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 Protocol 2.10:  (Kernel 2.6.31) Added a protocol for relaxed alignment
  54                 beyond the kernel_alignment added, new init_size and
  55                 pref_address fields.  Added extended boot loader IDs.
  56
  57 Protocol 2.11:  (Kernel 3.6) Added a field for offset of EFI handover
  58                 protocol entry point.
  59
  60 Protocol 2.12:  (Kernel 3.8) Added the xloadflags field and extension fields
  61                 to struct boot_params for for loading bzImage and ramdisk
  62                 above 4G in 64bit.
  63
  64 **** MEMORY LAYOUT
  65
  66 The traditional memory map for the kernel loader, used for Image or
  67 zImage kernels, typically looks like:
  68
  69         |                        |
  70 0A0000  +------------------------+
  71         |  Reserved for BIOS     |      Do not use.  Reserved for BIOS EBDA.
  72 09A000  +------------------------+
  73         |  Command line          |
  74         |  Stack/heap            |      For use by the kernel real-mode code.
  75 098000  +------------------------+
  76         |  Kernel setup          |      The kernel real-mode code.
  77 090200  +------------------------+
  78         |  Kernel boot sector    |      The kernel legacy boot sector.
  79 090000  +------------------------+
  80         |  Protected-mode kernel |      The bulk of the kernel image.
  81 010000  +------------------------+
  82         |  Boot loader           |      <- Boot sector entry point 0000:7C00
  83 001000  +------------------------+
  84         |  Reserved for MBR/BIOS |
  85 000800  +------------------------+
  86         |  Typically used by MBR |
  87 000600  +------------------------+
  88         |  BIOS use only         |
  89 000000  +------------------------+
  90
  91
  92 When using bzImage, the protected-mode kernel was relocated to
  93 0x100000 ("high memory"), and the kernel real-mode block (boot sector,
  94 setup, and stack/heap) was made relocatable to any address between
  95 0x10000 and end of low memory. Unfortunately, in protocols 2.00 and
  96 2.01 the 0x90000+ memory range is still used internally by the kernel;
  97 the 2.02 protocol resolves that problem.
  98
  99 It is desirable to keep the "memory ceiling" -- the highest point in
 100 low memory touched by the boot loader -- as low as possible, since
 101 some newer BIOSes have begun to allocate some rather large amounts of
 102 memory, called the Extended BIOS Data Area, near the top of low
 103 memory.  The boot loader should use the "INT 12h" BIOS call to verify
 104 how much low memory is available.
 105
 106 Unfortunately, if INT 12h reports that the amount of memory is too
 107 low, there is usually nothing the boot loader can do but to report an
 108 error to the user.  The boot loader should therefore be designed to
 109 take up as little space in low memory as it reasonably can.  For
 110 zImage or old bzImage kernels, which need data written into the
 111 0x90000 segment, the boot loader should make sure not to use memory
 112 above the 0x9A000 point; too many BIOSes will break above that point.
 113
 114 For a modern bzImage kernel with boot protocol version >= 2.02, a
 115 memory layout like the following is suggested:
 116
 117         ~                        ~
 118         |  Protected-mode kernel |
 119 100000  +------------------------+
 120         |  I/O memory hole       |
 121 0A0000  +------------------------+
 122         |  Reserved for BIOS     |      Leave as much as possible unused
 123         ~                        ~
 124         |  Command line          |      (Can also be below the X+10000 mark)
 125 X+10000 +------------------------+
 126         |  Stack/heap            |      For use by the kernel real-mode code.
 127 X+08000 +------------------------+
 128         |  Kernel setup          |      The kernel real-mode code.
 129         |  Kernel boot sector    |      The kernel legacy boot sector.
 130 X       +------------------------+
 131         |  Boot loader           |      <- Boot sector entry point 0000:7C00
 132 001000  +------------------------+
 133         |  Reserved for MBR/BIOS |
 134 000800  +------------------------+
 135         |  Typically used by MBR |
 136 000600  +------------------------+
 137         |  BIOS use only         |
 138 000000  +------------------------+
 139
 140 ... where the address X is as low as the design of the boot loader
 141 permits.
 142
 143
 144 **** THE REAL-MODE KERNEL HEADER
 145
 146 In the following text, and anywhere in the kernel boot sequence, "a
 147 sector" refers to 512 bytes.  It is independent of the actual sector
 148 size of the underlying medium.
 149
 150 The first step in loading a Linux kernel should be to load the
 151 real-mode code (boot sector and setup code) and then examine the
 152 following header at offset 0x01f1.  The real-mode code can total up to
 153 32K, although the boot loader may choose to load only the first two
 154 sectors (1K) and then examine the bootup sector size.
 155
 156 The header looks like:
 157
 158 Offset  Proto   Name            Meaning
 159 /Size
 160
 161 01F1/1  ALL(1   setup_sects     The size of the setup in sectors
 162 01F2/2  ALL     root_flags      If set, the root is mounted readonly
 163 01F4/4  2.04+(2 syssize         The size of the 32-bit code in 16-byte paras
 164 01F8/2  ALL     ram_size        DO NOT USE - for bootsect.S use only
 165 01FA/2  ALL     vid_mode        Video mode control
 166 01FC/2  ALL     root_dev        Default root device number
 167 01FE/2  ALL     boot_flag       0xAA55 magic number
 168 0200/2  2.00+   jump            Jump instruction
 169 0202/4  2.00+   header          Magic signature "HdrS"
 170 0206/2  2.00+   version         Boot protocol version supported
 171 0208/4  2.00+   realmode_swtch  Boot loader hook (see below)
 172 020C/2  2.00+   start_sys_seg   The load-low segment (0x1000) (obsolete)
 173 020E/2  2.00+   kernel_version  Pointer to kernel version string
 174 0210/1  2.00+   type_of_loader  Boot loader identifier
 175 0211/1  2.00+   loadflags       Boot protocol option flags
 176 0212/2  2.00+   setup_move_size Move to high memory size (used with hooks)
 177 0214/4  2.00+   code32_start    Boot loader hook (see below)
 178 0218/4  2.00+   ramdisk_image   initrd load address (set by boot loader)
 179 021C/4  2.00+   ramdisk_size    initrd size (set by boot loader)
 180 0220/4  2.00+   bootsect_kludge DO NOT USE - for bootsect.S use only
 181 0224/2  2.01+   heap_end_ptr    Free memory after setup end
 182 0226/1  2.02+(3 ext_loader_ver  Extended boot loader version
 183 0227/1  2.02+(3 ext_loader_type Extended boot loader ID
 184 0228/4  2.02+   cmd_line_ptr    32-bit pointer to the kernel command line
 185 022C/4  2.03+   ramdisk_max     Highest legal initrd address
 186 0230/4  2.05+   kernel_alignment Physical addr alignment required for kernel
 187 0234/1  2.05+   relocatable_kernel Whether kernel is relocatable or not
 188 0235/1  2.10+   min_alignment   Minimum alignment, as a power of two
 189 0236/2  2.12+   xloadflags      Boot protocol option flags
 190 0238/4  2.06+   cmdline_size    Maximum size of the kernel command line
 191 023C/4  2.07+   hardware_subarch Hardware subarchitecture
 192 0240/8  2.07+   hardware_subarch_data Subarchitecture-specific data
 193 0248/4  2.08+   payload_offset  Offset of kernel payload
 194 024C/4  2.08+   payload_length  Length of kernel payload
 195 0250/8  2.09+   setup_data      64-bit physical pointer to linked list
 196                                 of struct setup_data
 197 0258/8  2.10+   pref_address    Preferred loading address
 198 0260/4  2.10+   init_size       Linear memory required during initialization
 199 0264/4  2.11+   handover_offset Offset of handover entry point
 200
 201 (1) For backwards compatibility, if the setup_sects field contains 0, the
 202     real value is 4.
 203
 204 (2) For boot protocol prior to 2.04, the upper two bytes of the syssize
 205     field are unusable, which means the size of a bzImage kernel
 206     cannot be determined.
 207
 208 (3) Ignored, but safe to set, for boot protocols 2.02-2.09.
 209
 210 If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
 211 the boot protocol version is "old".  Loading an old kernel, the
 212 following parameters should be assumed:
 213
 214         Image type = zImage
 215         initrd not supported
 216         Real-mode kernel must be located at 0x90000.
 217
 218 Otherwise, the "version" field contains the protocol version,
 219 e.g. protocol version 2.01 will contain 0x0201 in this field.  When
 220 setting fields in the header, you must make sure only to set fields
 221 supported by the protocol version in use.
 222
 223
 224 **** DETAILS OF HEADER FIELDS
 225
 226 For each field, some are information from the kernel to the bootloader
 227 ("read"), some are expected to be filled out by the bootloader
 228 ("write"), and some are expected to be read and modified by the
 229 bootloader ("modify").
 230
 231 All general purpose boot loaders should write the fields marked
 232 (obligatory).  Boot loaders who want to load the kernel at a
 233 nonstandard address should fill in the fields marked (reloc); other
 234 boot loaders can ignore those fields.
 235
 236 The byte order of all fields is littleendian (this is x86, after all.)
 237
 238 Field name:     setup_sects
 239 Type:           read
 240 Offset/size:    0x1f1/1
 241 Protocol:       ALL
 242
 243   The size of the setup code in 512-byte sectors.  If this field is
 244   0, the real value is 4.  The real-mode code consists of the boot
 245   sector (always one 512-byte sector) plus the setup code.
 246
 247 Field name:      root_flags
 248 Type:            modify (optional)
 249 Offset/size:     0x1f2/2
 250 Protocol:        ALL
 251
 252   If this field is nonzero, the root defaults to readonly.  The use of
 253   this field is deprecated; use the "ro" or "rw" options on the
 254   command line instead.
 255
 256 Field name:     syssize
 257 Type:           read
 258 Offset/size:    0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
 259 Protocol:       2.04+
 260
 261   The size of the protected-mode code in units of 16-byte paragraphs.
 262   For protocol versions older than 2.04 this field is only two bytes
 263   wide, and therefore cannot be trusted for the size of a kernel if
 264   the LOAD_HIGH flag is set.
 265
 266 Field name:     ram_size
 267 Type:           kernel internal
 268 Offset/size:    0x1f8/2
 269 Protocol:       ALL
 270
 271   This field is obsolete.
 272
 273 Field name:     vid_mode
 274 Type:           modify (obligatory)
 275 Offset/size:    0x1fa/2
 276
 277   Please see the section on SPECIAL COMMAND LINE OPTIONS.
 278
 279 Field name:     root_dev
 280 Type:           modify (optional)
 281 Offset/size:    0x1fc/2
 282 Protocol:       ALL
 283
 284   The default root device device number.  The use of this field is
 285   deprecated, use the "root=" option on the command line instead.
 286
 287 Field name:     boot_flag
 288 Type:           read
 289 Offset/size:    0x1fe/2
 290 Protocol:       ALL
 291
 292   Contains 0xAA55.  This is the closest thing old Linux kernels have
 293   to a magic number.
 294
 295 Field name:     jump
 296 Type:           read
 297 Offset/size:    0x200/2
 298 Protocol:       2.00+
 299
 300   Contains an x86 jump instruction, 0xEB followed by a signed offset
 301   relative to byte 0x202.  This can be used to determine the size of
 302   the header.
 303
 304 Field name:     header
 305 Type:           read
 306 Offset/size:    0x202/4
 307 Protocol:       2.00+
 308
 309   Contains the magic number "HdrS" (0x53726448).
 310
 311 Field name:     version
 312 Type:           read
 313 Offset/size:    0x206/2
 314 Protocol:       2.00+
 315
 316   Contains the boot protocol version, in (major << 8)+minor format,
 317   e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
 318   10.17.
 319
 320 Field name:     realmode_swtch
 321 Type:           modify (optional)
 322 Offset/size:    0x208/4
 323 Protocol:       2.00+
 324
 325   Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
 326
 327 Field name:     start_sys_seg
 328 Type:           read
 329 Offset/size:    0x20c/2
 330 Protocol:       2.00+
 331
 332   The load low segment (0x1000).  Obsolete.
 333
 334 Field name:     kernel_version
 335 Type:           read
 336 Offset/size:    0x20e/2
 337 Protocol:       2.00+
 338
 339   If set to a nonzero value, contains a pointer to a NUL-terminated
 340   human-readable kernel version number string, less 0x200.  This can
 341   be used to display the kernel version to the user.  This value
 342   should be less than (0x200*setup_sects).
 343
 344   For example, if this value is set to 0x1c00, the kernel version
 345   number string can be found at offset 0x1e00 in the kernel file.
 346   This is a valid value if and only if the "setup_sects" field
 347   contains the value 15 or higher, as:
 348
 349         0x1c00  < 15*0x200 (= 0x1e00) but
 350         0x1c00 >= 14*0x200 (= 0x1c00)
 351
 352         0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
 353
 354 Field name:     type_of_loader
 355 Type:           write (obligatory)
 356 Offset/size:    0x210/1
 357 Protocol:       2.00+
 358
 359   If your boot loader has an assigned id (see table below), enter
 360   0xTV here, where T is an identifier for the boot loader and V is
 361   a version number.  Otherwise, enter 0xFF here.
 362
 363   For boot loader IDs above T = 0xD, write T = 0xE to this field and
 364   write the extended ID minus 0x10 to the ext_loader_type field.
 365   Similarly, the ext_loader_ver field can be used to provide more than
 366   four bits for the bootloader version.
 367
 368   For example, for T = 0x15, V = 0x234, write:
 369
 370   type_of_loader  <- 0xE4
 371   ext_loader_type <- 0x05
 372   ext_loader_ver  <- 0x23
 373
 374   Assigned boot loader ids (hexadecimal):
 375
 376         0  LILO                 (0x00 reserved for pre-2.00 bootloader)
 377         1  Loadlin
 378         2  bootsect-loader      (0x20, all other values reserved)
 379         3  Syslinux
 380         4  Etherboot/gPXE/iPXE
 381         5  ELILO
 382         7  GRUB
 383         8  U-Boot
 384         9  Xen
 385         A  Gujin
 386         B  Qemu
 387         C  Arcturus Networks uCbootloader
 388         D  kexec-tools
 389         E  Extended             (see ext_loader_type)
 390         F  Special              (0xFF = undefined)
 391        10  Reserved
 392        11  Minimal Linux Bootloader <http://sebastian-plotz.blogspot.de>
 393        12  OVMF UEFI virtualization stack
 394
 395   Please contact <hpa@zytor.com> if you need a bootloader ID
 396   value assigned.
 397
 398 Field name:     loadflags
 399 Type:           modify (obligatory)
 400 Offset/size:    0x211/1
 401 Protocol:       2.00+
 402
 403   This field is a bitmask.
 404
 405   Bit 0 (read): LOADED_HIGH
 406         - If 0, the protected-mode code is loaded at 0x10000.
 407         - If 1, the protected-mode code is loaded at 0x100000.
 408
 409   Bit 5 (write): QUIET_FLAG
 410         - If 0, print early messages.
 411         - If 1, suppress early messages.
 412                 This requests to the kernel (decompressor and early
 413                 kernel) to not write early messages that require
 414                 accessing the display hardware directly.
 415
 416   Bit 6 (write): KEEP_SEGMENTS
 417         Protocol: 2.07+
 418         - If 0, reload the segment registers in the 32bit entry point.
 419         - If 1, do not reload the segment registers in the 32bit entry point.
 420                 Assume that %cs %ds %ss %es are all set to flat segments with
 421                 a base of 0 (or the equivalent for their environment).
 422
 423   Bit 7 (write): CAN_USE_HEAP
 424         Set this bit to 1 to indicate that the value entered in the
 425         heap_end_ptr is valid.  If this field is clear, some setup code
 426         functionality will be disabled.
 427
 428 Field name:     setup_move_size
 429 Type:           modify (obligatory)
 430 Offset/size:    0x212/2
 431 Protocol:       2.00-2.01
 432
 433   When using protocol 2.00 or 2.01, if the real mode kernel is not
 434   loaded at 0x90000, it gets moved there later in the loading
 435   sequence.  Fill in this field if you want additional data (such as
 436   the kernel command line) moved in addition to the real-mode kernel
 437   itself.
 438
 439   The unit is bytes starting with the beginning of the boot sector.
 440
 441   This field is can be ignored when the protocol is 2.02 or higher, or
 442   if the real-mode code is loaded at 0x90000.
 443
 444 Field name:     code32_start
 445 Type:           modify (optional, reloc)
 446 Offset/size:    0x214/4
 447 Protocol:       2.00+
 448
 449   The address to jump to in protected mode.  This defaults to the load
 450   address of the kernel, and can be used by the boot loader to
 451   determine the proper load address.
 452
 453   This field can be modified for two purposes:
 454
 455   1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
 456
 457   2. if a bootloader which does not install a hook loads a
 458      relocatable kernel at a nonstandard address it will have to modify
 459      this field to point to the load address.
 460
 461 Field name:     ramdisk_image
 462 Type:           write (obligatory)
 463 Offset/size:    0x218/4
 464 Protocol:       2.00+
 465
 466   The 32-bit linear address of the initial ramdisk or ramfs.  Leave at
 467   zero if there is no initial ramdisk/ramfs.
 468
 469 Field name:     ramdisk_size
 470 Type:           write (obligatory)
 471 Offset/size:    0x21c/4
 472 Protocol:       2.00+
 473
 474   Size of the initial ramdisk or ramfs.  Leave at zero if there is no
 475   initial ramdisk/ramfs.
 476
 477 Field name:     bootsect_kludge
 478 Type:           kernel internal
 479 Offset/size:    0x220/4
 480 Protocol:       2.00+
 481
 482   This field is obsolete.
 483
 484 Field name:     heap_end_ptr
 485 Type:           write (obligatory)
 486 Offset/size:    0x224/2
 487 Protocol:       2.01+
 488
 489   Set this field to the offset (from the beginning of the real-mode
 490   code) of the end of the setup stack/heap, minus 0x0200.
 491
 492 Field name:     ext_loader_ver
 493 Type:           write (optional)
 494 Offset/size:    0x226/1
 495 Protocol:       2.02+
 496
 497   This field is used as an extension of the version number in the
 498   type_of_loader field.  The total version number is considered to be
 499   (type_of_loader & 0x0f) + (ext_loader_ver << 4).
 500
 501   The use of this field is boot loader specific.  If not written, it
 502   is zero.
 503
 504   Kernels prior to 2.6.31 did not recognize this field, but it is safe
 505   to write for protocol version 2.02 or higher.
 506
 507 Field name:     ext_loader_type
 508 Type:           write (obligatory if (type_of_loader & 0xf0) == 0xe0)
 509 Offset/size:    0x227/1
 510 Protocol:       2.02+
 511
 512   This field is used as an extension of the type number in
 513   type_of_loader field.  If the type in type_of_loader is 0xE, then
 514   the actual type is (ext_loader_type + 0x10).
 515
 516   This field is ignored if the type in type_of_loader is not 0xE.
 517
 518   Kernels prior to 2.6.31 did not recognize this field, but it is safe
 519   to write for protocol version 2.02 or higher.
 520
 521 Field name:     cmd_line_ptr
 522 Type:           write (obligatory)
 523 Offset/size:    0x228/4
 524 Protocol:       2.02+
 525
 526   Set this field to the linear address of the kernel command line.
 527   The kernel command line can be located anywhere between the end of
 528   the setup heap and 0xA0000; it does not have to be located in the
 529   same 64K segment as the real-mode code itself.
 530
 531   Fill in this field even if your boot loader does not support a
 532   command line, in which case you can point this to an empty string
 533   (or better yet, to the string "auto".)  If this field is left at
 534   zero, the kernel will assume that your boot loader does not support
 535   the 2.02+ protocol.
 536
 537 Field name:     ramdisk_max
 538 Type:           read
 539 Offset/size:    0x22c/4
 540 Protocol:       2.03+
 541
 542   The maximum address that may be occupied by the initial
 543   ramdisk/ramfs contents.  For boot protocols 2.02 or earlier, this
 544   field is not present, and the maximum address is 0x37FFFFFF.  (This
 545   address is defined as the address of the highest safe byte, so if
 546   your ramdisk is exactly 131072 bytes long and this field is
 547   0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
 548
 549 Field name:     kernel_alignment
 550 Type:           read/modify (reloc)
 551 Offset/size:    0x230/4
 552 Protocol:       2.05+ (read), 2.10+ (modify)
 553
 554   Alignment unit required by the kernel (if relocatable_kernel is
 555   true.)  A relocatable kernel that is loaded at an alignment
 556   incompatible with the value in this field will be realigned during
 557   kernel initialization.
 558
 559   Starting with protocol version 2.10, this reflects the kernel
 560   alignment preferred for optimal performance; it is possible for the
 561   loader to modify this field to permit a lesser alignment.  See the
 562   min_alignment and pref_address field below.
 563
 564 Field name:     relocatable_kernel
 565 Type:           read (reloc)
 566 Offset/size:    0x234/1
 567 Protocol:       2.05+
 568
 569   If this field is nonzero, the protected-mode part of the kernel can
 570   be loaded at any address that satisfies the kernel_alignment field.
 571   After loading, the boot loader must set the code32_start field to
 572   point to the loaded code, or to a boot loader hook.
 573
 574 Field name:     min_alignment
 575 Type:           read (reloc)
 576 Offset/size:    0x235/1
 577 Protocol:       2.10+
 578
 579   This field, if nonzero, indicates as a power of two the minimum
 580   alignment required, as opposed to preferred, by the kernel to boot.
 581   If a boot loader makes use of this field, it should update the
 582   kernel_alignment field with the alignment unit desired; typically:
 583
 584         kernel_alignment = 1 << min_alignment
 585
 586   There may be a considerable performance cost with an excessively
 587   misaligned kernel.  Therefore, a loader should typically try each
 588   power-of-two alignment from kernel_alignment down to this alignment.
 589
 590 Field name:     xloadflags
 591 Type:           read
 592 Offset/size:    0x236/2
 593 Protocol:       2.12+
 594
 595   This field is a bitmask.
 596
 597   Bit 0 (read): XLF_KERNEL_64
 598         - If 1, this kernel has the legacy 64-bit entry point at 0x200.
 599
 600   Bit 1 (read): XLF_CAN_BE_LOADED_ABOVE_4G
 601         - If 1, kernel/boot_params/cmdline/ramdisk can be above 4G.
 602
 603   Bit 2 (read): XLF_EFI_HANDOVER_32
 604         - If 1, the kernel supports the 32-bit EFI handoff entry point
 605           given at handover_offset.
 606
 607   Bit 3 (read): XLF_EFI_HANDOVER_64
 608         - If 1, the kernel supports the 64-bit EFI handoff entry point
 609           given at handover_offset + 0x200.
 610
 611 Field name:     cmdline_size
 612 Type:           read
 613 Offset/size:    0x238/4
 614 Protocol:       2.06+
 615
 616   The maximum size of the command line without the terminating
 617   zero. This means that the command line can contain at most
 618   cmdline_size characters. With protocol version 2.05 and earlier, the
 619   maximum size was 255.
 620
 621 Field name:     hardware_subarch
 622 Type:           write (optional, defaults to x86/PC)
 623 Offset/size:    0x23c/4
 624 Protocol:       2.07+
 625
 626   In a paravirtualized environment the hardware low level architectural
 627   pieces such as interrupt handling, page table handling, and
 628   accessing process control registers needs to be done differently.
 629
 630   This field allows the bootloader to inform the kernel we are in one
 631   one of those environments.
 632
 633   0x00000000    The default x86/PC environment
 634   0x00000001    lguest
 635   0x00000002    Xen
 636   0x00000003    Moorestown MID
 637   0x00000004    CE4100 TV Platform
 638
 639 Field name:     hardware_subarch_data
 640 Type:           write (subarch-dependent)
 641 Offset/size:    0x240/8
 642 Protocol:       2.07+
 643
 644   A pointer to data that is specific to hardware subarch
 645   This field is currently unused for the default x86/PC environment,
 646   do not modify.
 647
 648 Field name:     payload_offset
 649 Type:           read
 650 Offset/size:    0x248/4
 651 Protocol:       2.08+
 652
 653   If non-zero then this field contains the offset from the beginning
 654   of the protected-mode code to the payload.
 655
 656   The payload may be compressed. The format of both the compressed and
 657   uncompressed data should be determined using the standard magic
 658   numbers.  The currently supported compression formats are gzip
 659   (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA
 660   (magic number 5D 00), XZ (magic number FD 37), and LZ4 (magic number
 661   02 21).  The uncompressed payload is currently always ELF (magic
 662   number 7F 45 4C 46).
 663
 664 Field name:     payload_length
 665 Type:           read
 666 Offset/size:    0x24c/4
 667 Protocol:       2.08+
 668
 669   The length of the payload.
 670
 671 Field name:     setup_data
 672 Type:           write (special)
 673 Offset/size:    0x250/8
 674 Protocol:       2.09+
 675
 676   The 64-bit physical pointer to NULL terminated single linked list of
 677   struct setup_data. This is used to define a more extensible boot
 678   parameters passing mechanism. The definition of struct setup_data is
 679   as follow:
 680
 681   struct setup_data {
 682           u64 next;
 683           u32 type;
 684           u32 len;
 685           u8  data[0];
 686   };
 687
 688   Where, the next is a 64-bit physical pointer to the next node of
 689   linked list, the next field of the last node is 0; the type is used
 690   to identify the contents of data; the len is the length of data
 691   field; the data holds the real payload.
 692
 693   This list may be modified at a number of points during the bootup
 694   process.  Therefore, when modifying this list one should always make
 695   sure to consider the case where the linked list already contains
 696   entries.
 697
 698 Field name:     pref_address
 699 Type:           read (reloc)
 700 Offset/size:    0x258/8
 701 Protocol:       2.10+
 702
 703   This field, if nonzero, represents a preferred load address for the
 704   kernel.  A relocating bootloader should attempt to load at this
 705   address if possible.
 706
 707   A non-relocatable kernel will unconditionally move itself and to run
 708   at this address.
 709
 710 Field name:     init_size
 711 Type:           read
 712 Offset/size:    0x260/4
 713
 714   This field indicates the amount of linear contiguous memory starting
 715   at the kernel runtime start address that the kernel needs before it
 716   is capable of examining its memory map.  This is not the same thing
 717   as the total amount of memory the kernel needs to boot, but it can
 718   be used by a relocating boot loader to help select a safe load
 719   address for the kernel.
 720
 721   The kernel runtime start address is determined by the following algorithm:
 722
 723   if (relocatable_kernel)
 724         runtime_start = align_up(load_address, kernel_alignment)
 725   else
 726         runtime_start = pref_address
 727
 728 Field name:     handover_offset
 729 Type:           read
 730 Offset/size:    0x264/4
 731
 732   This field is the offset from the beginning of the kernel image to
 733   the EFI handover protocol entry point. Boot loaders using the EFI
 734   handover protocol to boot the kernel should jump to this offset.
 735
 736   See EFI HANDOVER PROTOCOL below for more details.
 737
 738
 739 **** THE IMAGE CHECKSUM
 740
 741 From boot protocol version 2.08 onwards the CRC-32 is calculated over
 742 the entire file using the characteristic polynomial 0x04C11DB7 and an
 743 initial remainder of 0xffffffff.  The checksum is appended to the
 744 file; therefore the CRC of the file up to the limit specified in the
 745 syssize field of the header is always 0.
 746
 747
 748 **** THE KERNEL COMMAND LINE
 749
 750 The kernel command line has become an important way for the boot
 751 loader to communicate with the kernel.  Some of its options are also
 752 relevant to the boot loader itself, see "special command line options"
 753 below.
 754
 755 The kernel command line is a null-terminated string. The maximum
 756 length can be retrieved from the field cmdline_size.  Before protocol
 757 version 2.06, the maximum was 255 characters.  A string that is too
 758 long will be automatically truncated by the kernel.
 759
 760 If the boot protocol version is 2.02 or later, the address of the
 761 kernel command line is given by the header field cmd_line_ptr (see
 762 above.)  This address can be anywhere between the end of the setup
 763 heap and 0xA0000.
 764
 765 If the protocol version is *not* 2.02 or higher, the kernel
 766 command line is entered using the following protocol:
 767
 768         At offset 0x0020 (word), "cmd_line_magic", enter the magic
 769         number 0xA33F.
 770
 771         At offset 0x0022 (word), "cmd_line_offset", enter the offset
 772         of the kernel command line (relative to the start of the
 773         real-mode kernel).
 774
 775         The kernel command line *must* be within the memory region
 776         covered by setup_move_size, so you may need to adjust this
 777         field.
 778
 779
 780 **** MEMORY LAYOUT OF THE REAL-MODE CODE
 781
 782 The real-mode code requires a stack/heap to be set up, as well as
 783 memory allocated for the kernel command line.  This needs to be done
 784 in the real-mode accessible memory in bottom megabyte.
 785
 786 It should be noted that modern machines often have a sizable Extended
 787 BIOS Data Area (EBDA).  As a result, it is advisable to use as little
 788 of the low megabyte as possible.
 789
 790 Unfortunately, under the following circumstances the 0x90000 memory
 791 segment has to be used:
 792
 793         - When loading a zImage kernel ((loadflags & 0x01) == 0).
 794         - When loading a 2.01 or earlier boot protocol kernel.
 795
 796           -> For the 2.00 and 2.01 boot protocols, the real-mode code
 797              can be loaded at another address, but it is internally
 798              relocated to 0x90000.  For the "old" protocol, the
 799              real-mode code must be loaded at 0x90000.
 800
 801 When loading at 0x90000, avoid using memory above 0x9a000.
 802
 803 For boot protocol 2.02 or higher, the command line does not have to be
 804 located in the same 64K segment as the real-mode setup code; it is
 805 thus permitted to give the stack/heap the full 64K segment and locate
 806 the command line above it.
 807
 808 The kernel command line should not be located below the real-mode
 809 code, nor should it be located in high memory.
 810
 811
 812 **** SAMPLE BOOT CONFIGURATION
 813
 814 As a sample configuration, assume the following layout of the real
 815 mode segment:
 816
 817     When loading below 0x90000, use the entire segment:
 818
 819         0x0000-0x7fff   Real mode kernel
 820         0x8000-0xdfff   Stack and heap
 821         0xe000-0xffff   Kernel command line
 822
 823     When loading at 0x90000 OR the protocol version is 2.01 or earlier:
 824
 825         0x0000-0x7fff   Real mode kernel
 826         0x8000-0x97ff   Stack and heap
 827         0x9800-0x9fff   Kernel command line
 828
 829 Such a boot loader should enter the following fields in the header:
 830
 831         unsigned long base_ptr; /* base address for real-mode segment */
 832
 833         if ( setup_sects == 0 ) {
 834                 setup_sects = 4;
 835         }
 836
 837         if ( protocol >= 0x0200 ) {
 838                 type_of_loader = <type code>;
 839                 if ( loading_initrd ) {
 840                         ramdisk_image = <initrd_address>;
 841                         ramdisk_size = <initrd_size>;
 842                 }
 843
 844                 if ( protocol >= 0x0202 && loadflags & 0x01 )
 845                         heap_end = 0xe000;
 846                 else
 847                         heap_end = 0x9800;
 848
 849                 if ( protocol >= 0x0201 ) {
 850                         heap_end_ptr = heap_end - 0x200;
 851                         loadflags |= 0x80; /* CAN_USE_HEAP */
 852                 }
 853
 854                 if ( protocol >= 0x0202 ) {
 855                         cmd_line_ptr = base_ptr + heap_end;
 856                         strcpy(cmd_line_ptr, cmdline);
 857                 } else {
 858                         cmd_line_magic  = 0xA33F;
 859                         cmd_line_offset = heap_end;
 860                         setup_move_size = heap_end + strlen(cmdline)+1;
 861                         strcpy(base_ptr+cmd_line_offset, cmdline);
 862                 }
 863         } else {
 864                 /* Very old kernel */
 865
 866                 heap_end = 0x9800;
 867
 868                 cmd_line_magic  = 0xA33F;
 869                 cmd_line_offset = heap_end;
 870
 871                 /* A very old kernel MUST have its real-mode code
 872                    loaded at 0x90000 */
 873
 874                 if ( base_ptr != 0x90000 ) {
 875                         /* Copy the real-mode kernel */
 876                         memcpy(0x90000, base_ptr, (setup_sects+1)*512);
 877                         base_ptr = 0x90000;              /* Relocated */
 878                 }
 879
 880                 strcpy(0x90000+cmd_line_offset, cmdline);
 881
 882                 /* It is recommended to clear memory up to the 32K mark */
 883                 memset(0x90000 + (setup_sects+1)*512, 0,
 884                        (64-(setup_sects+1))*512);
 885         }
 886
 887
 888 **** LOADING THE REST OF THE KERNEL
 889
 890 The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
 891 in the kernel file (again, if setup_sects == 0 the real value is 4.)
 892 It should be loaded at address 0x10000 for Image/zImage kernels and
 893 0x100000 for bzImage kernels.
 894
 895 The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
 896 bit (LOAD_HIGH) in the loadflags field is set:
 897
 898         is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
 899         load_address = is_bzImage ? 0x100000 : 0x10000;
 900
 901 Note that Image/zImage kernels can be up to 512K in size, and thus use
 902 the entire 0x10000-0x90000 range of memory.  This means it is pretty
 903 much a requirement for these kernels to load the real-mode part at
 904 0x90000.  bzImage kernels allow much more flexibility.
 905
 906
 907 **** SPECIAL COMMAND LINE OPTIONS
 908
 909 If the command line provided by the boot loader is entered by the
 910 user, the user may expect the following command line options to work.
 911 They should normally not be deleted from the kernel command line even
 912 though not all of them are actually meaningful to the kernel.  Boot
 913 loader authors who need additional command line options for the boot
 914 loader itself should get them registered in
 915 Documentation/kernel-parameters.txt to make sure they will not
 916 conflict with actual kernel options now or in the future.
 917
 918   vga=<mode>
 919         <mode> here is either an integer (in C notation, either
 920         decimal, octal, or hexadecimal) or one of the strings
 921         "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
 922         (meaning 0xFFFD).  This value should be entered into the
 923         vid_mode field, as it is used by the kernel before the command
 924         line is parsed.
 925
 926   mem=<size>
 927         <size> is an integer in C notation optionally followed by
 928         (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
 929         << 30, << 40, << 50 or << 60).  This specifies the end of
 930         memory to the kernel. This affects the possible placement of
 931         an initrd, since an initrd should be placed near end of
 932         memory.  Note that this is an option to *both* the kernel and
 933         the bootloader!
 934
 935   initrd=<file>
 936         An initrd should be loaded.  The meaning of <file> is
 937         obviously bootloader-dependent, and some boot loaders
 938         (e.g. LILO) do not have such a command.
 939
 940 In addition, some boot loaders add the following options to the
 941 user-specified command line:
 942
 943   BOOT_IMAGE=<file>
 944         The boot image which was loaded.  Again, the meaning of <file>
 945         is obviously bootloader-dependent.
 946
 947   auto
 948         The kernel was booted without explicit user intervention.
 949
 950 If these options are added by the boot loader, it is highly
 951 recommended that they are located *first*, before the user-specified
 952 or configuration-specified command line.  Otherwise, "init=/bin/sh"
 953 gets confused by the "auto" option.
 954
 955
 956 **** RUNNING THE KERNEL
 957
 958 The kernel is started by jumping to the kernel entry point, which is
 959 located at *segment* offset 0x20 from the start of the real mode
 960 kernel.  This means that if you loaded your real-mode kernel code at
 961 0x90000, the kernel entry point is 9020:0000.
 962
 963 At entry, ds = es = ss should point to the start of the real-mode
 964 kernel code (0x9000 if the code is loaded at 0x90000), sp should be
 965 set up properly, normally pointing to the top of the heap, and
 966 interrupts should be disabled.  Furthermore, to guard against bugs in
 967 the kernel, it is recommended that the boot loader sets fs = gs = ds =
 968 es = ss.
 969
 970 In our example from above, we would do:
 971
 972         /* Note: in the case of the "old" kernel protocol, base_ptr must
 973            be == 0x90000 at this point; see the previous sample code */
 974
 975         seg = base_ptr >> 4;
 976
 977         cli();  /* Enter with interrupts disabled! */
 978
 979         /* Set up the real-mode kernel stack */
 980         _SS = seg;
 981         _SP = heap_end;
 982
 983         _DS = _ES = _FS = _GS = seg;
 984         jmp_far(seg+0x20, 0);   /* Run the kernel */
 985
 986 If your boot sector accesses a floppy drive, it is recommended to
 987 switch off the floppy motor before running the kernel, since the
 988 kernel boot leaves interrupts off and thus the motor will not be
 989 switched off, especially if the loaded kernel has the floppy driver as
 990 a demand-loaded module!
 991
 992
 993 **** ADVANCED BOOT LOADER HOOKS
 994
 995 If the boot loader runs in a particularly hostile environment (such as
 996 LOADLIN, which runs under DOS) it may be impossible to follow the
 997 standard memory location requirements.  Such a boot loader may use the
 998 following hooks that, if set, are invoked by the kernel at the
 999 appropriate time.  The use of these hooks should probably be
1000 considered an absolutely last resort!
1001
1002 IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
1003 %edi across invocation.
1004
1005   realmode_swtch:
1006         A 16-bit real mode far subroutine invoked immediately before
1007         entering protected mode.  The default routine disables NMI, so
1008         your routine should probably do so, too.
1009
1010   code32_start:
1011         A 32-bit flat-mode routine *jumped* to immediately after the
1012         transition to protected mode, but before the kernel is
1013         uncompressed.  No segments, except CS, are guaranteed to be
1014         set up (current kernels do, but older ones do not); you should
1015         set them up to BOOT_DS (0x18) yourself.
1016
1017         After completing your hook, you should jump to the address
1018         that was in this field before your boot loader overwrote it
1019         (relocated, if appropriate.)
1020
1021
1022 **** 32-bit BOOT PROTOCOL
1023
1024 For machine with some new BIOS other than legacy BIOS, such as EFI,
1025 LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
1026 based on legacy BIOS can not be used, so a 32-bit boot protocol needs
1027 to be defined.
1028
1029 In 32-bit boot protocol, the first step in loading a Linux kernel
1030 should be to setup the boot parameters (struct boot_params,
1031 traditionally known as "zero page"). The memory for struct boot_params
1032 should be allocated and initialized to all zero. Then the setup header
1033 from offset 0x01f1 of kernel image on should be loaded into struct
1034 boot_params and examined. The end of setup header can be calculated as
1035 follow:
1036
1037         0x0202 + byte value at offset 0x0201
1038
1039 In addition to read/modify/write the setup header of the struct
1040 boot_params as that of 16-bit boot protocol, the boot loader should
1041 also fill the additional fields of the struct boot_params as that
1042 described in zero-page.txt.
1043
1044 After setting up the struct boot_params, the boot loader can load the
1045 32/64-bit kernel in the same way as that of 16-bit boot protocol.
1046
1047 In 32-bit boot protocol, the kernel is started by jumping to the
1048 32-bit kernel entry point, which is the start address of loaded
1049 32/64-bit kernel.
1050
1051 At entry, the CPU must be in 32-bit protected mode with paging
1052 disabled; a GDT must be loaded with the descriptors for selectors
1053 __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1054 segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1055 must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1056 must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
1057 address of the struct boot_params; %ebp, %edi and %ebx must be zero.
1058
1059 **** 64-bit BOOT PROTOCOL
1060
1061 For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader
1062 and we need a 64-bit boot protocol.
1063
1064 In 64-bit boot protocol, the first step in loading a Linux kernel
1065 should be to setup the boot parameters (struct boot_params,
1066 traditionally known as "zero page"). The memory for struct boot_params
1067 could be allocated anywhere (even above 4G) and initialized to all zero.
1068 Then, the setup header at offset 0x01f1 of kernel image on should be
1069 loaded into struct boot_params and examined. The end of setup header
1070 can be calculated as follows:
1071
1072         0x0202 + byte value at offset 0x0201
1073
1074 In addition to read/modify/write the setup header of the struct
1075 boot_params as that of 16-bit boot protocol, the boot loader should
1076 also fill the additional fields of the struct boot_params as described
1077 in zero-page.txt.
1078
1079 After setting up the struct boot_params, the boot loader can load
1080 64-bit kernel in the same way as that of 16-bit boot protocol, but
1081 kernel could be loaded above 4G.
1082
1083 In 64-bit boot protocol, the kernel is started by jumping to the
1084 64-bit kernel entry point, which is the start address of loaded
1085 64-bit kernel plus 0x200.
1086
1087 At entry, the CPU must be in 64-bit mode with paging enabled.
1088 The range with setup_header.init_size from start address of loaded
1089 kernel and zero page and command line buffer get ident mapping;
1090 a GDT must be loaded with the descriptors for selectors
1091 __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1092 segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1093 must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1094 must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base
1095 address of the struct boot_params.
1096
1097 **** EFI HANDOVER PROTOCOL
1098
1099 This protocol allows boot loaders to defer initialisation to the EFI
1100 boot stub. The boot loader is required to load the kernel/initrd(s)
1101 from the boot media and jump to the EFI handover protocol entry point
1102 which is hdr->handover_offset bytes from the beginning of
1103 startup_{32,64}.
1104
1105 The function prototype for the handover entry point looks like this,
1106
1107     efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp)
1108
1109 'handle' is the EFI image handle passed to the boot loader by the EFI
1110 firmware, 'table' is the EFI system table - these are the first two
1111 arguments of the "handoff state" as described in section 2.3 of the
1112 UEFI specification. 'bp' is the boot loader-allocated boot params.
1113
1114 The boot loader *must* fill out the following fields in bp,
1115
1116     o hdr.code32_start
1117     o hdr.cmd_line_ptr
1118     o hdr.cmdline_size
1119     o hdr.ramdisk_image (if applicable)
1120     o hdr.ramdisk_size  (if applicable)
1121
1122 All other fields should be zero.