linux/Documentation/i386/boot.txt
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   1                     THE LINUX/I386 BOOT PROTOCOL
   2                     ----------------------------
   3
   4                    H. Peter Anvin <hpa@zytor.com>
   5                        Last update 2007-05-23
   6
   7On the i386 platform, the Linux kernel uses a rather complicated boot
   8convention.  This has evolved partially due to historical aspects, as
   9well as the desire in the early days to have the kernel itself be a
  10bootable image, the complicated PC memory model and due to changed
  11expectations in the PC industry caused by the effective demise of
  12real-mode DOS as a mainstream operating system.
  13
  14Currently, the following versions of the Linux/i386 boot protocol exist.
  15
  16Old kernels:    zImage/Image support only.  Some very early kernels
  17                may not even support a command line.
  18
  19Protocol 2.00:  (Kernel 1.3.73) Added bzImage and initrd support, as
  20                well as a formalized way to communicate between the
  21                boot loader and the kernel.  setup.S made relocatable,
  22                although the traditional setup area still assumed
  23                writable.
  24
  25Protocol 2.01:  (Kernel 1.3.76) Added a heap overrun warning.
  26
  27Protocol 2.02:  (Kernel 2.4.0-test3-pre3) New command line protocol.
  28                Lower the conventional memory ceiling.  No overwrite
  29                of the traditional setup area, thus making booting
  30                safe for systems which use the EBDA from SMM or 32-bit
  31                BIOS entry points.  zImage deprecated but still
  32                supported.
  33
  34Protocol 2.03:  (Kernel 2.4.18-pre1) Explicitly makes the highest possible
  35                initrd address available to the bootloader.
  36
  37Protocol 2.04:  (Kernel 2.6.14) Extend the syssize field to four bytes.
  38
  39Protocol 2.05:  (Kernel 2.6.20) Make protected mode kernel relocatable.
  40                Introduce relocatable_kernel and kernel_alignment fields.
  41
  42Protocol 2.06:  (Kernel 2.6.22) Added a field that contains the size of
  43                the boot command line
  44
  45
  46**** MEMORY LAYOUT
  47
  48The traditional memory map for the kernel loader, used for Image or
  49zImage kernels, typically looks like:
  50
  51        |                        |
  520A0000  +------------------------+
  53        |  Reserved for BIOS     |      Do not use.  Reserved for BIOS EBDA.
  5409A000  +------------------------+
  55        |  Command line          |
  56        |  Stack/heap            |      For use by the kernel real-mode code.
  57098000  +------------------------+      
  58        |  Kernel setup          |      The kernel real-mode code.
  59090200  +------------------------+
  60        |  Kernel boot sector    |      The kernel legacy boot sector.
  61090000  +------------------------+
  62        |  Protected-mode kernel |      The bulk of the kernel image.
  63010000  +------------------------+
  64        |  Boot loader           |      <- Boot sector entry point 0000:7C00
  65001000  +------------------------+
  66        |  Reserved for MBR/BIOS |
  67000800  +------------------------+
  68        |  Typically used by MBR |
  69000600  +------------------------+ 
  70        |  BIOS use only         |
  71000000  +------------------------+
  72
  73
  74When using bzImage, the protected-mode kernel was relocated to
  750x100000 ("high memory"), and the kernel real-mode block (boot sector,
  76setup, and stack/heap) was made relocatable to any address between
  770x10000 and end of low memory. Unfortunately, in protocols 2.00 and
  782.01 the 0x90000+ memory range is still used internally by the kernel;
  79the 2.02 protocol resolves that problem.
  80
  81It is desirable to keep the "memory ceiling" -- the highest point in
  82low memory touched by the boot loader -- as low as possible, since
  83some newer BIOSes have begun to allocate some rather large amounts of
  84memory, called the Extended BIOS Data Area, near the top of low
  85memory.  The boot loader should use the "INT 12h" BIOS call to verify
  86how much low memory is available.
  87
  88Unfortunately, if INT 12h reports that the amount of memory is too
  89low, there is usually nothing the boot loader can do but to report an
  90error to the user.  The boot loader should therefore be designed to
  91take up as little space in low memory as it reasonably can.  For
  92zImage or old bzImage kernels, which need data written into the
  930x90000 segment, the boot loader should make sure not to use memory
  94above the 0x9A000 point; too many BIOSes will break above that point.
  95
  96For a modern bzImage kernel with boot protocol version >= 2.02, a
  97memory layout like the following is suggested:
  98
  99        ~                        ~
 100        |  Protected-mode kernel |
 101100000  +------------------------+
 102        |  I/O memory hole       |
 1030A0000  +------------------------+
 104        |  Reserved for BIOS     |      Leave as much as possible unused
 105        ~                        ~
 106        |  Command line          |      (Can also be below the X+10000 mark)
 107X+10000 +------------------------+
 108        |  Stack/heap            |      For use by the kernel real-mode code.
 109X+08000 +------------------------+      
 110        |  Kernel setup          |      The kernel real-mode code.
 111        |  Kernel boot sector    |      The kernel legacy boot sector.
 112X       +------------------------+
 113        |  Boot loader           |      <- Boot sector entry point 0000:7C00
 114001000  +------------------------+
 115        |  Reserved for MBR/BIOS |
 116000800  +------------------------+
 117        |  Typically used by MBR |
 118000600  +------------------------+ 
 119        |  BIOS use only         |
 120000000  +------------------------+
 121
 122... where the address X is as low as the design of the boot loader
 123permits.
 124
 125
 126**** THE REAL-MODE KERNEL HEADER
 127
 128In the following text, and anywhere in the kernel boot sequence, "a
 129sector" refers to 512 bytes.  It is independent of the actual sector
 130size of the underlying medium.
 131
 132The first step in loading a Linux kernel should be to load the
 133real-mode code (boot sector and setup code) and then examine the
 134following header at offset 0x01f1.  The real-mode code can total up to
 13532K, although the boot loader may choose to load only the first two
 136sectors (1K) and then examine the bootup sector size.
 137
 138The header looks like:
 139
 140Offset  Proto   Name            Meaning
 141/Size
 142
 14301F1/1  ALL(1   setup_sects     The size of the setup in sectors
 14401F2/2  ALL     root_flags      If set, the root is mounted readonly
 14501F4/4  2.04+(2 syssize         The size of the 32-bit code in 16-byte paras
 14601F8/2  ALL     ram_size        DO NOT USE - for bootsect.S use only
 14701FA/2  ALL     vid_mode        Video mode control
 14801FC/2  ALL     root_dev        Default root device number
 14901FE/2  ALL     boot_flag       0xAA55 magic number
 1500200/2  2.00+   jump            Jump instruction
 1510202/4  2.00+   header          Magic signature "HdrS"
 1520206/2  2.00+   version         Boot protocol version supported
 1530208/4  2.00+   realmode_swtch  Boot loader hook (see below)
 154020C/2  2.00+   start_sys       The load-low segment (0x1000) (obsolete)
 155020E/2  2.00+   kernel_version  Pointer to kernel version string
 1560210/1  2.00+   type_of_loader  Boot loader identifier
 1570211/1  2.00+   loadflags       Boot protocol option flags
 1580212/2  2.00+   setup_move_size Move to high memory size (used with hooks)
 1590214/4  2.00+   code32_start    Boot loader hook (see below)
 1600218/4  2.00+   ramdisk_image   initrd load address (set by boot loader)
 161021C/4  2.00+   ramdisk_size    initrd size (set by boot loader)
 1620220/4  2.00+   bootsect_kludge DO NOT USE - for bootsect.S use only
 1630224/2  2.01+   heap_end_ptr    Free memory after setup end
 1640226/2  N/A     pad1            Unused
 1650228/4  2.02+   cmd_line_ptr    32-bit pointer to the kernel command line
 166022C/4  2.03+   initrd_addr_max Highest legal initrd address
 1670230/4  2.05+   kernel_alignment Physical addr alignment required for kernel
 1680234/1  2.05+   relocatable_kernel Whether kernel is relocatable or not
 1690235/3  N/A     pad2            Unused
 1700238/4  2.06+   cmdline_size    Maximum size of the kernel command line
 171023C/4  2.07+   hardware_subarch Hardware subarchitecture
 1720240/8  2.07+   hardware_subarch_data Subarchitecture-specific data
 173
 174(1) For backwards compatibility, if the setup_sects field contains 0, the
 175    real value is 4.
 176
 177(2) For boot protocol prior to 2.04, the upper two bytes of the syssize
 178    field are unusable, which means the size of a bzImage kernel
 179    cannot be determined.
 180
 181If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
 182the boot protocol version is "old".  Loading an old kernel, the
 183following parameters should be assumed:
 184
 185        Image type = zImage
 186        initrd not supported
 187        Real-mode kernel must be located at 0x90000.
 188
 189Otherwise, the "version" field contains the protocol version,
 190e.g. protocol version 2.01 will contain 0x0201 in this field.  When
 191setting fields in the header, you must make sure only to set fields
 192supported by the protocol version in use.
 193
 194
 195**** DETAILS OF HEADER FIELDS
 196
 197For each field, some are information from the kernel to the bootloader
 198("read"), some are expected to be filled out by the bootloader
 199("write"), and some are expected to be read and modified by the
 200bootloader ("modify").
 201
 202All general purpose boot loaders should write the fields marked
 203(obligatory).  Boot loaders who want to load the kernel at a
 204nonstandard address should fill in the fields marked (reloc); other
 205boot loaders can ignore those fields.
 206
 207The byte order of all fields is littleendian (this is x86, after all.)
 208
 209Field name:     setup_sects
 210Type:           read
 211Offset/size:    0x1f1/1
 212Protocol:       ALL
 213
 214  The size of the setup code in 512-byte sectors.  If this field is
 215  0, the real value is 4.  The real-mode code consists of the boot
 216  sector (always one 512-byte sector) plus the setup code.
 217
 218Field name:      root_flags
 219Type:            modify (optional)
 220Offset/size:     0x1f2/2
 221Protocol:        ALL
 222
 223  If this field is nonzero, the root defaults to readonly.  The use of
 224  this field is deprecated; use the "ro" or "rw" options on the
 225  command line instead.
 226
 227Field name:     syssize
 228Type:           read
 229Offset/size:    0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
 230Protocol:       2.04+
 231
 232  The size of the protected-mode code in units of 16-byte paragraphs.
 233  For protocol versions older than 2.04 this field is only two bytes
 234  wide, and therefore cannot be trusted for the size of a kernel if
 235  the LOAD_HIGH flag is set.
 236
 237Field name:     ram_size
 238Type:           kernel internal
 239Offset/size:    0x1f8/2
 240Protocol:       ALL
 241
 242  This field is obsolete.
 243
 244Field name:     vid_mode
 245Type:           modify (obligatory)
 246Offset/size:    0x1fa/2
 247
 248  Please see the section on SPECIAL COMMAND LINE OPTIONS.
 249
 250Field name:     root_dev
 251Type:           modify (optional)
 252Offset/size:    0x1fc/2
 253Protocol:       ALL
 254
 255  The default root device device number.  The use of this field is
 256  deprecated, use the "root=" option on the command line instead.
 257
 258Field name:     boot_flag
 259Type:           read
 260Offset/size:    0x1fe/2
 261Protocol:       ALL
 262
 263  Contains 0xAA55.  This is the closest thing old Linux kernels have
 264  to a magic number.
 265
 266Field name:     jump
 267Type:           read
 268Offset/size:    0x200/2
 269Protocol:       2.00+
 270
 271  Contains an x86 jump instruction, 0xEB followed by a signed offset
 272  relative to byte 0x202.  This can be used to determine the size of
 273  the header.
 274
 275Field name:     header
 276Type:           read
 277Offset/size:    0x202/4
 278Protocol:       2.00+
 279
 280  Contains the magic number "HdrS" (0x53726448).
 281
 282Field name:     version
 283Type:           read
 284Offset/size:    0x206/2
 285Protocol:       2.00+
 286
 287  Contains the boot protocol version, in (major << 8)+minor format,
 288  e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
 289  10.17.
 290
 291Field name:     readmode_swtch
 292Type:           modify (optional)
 293Offset/size:    0x208/4
 294Protocol:       2.00+
 295
 296  Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
 297
 298Field name:     start_sys
 299Type:           read
 300Offset/size:    0x20c/4
 301Protocol:       2.00+
 302
 303  The load low segment (0x1000).  Obsolete.
 304
 305Field name:     kernel_version
 306Type:           read
 307Offset/size:    0x20e/2
 308Protocol:       2.00+
 309
 310  If set to a nonzero value, contains a pointer to a NUL-terminated
 311  human-readable kernel version number string, less 0x200.  This can
 312  be used to display the kernel version to the user.  This value
 313  should be less than (0x200*setup_sects).
 314
 315  For example, if this value is set to 0x1c00, the kernel version
 316  number string can be found at offset 0x1e00 in the kernel file.
 317  This is a valid value if and only if the "setup_sects" field
 318  contains the value 15 or higher, as:
 319
 320        0x1c00  < 15*0x200 (= 0x1e00) but
 321        0x1c00 >= 14*0x200 (= 0x1c00)
 322
 323        0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
 324
 325Field name:     type_of_loader
 326Type:           write (obligatory)
 327Offset/size:    0x210/1
 328Protocol:       2.00+
 329
 330  If your boot loader has an assigned id (see table below), enter
 331  0xTV here, where T is an identifier for the boot loader and V is
 332  a version number.  Otherwise, enter 0xFF here.
 333
 334  Assigned boot loader ids:
 335        0  LILO                 (0x00 reserved for pre-2.00 bootloader)
 336        1  Loadlin
 337        2  bootsect-loader      (0x20, all other values reserved)
 338        3  SYSLINUX
 339        4  EtherBoot
 340        5  ELILO
 341        7  GRuB
 342        8  U-BOOT
 343        9  Xen
 344        A  Gujin
 345        B  Qemu
 346
 347  Please contact <hpa@zytor.com> if you need a bootloader ID
 348  value assigned.
 349
 350Field name:     loadflags
 351Type:           modify (obligatory)
 352Offset/size:    0x211/1
 353Protocol:       2.00+
 354
 355  This field is a bitmask.
 356
 357  Bit 0 (read): LOADED_HIGH
 358        - If 0, the protected-mode code is loaded at 0x10000.
 359        - If 1, the protected-mode code is loaded at 0x100000.
 360
 361  Bit 6 (write): KEEP_SEGMENTS
 362        Protocol: 2.07+
 363        - if 0, reload the segment registers in the 32bit entry point.
 364        - if 1, do not reload the segment registers in the 32bit entry point.
 365                Assume that %cs %ds %ss %es are all set to flat segments with
 366                a base of 0 (or the equivalent for their environment).
 367
 368  Bit 7 (write): CAN_USE_HEAP
 369        Set this bit to 1 to indicate that the value entered in the
 370        heap_end_ptr is valid.  If this field is clear, some setup code
 371        functionality will be disabled.
 372
 373Field name:     setup_move_size
 374Type:           modify (obligatory)
 375Offset/size:    0x212/2
 376Protocol:       2.00-2.01
 377
 378  When using protocol 2.00 or 2.01, if the real mode kernel is not
 379  loaded at 0x90000, it gets moved there later in the loading
 380  sequence.  Fill in this field if you want additional data (such as
 381  the kernel command line) moved in addition to the real-mode kernel
 382  itself.
 383
 384  The unit is bytes starting with the beginning of the boot sector.
 385  
 386  This field is can be ignored when the protocol is 2.02 or higher, or
 387  if the real-mode code is loaded at 0x90000.
 388
 389Field name:     code32_start
 390Type:           modify (optional, reloc)
 391Offset/size:    0x214/4
 392Protocol:       2.00+
 393
 394  The address to jump to in protected mode.  This defaults to the load
 395  address of the kernel, and can be used by the boot loader to
 396  determine the proper load address.
 397
 398  This field can be modified for two purposes:
 399
 400  1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
 401
 402  2. if a bootloader which does not install a hook loads a
 403     relocatable kernel at a nonstandard address it will have to modify
 404     this field to point to the load address.
 405
 406Field name:     ramdisk_image
 407Type:           write (obligatory)
 408Offset/size:    0x218/4
 409Protocol:       2.00+
 410
 411  The 32-bit linear address of the initial ramdisk or ramfs.  Leave at
 412  zero if there is no initial ramdisk/ramfs.
 413
 414Field name:     ramdisk_size
 415Type:           write (obligatory)
 416Offset/size:    0x21c/4
 417Protocol:       2.00+
 418
 419  Size of the initial ramdisk or ramfs.  Leave at zero if there is no
 420  initial ramdisk/ramfs.
 421
 422Field name:     bootsect_kludge
 423Type:           kernel internal
 424Offset/size:    0x220/4
 425Protocol:       2.00+
 426
 427  This field is obsolete.
 428
 429Field name:     heap_end_ptr
 430Type:           write (obligatory)
 431Offset/size:    0x224/2
 432Protocol:       2.01+
 433
 434  Set this field to the offset (from the beginning of the real-mode
 435  code) of the end of the setup stack/heap, minus 0x0200.
 436
 437Field name:     cmd_line_ptr
 438Type:           write (obligatory)
 439Offset/size:    0x228/4
 440Protocol:       2.02+
 441
 442  Set this field to the linear address of the kernel command line.
 443  The kernel command line can be located anywhere between the end of
 444  the setup heap and 0xA0000; it does not have to be located in the
 445  same 64K segment as the real-mode code itself.
 446
 447  Fill in this field even if your boot loader does not support a
 448  command line, in which case you can point this to an empty string
 449  (or better yet, to the string "auto".)  If this field is left at
 450  zero, the kernel will assume that your boot loader does not support
 451  the 2.02+ protocol.
 452
 453Field name:     initrd_addr_max
 454Type:           read
 455Offset/size:    0x22c/4
 456Protocol:       2.03+
 457
 458  The maximum address that may be occupied by the initial
 459  ramdisk/ramfs contents.  For boot protocols 2.02 or earlier, this
 460  field is not present, and the maximum address is 0x37FFFFFF.  (This
 461  address is defined as the address of the highest safe byte, so if
 462  your ramdisk is exactly 131072 bytes long and this field is
 463  0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
 464
 465Field name:     kernel_alignment
 466Type:           read (reloc)
 467Offset/size:    0x230/4
 468Protocol:       2.05+
 469
 470  Alignment unit required by the kernel (if relocatable_kernel is true.)
 471
 472Field name:     relocatable_kernel
 473Type:           read (reloc)
 474Offset/size:    0x234/1
 475Protocol:       2.05+
 476
 477  If this field is nonzero, the protected-mode part of the kernel can
 478  be loaded at any address that satisfies the kernel_alignment field.
 479  After loading, the boot loader must set the code32_start field to
 480  point to the loaded code, or to a boot loader hook.
 481
 482Field name:     cmdline_size
 483Type:           read
 484Offset/size:    0x238/4
 485Protocol:       2.06+
 486
 487  The maximum size of the command line without the terminating
 488  zero. This means that the command line can contain at most
 489  cmdline_size characters. With protocol version 2.05 and earlier, the
 490  maximum size was 255.
 491
 492Field name:     hardware_subarch
 493Type:           write
 494Offset/size:    0x23c/4
 495Protocol:       2.07+
 496
 497  In a paravirtualized environment the hardware low level architectural
 498  pieces such as interrupt handling, page table handling, and
 499  accessing process control registers needs to be done differently.
 500
 501  This field allows the bootloader to inform the kernel we are in one
 502  one of those environments.
 503
 504  0x00000000    The default x86/PC environment
 505  0x00000001    lguest
 506  0x00000002    Xen
 507
 508Field name:     hardware_subarch_data
 509Type:           write
 510Offset/size:    0x240/8
 511Protocol:       2.07+
 512
 513  A pointer to data that is specific to hardware subarch
 514
 515
 516**** THE KERNEL COMMAND LINE
 517
 518The kernel command line has become an important way for the boot
 519loader to communicate with the kernel.  Some of its options are also
 520relevant to the boot loader itself, see "special command line options"
 521below.
 522
 523The kernel command line is a null-terminated string. The maximum
 524length can be retrieved from the field cmdline_size.  Before protocol
 525version 2.06, the maximum was 255 characters.  A string that is too
 526long will be automatically truncated by the kernel.
 527
 528If the boot protocol version is 2.02 or later, the address of the
 529kernel command line is given by the header field cmd_line_ptr (see
 530above.)  This address can be anywhere between the end of the setup
 531heap and 0xA0000.
 532
 533If the protocol version is *not* 2.02 or higher, the kernel
 534command line is entered using the following protocol:
 535
 536        At offset 0x0020 (word), "cmd_line_magic", enter the magic
 537        number 0xA33F.
 538
 539        At offset 0x0022 (word), "cmd_line_offset", enter the offset
 540        of the kernel command line (relative to the start of the
 541        real-mode kernel).
 542        
 543        The kernel command line *must* be within the memory region
 544        covered by setup_move_size, so you may need to adjust this
 545        field.
 546
 547
 548**** MEMORY LAYOUT OF THE REAL-MODE CODE
 549
 550The real-mode code requires a stack/heap to be set up, as well as
 551memory allocated for the kernel command line.  This needs to be done
 552in the real-mode accessible memory in bottom megabyte.
 553
 554It should be noted that modern machines often have a sizable Extended
 555BIOS Data Area (EBDA).  As a result, it is advisable to use as little
 556of the low megabyte as possible.
 557
 558Unfortunately, under the following circumstances the 0x90000 memory
 559segment has to be used:
 560
 561        - When loading a zImage kernel ((loadflags & 0x01) == 0).
 562        - When loading a 2.01 or earlier boot protocol kernel.
 563
 564          -> For the 2.00 and 2.01 boot protocols, the real-mode code
 565             can be loaded at another address, but it is internally
 566             relocated to 0x90000.  For the "old" protocol, the
 567             real-mode code must be loaded at 0x90000.
 568
 569When loading at 0x90000, avoid using memory above 0x9a000.
 570
 571For boot protocol 2.02 or higher, the command line does not have to be
 572located in the same 64K segment as the real-mode setup code; it is
 573thus permitted to give the stack/heap the full 64K segment and locate
 574the command line above it.
 575
 576The kernel command line should not be located below the real-mode
 577code, nor should it be located in high memory.
 578
 579
 580**** SAMPLE BOOT CONFIGURATION
 581
 582As a sample configuration, assume the following layout of the real
 583mode segment:
 584
 585    When loading below 0x90000, use the entire segment:
 586
 587        0x0000-0x7fff   Real mode kernel
 588        0x8000-0xdfff   Stack and heap
 589        0xe000-0xffff   Kernel command line
 590
 591    When loading at 0x90000 OR the protocol version is 2.01 or earlier:
 592
 593        0x0000-0x7fff   Real mode kernel
 594        0x8000-0x97ff   Stack and heap
 595        0x9800-0x9fff   Kernel command line
 596
 597Such a boot loader should enter the following fields in the header:
 598
 599        unsigned long base_ptr; /* base address for real-mode segment */
 600
 601        if ( setup_sects == 0 ) {
 602                setup_sects = 4;
 603        }
 604
 605        if ( protocol >= 0x0200 ) {
 606                type_of_loader = <type code>;
 607                if ( loading_initrd ) {
 608                        ramdisk_image = <initrd_address>;
 609                        ramdisk_size = <initrd_size>;
 610                }
 611
 612                if ( protocol >= 0x0202 && loadflags & 0x01 )
 613                        heap_end = 0xe000;
 614                else
 615                        heap_end = 0x9800;
 616
 617                if ( protocol >= 0x0201 ) {
 618                        heap_end_ptr = heap_end - 0x200;
 619                        loadflags |= 0x80; /* CAN_USE_HEAP */
 620                }
 621
 622                if ( protocol >= 0x0202 ) {
 623                        cmd_line_ptr = base_ptr + heap_end;
 624                        strcpy(cmd_line_ptr, cmdline);
 625                } else {
 626                        cmd_line_magic  = 0xA33F;
 627                        cmd_line_offset = heap_end;
 628                        setup_move_size = heap_end + strlen(cmdline)+1;
 629                        strcpy(base_ptr+cmd_line_offset, cmdline);
 630                }
 631        } else {
 632                /* Very old kernel */
 633
 634                heap_end = 0x9800;
 635
 636                cmd_line_magic  = 0xA33F;
 637                cmd_line_offset = heap_end;
 638
 639                /* A very old kernel MUST have its real-mode code
 640                   loaded at 0x90000 */
 641
 642                if ( base_ptr != 0x90000 ) {
 643                        /* Copy the real-mode kernel */
 644                        memcpy(0x90000, base_ptr, (setup_sects+1)*512);
 645                        base_ptr = 0x90000;              /* Relocated */
 646                }
 647
 648                strcpy(0x90000+cmd_line_offset, cmdline);
 649
 650                /* It is recommended to clear memory up to the 32K mark */
 651                memset(0x90000 + (setup_sects+1)*512, 0,
 652                       (64-(setup_sects+1))*512);
 653        }
 654
 655
 656**** LOADING THE REST OF THE KERNEL
 657
 658The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
 659in the kernel file (again, if setup_sects == 0 the real value is 4.)
 660It should be loaded at address 0x10000 for Image/zImage kernels and
 6610x100000 for bzImage kernels.
 662
 663The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
 664bit (LOAD_HIGH) in the loadflags field is set:
 665
 666        is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
 667        load_address = is_bzImage ? 0x100000 : 0x10000;
 668
 669Note that Image/zImage kernels can be up to 512K in size, and thus use
 670the entire 0x10000-0x90000 range of memory.  This means it is pretty
 671much a requirement for these kernels to load the real-mode part at
 6720x90000.  bzImage kernels allow much more flexibility.
 673
 674
 675**** SPECIAL COMMAND LINE OPTIONS
 676
 677If the command line provided by the boot loader is entered by the
 678user, the user may expect the following command line options to work.
 679They should normally not be deleted from the kernel command line even
 680though not all of them are actually meaningful to the kernel.  Boot
 681loader authors who need additional command line options for the boot
 682loader itself should get them registered in
 683Documentation/kernel-parameters.txt to make sure they will not
 684conflict with actual kernel options now or in the future.
 685
 686  vga=<mode>
 687        <mode> here is either an integer (in C notation, either
 688        decimal, octal, or hexadecimal) or one of the strings
 689        "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
 690        (meaning 0xFFFD).  This value should be entered into the
 691        vid_mode field, as it is used by the kernel before the command
 692        line is parsed.
 693
 694  mem=<size>
 695        <size> is an integer in C notation optionally followed by
 696        (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
 697        << 30, << 40, << 50 or << 60).  This specifies the end of
 698        memory to the kernel. This affects the possible placement of
 699        an initrd, since an initrd should be placed near end of
 700        memory.  Note that this is an option to *both* the kernel and
 701        the bootloader!
 702
 703  initrd=<file>
 704        An initrd should be loaded.  The meaning of <file> is
 705        obviously bootloader-dependent, and some boot loaders
 706        (e.g. LILO) do not have such a command.
 707
 708In addition, some boot loaders add the following options to the
 709user-specified command line:
 710
 711  BOOT_IMAGE=<file>
 712        The boot image which was loaded.  Again, the meaning of <file>
 713        is obviously bootloader-dependent.
 714
 715  auto
 716        The kernel was booted without explicit user intervention.
 717
 718If these options are added by the boot loader, it is highly
 719recommended that they are located *first*, before the user-specified
 720or configuration-specified command line.  Otherwise, "init=/bin/sh"
 721gets confused by the "auto" option.
 722
 723
 724**** RUNNING THE KERNEL
 725
 726The kernel is started by jumping to the kernel entry point, which is
 727located at *segment* offset 0x20 from the start of the real mode
 728kernel.  This means that if you loaded your real-mode kernel code at
 7290x90000, the kernel entry point is 9020:0000.
 730
 731At entry, ds = es = ss should point to the start of the real-mode
 732kernel code (0x9000 if the code is loaded at 0x90000), sp should be
 733set up properly, normally pointing to the top of the heap, and
 734interrupts should be disabled.  Furthermore, to guard against bugs in
 735the kernel, it is recommended that the boot loader sets fs = gs = ds =
 736es = ss.
 737
 738In our example from above, we would do:
 739
 740        /* Note: in the case of the "old" kernel protocol, base_ptr must
 741           be == 0x90000 at this point; see the previous sample code */
 742
 743        seg = base_ptr >> 4;
 744
 745        cli();  /* Enter with interrupts disabled! */
 746
 747        /* Set up the real-mode kernel stack */
 748        _SS = seg;
 749        _SP = heap_end;
 750
 751        _DS = _ES = _FS = _GS = seg;
 752        jmp_far(seg+0x20, 0);   /* Run the kernel */
 753
 754If your boot sector accesses a floppy drive, it is recommended to
 755switch off the floppy motor before running the kernel, since the
 756kernel boot leaves interrupts off and thus the motor will not be
 757switched off, especially if the loaded kernel has the floppy driver as
 758a demand-loaded module!
 759
 760
 761**** ADVANCED BOOT LOADER HOOKS
 762
 763If the boot loader runs in a particularly hostile environment (such as
 764LOADLIN, which runs under DOS) it may be impossible to follow the
 765standard memory location requirements.  Such a boot loader may use the
 766following hooks that, if set, are invoked by the kernel at the
 767appropriate time.  The use of these hooks should probably be
 768considered an absolutely last resort!
 769
 770IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
 771%edi across invocation.
 772
 773  realmode_swtch:
 774        A 16-bit real mode far subroutine invoked immediately before
 775        entering protected mode.  The default routine disables NMI, so
 776        your routine should probably do so, too.
 777
 778  code32_start:
 779        A 32-bit flat-mode routine *jumped* to immediately after the
 780        transition to protected mode, but before the kernel is
 781        uncompressed.  No segments, except CS, are guaranteed to be
 782        set up (current kernels do, but older ones do not); you should
 783        set them up to BOOT_DS (0x18) yourself.
 784
 785        After completing your hook, you should jump to the address
 786        that was in this field before your boot loader overwrote it
 787        (relocated, if appropriate.)
 788
 789
 790**** 32-bit BOOT PROTOCOL
 791
 792For machine with some new BIOS other than legacy BIOS, such as EFI,
 793LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
 794based on legacy BIOS can not be used, so a 32-bit boot protocol needs
 795to be defined.
 796
 797In 32-bit boot protocol, the first step in loading a Linux kernel
 798should be to setup the boot parameters (struct boot_params,
 799traditionally known as "zero page"). The memory for struct boot_params
 800should be allocated and initialized to all zero. Then the setup header
 801from offset 0x01f1 of kernel image on should be loaded into struct
 802boot_params and examined. The end of setup header can be calculated as
 803follow:
 804
 805        0x0202 + byte value at offset 0x0201
 806
 807In addition to read/modify/write the setup header of the struct
 808boot_params as that of 16-bit boot protocol, the boot loader should
 809also fill the additional fields of the struct boot_params as that
 810described in zero-page.txt.
 811
 812After setupping the struct boot_params, the boot loader can load the
 81332/64-bit kernel in the same way as that of 16-bit boot protocol.
 814
 815In 32-bit boot protocol, the kernel is started by jumping to the
 81632-bit kernel entry point, which is the start address of loaded
 81732/64-bit kernel.
 818
 819At entry, the CPU must be in 32-bit protected mode with paging
 820disabled; a GDT must be loaded with the descriptors for selectors
 821__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
 822segment; __BOOS_CS must have execute/read permission, and __BOOT_DS
 823must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
 824must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
 825address of the struct boot_params; %ebp, %edi and %ebx must be zero.
 826
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