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
  45Protocol 2.07:  (Kernel 2.6.24) Added paravirtualised boot protocol.
  46                Introduced hardware_subarch and hardware_subarch_data
  47                and KEEP_SEGMENTS flag in load_flags.
  48
  49Protocol 2.08:  (Kernel 2.6.26) Added crc32 checksum and ELF format
  50                payload. Introduced payload_offset and payload length
  51                fields to aid in locating the payload.
  52
  53Protocol 2.09:  (Kernel 2.6.26) Added a field of 64-bit physical
  54                pointer to single linked list of struct setup_data.
  55
  56**** MEMORY LAYOUT
  57
  58The traditional memory map for the kernel loader, used for Image or
  59zImage kernels, typically looks like:
  60
  61        |                        |
  620A0000  +------------------------+
  63        |  Reserved for BIOS     |      Do not use.  Reserved for BIOS EBDA.
  6409A000  +------------------------+
  65        |  Command line          |
  66        |  Stack/heap            |      For use by the kernel real-mode code.
  67098000  +------------------------+      
  68        |  Kernel setup          |      The kernel real-mode code.
  69090200  +------------------------+
  70        |  Kernel boot sector    |      The kernel legacy boot sector.
  71090000  +------------------------+
  72        |  Protected-mode kernel |      The bulk of the kernel image.
  73010000  +------------------------+
  74        |  Boot loader           |      <- Boot sector entry point 0000:7C00
  75001000  +------------------------+
  76        |  Reserved for MBR/BIOS |
  77000800  +------------------------+
  78        |  Typically used by MBR |
  79000600  +------------------------+ 
  80        |  BIOS use only         |
  81000000  +------------------------+
  82
  83
  84When using bzImage, the protected-mode kernel was relocated to
  850x100000 ("high memory"), and the kernel real-mode block (boot sector,
  86setup, and stack/heap) was made relocatable to any address between
  870x10000 and end of low memory. Unfortunately, in protocols 2.00 and
  882.01 the 0x90000+ memory range is still used internally by the kernel;
  89the 2.02 protocol resolves that problem.
  90
  91It is desirable to keep the "memory ceiling" -- the highest point in
  92low memory touched by the boot loader -- as low as possible, since
  93some newer BIOSes have begun to allocate some rather large amounts of
  94memory, called the Extended BIOS Data Area, near the top of low
  95memory.  The boot loader should use the "INT 12h" BIOS call to verify
  96how much low memory is available.
  97
  98Unfortunately, if INT 12h reports that the amount of memory is too
  99low, there is usually nothing the boot loader can do but to report an
 100error to the user.  The boot loader should therefore be designed to
 101take up as little space in low memory as it reasonably can.  For
 102zImage or old bzImage kernels, which need data written into the
 1030x90000 segment, the boot loader should make sure not to use memory
 104above the 0x9A000 point; too many BIOSes will break above that point.
 105
 106For a modern bzImage kernel with boot protocol version >= 2.02, a
 107memory layout like the following is suggested:
 108
 109        ~                        ~
 110        |  Protected-mode kernel |
 111100000  +------------------------+
 112        |  I/O memory hole       |
 1130A0000  +------------------------+
 114        |  Reserved for BIOS     |      Leave as much as possible unused
 115        ~                        ~
 116        |  Command line          |      (Can also be below the X+10000 mark)
 117X+10000 +------------------------+
 118        |  Stack/heap            |      For use by the kernel real-mode code.
 119X+08000 +------------------------+      
 120        |  Kernel setup          |      The kernel real-mode code.
 121        |  Kernel boot sector    |      The kernel legacy boot sector.
 122X       +------------------------+
 123        |  Boot loader           |      <- Boot sector entry point 0000:7C00
 124001000  +------------------------+
 125        |  Reserved for MBR/BIOS |
 126000800  +------------------------+
 127        |  Typically used by MBR |
 128000600  +------------------------+ 
 129        |  BIOS use only         |
 130000000  +------------------------+
 131
 132... where the address X is as low as the design of the boot loader
 133permits.
 134
 135
 136**** THE REAL-MODE KERNEL HEADER
 137
 138In the following text, and anywhere in the kernel boot sequence, "a
 139sector" refers to 512 bytes.  It is independent of the actual sector
 140size of the underlying medium.
 141
 142The first step in loading a Linux kernel should be to load the
 143real-mode code (boot sector and setup code) and then examine the
 144following header at offset 0x01f1.  The real-mode code can total up to
 14532K, although the boot loader may choose to load only the first two
 146sectors (1K) and then examine the bootup sector size.
 147
 148The header looks like:
 149
 150Offset  Proto   Name            Meaning
 151/Size
 152
 15301F1/1  ALL(1   setup_sects     The size of the setup in sectors
 15401F2/2  ALL     root_flags      If set, the root is mounted readonly
 15501F4/4  2.04+(2 syssize         The size of the 32-bit code in 16-byte paras
 15601F8/2  ALL     ram_size        DO NOT USE - for bootsect.S use only
 15701FA/2  ALL     vid_mode        Video mode control
 15801FC/2  ALL     root_dev        Default root device number
 15901FE/2  ALL     boot_flag       0xAA55 magic number
 1600200/2  2.00+   jump            Jump instruction
 1610202/4  2.00+   header          Magic signature "HdrS"
 1620206/2  2.00+   version         Boot protocol version supported
 1630208/4  2.00+   realmode_swtch  Boot loader hook (see below)
 164020C/2  2.00+   start_sys       The load-low segment (0x1000) (obsolete)
 165020E/2  2.00+   kernel_version  Pointer to kernel version string
 1660210/1  2.00+   type_of_loader  Boot loader identifier
 1670211/1  2.00+   loadflags       Boot protocol option flags
 1680212/2  2.00+   setup_move_size Move to high memory size (used with hooks)
 1690214/4  2.00+   code32_start    Boot loader hook (see below)
 1700218/4  2.00+   ramdisk_image   initrd load address (set by boot loader)
 171021C/4  2.00+   ramdisk_size    initrd size (set by boot loader)
 1720220/4  2.00+   bootsect_kludge DO NOT USE - for bootsect.S use only
 1730224/2  2.01+   heap_end_ptr    Free memory after setup end
 1740226/2  N/A     pad1            Unused
 1750228/4  2.02+   cmd_line_ptr    32-bit pointer to the kernel command line
 176022C/4  2.03+   initrd_addr_max Highest legal initrd address
 1770230/4  2.05+   kernel_alignment Physical addr alignment required for kernel
 1780234/1  2.05+   relocatable_kernel Whether kernel is relocatable or not
 1790235/3  N/A     pad2            Unused
 1800238/4  2.06+   cmdline_size    Maximum size of the kernel command line
 181023C/4  2.07+   hardware_subarch Hardware subarchitecture
 1820240/8  2.07+   hardware_subarch_data Subarchitecture-specific data
 1830248/4  2.08+   payload_offset  Offset of kernel payload
 184024C/4  2.08+   payload_length  Length of kernel payload
 1850250/8  2.09+   setup_data      64-bit physical pointer to linked list
 186                                of struct setup_data
 187
 188(1) For backwards compatibility, if the setup_sects field contains 0, the
 189    real value is 4.
 190
 191(2) For boot protocol prior to 2.04, the upper two bytes of the syssize
 192    field are unusable, which means the size of a bzImage kernel
 193    cannot be determined.
 194
 195If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
 196the boot protocol version is "old".  Loading an old kernel, the
 197following parameters should be assumed:
 198
 199        Image type = zImage
 200        initrd not supported
 201        Real-mode kernel must be located at 0x90000.
 202
 203Otherwise, the "version" field contains the protocol version,
 204e.g. protocol version 2.01 will contain 0x0201 in this field.  When
 205setting fields in the header, you must make sure only to set fields
 206supported by the protocol version in use.
 207
 208
 209**** DETAILS OF HEADER FIELDS
 210
 211For each field, some are information from the kernel to the bootloader
 212("read"), some are expected to be filled out by the bootloader
 213("write"), and some are expected to be read and modified by the
 214bootloader ("modify").
 215
 216All general purpose boot loaders should write the fields marked
 217(obligatory).  Boot loaders who want to load the kernel at a
 218nonstandard address should fill in the fields marked (reloc); other
 219boot loaders can ignore those fields.
 220
 221The byte order of all fields is littleendian (this is x86, after all.)
 222
 223Field name:     setup_sects
 224Type:           read
 225Offset/size:    0x1f1/1
 226Protocol:       ALL
 227
 228  The size of the setup code in 512-byte sectors.  If this field is
 229  0, the real value is 4.  The real-mode code consists of the boot
 230  sector (always one 512-byte sector) plus the setup code.
 231
 232Field name:      root_flags
 233Type:            modify (optional)
 234Offset/size:     0x1f2/2
 235Protocol:        ALL
 236
 237  If this field is nonzero, the root defaults to readonly.  The use of
 238  this field is deprecated; use the "ro" or "rw" options on the
 239  command line instead.
 240
 241Field name:     syssize
 242Type:           read
 243Offset/size:    0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
 244Protocol:       2.04+
 245
 246  The size of the protected-mode code in units of 16-byte paragraphs.
 247  For protocol versions older than 2.04 this field is only two bytes
 248  wide, and therefore cannot be trusted for the size of a kernel if
 249  the LOAD_HIGH flag is set.
 250
 251Field name:     ram_size
 252Type:           kernel internal
 253Offset/size:    0x1f8/2
 254Protocol:       ALL
 255
 256  This field is obsolete.
 257
 258Field name:     vid_mode
 259Type:           modify (obligatory)
 260Offset/size:    0x1fa/2
 261
 262  Please see the section on SPECIAL COMMAND LINE OPTIONS.
 263
 264Field name:     root_dev
 265Type:           modify (optional)
 266Offset/size:    0x1fc/2
 267Protocol:       ALL
 268
 269  The default root device device number.  The use of this field is
 270  deprecated, use the "root=" option on the command line instead.
 271
 272Field name:     boot_flag
 273Type:           read
 274Offset/size:    0x1fe/2
 275Protocol:       ALL
 276
 277  Contains 0xAA55.  This is the closest thing old Linux kernels have
 278  to a magic number.
 279
 280Field name:     jump
 281Type:           read
 282Offset/size:    0x200/2
 283Protocol:       2.00+
 284
 285  Contains an x86 jump instruction, 0xEB followed by a signed offset
 286  relative to byte 0x202.  This can be used to determine the size of
 287  the header.
 288
 289Field name:     header
 290Type:           read
 291Offset/size:    0x202/4
 292Protocol:       2.00+
 293
 294  Contains the magic number "HdrS" (0x53726448).
 295
 296Field name:     version
 297Type:           read
 298Offset/size:    0x206/2
 299Protocol:       2.00+
 300
 301  Contains the boot protocol version, in (major << 8)+minor format,
 302  e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
 303  10.17.
 304
 305Field name:     readmode_swtch
 306Type:           modify (optional)
 307Offset/size:    0x208/4
 308Protocol:       2.00+
 309
 310  Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
 311
 312Field name:     start_sys
 313Type:           read
 314Offset/size:    0x20c/4
 315Protocol:       2.00+
 316
 317  The load low segment (0x1000).  Obsolete.
 318
 319Field name:     kernel_version
 320Type:           read
 321Offset/size:    0x20e/2
 322Protocol:       2.00+
 323
 324  If set to a nonzero value, contains a pointer to a NUL-terminated
 325  human-readable kernel version number string, less 0x200.  This can
 326  be used to display the kernel version to the user.  This value
 327  should be less than (0x200*setup_sects).
 328
 329  For example, if this value is set to 0x1c00, the kernel version
 330  number string can be found at offset 0x1e00 in the kernel file.
 331  This is a valid value if and only if the "setup_sects" field
 332  contains the value 15 or higher, as:
 333
 334        0x1c00  < 15*0x200 (= 0x1e00) but
 335        0x1c00 >= 14*0x200 (= 0x1c00)
 336
 337        0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
 338
 339Field name:     type_of_loader
 340Type:           write (obligatory)
 341Offset/size:    0x210/1
 342Protocol:       2.00+
 343
 344  If your boot loader has an assigned id (see table below), enter
 345  0xTV here, where T is an identifier for the boot loader and V is
 346  a version number.  Otherwise, enter 0xFF here.
 347
 348  Assigned boot loader ids:
 349        0  LILO                 (0x00 reserved for pre-2.00 bootloader)
 350        1  Loadlin
 351        2  bootsect-loader      (0x20, all other values reserved)
 352        3  SYSLINUX
 353        4  EtherBoot
 354        5  ELILO
 355        7  GRuB
 356        8  U-BOOT
 357        9  Xen
 358        A  Gujin
 359        B  Qemu
 360
 361  Please contact <hpa@zytor.com> if you need a bootloader ID
 362  value assigned.
 363
 364Field name:     loadflags
 365Type:           modify (obligatory)
 366Offset/size:    0x211/1
 367Protocol:       2.00+
 368
 369  This field is a bitmask.
 370
 371  Bit 0 (read): LOADED_HIGH
 372        - If 0, the protected-mode code is loaded at 0x10000.
 373        - If 1, the protected-mode code is loaded at 0x100000.
 374
 375  Bit 6 (write): KEEP_SEGMENTS
 376        Protocol: 2.07+
 377        - if 0, reload the segment registers in the 32bit entry point.
 378        - if 1, do not reload the segment registers in the 32bit entry point.
 379                Assume that %cs %ds %ss %es are all set to flat segments with
 380                a base of 0 (or the equivalent for their environment).
 381
 382  Bit 7 (write): CAN_USE_HEAP
 383        Set this bit to 1 to indicate that the value entered in the
 384        heap_end_ptr is valid.  If this field is clear, some setup code
 385        functionality will be disabled.
 386
 387Field name:     setup_move_size
 388Type:           modify (obligatory)
 389Offset/size:    0x212/2
 390Protocol:       2.00-2.01
 391
 392  When using protocol 2.00 or 2.01, if the real mode kernel is not
 393  loaded at 0x90000, it gets moved there later in the loading
 394  sequence.  Fill in this field if you want additional data (such as
 395  the kernel command line) moved in addition to the real-mode kernel
 396  itself.
 397
 398  The unit is bytes starting with the beginning of the boot sector.
 399  
 400  This field is can be ignored when the protocol is 2.02 or higher, or
 401  if the real-mode code is loaded at 0x90000.
 402
 403Field name:     code32_start
 404Type:           modify (optional, reloc)
 405Offset/size:    0x214/4
 406Protocol:       2.00+
 407
 408  The address to jump to in protected mode.  This defaults to the load
 409  address of the kernel, and can be used by the boot loader to
 410  determine the proper load address.
 411
 412  This field can be modified for two purposes:
 413
 414  1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
 415
 416  2. if a bootloader which does not install a hook loads a
 417     relocatable kernel at a nonstandard address it will have to modify
 418     this field to point to the load address.
 419
 420Field name:     ramdisk_image
 421Type:           write (obligatory)
 422Offset/size:    0x218/4
 423Protocol:       2.00+
 424
 425  The 32-bit linear address of the initial ramdisk or ramfs.  Leave at
 426  zero if there is no initial ramdisk/ramfs.
 427
 428Field name:     ramdisk_size
 429Type:           write (obligatory)
 430Offset/size:    0x21c/4
 431Protocol:       2.00+
 432
 433  Size of the initial ramdisk or ramfs.  Leave at zero if there is no
 434  initial ramdisk/ramfs.
 435
 436Field name:     bootsect_kludge
 437Type:           kernel internal
 438Offset/size:    0x220/4
 439Protocol:       2.00+
 440
 441  This field is obsolete.
 442
 443Field name:     heap_end_ptr
 444Type:           write (obligatory)
 445Offset/size:    0x224/2
 446Protocol:       2.01+
 447
 448  Set this field to the offset (from the beginning of the real-mode
 449  code) of the end of the setup stack/heap, minus 0x0200.
 450
 451Field name:     cmd_line_ptr
 452Type:           write (obligatory)
 453Offset/size:    0x228/4
 454Protocol:       2.02+
 455
 456  Set this field to the linear address of the kernel command line.
 457  The kernel command line can be located anywhere between the end of
 458  the setup heap and 0xA0000; it does not have to be located in the
 459  same 64K segment as the real-mode code itself.
 460
 461  Fill in this field even if your boot loader does not support a
 462  command line, in which case you can point this to an empty string
 463  (or better yet, to the string "auto".)  If this field is left at
 464  zero, the kernel will assume that your boot loader does not support
 465  the 2.02+ protocol.
 466
 467Field name:     initrd_addr_max
 468Type:           read
 469Offset/size:    0x22c/4
 470Protocol:       2.03+
 471
 472  The maximum address that may be occupied by the initial
 473  ramdisk/ramfs contents.  For boot protocols 2.02 or earlier, this
 474  field is not present, and the maximum address is 0x37FFFFFF.  (This
 475  address is defined as the address of the highest safe byte, so if
 476  your ramdisk is exactly 131072 bytes long and this field is
 477  0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
 478
 479Field name:     kernel_alignment
 480Type:           read (reloc)
 481Offset/size:    0x230/4
 482Protocol:       2.05+
 483
 484  Alignment unit required by the kernel (if relocatable_kernel is true.)
 485
 486Field name:     relocatable_kernel
 487Type:           read (reloc)
 488Offset/size:    0x234/1
 489Protocol:       2.05+
 490
 491  If this field is nonzero, the protected-mode part of the kernel can
 492  be loaded at any address that satisfies the kernel_alignment field.
 493  After loading, the boot loader must set the code32_start field to
 494  point to the loaded code, or to a boot loader hook.
 495
 496Field name:     cmdline_size
 497Type:           read
 498Offset/size:    0x238/4
 499Protocol:       2.06+
 500
 501  The maximum size of the command line without the terminating
 502  zero. This means that the command line can contain at most
 503  cmdline_size characters. With protocol version 2.05 and earlier, the
 504  maximum size was 255.
 505
 506Field name:     hardware_subarch
 507Type:           write
 508Offset/size:    0x23c/4
 509Protocol:       2.07+
 510
 511  In a paravirtualized environment the hardware low level architectural
 512  pieces such as interrupt handling, page table handling, and
 513  accessing process control registers needs to be done differently.
 514
 515  This field allows the bootloader to inform the kernel we are in one
 516  one of those environments.
 517
 518  0x00000000    The default x86/PC environment
 519  0x00000001    lguest
 520  0x00000002    Xen
 521
 522Field name:     hardware_subarch_data
 523Type:           write
 524Offset/size:    0x240/8
 525Protocol:       2.07+
 526
 527  A pointer to data that is specific to hardware subarch
 528
 529Field name:     payload_offset
 530Type:           read
 531Offset/size:    0x248/4
 532Protocol:       2.08+
 533
 534  If non-zero then this field contains the offset from the end of the
 535  real-mode code to the payload.
 536
 537  The payload may be compressed. The format of both the compressed and
 538  uncompressed data should be determined using the standard magic
 539  numbers. Currently only gzip compressed ELF is used.
 540  
 541Field name:     payload_length
 542Type:           read
 543Offset/size:    0x24c/4
 544Protocol:       2.08+
 545
 546  The length of the payload.
 547
 548**** THE IMAGE CHECKSUM
 549
 550From boot protocol version 2.08 onwards the CRC-32 is calculated over
 551the entire file using the characteristic polynomial 0x04C11DB7 and an
 552initial remainder of 0xffffffff.  The checksum is appended to the
 553file; therefore the CRC of the file up to the limit specified in the
 554syssize field of the header is always 0.
 555
 556**** THE KERNEL COMMAND LINE
 557
 558The kernel command line has become an important way for the boot
 559loader to communicate with the kernel.  Some of its options are also
 560relevant to the boot loader itself, see "special command line options"
 561below.
 562
 563The kernel command line is a null-terminated string. The maximum
 564length can be retrieved from the field cmdline_size.  Before protocol
 565version 2.06, the maximum was 255 characters.  A string that is too
 566long will be automatically truncated by the kernel.
 567
 568If the boot protocol version is 2.02 or later, the address of the
 569kernel command line is given by the header field cmd_line_ptr (see
 570above.)  This address can be anywhere between the end of the setup
 571heap and 0xA0000.
 572
 573If the protocol version is *not* 2.02 or higher, the kernel
 574command line is entered using the following protocol:
 575
 576        At offset 0x0020 (word), "cmd_line_magic", enter the magic
 577        number 0xA33F.
 578
 579        At offset 0x0022 (word), "cmd_line_offset", enter the offset
 580        of the kernel command line (relative to the start of the
 581        real-mode kernel).
 582        
 583        The kernel command line *must* be within the memory region
 584        covered by setup_move_size, so you may need to adjust this
 585        field.
 586
 587Field name:     setup_data
 588Type:           write (obligatory)
 589Offset/size:    0x250/8
 590Protocol:       2.09+
 591
 592  The 64-bit physical pointer to NULL terminated single linked list of
 593  struct setup_data. This is used to define a more extensible boot
 594  parameters passing mechanism. The definition of struct setup_data is
 595  as follow:
 596
 597  struct setup_data {
 598          u64 next;
 599          u32 type;
 600          u32 len;
 601          u8  data[0];
 602  };
 603
 604  Where, the next is a 64-bit physical pointer to the next node of
 605  linked list, the next field of the last node is 0; the type is used
 606  to identify the contents of data; the len is the length of data
 607  field; the data holds the real payload.
 608
 609
 610**** MEMORY LAYOUT OF THE REAL-MODE CODE
 611
 612The real-mode code requires a stack/heap to be set up, as well as
 613memory allocated for the kernel command line.  This needs to be done
 614in the real-mode accessible memory in bottom megabyte.
 615
 616It should be noted that modern machines often have a sizable Extended
 617BIOS Data Area (EBDA).  As a result, it is advisable to use as little
 618of the low megabyte as possible.
 619
 620Unfortunately, under the following circumstances the 0x90000 memory
 621segment has to be used:
 622
 623        - When loading a zImage kernel ((loadflags & 0x01) == 0).
 624        - When loading a 2.01 or earlier boot protocol kernel.
 625
 626          -> For the 2.00 and 2.01 boot protocols, the real-mode code
 627             can be loaded at another address, but it is internally
 628             relocated to 0x90000.  For the "old" protocol, the
 629             real-mode code must be loaded at 0x90000.
 630
 631When loading at 0x90000, avoid using memory above 0x9a000.
 632
 633For boot protocol 2.02 or higher, the command line does not have to be
 634located in the same 64K segment as the real-mode setup code; it is
 635thus permitted to give the stack/heap the full 64K segment and locate
 636the command line above it.
 637
 638The kernel command line should not be located below the real-mode
 639code, nor should it be located in high memory.
 640
 641
 642**** SAMPLE BOOT CONFIGURATION
 643
 644As a sample configuration, assume the following layout of the real
 645mode segment:
 646
 647    When loading below 0x90000, use the entire segment:
 648
 649        0x0000-0x7fff   Real mode kernel
 650        0x8000-0xdfff   Stack and heap
 651        0xe000-0xffff   Kernel command line
 652
 653    When loading at 0x90000 OR the protocol version is 2.01 or earlier:
 654
 655        0x0000-0x7fff   Real mode kernel
 656        0x8000-0x97ff   Stack and heap
 657        0x9800-0x9fff   Kernel command line
 658
 659Such a boot loader should enter the following fields in the header:
 660
 661        unsigned long base_ptr; /* base address for real-mode segment */
 662
 663        if ( setup_sects == 0 ) {
 664                setup_sects = 4;
 665        }
 666
 667        if ( protocol >= 0x0200 ) {
 668                type_of_loader = <type code>;
 669                if ( loading_initrd ) {
 670                        ramdisk_image = <initrd_address>;
 671                        ramdisk_size = <initrd_size>;
 672                }
 673
 674                if ( protocol >= 0x0202 && loadflags & 0x01 )
 675                        heap_end = 0xe000;
 676                else
 677                        heap_end = 0x9800;
 678
 679                if ( protocol >= 0x0201 ) {
 680                        heap_end_ptr = heap_end - 0x200;
 681                        loadflags |= 0x80; /* CAN_USE_HEAP */
 682                }
 683
 684                if ( protocol >= 0x0202 ) {
 685                        cmd_line_ptr = base_ptr + heap_end;
 686                        strcpy(cmd_line_ptr, cmdline);
 687                } else {
 688                        cmd_line_magic  = 0xA33F;
 689                        cmd_line_offset = heap_end;
 690                        setup_move_size = heap_end + strlen(cmdline)+1;
 691                        strcpy(base_ptr+cmd_line_offset, cmdline);
 692                }
 693        } else {
 694                /* Very old kernel */
 695
 696                heap_end = 0x9800;
 697
 698                cmd_line_magic  = 0xA33F;
 699                cmd_line_offset = heap_end;
 700
 701                /* A very old kernel MUST have its real-mode code
 702                   loaded at 0x90000 */
 703
 704                if ( base_ptr != 0x90000 ) {
 705                        /* Copy the real-mode kernel */
 706                        memcpy(0x90000, base_ptr, (setup_sects+1)*512);
 707                        base_ptr = 0x90000;              /* Relocated */
 708                }
 709
 710                strcpy(0x90000+cmd_line_offset, cmdline);
 711
 712                /* It is recommended to clear memory up to the 32K mark */
 713                memset(0x90000 + (setup_sects+1)*512, 0,
 714                       (64-(setup_sects+1))*512);
 715        }
 716
 717
 718**** LOADING THE REST OF THE KERNEL
 719
 720The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
 721in the kernel file (again, if setup_sects == 0 the real value is 4.)
 722It should be loaded at address 0x10000 for Image/zImage kernels and
 7230x100000 for bzImage kernels.
 724
 725The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
 726bit (LOAD_HIGH) in the loadflags field is set:
 727
 728        is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
 729        load_address = is_bzImage ? 0x100000 : 0x10000;
 730
 731Note that Image/zImage kernels can be up to 512K in size, and thus use
 732the entire 0x10000-0x90000 range of memory.  This means it is pretty
 733much a requirement for these kernels to load the real-mode part at
 7340x90000.  bzImage kernels allow much more flexibility.
 735
 736
 737**** SPECIAL COMMAND LINE OPTIONS
 738
 739If the command line provided by the boot loader is entered by the
 740user, the user may expect the following command line options to work.
 741They should normally not be deleted from the kernel command line even
 742though not all of them are actually meaningful to the kernel.  Boot
 743loader authors who need additional command line options for the boot
 744loader itself should get them registered in
 745Documentation/kernel-parameters.txt to make sure they will not
 746conflict with actual kernel options now or in the future.
 747
 748  vga=<mode>
 749        <mode> here is either an integer (in C notation, either
 750        decimal, octal, or hexadecimal) or one of the strings
 751        "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
 752        (meaning 0xFFFD).  This value should be entered into the
 753        vid_mode field, as it is used by the kernel before the command
 754        line is parsed.
 755
 756  mem=<size>
 757        <size> is an integer in C notation optionally followed by
 758        (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
 759        << 30, << 40, << 50 or << 60).  This specifies the end of
 760        memory to the kernel. This affects the possible placement of
 761        an initrd, since an initrd should be placed near end of
 762        memory.  Note that this is an option to *both* the kernel and
 763        the bootloader!
 764
 765  initrd=<file>
 766        An initrd should be loaded.  The meaning of <file> is
 767        obviously bootloader-dependent, and some boot loaders
 768        (e.g. LILO) do not have such a command.
 769
 770In addition, some boot loaders add the following options to the
 771user-specified command line:
 772
 773  BOOT_IMAGE=<file>
 774        The boot image which was loaded.  Again, the meaning of <file>
 775        is obviously bootloader-dependent.
 776
 777  auto
 778        The kernel was booted without explicit user intervention.
 779
 780If these options are added by the boot loader, it is highly
 781recommended that they are located *first*, before the user-specified
 782or configuration-specified command line.  Otherwise, "init=/bin/sh"
 783gets confused by the "auto" option.
 784
 785
 786**** RUNNING THE KERNEL
 787
 788The kernel is started by jumping to the kernel entry point, which is
 789located at *segment* offset 0x20 from the start of the real mode
 790kernel.  This means that if you loaded your real-mode kernel code at
 7910x90000, the kernel entry point is 9020:0000.
 792
 793At entry, ds = es = ss should point to the start of the real-mode
 794kernel code (0x9000 if the code is loaded at 0x90000), sp should be
 795set up properly, normally pointing to the top of the heap, and
 796interrupts should be disabled.  Furthermore, to guard against bugs in
 797the kernel, it is recommended that the boot loader sets fs = gs = ds =
 798es = ss.
 799
 800In our example from above, we would do:
 801
 802        /* Note: in the case of the "old" kernel protocol, base_ptr must
 803           be == 0x90000 at this point; see the previous sample code */
 804
 805        seg = base_ptr >> 4;
 806
 807        cli();  /* Enter with interrupts disabled! */
 808
 809        /* Set up the real-mode kernel stack */
 810        _SS = seg;
 811        _SP = heap_end;
 812
 813        _DS = _ES = _FS = _GS = seg;
 814        jmp_far(seg+0x20, 0);   /* Run the kernel */
 815
 816If your boot sector accesses a floppy drive, it is recommended to
 817switch off the floppy motor before running the kernel, since the
 818kernel boot leaves interrupts off and thus the motor will not be
 819switched off, especially if the loaded kernel has the floppy driver as
 820a demand-loaded module!
 821
 822
 823**** ADVANCED BOOT LOADER HOOKS
 824
 825If the boot loader runs in a particularly hostile environment (such as
 826LOADLIN, which runs under DOS) it may be impossible to follow the
 827standard memory location requirements.  Such a boot loader may use the
 828following hooks that, if set, are invoked by the kernel at the
 829appropriate time.  The use of these hooks should probably be
 830considered an absolutely last resort!
 831
 832IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
 833%edi across invocation.
 834
 835  realmode_swtch:
 836        A 16-bit real mode far subroutine invoked immediately before
 837        entering protected mode.  The default routine disables NMI, so
 838        your routine should probably do so, too.
 839
 840  code32_start:
 841        A 32-bit flat-mode routine *jumped* to immediately after the
 842        transition to protected mode, but before the kernel is
 843        uncompressed.  No segments, except CS, are guaranteed to be
 844        set up (current kernels do, but older ones do not); you should
 845        set them up to BOOT_DS (0x18) yourself.
 846
 847        After completing your hook, you should jump to the address
 848        that was in this field before your boot loader overwrote it
 849        (relocated, if appropriate.)
 850
 851
 852**** 32-bit BOOT PROTOCOL
 853
 854For machine with some new BIOS other than legacy BIOS, such as EFI,
 855LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
 856based on legacy BIOS can not be used, so a 32-bit boot protocol needs
 857to be defined.
 858
 859In 32-bit boot protocol, the first step in loading a Linux kernel
 860should be to setup the boot parameters (struct boot_params,
 861traditionally known as "zero page"). The memory for struct boot_params
 862should be allocated and initialized to all zero. Then the setup header
 863from offset 0x01f1 of kernel image on should be loaded into struct
 864boot_params and examined. The end of setup header can be calculated as
 865follow:
 866
 867        0x0202 + byte value at offset 0x0201
 868
 869In addition to read/modify/write the setup header of the struct
 870boot_params as that of 16-bit boot protocol, the boot loader should
 871also fill the additional fields of the struct boot_params as that
 872described in zero-page.txt.
 873
 874After setupping the struct boot_params, the boot loader can load the
 87532/64-bit kernel in the same way as that of 16-bit boot protocol.
 876
 877In 32-bit boot protocol, the kernel is started by jumping to the
 87832-bit kernel entry point, which is the start address of loaded
 87932/64-bit kernel.
 880
 881At entry, the CPU must be in 32-bit protected mode with paging
 882disabled; a GDT must be loaded with the descriptors for selectors
 883__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
 884segment; __BOOS_CS must have execute/read permission, and __BOOT_DS
 885must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
 886must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
 887address of the struct boot_params; %ebp, %edi and %ebx must be zero.
 888
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