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 **** MEMORY LAYOUT
  61
  62 The traditional memory map for the kernel loader, used for Image or
  63 zImage kernels, typically looks like:
  64
  65         |                        |
  66 0A0000  +------------------------+
  67         |  Reserved for BIOS     |      Do not use.  Reserved for BIOS EBDA.
  68 09A000  +------------------------+
  69         |  Command line          |
  70         |  Stack/heap            |      For use by the kernel real-mode code.
  71 098000  +------------------------+
  72         |  Kernel setup          |      The kernel real-mode code.
  73 090200  +------------------------+
  74         |  Kernel boot sector    |      The kernel legacy boot sector.
  75 090000  +------------------------+
  76         |  Protected-mode kernel |      The bulk of the kernel image.
  77 010000  +------------------------+
  78         |  Boot loader           |      <- Boot sector entry point 0000:7C00
  79 001000  +------------------------+
  80         |  Reserved for MBR/BIOS |
  81 000800  +------------------------+
  82         |  Typically used by MBR |
  83 000600  +------------------------+
  84         |  BIOS use only         |
  85 000000  +------------------------+
  86
  87
  88 When using bzImage, the protected-mode kernel was relocated to
  89 0x100000 ("high memory"), and the kernel real-mode block (boot sector,
  90 setup, and stack/heap) was made relocatable to any address between
  91 0x10000 and end of low memory. Unfortunately, in protocols 2.00 and
  92 2.01 the 0x90000+ memory range is still used internally by the kernel;
  93 the 2.02 protocol resolves that problem.
  94
  95 It is desirable to keep the "memory ceiling" -- the highest point in
  96 low memory touched by the boot loader -- as low as possible, since
  97 some newer BIOSes have begun to allocate some rather large amounts of
  98 memory, called the Extended BIOS Data Area, near the top of low
  99 memory.  The boot loader should use the "INT 12h" BIOS call to verify
 100 how much low memory is available.
 101
 102 Unfortunately, if INT 12h reports that the amount of memory is too
 103 low, there is usually nothing the boot loader can do but to report an
 104 error to the user.  The boot loader should therefore be designed to
 105 take up as little space in low memory as it reasonably can.  For
 106 zImage or old bzImage kernels, which need data written into the
 107 0x90000 segment, the boot loader should make sure not to use memory
 108 above the 0x9A000 point; too many BIOSes will break above that point.
 109
 110 For a modern bzImage kernel with boot protocol version >= 2.02, a
 111 memory layout like the following is suggested:
 112
 113         ~                        ~
 114         |  Protected-mode kernel |
 115 100000  +------------------------+
 116         |  I/O memory hole       |
 117 0A0000  +------------------------+
 118         |  Reserved for BIOS     |      Leave as much as possible unused
 119         ~                        ~
 120         |  Command line          |      (Can also be below the X+10000 mark)
 121 X+10000 +------------------------+
 122         |  Stack/heap            |      For use by the kernel real-mode code.
 123 X+08000 +------------------------+
 124         |  Kernel setup          |      The kernel real-mode code.
 125         |  Kernel boot sector    |      The kernel legacy boot sector.
 126 X       +------------------------+
 127         |  Boot loader           |      <- Boot sector entry point 0000:7C00
 128 001000  +------------------------+
 129         |  Reserved for MBR/BIOS |
 130 000800  +------------------------+
 131         |  Typically used by MBR |
 132 000600  +------------------------+
 133         |  BIOS use only         |
 134 000000  +------------------------+
 135
 136 ... where the address X is as low as the design of the boot loader
 137 permits.
 138
 139
 140 **** THE REAL-MODE KERNEL HEADER
 141
 142 In the following text, and anywhere in the kernel boot sequence, "a
 143 sector" refers to 512 bytes.  It is independent of the actual sector
 144 size of the underlying medium.
 145
 146 The first step in loading a Linux kernel should be to load the
 147 real-mode code (boot sector and setup code) and then examine the
 148 following header at offset 0x01f1.  The real-mode code can total up to
 149 32K, although the boot loader may choose to load only the first two
 150 sectors (1K) and then examine the bootup sector size.
 151
 152 The header looks like:
 153
 154 Offset  Proto   Name            Meaning
 155 /Size
 156
 157 01F1/1  ALL(1   setup_sects     The size of the setup in sectors
 158 01F2/2  ALL     root_flags      If set, the root is mounted readonly
 159 01F4/4  2.04+(2 syssize         The size of the 32-bit code in 16-byte paras
 160 01F8/2  ALL     ram_size        DO NOT USE - for bootsect.S use only
 161 01FA/2  ALL     vid_mode        Video mode control
 162 01FC/2  ALL     root_dev        Default root device number
 163 01FE/2  ALL     boot_flag       0xAA55 magic number
 164 0200/2  2.00+   jump            Jump instruction
 165 0202/4  2.00+   header          Magic signature "HdrS"
 166 0206/2  2.00+   version         Boot protocol version supported
 167 0208/4  2.00+   realmode_swtch  Boot loader hook (see below)
 168 020C/2  2.00+   start_sys_seg   The load-low segment (0x1000) (obsolete)
 169 020E/2  2.00+   kernel_version  Pointer to kernel version string
 170 0210/1  2.00+   type_of_loader  Boot loader identifier
 171 0211/1  2.00+   loadflags       Boot protocol option flags
 172 0212/2  2.00+   setup_move_size Move to high memory size (used with hooks)
 173 0214/4  2.00+   code32_start    Boot loader hook (see below)
 174 0218/4  2.00+   ramdisk_image   initrd load address (set by boot loader)
 175 021C/4  2.00+   ramdisk_size    initrd size (set by boot loader)
 176 0220/4  2.00+   bootsect_kludge DO NOT USE - for bootsect.S use only
 177 0224/2  2.01+   heap_end_ptr    Free memory after setup end
 178 0226/1  2.02+(3 ext_loader_ver  Extended boot loader version
 179 0227/1  2.02+(3 ext_loader_type Extended boot loader ID
 180 0228/4  2.02+   cmd_line_ptr    32-bit pointer to the kernel command line
 181 022C/4  2.03+   ramdisk_max     Highest legal initrd address
 182 0230/4  2.05+   kernel_alignment Physical addr alignment required for kernel
 183 0234/1  2.05+   relocatable_kernel Whether kernel is relocatable or not
 184 0235/1  2.10+   min_alignment   Minimum alignment, as a power of two
 185 0236/2  N/A     pad3            Unused
 186 0238/4  2.06+   cmdline_size    Maximum size of the kernel command line
 187 023C/4  2.07+   hardware_subarch Hardware subarchitecture
 188 0240/8  2.07+   hardware_subarch_data Subarchitecture-specific data
 189 0248/4  2.08+   payload_offset  Offset of kernel payload
 190 024C/4  2.08+   payload_length  Length of kernel payload
 191 0250/8  2.09+   setup_data      64-bit physical pointer to linked list
 192                                 of struct setup_data
 193 0258/8  2.10+   pref_address    Preferred loading address
 194 0260/4  2.10+   init_size       Linear memory required during initialization
 195 0264/4  2.11+   handover_offset Offset of handover entry point
 196
 197 (1) For backwards compatibility, if the setup_sects field contains 0, the
 198     real value is 4.
 199
 200 (2) For boot protocol prior to 2.04, the upper two bytes of the syssize
 201     field are unusable, which means the size of a bzImage kernel
 202     cannot be determined.
 203
 204 (3) Ignored, but safe to set, for boot protocols 2.02-2.09.
 205
 206 If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
 207 the boot protocol version is "old".  Loading an old kernel, the
 208 following parameters should be assumed:
 209
 210         Image type = zImage
 211         initrd not supported
 212         Real-mode kernel must be located at 0x90000.
 213
 214 Otherwise, the "version" field contains the protocol version,
 215 e.g. protocol version 2.01 will contain 0x0201 in this field.  When
 216 setting fields in the header, you must make sure only to set fields
 217 supported by the protocol version in use.
 218
 219
 220 **** DETAILS OF HEADER FIELDS
 221
 222 For each field, some are information from the kernel to the bootloader
 223 ("read"), some are expected to be filled out by the bootloader
 224 ("write"), and some are expected to be read and modified by the
 225 bootloader ("modify").
 226
 227 All general purpose boot loaders should write the fields marked
 228 (obligatory).  Boot loaders who want to load the kernel at a
 229 nonstandard address should fill in the fields marked (reloc); other
 230 boot loaders can ignore those fields.
 231
 232 The byte order of all fields is littleendian (this is x86, after all.)
 233
 234 Field name:     setup_sects
 235 Type:           read
 236 Offset/size:    0x1f1/1
 237 Protocol:       ALL
 238
 239   The size of the setup code in 512-byte sectors.  If this field is
 240   0, the real value is 4.  The real-mode code consists of the boot
 241   sector (always one 512-byte sector) plus the setup code.
 242
 243 Field name:      root_flags
 244 Type:            modify (optional)
 245 Offset/size:     0x1f2/2
 246 Protocol:        ALL
 247
 248   If this field is nonzero, the root defaults to readonly.  The use of
 249   this field is deprecated; use the "ro" or "rw" options on the
 250   command line instead.
 251
 252 Field name:     syssize
 253 Type:           read
 254 Offset/size:    0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
 255 Protocol:       2.04+
 256
 257   The size of the protected-mode code in units of 16-byte paragraphs.
 258   For protocol versions older than 2.04 this field is only two bytes
 259   wide, and therefore cannot be trusted for the size of a kernel if
 260   the LOAD_HIGH flag is set.
 261
 262 Field name:     ram_size
 263 Type:           kernel internal
 264 Offset/size:    0x1f8/2
 265 Protocol:       ALL
 266
 267   This field is obsolete.
 268
 269 Field name:     vid_mode
 270 Type:           modify (obligatory)
 271 Offset/size:    0x1fa/2
 272
 273   Please see the section on SPECIAL COMMAND LINE OPTIONS.
 274
 275 Field name:     root_dev
 276 Type:           modify (optional)
 277 Offset/size:    0x1fc/2
 278 Protocol:       ALL
 279
 280   The default root device device number.  The use of this field is
 281   deprecated, use the "root=" option on the command line instead.
 282
 283 Field name:     boot_flag
 284 Type:           read
 285 Offset/size:    0x1fe/2
 286 Protocol:       ALL
 287
 288   Contains 0xAA55.  This is the closest thing old Linux kernels have
 289   to a magic number.
 290
 291 Field name:     jump
 292 Type:           read
 293 Offset/size:    0x200/2
 294 Protocol:       2.00+
 295
 296   Contains an x86 jump instruction, 0xEB followed by a signed offset
 297   relative to byte 0x202.  This can be used to determine the size of
 298   the header.
 299
 300 Field name:     header
 301 Type:           read
 302 Offset/size:    0x202/4
 303 Protocol:       2.00+
 304
 305   Contains the magic number "HdrS" (0x53726448).
 306
 307 Field name:     version
 308 Type:           read
 309 Offset/size:    0x206/2
 310 Protocol:       2.00+
 311
 312   Contains the boot protocol version, in (major << 8)+minor format,
 313   e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
 314   10.17.
 315
 316 Field name:     realmode_swtch
 317 Type:           modify (optional)
 318 Offset/size:    0x208/4
 319 Protocol:       2.00+
 320
 321   Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
 322
 323 Field name:     start_sys_seg
 324 Type:           read
 325 Offset/size:    0x20c/2
 326 Protocol:       2.00+
 327
 328   The load low segment (0x1000).  Obsolete.
 329
 330 Field name:     kernel_version
 331 Type:           read
 332 Offset/size:    0x20e/2
 333 Protocol:       2.00+
 334
 335   If set to a nonzero value, contains a pointer to a NUL-terminated
 336   human-readable kernel version number string, less 0x200.  This can
 337   be used to display the kernel version to the user.  This value
 338   should be less than (0x200*setup_sects).
 339
 340   For example, if this value is set to 0x1c00, the kernel version
 341   number string can be found at offset 0x1e00 in the kernel file.
 342   This is a valid value if and only if the "setup_sects" field
 343   contains the value 15 or higher, as:
 344
 345         0x1c00  < 15*0x200 (= 0x1e00) but
 346         0x1c00 >= 14*0x200 (= 0x1c00)
 347
 348         0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
 349
 350 Field name:     type_of_loader
 351 Type:           write (obligatory)
 352 Offset/size:    0x210/1
 353 Protocol:       2.00+
 354
 355   If your boot loader has an assigned id (see table below), enter
 356   0xTV here, where T is an identifier for the boot loader and V is
 357   a version number.  Otherwise, enter 0xFF here.
 358
 359   For boot loader IDs above T = 0xD, write T = 0xE to this field and
 360   write the extended ID minus 0x10 to the ext_loader_type field.
 361   Similarly, the ext_loader_ver field can be used to provide more than
 362   four bits for the bootloader version.
 363
 364   For example, for T = 0x15, V = 0x234, write:
 365
 366   type_of_loader  <- 0xE4
 367   ext_loader_type <- 0x05
 368   ext_loader_ver  <- 0x23
 369
 370   Assigned boot loader ids (hexadecimal):
 371
 372         0  LILO                 (0x00 reserved for pre-2.00 bootloader)
 373         1  Loadlin
 374         2  bootsect-loader      (0x20, all other values reserved)
 375         3  Syslinux
 376         4  Etherboot/gPXE/iPXE
 377         5  ELILO
 378         7  GRUB
 379         8  U-Boot
 380         9  Xen
 381         A  Gujin
 382         B  Qemu
 383         C  Arcturus Networks uCbootloader
 384         D  kexec-tools
 385         E  Extended             (see ext_loader_type)
 386         F  Special              (0xFF = undefined)
 387        10  Reserved
 388        11  Minimal Linux Bootloader <http://sebastian-plotz.blogspot.de>
 389
 390   Please contact <hpa@zytor.com> if you need a bootloader ID
 391   value assigned.
 392
 393 Field name:     loadflags
 394 Type:           modify (obligatory)
 395 Offset/size:    0x211/1
 396 Protocol:       2.00+
 397
 398   This field is a bitmask.
 399
 400   Bit 0 (read): LOADED_HIGH
 401         - If 0, the protected-mode code is loaded at 0x10000.
 402         - If 1, the protected-mode code is loaded at 0x100000.
 403
 404   Bit 5 (write): QUIET_FLAG
 405         - If 0, print early messages.
 406         - If 1, suppress early messages.
 407                 This requests to the kernel (decompressor and early
 408                 kernel) to not write early messages that require
 409                 accessing the display hardware directly.
 410
 411   Bit 6 (write): KEEP_SEGMENTS
 412         Protocol: 2.07+
 413         - If 0, reload the segment registers in the 32bit entry point.
 414         - If 1, do not reload the segment registers in the 32bit entry point.
 415                 Assume that %cs %ds %ss %es are all set to flat segments with
 416                 a base of 0 (or the equivalent for their environment).
 417
 418   Bit 7 (write): CAN_USE_HEAP
 419         Set this bit to 1 to indicate that the value entered in the
 420         heap_end_ptr is valid.  If this field is clear, some setup code
 421         functionality will be disabled.
 422
 423 Field name:     setup_move_size
 424 Type:           modify (obligatory)
 425 Offset/size:    0x212/2
 426 Protocol:       2.00-2.01
 427
 428   When using protocol 2.00 or 2.01, if the real mode kernel is not
 429   loaded at 0x90000, it gets moved there later in the loading
 430   sequence.  Fill in this field if you want additional data (such as
 431   the kernel command line) moved in addition to the real-mode kernel
 432   itself.
 433
 434   The unit is bytes starting with the beginning of the boot sector.
 435
 436   This field is can be ignored when the protocol is 2.02 or higher, or
 437   if the real-mode code is loaded at 0x90000.
 438
 439 Field name:     code32_start
 440 Type:           modify (optional, reloc)
 441 Offset/size:    0x214/4
 442 Protocol:       2.00+
 443
 444   The address to jump to in protected mode.  This defaults to the load
 445   address of the kernel, and can be used by the boot loader to
 446   determine the proper load address.
 447
 448   This field can be modified for two purposes:
 449
 450   1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
 451
 452   2. if a bootloader which does not install a hook loads a
 453      relocatable kernel at a nonstandard address it will have to modify
 454      this field to point to the load address.
 455
 456 Field name:     ramdisk_image
 457 Type:           write (obligatory)
 458 Offset/size:    0x218/4
 459 Protocol:       2.00+
 460
 461   The 32-bit linear address of the initial ramdisk or ramfs.  Leave at
 462   zero if there is no initial ramdisk/ramfs.
 463
 464 Field name:     ramdisk_size
 465 Type:           write (obligatory)
 466 Offset/size:    0x21c/4
 467 Protocol:       2.00+
 468
 469   Size of the initial ramdisk or ramfs.  Leave at zero if there is no
 470   initial ramdisk/ramfs.
 471
 472 Field name:     bootsect_kludge
 473 Type:           kernel internal
 474 Offset/size:    0x220/4
 475 Protocol:       2.00+
 476
 477   This field is obsolete.
 478
 479 Field name:     heap_end_ptr
 480 Type:           write (obligatory)
 481 Offset/size:    0x224/2
 482 Protocol:       2.01+
 483
 484   Set this field to the offset (from the beginning of the real-mode
 485   code) of the end of the setup stack/heap, minus 0x0200.
 486
 487 Field name:     ext_loader_ver
 488 Type:           write (optional)
 489 Offset/size:    0x226/1
 490 Protocol:       2.02+
 491
 492   This field is used as an extension of the version number in the
 493   type_of_loader field.  The total version number is considered to be
 494   (type_of_loader & 0x0f) + (ext_loader_ver << 4).
 495
 496   The use of this field is boot loader specific.  If not written, it
 497   is zero.
 498
 499   Kernels prior to 2.6.31 did not recognize this field, but it is safe
 500   to write for protocol version 2.02 or higher.
 501
 502 Field name:     ext_loader_type
 503 Type:           write (obligatory if (type_of_loader & 0xf0) == 0xe0)
 504 Offset/size:    0x227/1
 505 Protocol:       2.02+
 506
 507   This field is used as an extension of the type number in
 508   type_of_loader field.  If the type in type_of_loader is 0xE, then
 509   the actual type is (ext_loader_type + 0x10).
 510
 511   This field is ignored if the type in type_of_loader is not 0xE.
 512
 513   Kernels prior to 2.6.31 did not recognize this field, but it is safe
 514   to write for protocol version 2.02 or higher.
 515
 516 Field name:     cmd_line_ptr
 517 Type:           write (obligatory)
 518 Offset/size:    0x228/4
 519 Protocol:       2.02+
 520
 521   Set this field to the linear address of the kernel command line.
 522   The kernel command line can be located anywhere between the end of
 523   the setup heap and 0xA0000; it does not have to be located in the
 524   same 64K segment as the real-mode code itself.
 525
 526   Fill in this field even if your boot loader does not support a
 527   command line, in which case you can point this to an empty string
 528   (or better yet, to the string "auto".)  If this field is left at
 529   zero, the kernel will assume that your boot loader does not support
 530   the 2.02+ protocol.
 531
 532 Field name:     ramdisk_max
 533 Type:           read
 534 Offset/size:    0x22c/4
 535 Protocol:       2.03+
 536
 537   The maximum address that may be occupied by the initial
 538   ramdisk/ramfs contents.  For boot protocols 2.02 or earlier, this
 539   field is not present, and the maximum address is 0x37FFFFFF.  (This
 540   address is defined as the address of the highest safe byte, so if
 541   your ramdisk is exactly 131072 bytes long and this field is
 542   0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
 543
 544 Field name:     kernel_alignment
 545 Type:           read/modify (reloc)
 546 Offset/size:    0x230/4
 547 Protocol:       2.05+ (read), 2.10+ (modify)
 548
 549   Alignment unit required by the kernel (if relocatable_kernel is
 550   true.)  A relocatable kernel that is loaded at an alignment
 551   incompatible with the value in this field will be realigned during
 552   kernel initialization.
 553
 554   Starting with protocol version 2.10, this reflects the kernel
 555   alignment preferred for optimal performance; it is possible for the
 556   loader to modify this field to permit a lesser alignment.  See the
 557   min_alignment and pref_address field below.
 558
 559 Field name:     relocatable_kernel
 560 Type:           read (reloc)
 561 Offset/size:    0x234/1
 562 Protocol:       2.05+
 563
 564   If this field is nonzero, the protected-mode part of the kernel can
 565   be loaded at any address that satisfies the kernel_alignment field.
 566   After loading, the boot loader must set the code32_start field to
 567   point to the loaded code, or to a boot loader hook.
 568
 569 Field name:     min_alignment
 570 Type:           read (reloc)
 571 Offset/size:    0x235/1
 572 Protocol:       2.10+
 573
 574   This field, if nonzero, indicates as a power of two the minimum
 575   alignment required, as opposed to preferred, by the kernel to boot.
 576   If a boot loader makes use of this field, it should update the
 577   kernel_alignment field with the alignment unit desired; typically:
 578
 579         kernel_alignment = 1 << min_alignment
 580
 581   There may be a considerable performance cost with an excessively
 582   misaligned kernel.  Therefore, a loader should typically try each
 583   power-of-two alignment from kernel_alignment down to this alignment.
 584
 585 Field name:     cmdline_size
 586 Type:           read
 587 Offset/size:    0x238/4
 588 Protocol:       2.06+
 589
 590   The maximum size of the command line without the terminating
 591   zero. This means that the command line can contain at most
 592   cmdline_size characters. With protocol version 2.05 and earlier, the
 593   maximum size was 255.
 594
 595 Field name:     hardware_subarch
 596 Type:           write (optional, defaults to x86/PC)
 597 Offset/size:    0x23c/4
 598 Protocol:       2.07+
 599
 600   In a paravirtualized environment the hardware low level architectural
 601   pieces such as interrupt handling, page table handling, and
 602   accessing process control registers needs to be done differently.
 603
 604   This field allows the bootloader to inform the kernel we are in one
 605   one of those environments.
 606
 607   0x00000000    The default x86/PC environment
 608   0x00000001    lguest
 609   0x00000002    Xen
 610   0x00000003    Moorestown MID
 611   0x00000004    CE4100 TV Platform
 612
 613 Field name:     hardware_subarch_data
 614 Type:           write (subarch-dependent)
 615 Offset/size:    0x240/8
 616 Protocol:       2.07+
 617
 618   A pointer to data that is specific to hardware subarch
 619   This field is currently unused for the default x86/PC environment,
 620   do not modify.
 621
 622 Field name:     payload_offset
 623 Type:           read
 624 Offset/size:    0x248/4
 625 Protocol:       2.08+
 626
 627   If non-zero then this field contains the offset from the beginning
 628   of the protected-mode code to the payload.
 629
 630   The payload may be compressed. The format of both the compressed and
 631   uncompressed data should be determined using the standard magic
 632   numbers.  The currently supported compression formats are gzip
 633   (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA
 634   (magic number 5D 00), and XZ (magic number FD 37).  The uncompressed
 635   payload is currently always ELF (magic number 7F 45 4C 46).
 636
 637 Field name:     payload_length
 638 Type:           read
 639 Offset/size:    0x24c/4
 640 Protocol:       2.08+
 641
 642   The length of the payload.
 643
 644 Field name:     setup_data
 645 Type:           write (special)
 646 Offset/size:    0x250/8
 647 Protocol:       2.09+
 648
 649   The 64-bit physical pointer to NULL terminated single linked list of
 650   struct setup_data. This is used to define a more extensible boot
 651   parameters passing mechanism. The definition of struct setup_data is
 652   as follow:
 653
 654   struct setup_data {
 655           u64 next;
 656           u32 type;
 657           u32 len;
 658           u8  data[0];
 659   };
 660
 661   Where, the next is a 64-bit physical pointer to the next node of
 662   linked list, the next field of the last node is 0; the type is used
 663   to identify the contents of data; the len is the length of data
 664   field; the data holds the real payload.
 665
 666   This list may be modified at a number of points during the bootup
 667   process.  Therefore, when modifying this list one should always make
 668   sure to consider the case where the linked list already contains
 669   entries.
 670
 671 Field name:     pref_address
 672 Type:           read (reloc)
 673 Offset/size:    0x258/8
 674 Protocol:       2.10+
 675
 676   This field, if nonzero, represents a preferred load address for the
 677   kernel.  A relocating bootloader should attempt to load at this
 678   address if possible.
 679
 680   A non-relocatable kernel will unconditionally move itself and to run
 681   at this address.
 682
 683 Field name:     init_size
 684 Type:           read
 685 Offset/size:    0x260/4
 686
 687   This field indicates the amount of linear contiguous memory starting
 688   at the kernel runtime start address that the kernel needs before it
 689   is capable of examining its memory map.  This is not the same thing
 690   as the total amount of memory the kernel needs to boot, but it can
 691   be used by a relocating boot loader to help select a safe load
 692   address for the kernel.
 693
 694   The kernel runtime start address is determined by the following algorithm:
 695
 696   if (relocatable_kernel)
 697         runtime_start = align_up(load_address, kernel_alignment)
 698   else
 699         runtime_start = pref_address
 700
 701 Field name:     handover_offset
 702 Type:           read
 703 Offset/size:    0x264/4
 704
 705   This field is the offset from the beginning of the kernel image to
 706   the EFI handover protocol entry point. Boot loaders using the EFI
 707   handover protocol to boot the kernel should jump to this offset.
 708
 709   See EFI HANDOVER PROTOCOL below for more details.
 710
 711
 712 **** THE IMAGE CHECKSUM
 713
 714 From boot protocol version 2.08 onwards the CRC-32 is calculated over
 715 the entire file using the characteristic polynomial 0x04C11DB7 and an
 716 initial remainder of 0xffffffff.  The checksum is appended to the
 717 file; therefore the CRC of the file up to the limit specified in the
 718 syssize field of the header is always 0.
 719
 720
 721 **** THE KERNEL COMMAND LINE
 722
 723 The kernel command line has become an important way for the boot
 724 loader to communicate with the kernel.  Some of its options are also
 725 relevant to the boot loader itself, see "special command line options"
 726 below.
 727
 728 The kernel command line is a null-terminated string. The maximum
 729 length can be retrieved from the field cmdline_size.  Before protocol
 730 version 2.06, the maximum was 255 characters.  A string that is too
 731 long will be automatically truncated by the kernel.
 732
 733 If the boot protocol version is 2.02 or later, the address of the
 734 kernel command line is given by the header field cmd_line_ptr (see
 735 above.)  This address can be anywhere between the end of the setup
 736 heap and 0xA0000.
 737
 738 If the protocol version is *not* 2.02 or higher, the kernel
 739 command line is entered using the following protocol:
 740
 741         At offset 0x0020 (word), "cmd_line_magic", enter the magic
 742         number 0xA33F.
 743
 744         At offset 0x0022 (word), "cmd_line_offset", enter the offset
 745         of the kernel command line (relative to the start of the
 746         real-mode kernel).
 747
 748         The kernel command line *must* be within the memory region
 749         covered by setup_move_size, so you may need to adjust this
 750         field.
 751
 752
 753 **** MEMORY LAYOUT OF THE REAL-MODE CODE
 754
 755 The real-mode code requires a stack/heap to be set up, as well as
 756 memory allocated for the kernel command line.  This needs to be done
 757 in the real-mode accessible memory in bottom megabyte.
 758
 759 It should be noted that modern machines often have a sizable Extended
 760 BIOS Data Area (EBDA).  As a result, it is advisable to use as little
 761 of the low megabyte as possible.
 762
 763 Unfortunately, under the following circumstances the 0x90000 memory
 764 segment has to be used:
 765
 766         - When loading a zImage kernel ((loadflags & 0x01) == 0).
 767         - When loading a 2.01 or earlier boot protocol kernel.
 768
 769           -> For the 2.00 and 2.01 boot protocols, the real-mode code
 770              can be loaded at another address, but it is internally
 771              relocated to 0x90000.  For the "old" protocol, the
 772              real-mode code must be loaded at 0x90000.
 773
 774 When loading at 0x90000, avoid using memory above 0x9a000.
 775
 776 For boot protocol 2.02 or higher, the command line does not have to be
 777 located in the same 64K segment as the real-mode setup code; it is
 778 thus permitted to give the stack/heap the full 64K segment and locate
 779 the command line above it.
 780
 781 The kernel command line should not be located below the real-mode
 782 code, nor should it be located in high memory.
 783
 784
 785 **** SAMPLE BOOT CONFIGURATION
 786
 787 As a sample configuration, assume the following layout of the real
 788 mode segment:
 789
 790     When loading below 0x90000, use the entire segment:
 791
 792         0x0000-0x7fff   Real mode kernel
 793         0x8000-0xdfff   Stack and heap
 794         0xe000-0xffff   Kernel command line
 795
 796     When loading at 0x90000 OR the protocol version is 2.01 or earlier:
 797
 798         0x0000-0x7fff   Real mode kernel
 799         0x8000-0x97ff   Stack and heap
 800         0x9800-0x9fff   Kernel command line
 801
 802 Such a boot loader should enter the following fields in the header:
 803
 804         unsigned long base_ptr; /* base address for real-mode segment */
 805
 806         if ( setup_sects == 0 ) {
 807                 setup_sects = 4;
 808         }
 809
 810         if ( protocol >= 0x0200 ) {
 811                 type_of_loader = <type code>;
 812                 if ( loading_initrd ) {
 813                         ramdisk_image = <initrd_address>;
 814                         ramdisk_size = <initrd_size>;
 815                 }
 816
 817                 if ( protocol >= 0x0202 && loadflags & 0x01 )
 818                         heap_end = 0xe000;
 819                 else
 820                         heap_end = 0x9800;
 821
 822                 if ( protocol >= 0x0201 ) {
 823                         heap_end_ptr = heap_end - 0x200;
 824                         loadflags |= 0x80; /* CAN_USE_HEAP */
 825                 }
 826
 827                 if ( protocol >= 0x0202 ) {
 828                         cmd_line_ptr = base_ptr + heap_end;
 829                         strcpy(cmd_line_ptr, cmdline);
 830                 } else {
 831                         cmd_line_magic  = 0xA33F;
 832                         cmd_line_offset = heap_end;
 833                         setup_move_size = heap_end + strlen(cmdline)+1;
 834                         strcpy(base_ptr+cmd_line_offset, cmdline);
 835                 }
 836         } else {
 837                 /* Very old kernel */
 838
 839                 heap_end = 0x9800;
 840
 841                 cmd_line_magic  = 0xA33F;
 842                 cmd_line_offset = heap_end;
 843
 844                 /* A very old kernel MUST have its real-mode code
 845                    loaded at 0x90000 */
 846
 847                 if ( base_ptr != 0x90000 ) {
 848                         /* Copy the real-mode kernel */
 849                         memcpy(0x90000, base_ptr, (setup_sects+1)*512);
 850                         base_ptr = 0x90000;              /* Relocated */
 851                 }
 852
 853                 strcpy(0x90000+cmd_line_offset, cmdline);
 854
 855                 /* It is recommended to clear memory up to the 32K mark */
 856                 memset(0x90000 + (setup_sects+1)*512, 0,
 857                        (64-(setup_sects+1))*512);
 858         }
 859
 860
 861 **** LOADING THE REST OF THE KERNEL
 862
 863 The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
 864 in the kernel file (again, if setup_sects == 0 the real value is 4.)
 865 It should be loaded at address 0x10000 for Image/zImage kernels and
 866 0x100000 for bzImage kernels.
 867
 868 The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
 869 bit (LOAD_HIGH) in the loadflags field is set:
 870
 871         is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
 872         load_address = is_bzImage ? 0x100000 : 0x10000;
 873
 874 Note that Image/zImage kernels can be up to 512K in size, and thus use
 875 the entire 0x10000-0x90000 range of memory.  This means it is pretty
 876 much a requirement for these kernels to load the real-mode part at
 877 0x90000.  bzImage kernels allow much more flexibility.
 878
 879
 880 **** SPECIAL COMMAND LINE OPTIONS
 881
 882 If the command line provided by the boot loader is entered by the
 883 user, the user may expect the following command line options to work.
 884 They should normally not be deleted from the kernel command line even
 885 though not all of them are actually meaningful to the kernel.  Boot
 886 loader authors who need additional command line options for the boot
 887 loader itself should get them registered in
 888 Documentation/kernel-parameters.txt to make sure they will not
 889 conflict with actual kernel options now or in the future.
 890
 891   vga=<mode>
 892         <mode> here is either an integer (in C notation, either
 893         decimal, octal, or hexadecimal) or one of the strings
 894         "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
 895         (meaning 0xFFFD).  This value should be entered into the
 896         vid_mode field, as it is used by the kernel before the command
 897         line is parsed.
 898
 899   mem=<size>
 900         <size> is an integer in C notation optionally followed by
 901         (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
 902         << 30, << 40, << 50 or << 60).  This specifies the end of
 903         memory to the kernel. This affects the possible placement of
 904         an initrd, since an initrd should be placed near end of
 905         memory.  Note that this is an option to *both* the kernel and
 906         the bootloader!
 907
 908   initrd=<file>
 909         An initrd should be loaded.  The meaning of <file> is
 910         obviously bootloader-dependent, and some boot loaders
 911         (e.g. LILO) do not have such a command.
 912
 913 In addition, some boot loaders add the following options to the
 914 user-specified command line:
 915
 916   BOOT_IMAGE=<file>
 917         The boot image which was loaded.  Again, the meaning of <file>
 918         is obviously bootloader-dependent.
 919
 920   auto
 921         The kernel was booted without explicit user intervention.
 922
 923 If these options are added by the boot loader, it is highly
 924 recommended that they are located *first*, before the user-specified
 925 or configuration-specified command line.  Otherwise, "init=/bin/sh"
 926 gets confused by the "auto" option.
 927
 928
 929 **** RUNNING THE KERNEL
 930
 931 The kernel is started by jumping to the kernel entry point, which is
 932 located at *segment* offset 0x20 from the start of the real mode
 933 kernel.  This means that if you loaded your real-mode kernel code at
 934 0x90000, the kernel entry point is 9020:0000.
 935
 936 At entry, ds = es = ss should point to the start of the real-mode
 937 kernel code (0x9000 if the code is loaded at 0x90000), sp should be
 938 set up properly, normally pointing to the top of the heap, and
 939 interrupts should be disabled.  Furthermore, to guard against bugs in
 940 the kernel, it is recommended that the boot loader sets fs = gs = ds =
 941 es = ss.
 942
 943 In our example from above, we would do:
 944
 945         /* Note: in the case of the "old" kernel protocol, base_ptr must
 946            be == 0x90000 at this point; see the previous sample code */
 947
 948         seg = base_ptr >> 4;
 949
 950         cli();  /* Enter with interrupts disabled! */
 951
 952         /* Set up the real-mode kernel stack */
 953         _SS = seg;
 954         _SP = heap_end;
 955
 956         _DS = _ES = _FS = _GS = seg;
 957         jmp_far(seg+0x20, 0);   /* Run the kernel */
 958
 959 If your boot sector accesses a floppy drive, it is recommended to
 960 switch off the floppy motor before running the kernel, since the
 961 kernel boot leaves interrupts off and thus the motor will not be
 962 switched off, especially if the loaded kernel has the floppy driver as
 963 a demand-loaded module!
 964
 965
 966 **** ADVANCED BOOT LOADER HOOKS
 967
 968 If the boot loader runs in a particularly hostile environment (such as
 969 LOADLIN, which runs under DOS) it may be impossible to follow the
 970 standard memory location requirements.  Such a boot loader may use the
 971 following hooks that, if set, are invoked by the kernel at the
 972 appropriate time.  The use of these hooks should probably be
 973 considered an absolutely last resort!
 974
 975 IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
 976 %edi across invocation.
 977
 978   realmode_swtch:
 979         A 16-bit real mode far subroutine invoked immediately before
 980         entering protected mode.  The default routine disables NMI, so
 981         your routine should probably do so, too.
 982
 983   code32_start:
 984         A 32-bit flat-mode routine *jumped* to immediately after the
 985         transition to protected mode, but before the kernel is
 986         uncompressed.  No segments, except CS, are guaranteed to be
 987         set up (current kernels do, but older ones do not); you should
 988         set them up to BOOT_DS (0x18) yourself.
 989
 990         After completing your hook, you should jump to the address
 991         that was in this field before your boot loader overwrote it
 992         (relocated, if appropriate.)
 993
 994
 995 **** 32-bit BOOT PROTOCOL
 996
 997 For machine with some new BIOS other than legacy BIOS, such as EFI,
 998 LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
 999 based on legacy BIOS can not be used, so a 32-bit boot protocol needs
1000 to be defined.
1001
1002 In 32-bit boot protocol, the first step in loading a Linux kernel
1003 should be to setup the boot parameters (struct boot_params,
1004 traditionally known as "zero page"). The memory for struct boot_params
1005 should be allocated and initialized to all zero. Then the setup header
1006 from offset 0x01f1 of kernel image on should be loaded into struct
1007 boot_params and examined. The end of setup header can be calculated as
1008 follow:
1009
1010         0x0202 + byte value at offset 0x0201
1011
1012 In addition to read/modify/write the setup header of the struct
1013 boot_params as that of 16-bit boot protocol, the boot loader should
1014 also fill the additional fields of the struct boot_params as that
1015 described in zero-page.txt.
1016
1017 After setting up the struct boot_params, the boot loader can load the
1018 32/64-bit kernel in the same way as that of 16-bit boot protocol.
1019
1020 In 32-bit boot protocol, the kernel is started by jumping to the
1021 32-bit kernel entry point, which is the start address of loaded
1022 32/64-bit kernel.
1023
1024 At entry, the CPU must be in 32-bit protected mode with paging
1025 disabled; a GDT must be loaded with the descriptors for selectors
1026 __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1027 segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1028 must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1029 must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
1030 address of the struct boot_params; %ebp, %edi and %ebx must be zero.
1031
1032 **** EFI HANDOVER PROTOCOL
1033
1034 This protocol allows boot loaders to defer initialisation to the EFI
1035 boot stub. The boot loader is required to load the kernel/initrd(s)
1036 from the boot media and jump to the EFI handover protocol entry point
1037 which is hdr->handover_offset bytes from the beginning of
1038 startup_{32,64}.
1039
1040 The function prototype for the handover entry point looks like this,
1041
1042     efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp)
1043
1044 'handle' is the EFI image handle passed to the boot loader by the EFI
1045 firmware, 'table' is the EFI system table - these are the first two
1046 arguments of the "handoff state" as described in section 2.3 of the
1047 UEFI specification. 'bp' is the boot loader-allocated boot params.
1048
1049 The boot loader *must* fill out the following fields in bp,
1050
1051     o hdr.code32_start
1052     o hdr.cmd_line_ptr
1053     o hdr.cmdline_size
1054     o hdr.ramdisk_image (if applicable)
1055     o hdr.ramdisk_size  (if applicable)
1056
1057 All other fields should be zero.