Documentation/x86/boot.txt

   1                      THE LINUX/x86 BOOT PROTOCOL
   2                      ---------------------------
   3
   4 On the x86 platform, the Linux kernel uses a rather complicated boot
   5 convention.  This has evolved partially due to historical aspects, as
   6 well as the desire in the early days to have the kernel itself be a
   7 bootable image, the complicated PC memory model and due to changed
   8 expectations in the PC industry caused by the effective demise of
   9 real-mode DOS as a mainstream operating system.
  10
  11 Currently, the following versions of the Linux/x86 boot protocol exist.
  12
  13 Old kernels:    zImage/Image support only.  Some very early kernels
  14                 may not even support a command line.
  15
  16 Protocol 2.00:  (Kernel 1.3.73) Added bzImage and initrd support, as
  17                 well as a formalized way to communicate between the
  18                 boot loader and the kernel.  setup.S made relocatable,
  19                 although the traditional setup area still assumed
  20                 writable.
  21
  22 Protocol 2.01:  (Kernel 1.3.76) Added a heap overrun warning.
  23
  24 Protocol 2.02:  (Kernel 2.4.0-test3-pre3) New command line protocol.
  25                 Lower the conventional memory ceiling.  No overwrite
  26                 of the traditional setup area, thus making booting
  27                 safe for systems which use the EBDA from SMM or 32-bit
  28                 BIOS entry points.  zImage deprecated but still
  29                 supported.
  30
  31 Protocol 2.03:  (Kernel 2.4.18-pre1) Explicitly makes the highest possible
  32                 initrd address available to the bootloader.
  33
  34 Protocol 2.04:  (Kernel 2.6.14) Extend the syssize field to four bytes.
  35
  36 Protocol 2.05:  (Kernel 2.6.20) Make protected mode kernel relocatable.
  37                 Introduce relocatable_kernel and kernel_alignment fields.
  38
  39 Protocol 2.06:  (Kernel 2.6.22) Added a field that contains the size of
  40                 the boot command line.
  41
  42 Protocol 2.07:  (Kernel 2.6.24) Added paravirtualised boot protocol.
  43                 Introduced hardware_subarch and hardware_subarch_data
  44                 and KEEP_SEGMENTS flag in load_flags.
  45
  46 Protocol 2.08:  (Kernel 2.6.26) Added crc32 checksum and ELF format
  47                 payload. Introduced payload_offset and payload_length
  48                 fields to aid in locating the payload.
  49
  50 Protocol 2.09:  (Kernel 2.6.26) Added a field of 64-bit physical
  51                 pointer to single linked list of struct setup_data.
  52
  53 **** MEMORY LAYOUT
  54
  55 The traditional memory map for the kernel loader, used for Image or
  56 zImage kernels, typically looks like:
  57
  58         |                        |
  59 0A0000  +------------------------+
  60         |  Reserved for BIOS     |      Do not use.  Reserved for BIOS EBDA.
  61 09A000  +------------------------+
  62         |  Command line          |
  63         |  Stack/heap            |      For use by the kernel real-mode code.
  64 098000  +------------------------+
  65         |  Kernel setup          |      The kernel real-mode code.
  66 090200  +------------------------+
  67         |  Kernel boot sector    |      The kernel legacy boot sector.
  68 090000  +------------------------+
  69         |  Protected-mode kernel |      The bulk of the kernel image.
  70 010000  +------------------------+
  71         |  Boot loader           |      <- Boot sector entry point 0000:7C00
  72 001000  +------------------------+
  73         |  Reserved for MBR/BIOS |
  74 000800  +------------------------+
  75         |  Typically used by MBR |
  76 000600  +------------------------+
  77         |  BIOS use only         |
  78 000000  +------------------------+
  79
  80
  81 When using bzImage, the protected-mode kernel was relocated to
  82 0x100000 ("high memory"), and the kernel real-mode block (boot sector,
  83 setup, and stack/heap) was made relocatable to any address between
  84 0x10000 and end of low memory. Unfortunately, in protocols 2.00 and
  85 2.01 the 0x90000+ memory range is still used internally by the kernel;
  86 the 2.02 protocol resolves that problem.
  87
  88 It is desirable to keep the "memory ceiling" -- the highest point in
  89 low memory touched by the boot loader -- as low as possible, since
  90 some newer BIOSes have begun to allocate some rather large amounts of
  91 memory, called the Extended BIOS Data Area, near the top of low
  92 memory.  The boot loader should use the "INT 12h" BIOS call to verify
  93 how much low memory is available.
  94
  95 Unfortunately, if INT 12h reports that the amount of memory is too
  96 low, there is usually nothing the boot loader can do but to report an
  97 error to the user.  The boot loader should therefore be designed to
  98 take up as little space in low memory as it reasonably can.  For
  99 zImage or old bzImage kernels, which need data written into the
 100 0x90000 segment, the boot loader should make sure not to use memory
 101 above the 0x9A000 point; too many BIOSes will break above that point.
 102
 103 For a modern bzImage kernel with boot protocol version >= 2.02, a
 104 memory layout like the following is suggested:
 105
 106         ~                        ~
 107         |  Protected-mode kernel |
 108 100000  +------------------------+
 109         |  I/O memory hole       |
 110 0A0000  +------------------------+
 111         |  Reserved for BIOS     |      Leave as much as possible unused
 112         ~                        ~
 113         |  Command line          |      (Can also be below the X+10000 mark)
 114 X+10000 +------------------------+
 115         |  Stack/heap            |      For use by the kernel real-mode code.
 116 X+08000 +------------------------+
 117         |  Kernel setup          |      The kernel real-mode code.
 118         |  Kernel boot sector    |      The kernel legacy boot sector.
 119 X       +------------------------+
 120         |  Boot loader           |      <- Boot sector entry point 0000:7C00
 121 001000  +------------------------+
 122         |  Reserved for MBR/BIOS |
 123 000800  +------------------------+
 124         |  Typically used by MBR |
 125 000600  +------------------------+
 126         |  BIOS use only         |
 127 000000  +------------------------+
 128
 129 ... where the address X is as low as the design of the boot loader
 130 permits.
 131
 132
 133 **** THE REAL-MODE KERNEL HEADER
 134
 135 In the following text, and anywhere in the kernel boot sequence, "a
 136 sector" refers to 512 bytes.  It is independent of the actual sector
 137 size of the underlying medium.
 138
 139 The first step in loading a Linux kernel should be to load the
 140 real-mode code (boot sector and setup code) and then examine the
 141 following header at offset 0x01f1.  The real-mode code can total up to
 142 32K, although the boot loader may choose to load only the first two
 143 sectors (1K) and then examine the bootup sector size.
 144
 145 The header looks like:
 146
 147 Offset  Proto   Name            Meaning
 148 /Size
 149
 150 01F1/1  ALL(1   setup_sects     The size of the setup in sectors
 151 01F2/2  ALL     root_flags      If set, the root is mounted readonly
 152 01F4/4  2.04+(2 syssize         The size of the 32-bit code in 16-byte paras
 153 01F8/2  ALL     ram_size        DO NOT USE - for bootsect.S use only
 154 01FA/2  ALL     vid_mode        Video mode control
 155 01FC/2  ALL     root_dev        Default root device number
 156 01FE/2  ALL     boot_flag       0xAA55 magic number
 157 0200/2  2.00+   jump            Jump instruction
 158 0202/4  2.00+   header          Magic signature "HdrS"
 159 0206/2  2.00+   version         Boot protocol version supported
 160 0208/4  2.00+   realmode_swtch  Boot loader hook (see below)
 161 020C/2  2.00+   start_sys_seg   The load-low segment (0x1000) (obsolete)
 162 020E/2  2.00+   kernel_version  Pointer to kernel version string
 163 0210/1  2.00+   type_of_loader  Boot loader identifier
 164 0211/1  2.00+   loadflags       Boot protocol option flags
 165 0212/2  2.00+   setup_move_size Move to high memory size (used with hooks)
 166 0214/4  2.00+   code32_start    Boot loader hook (see below)
 167 0218/4  2.00+   ramdisk_image   initrd load address (set by boot loader)
 168 021C/4  2.00+   ramdisk_size    initrd size (set by boot loader)
 169 0220/4  2.00+   bootsect_kludge DO NOT USE - for bootsect.S use only
 170 0224/2  2.01+   heap_end_ptr    Free memory after setup end
 171 0226/2  N/A     pad1            Unused
 172 0228/4  2.02+   cmd_line_ptr    32-bit pointer to the kernel command line
 173 022C/4  2.03+   ramdisk_max     Highest legal initrd address
 174 0230/4  2.05+   kernel_alignment Physical addr alignment required for kernel
 175 0234/1  2.05+   relocatable_kernel Whether kernel is relocatable or not
 176 0235/1  N/A     pad2            Unused
 177 0236/2  N/A     pad3            Unused
 178 0238/4  2.06+   cmdline_size    Maximum size of the kernel command line
 179 023C/4  2.07+   hardware_subarch Hardware subarchitecture
 180 0240/8  2.07+   hardware_subarch_data Subarchitecture-specific data
 181 0248/4  2.08+   payload_offset  Offset of kernel payload
 182 024C/4  2.08+   payload_length  Length of kernel payload
 183 0250/8  2.09+   setup_data      64-bit physical pointer to linked list
 184                                 of struct setup_data
 185
 186 (1) For backwards compatibility, if the setup_sects field contains 0, the
 187     real value is 4.
 188
 189 (2) For boot protocol prior to 2.04, the upper two bytes of the syssize
 190     field are unusable, which means the size of a bzImage kernel
 191     cannot be determined.
 192
 193 If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
 194 the boot protocol version is "old".  Loading an old kernel, the
 195 following parameters should be assumed:
 196
 197         Image type = zImage
 198         initrd not supported
 199         Real-mode kernel must be located at 0x90000.
 200
 201 Otherwise, the "version" field contains the protocol version,
 202 e.g. protocol version 2.01 will contain 0x0201 in this field.  When
 203 setting fields in the header, you must make sure only to set fields
 204 supported by the protocol version in use.
 205
 206
 207 **** DETAILS OF HEADER FIELDS
 208
 209 For each field, some are information from the kernel to the bootloader
 210 ("read"), some are expected to be filled out by the bootloader
 211 ("write"), and some are expected to be read and modified by the
 212 bootloader ("modify").
 213
 214 All general purpose boot loaders should write the fields marked
 215 (obligatory).  Boot loaders who want to load the kernel at a
 216 nonstandard address should fill in the fields marked (reloc); other
 217 boot loaders can ignore those fields.
 218
 219 The byte order of all fields is littleendian (this is x86, after all.)
 220
 221 Field name:     setup_sects
 222 Type:           read
 223 Offset/size:    0x1f1/1
 224 Protocol:       ALL
 225
 226   The size of the setup code in 512-byte sectors.  If this field is
 227   0, the real value is 4.  The real-mode code consists of the boot
 228   sector (always one 512-byte sector) plus the setup code.
 229
 230 Field name:      root_flags
 231 Type:            modify (optional)
 232 Offset/size:     0x1f2/2
 233 Protocol:        ALL
 234
 235   If this field is nonzero, the root defaults to readonly.  The use of
 236   this field is deprecated; use the "ro" or "rw" options on the
 237   command line instead.
 238
 239 Field name:     syssize
 240 Type:           read
 241 Offset/size:    0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
 242 Protocol:       2.04+
 243
 244   The size of the protected-mode code in units of 16-byte paragraphs.
 245   For protocol versions older than 2.04 this field is only two bytes
 246   wide, and therefore cannot be trusted for the size of a kernel if
 247   the LOAD_HIGH flag is set.
 248
 249 Field name:     ram_size
 250 Type:           kernel internal
 251 Offset/size:    0x1f8/2
 252 Protocol:       ALL
 253
 254   This field is obsolete.
 255
 256 Field name:     vid_mode
 257 Type:           modify (obligatory)
 258 Offset/size:    0x1fa/2
 259
 260   Please see the section on SPECIAL COMMAND LINE OPTIONS.
 261
 262 Field name:     root_dev
 263 Type:           modify (optional)
 264 Offset/size:    0x1fc/2
 265 Protocol:       ALL
 266
 267   The default root device device number.  The use of this field is
 268   deprecated, use the "root=" option on the command line instead.
 269
 270 Field name:     boot_flag
 271 Type:           read
 272 Offset/size:    0x1fe/2
 273 Protocol:       ALL
 274
 275   Contains 0xAA55.  This is the closest thing old Linux kernels have
 276   to a magic number.
 277
 278 Field name:     jump
 279 Type:           read
 280 Offset/size:    0x200/2
 281 Protocol:       2.00+
 282
 283   Contains an x86 jump instruction, 0xEB followed by a signed offset
 284   relative to byte 0x202.  This can be used to determine the size of
 285   the header.
 286
 287 Field name:     header
 288 Type:           read
 289 Offset/size:    0x202/4
 290 Protocol:       2.00+
 291
 292   Contains the magic number "HdrS" (0x53726448).
 293
 294 Field name:     version
 295 Type:           read
 296 Offset/size:    0x206/2
 297 Protocol:       2.00+
 298
 299   Contains the boot protocol version, in (major << 8)+minor format,
 300   e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
 301   10.17.
 302
 303 Field name:     realmode_swtch
 304 Type:           modify (optional)
 305 Offset/size:    0x208/4
 306 Protocol:       2.00+
 307
 308   Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
 309
 310 Field name:     start_sys_seg
 311 Type:           read
 312 Offset/size:    0x20c/2
 313 Protocol:       2.00+
 314
 315   The load low segment (0x1000).  Obsolete.
 316
 317 Field name:     kernel_version
 318 Type:           read
 319 Offset/size:    0x20e/2
 320 Protocol:       2.00+
 321
 322   If set to a nonzero value, contains a pointer to a NUL-terminated
 323   human-readable kernel version number string, less 0x200.  This can
 324   be used to display the kernel version to the user.  This value
 325   should be less than (0x200*setup_sects).
 326
 327   For example, if this value is set to 0x1c00, the kernel version
 328   number string can be found at offset 0x1e00 in the kernel file.
 329   This is a valid value if and only if the "setup_sects" field
 330   contains the value 15 or higher, as:
 331
 332         0x1c00  < 15*0x200 (= 0x1e00) but
 333         0x1c00 >= 14*0x200 (= 0x1c00)
 334
 335         0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
 336
 337 Field name:     type_of_loader
 338 Type:           write (obligatory)
 339 Offset/size:    0x210/1
 340 Protocol:       2.00+
 341
 342   If your boot loader has an assigned id (see table below), enter
 343   0xTV here, where T is an identifier for the boot loader and V is
 344   a version number.  Otherwise, enter 0xFF here.
 345
 346   Assigned boot loader ids:
 347         0  LILO                 (0x00 reserved for pre-2.00 bootloader)
 348         1  Loadlin
 349         2  bootsect-loader      (0x20, all other values reserved)
 350         3  SYSLINUX
 351         4  EtherBoot
 352         5  ELILO
 353         7  GRUB
 354         8  U-BOOT
 355         9  Xen
 356         A  Gujin
 357         B  Qemu
 358
 359   Please contact <hpa@zytor.com> if you need a bootloader ID
 360   value assigned.
 361
 362 Field name:     loadflags
 363 Type:           modify (obligatory)
 364 Offset/size:    0x211/1
 365 Protocol:       2.00+
 366
 367   This field is a bitmask.
 368
 369   Bit 0 (read): LOADED_HIGH
 370         - If 0, the protected-mode code is loaded at 0x10000.
 371         - If 1, the protected-mode code is loaded at 0x100000.
 372
 373   Bit 5 (write): QUIET_FLAG
 374         - If 0, print early messages.
 375         - If 1, suppress early messages.
 376                 This requests to the kernel (decompressor and early
 377                 kernel) to not write early messages that require
 378                 accessing the display hardware directly.
 379
 380   Bit 6 (write): KEEP_SEGMENTS
 381         Protocol: 2.07+
 382         - If 0, reload the segment registers in the 32bit entry point.
 383         - If 1, do not reload the segment registers in the 32bit entry point.
 384                 Assume that %cs %ds %ss %es are all set to flat segments with
 385                 a base of 0 (or the equivalent for their environment).
 386
 387   Bit 7 (write): CAN_USE_HEAP
 388         Set this bit to 1 to indicate that the value entered in the
 389         heap_end_ptr is valid.  If this field is clear, some setup code
 390         functionality will be disabled.
 391
 392 Field name:     setup_move_size
 393 Type:           modify (obligatory)
 394 Offset/size:    0x212/2
 395 Protocol:       2.00-2.01
 396
 397   When using protocol 2.00 or 2.01, if the real mode kernel is not
 398   loaded at 0x90000, it gets moved there later in the loading
 399   sequence.  Fill in this field if you want additional data (such as
 400   the kernel command line) moved in addition to the real-mode kernel
 401   itself.
 402
 403   The unit is bytes starting with the beginning of the boot sector.
 404
 405   This field is can be ignored when the protocol is 2.02 or higher, or
 406   if the real-mode code is loaded at 0x90000.
 407
 408 Field name:     code32_start
 409 Type:           modify (optional, reloc)
 410 Offset/size:    0x214/4
 411 Protocol:       2.00+
 412
 413   The address to jump to in protected mode.  This defaults to the load
 414   address of the kernel, and can be used by the boot loader to
 415   determine the proper load address.
 416
 417   This field can be modified for two purposes:
 418
 419   1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
 420
 421   2. if a bootloader which does not install a hook loads a
 422      relocatable kernel at a nonstandard address it will have to modify
 423      this field to point to the load address.
 424
 425 Field name:     ramdisk_image
 426 Type:           write (obligatory)
 427 Offset/size:    0x218/4
 428 Protocol:       2.00+
 429
 430   The 32-bit linear address of the initial ramdisk or ramfs.  Leave at
 431   zero if there is no initial ramdisk/ramfs.
 432
 433 Field name:     ramdisk_size
 434 Type:           write (obligatory)
 435 Offset/size:    0x21c/4
 436 Protocol:       2.00+
 437
 438   Size of the initial ramdisk or ramfs.  Leave at zero if there is no
 439   initial ramdisk/ramfs.
 440
 441 Field name:     bootsect_kludge
 442 Type:           kernel internal
 443 Offset/size:    0x220/4
 444 Protocol:       2.00+
 445
 446   This field is obsolete.
 447
 448 Field name:     heap_end_ptr
 449 Type:           write (obligatory)
 450 Offset/size:    0x224/2
 451 Protocol:       2.01+
 452
 453   Set this field to the offset (from the beginning of the real-mode
 454   code) of the end of the setup stack/heap, minus 0x0200.
 455
 456 Field name:     cmd_line_ptr
 457 Type:           write (obligatory)
 458 Offset/size:    0x228/4
 459 Protocol:       2.02+
 460
 461   Set this field to the linear address of the kernel command line.
 462   The kernel command line can be located anywhere between the end of
 463   the setup heap and 0xA0000; it does not have to be located in the
 464   same 64K segment as the real-mode code itself.
 465
 466   Fill in this field even if your boot loader does not support a
 467   command line, in which case you can point this to an empty string
 468   (or better yet, to the string "auto".)  If this field is left at
 469   zero, the kernel will assume that your boot loader does not support
 470   the 2.02+ protocol.
 471
 472 Field name:     ramdisk_max
 473 Type:           read
 474 Offset/size:    0x22c/4
 475 Protocol:       2.03+
 476
 477   The maximum address that may be occupied by the initial
 478   ramdisk/ramfs contents.  For boot protocols 2.02 or earlier, this
 479   field is not present, and the maximum address is 0x37FFFFFF.  (This
 480   address is defined as the address of the highest safe byte, so if
 481   your ramdisk is exactly 131072 bytes long and this field is
 482   0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
 483
 484 Field name:     kernel_alignment
 485 Type:           read (reloc)
 486 Offset/size:    0x230/4
 487 Protocol:       2.05+
 488
 489   Alignment unit required by the kernel (if relocatable_kernel is true.)
 490
 491 Field name:     relocatable_kernel
 492 Type:           read (reloc)
 493 Offset/size:    0x234/1
 494 Protocol:       2.05+
 495
 496   If this field is nonzero, the protected-mode part of the kernel can
 497   be loaded at any address that satisfies the kernel_alignment field.
 498   After loading, the boot loader must set the code32_start field to
 499   point to the loaded code, or to a boot loader hook.
 500
 501 Field name:     cmdline_size
 502 Type:           read
 503 Offset/size:    0x238/4
 504 Protocol:       2.06+
 505
 506   The maximum size of the command line without the terminating
 507   zero. This means that the command line can contain at most
 508   cmdline_size characters. With protocol version 2.05 and earlier, the
 509   maximum size was 255.
 510
 511 Field name:     hardware_subarch
 512 Type:           write (optional, defaults to x86/PC)
 513 Offset/size:    0x23c/4
 514 Protocol:       2.07+
 515
 516   In a paravirtualized environment the hardware low level architectural
 517   pieces such as interrupt handling, page table handling, and
 518   accessing process control registers needs to be done differently.
 519
 520   This field allows the bootloader to inform the kernel we are in one
 521   one of those environments.
 522
 523   0x00000000    The default x86/PC environment
 524   0x00000001    lguest
 525   0x00000002    Xen
 526
 527 Field name:     hardware_subarch_data
 528 Type:           write (subarch-dependent)
 529 Offset/size:    0x240/8
 530 Protocol:       2.07+
 531
 532   A pointer to data that is specific to hardware subarch
 533   This field is currently unused for the default x86/PC environment,
 534   do not modify.
 535
 536 Field name:     payload_offset
 537 Type:           read
 538 Offset/size:    0x248/4
 539 Protocol:       2.08+
 540
 541   If non-zero then this field contains the offset from the beginning
 542   of the protected-mode code to the payload.
 543
 544   The payload may be compressed. The format of both the compressed and
 545   uncompressed data should be determined using the standard magic
 546   numbers.  The currently supported compression formats are gzip
 547   (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A) and LZMA
 548   (magic number 5D 00).  The uncompressed payload is currently always ELF
 549   (magic number 7F 45 4C 46).
 550
 551 Field name:     payload_length
 552 Type:           read
 553 Offset/size:    0x24c/4
 554 Protocol:       2.08+
 555
 556   The length of the payload.
 557
 558 Field name:     setup_data
 559 Type:           write (special)
 560 Offset/size:    0x250/8
 561 Protocol:       2.09+
 562
 563   The 64-bit physical pointer to NULL terminated single linked list of
 564   struct setup_data. This is used to define a more extensible boot
 565   parameters passing mechanism. The definition of struct setup_data is
 566   as follow:
 567
 568   struct setup_data {
 569           u64 next;
 570           u32 type;
 571           u32 len;
 572           u8  data[0];
 573   };
 574
 575   Where, the next is a 64-bit physical pointer to the next node of
 576   linked list, the next field of the last node is 0; the type is used
 577   to identify the contents of data; the len is the length of data
 578   field; the data holds the real payload.
 579
 580   This list may be modified at a number of points during the bootup
 581   process.  Therefore, when modifying this list one should always make
 582   sure to consider the case where the linked list already contains
 583   entries.
 584
 585
 586 **** THE IMAGE CHECKSUM
 587
 588 From boot protocol version 2.08 onwards the CRC-32 is calculated over
 589 the entire file using the characteristic polynomial 0x04C11DB7 and an
 590 initial remainder of 0xffffffff.  The checksum is appended to the
 591 file; therefore the CRC of the file up to the limit specified in the
 592 syssize field of the header is always 0.
 593
 594
 595 **** THE KERNEL COMMAND LINE
 596
 597 The kernel command line has become an important way for the boot
 598 loader to communicate with the kernel.  Some of its options are also
 599 relevant to the boot loader itself, see "special command line options"
 600 below.
 601
 602 The kernel command line is a null-terminated string. The maximum
 603 length can be retrieved from the field cmdline_size.  Before protocol
 604 version 2.06, the maximum was 255 characters.  A string that is too
 605 long will be automatically truncated by the kernel.
 606
 607 If the boot protocol version is 2.02 or later, the address of the
 608 kernel command line is given by the header field cmd_line_ptr (see
 609 above.)  This address can be anywhere between the end of the setup
 610 heap and 0xA0000.
 611
 612 If the protocol version is *not* 2.02 or higher, the kernel
 613 command line is entered using the following protocol:
 614
 615         At offset 0x0020 (word), "cmd_line_magic", enter the magic
 616         number 0xA33F.
 617
 618         At offset 0x0022 (word), "cmd_line_offset", enter the offset
 619         of the kernel command line (relative to the start of the
 620         real-mode kernel).
 621
 622         The kernel command line *must* be within the memory region
 623         covered by setup_move_size, so you may need to adjust this
 624         field.
 625
 626
 627 **** MEMORY LAYOUT OF THE REAL-MODE CODE
 628
 629 The real-mode code requires a stack/heap to be set up, as well as
 630 memory allocated for the kernel command line.  This needs to be done
 631 in the real-mode accessible memory in bottom megabyte.
 632
 633 It should be noted that modern machines often have a sizable Extended
 634 BIOS Data Area (EBDA).  As a result, it is advisable to use as little
 635 of the low megabyte as possible.
 636
 637 Unfortunately, under the following circumstances the 0x90000 memory
 638 segment has to be used:
 639
 640         - When loading a zImage kernel ((loadflags & 0x01) == 0).
 641         - When loading a 2.01 or earlier boot protocol kernel.
 642
 643           -> For the 2.00 and 2.01 boot protocols, the real-mode code
 644              can be loaded at another address, but it is internally
 645              relocated to 0x90000.  For the "old" protocol, the
 646              real-mode code must be loaded at 0x90000.
 647
 648 When loading at 0x90000, avoid using memory above 0x9a000.
 649
 650 For boot protocol 2.02 or higher, the command line does not have to be
 651 located in the same 64K segment as the real-mode setup code; it is
 652 thus permitted to give the stack/heap the full 64K segment and locate
 653 the command line above it.
 654
 655 The kernel command line should not be located below the real-mode
 656 code, nor should it be located in high memory.
 657
 658
 659 **** SAMPLE BOOT CONFIGURATION
 660
 661 As a sample configuration, assume the following layout of the real
 662 mode segment:
 663
 664     When loading below 0x90000, use the entire segment:
 665
 666         0x0000-0x7fff   Real mode kernel
 667         0x8000-0xdfff   Stack and heap
 668         0xe000-0xffff   Kernel command line
 669
 670     When loading at 0x90000 OR the protocol version is 2.01 or earlier:
 671
 672         0x0000-0x7fff   Real mode kernel
 673         0x8000-0x97ff   Stack and heap
 674         0x9800-0x9fff   Kernel command line
 675
 676 Such a boot loader should enter the following fields in the header:
 677
 678         unsigned long base_ptr; /* base address for real-mode segment */
 679
 680         if ( setup_sects == 0 ) {
 681                 setup_sects = 4;
 682         }
 683
 684         if ( protocol >= 0x0200 ) {
 685                 type_of_loader = <type code>;
 686                 if ( loading_initrd ) {
 687                         ramdisk_image = <initrd_address>;
 688                         ramdisk_size = <initrd_size>;
 689                 }
 690
 691                 if ( protocol >= 0x0202 && loadflags & 0x01 )
 692                         heap_end = 0xe000;
 693                 else
 694                         heap_end = 0x9800;
 695
 696                 if ( protocol >= 0x0201 ) {
 697                         heap_end_ptr = heap_end - 0x200;
 698                         loadflags |= 0x80; /* CAN_USE_HEAP */
 699                 }
 700
 701                 if ( protocol >= 0x0202 ) {
 702                         cmd_line_ptr = base_ptr + heap_end;
 703                         strcpy(cmd_line_ptr, cmdline);
 704                 } else {
 705                         cmd_line_magic  = 0xA33F;
 706                         cmd_line_offset = heap_end;
 707                         setup_move_size = heap_end + strlen(cmdline)+1;
 708                         strcpy(base_ptr+cmd_line_offset, cmdline);
 709                 }
 710         } else {
 711                 /* Very old kernel */
 712
 713                 heap_end = 0x9800;
 714
 715                 cmd_line_magic  = 0xA33F;
 716                 cmd_line_offset = heap_end;
 717
 718                 /* A very old kernel MUST have its real-mode code
 719                    loaded at 0x90000 */
 720
 721                 if ( base_ptr != 0x90000 ) {
 722                         /* Copy the real-mode kernel */
 723                         memcpy(0x90000, base_ptr, (setup_sects+1)*512);
 724                         base_ptr = 0x90000;              /* Relocated */
 725                 }
 726
 727                 strcpy(0x90000+cmd_line_offset, cmdline);
 728
 729                 /* It is recommended to clear memory up to the 32K mark */
 730                 memset(0x90000 + (setup_sects+1)*512, 0,
 731                        (64-(setup_sects+1))*512);
 732         }
 733
 734
 735 **** LOADING THE REST OF THE KERNEL
 736
 737 The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
 738 in the kernel file (again, if setup_sects == 0 the real value is 4.)
 739 It should be loaded at address 0x10000 for Image/zImage kernels and
 740 0x100000 for bzImage kernels.
 741
 742 The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
 743 bit (LOAD_HIGH) in the loadflags field is set:
 744
 745         is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
 746         load_address = is_bzImage ? 0x100000 : 0x10000;
 747
 748 Note that Image/zImage kernels can be up to 512K in size, and thus use
 749 the entire 0x10000-0x90000 range of memory.  This means it is pretty
 750 much a requirement for these kernels to load the real-mode part at
 751 0x90000.  bzImage kernels allow much more flexibility.
 752
 753
 754 **** SPECIAL COMMAND LINE OPTIONS
 755
 756 If the command line provided by the boot loader is entered by the
 757 user, the user may expect the following command line options to work.
 758 They should normally not be deleted from the kernel command line even
 759 though not all of them are actually meaningful to the kernel.  Boot
 760 loader authors who need additional command line options for the boot
 761 loader itself should get them registered in
 762 Documentation/kernel-parameters.txt to make sure they will not
 763 conflict with actual kernel options now or in the future.
 764
 765   vga=<mode>
 766         <mode> here is either an integer (in C notation, either
 767         decimal, octal, or hexadecimal) or one of the strings
 768         "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
 769         (meaning 0xFFFD).  This value should be entered into the
 770         vid_mode field, as it is used by the kernel before the command
 771         line is parsed.
 772
 773   mem=<size>
 774         <size> is an integer in C notation optionally followed by
 775         (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
 776         << 30, << 40, << 50 or << 60).  This specifies the end of
 777         memory to the kernel. This affects the possible placement of
 778         an initrd, since an initrd should be placed near end of
 779         memory.  Note that this is an option to *both* the kernel and
 780         the bootloader!
 781
 782   initrd=<file>
 783         An initrd should be loaded.  The meaning of <file> is
 784         obviously bootloader-dependent, and some boot loaders
 785         (e.g. LILO) do not have such a command.
 786
 787 In addition, some boot loaders add the following options to the
 788 user-specified command line:
 789
 790   BOOT_IMAGE=<file>
 791         The boot image which was loaded.  Again, the meaning of <file>
 792         is obviously bootloader-dependent.
 793
 794   auto
 795         The kernel was booted without explicit user intervention.
 796
 797 If these options are added by the boot loader, it is highly
 798 recommended that they are located *first*, before the user-specified
 799 or configuration-specified command line.  Otherwise, "init=/bin/sh"
 800 gets confused by the "auto" option.
 801
 802
 803 **** RUNNING THE KERNEL
 804
 805 The kernel is started by jumping to the kernel entry point, which is
 806 located at *segment* offset 0x20 from the start of the real mode
 807 kernel.  This means that if you loaded your real-mode kernel code at
 808 0x90000, the kernel entry point is 9020:0000.
 809
 810 At entry, ds = es = ss should point to the start of the real-mode
 811 kernel code (0x9000 if the code is loaded at 0x90000), sp should be
 812 set up properly, normally pointing to the top of the heap, and
 813 interrupts should be disabled.  Furthermore, to guard against bugs in
 814 the kernel, it is recommended that the boot loader sets fs = gs = ds =
 815 es = ss.
 816
 817 In our example from above, we would do:
 818
 819         /* Note: in the case of the "old" kernel protocol, base_ptr must
 820            be == 0x90000 at this point; see the previous sample code */
 821
 822         seg = base_ptr >> 4;
 823
 824         cli();  /* Enter with interrupts disabled! */
 825
 826         /* Set up the real-mode kernel stack */
 827         _SS = seg;
 828         _SP = heap_end;
 829
 830         _DS = _ES = _FS = _GS = seg;
 831         jmp_far(seg+0x20, 0);   /* Run the kernel */
 832
 833 If your boot sector accesses a floppy drive, it is recommended to
 834 switch off the floppy motor before running the kernel, since the
 835 kernel boot leaves interrupts off and thus the motor will not be
 836 switched off, especially if the loaded kernel has the floppy driver as
 837 a demand-loaded module!
 838
 839
 840 **** ADVANCED BOOT LOADER HOOKS
 841
 842 If the boot loader runs in a particularly hostile environment (such as
 843 LOADLIN, which runs under DOS) it may be impossible to follow the
 844 standard memory location requirements.  Such a boot loader may use the
 845 following hooks that, if set, are invoked by the kernel at the
 846 appropriate time.  The use of these hooks should probably be
 847 considered an absolutely last resort!
 848
 849 IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
 850 %edi across invocation.
 851
 852   realmode_swtch:
 853         A 16-bit real mode far subroutine invoked immediately before
 854         entering protected mode.  The default routine disables NMI, so
 855         your routine should probably do so, too.
 856
 857   code32_start:
 858         A 32-bit flat-mode routine *jumped* to immediately after the
 859         transition to protected mode, but before the kernel is
 860         uncompressed.  No segments, except CS, are guaranteed to be
 861         set up (current kernels do, but older ones do not); you should
 862         set them up to BOOT_DS (0x18) yourself.
 863
 864         After completing your hook, you should jump to the address
 865         that was in this field before your boot loader overwrote it
 866         (relocated, if appropriate.)
 867
 868
 869 **** 32-bit BOOT PROTOCOL
 870
 871 For machine with some new BIOS other than legacy BIOS, such as EFI,
 872 LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
 873 based on legacy BIOS can not be used, so a 32-bit boot protocol needs
 874 to be defined.
 875
 876 In 32-bit boot protocol, the first step in loading a Linux kernel
 877 should be to setup the boot parameters (struct boot_params,
 878 traditionally known as "zero page"). The memory for struct boot_params
 879 should be allocated and initialized to all zero. Then the setup header
 880 from offset 0x01f1 of kernel image on should be loaded into struct
 881 boot_params and examined. The end of setup header can be calculated as
 882 follow:
 883
 884         0x0202 + byte value at offset 0x0201
 885
 886 In addition to read/modify/write the setup header of the struct
 887 boot_params as that of 16-bit boot protocol, the boot loader should
 888 also fill the additional fields of the struct boot_params as that
 889 described in zero-page.txt.
 890
 891 After setupping the struct boot_params, the boot loader can load the
 892 32/64-bit kernel in the same way as that of 16-bit boot protocol.
 893
 894 In 32-bit boot protocol, the kernel is started by jumping to the
 895 32-bit kernel entry point, which is the start address of loaded
 896 32/64-bit kernel.
 897
 898 At entry, the CPU must be in 32-bit protected mode with paging
 899 disabled; a GDT must be loaded with the descriptors for selectors
 900 __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
 901 segment; __BOOS_CS must have execute/read permission, and __BOOT_DS
 902 must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
 903 must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
 904 address of the struct boot_params; %ebp, %edi and %ebx must be zero.