvin <hpa@zytor.com> - Last update 2007-05-07 + Last update 2007-05-23 On the i386 platform, the Linux kernel uses a rather complicated boot convention. This has evolved partially due to historical aspects, as @@ -52,7 +52,8 @@ 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. + | Command line | + | Stack/heap | For use by the kernel real-mode code. 098000 +------------------------+ | Kernel setup | The kernel real-mode code. 090200 +------------------------+ @@ -73,10 +74,9 @@ zImage kernels, typically looks like: 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. +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 @@ -93,6 +93,35 @@ 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 @@ -160,29 +189,147 @@ 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: + +**** 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_secs +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: readmode_swtch +Type: modify (optional) +Offset/size: 0x208/4 +Protocol: 2.00+ + + Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.) + +Field name: start_sys +Type: read +Offset/size: 0x20c/4 +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. + + Assigned boot loader ids: 0 LILO (0x00 reserved for pre-2.00 bootloader) 1 Loadlin 2 bootsect-loader (0x20, all other values reserved) @@ -193,60 +340,145 @@ filled out, however: 8 U-BOOT 9 Xen A Gujin + B Qemu + + 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 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: 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: initrd_addr_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 (reloc) +Offset/size: 0x230/4 +Protocol: 2.05+ + + Alignment unit required by the kernel (if relocatable_kernel is true.) + +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: cmdline_size +Type: read +Offset/size: 0x238/4 +Protocol: 2.06+ - Please contact <hpa@zytor.com> 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. - - The unit is bytes starting with the beginning of the boot - sector. - - 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.) - - cmdline_size: - 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. + 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. **** THE KERNEL COMMAND LINE @@ -494,7 +726,7 @@ switched off, especially if the loaded kernel has the floppy driver as a demand-loaded module! -**** ADVANCED BOOT TIME HOOKS +**** 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 @@ -519,4 +751,5 @@ IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and 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. + that was in this field before your boot loader overwrote it + (relocated, if appropriate.) |