Documentation/i386/boot.txt

   1                      THE LINUX/I386 BOOT PROTOCOL
   2                      ----------------------------
   3
   4                     H. Peter Anvin <hpa@zytor.com>
   5                         Last update 2007-05-23
   6
   7 On the i386 platform, the Linux kernel uses a rather complicated boot
   8 convention.  This has evolved partially due to historical aspects, as
   9 well as the desire in the early days to have the kernel itself be a
  10 bootable image, the complicated PC memory model and due to changed
  11 expectations in the PC industry caused by the effective demise of
  12 real-mode DOS as a mainstream operating system.
  13
  14 Currently, the following versions of the Linux/i386 boot protocol exist.
  15
  16 Old kernels:    zImage/Image support only.  Some very early kernels
  17                 may not even support a command line.
  18
  19 Protocol 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
  25 Protocol 2.01:  (Kernel 1.3.76) Added a heap overrun warning.
  26
  27 Protocol 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
  34 Protocol 2.03:  (Kernel 2.4.18-pre1) Explicitly makes the highest possible
  35                 initrd address available to the bootloader.
  36
  37 Protocol 2.04:  (Kernel 2.6.14) Extend the syssize field to four bytes.
  38
  39 Protocol 2.05:  (Kernel 2.6.20) Make protected mode kernel relocatable.
  40                 Introduce relocatable_kernel and kernel_alignment fields.
  41
  42 Protocol 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
  48 The traditional memory map for the kernel loader, used for Image or
  49 zImage kernels, typically looks like:
  50
  51         |                        |
  52 0A0000  +------------------------+
  53         |  Reserved for BIOS     |      Do not use.  Reserved for BIOS EBDA.
  54 09A000  +------------------------+
  55         |  Command line          |
  56         |  Stack/heap            |      For use by the kernel real-mode code.
  57 098000  +------------------------+
  58         |  Kernel setup          |      The kernel real-mode code.
  59 090200  +------------------------+
  60         |  Kernel boot sector    |      The kernel legacy boot sector.
  61 090000  +------------------------+
  62         |  Protected-mode kernel |      The bulk of the kernel image.
  63 010000  +------------------------+
  64         |  Boot loader           |      <- Boot sector entry point 0000:7C00
  65 001000  +------------------------+
  66         |  Reserved for MBR/BIOS |
  67 000800  +------------------------+
  68         |  Typically used by MBR |
  69 000600  +------------------------+
  70         |  BIOS use only         |
  71 000000  +------------------------+
  72
  73
  74 When using bzImage, the protected-mode kernel was relocated to
  75 0x100000 ("high memory"), and the kernel real-mode block (boot sector,
  76 setup, and stack/heap) was made relocatable to any address between
  77 0x10000 and end of low memory. Unfortunately, in protocols 2.00 and
  78 2.01 the 0x90000+ memory range is still used internally by the kernel;
  79 the 2.02 protocol resolves that problem.
  80
  81 It is desirable to keep the "memory ceiling" -- the highest point in
  82 low memory touched by the boot loader -- as low as possible, since
  83 some newer BIOSes have begun to allocate some rather large amounts of
  84 memory, called the Extended BIOS Data Area, near the top of low
  85 memory.  The boot loader should use the "INT 12h" BIOS call to verify
  86 how much low memory is available.
  87
  88 Unfortunately, if INT 12h reports that the amount of memory is too
  89 low, there is usually nothing the boot loader can do but to report an
  90 error to the user.  The boot loader should therefore be designed to
  91 take up as little space in low memory as it reasonably can.  For
  92 zImage or old bzImage kernels, which need data written into the
  93 0x90000 segment, the boot loader should make sure not to use memory
  94 above the 0x9A000 point; too many BIOSes will break above that point.
  95
  96 For a modern bzImage kernel with boot protocol version >= 2.02, a
  97 memory layout like the following is suggested:
  98
  99         ~                        ~
 100         |  Protected-mode kernel |
 101 100000  +------------------------+
 102         |  I/O memory hole       |
 103 0A0000  +------------------------+
 104         |  Reserved for BIOS     |      Leave as much as possible unused
 105         ~                        ~
 106         |  Command line          |      (Can also be below the X+10000 mark)
 107 X+10000 +------------------------+
 108         |  Stack/heap            |      For use by the kernel real-mode code.
 109 X+08000 +------------------------+
 110         |  Kernel setup          |      The kernel real-mode code.
 111         |  Kernel boot sector    |      The kernel legacy boot sector.
 112 X       +------------------------+
 113         |  Boot loader           |      <- Boot sector entry point 0000:7C00
 114 001000  +------------------------+
 115         |  Reserved for MBR/BIOS |
 116 000800  +------------------------+
 117         |  Typically used by MBR |
 118 000600  +------------------------+
 119         |  BIOS use only         |
 120 000000  +------------------------+
 121
 122 ... where the address X is as low as the design of the boot loader
 123 permits.
 124
 125
 126 **** THE REAL-MODE KERNEL HEADER
 127
 128 In the following text, and anywhere in the kernel boot sequence, "a
 129 sector" refers to 512 bytes.  It is independent of the actual sector
 130 size of the underlying medium.
 131
 132 The first step in loading a Linux kernel should be to load the
 133 real-mode code (boot sector and setup code) and then examine the
 134 following header at offset 0x01f1.  The real-mode code can total up to
 135 32K, although the boot loader may choose to load only the first two
 136 sectors (1K) and then examine the bootup sector size.
 137
 138 The header looks like:
 139
 140 Offset  Proto   Name            Meaning
 141 /Size
 142
 143 01F1/1  ALL(1   setup_sects     The size of the setup in sectors
 144 01F2/2  ALL     root_flags      If set, the root is mounted readonly
 145 01F4/4  2.04+(2 syssize         The size of the 32-bit code in 16-byte paras
 146 01F8/2  ALL     ram_size        DO NOT USE - for bootsect.S use only
 147 01FA/2  ALL     vid_mode        Video mode control
 148 01FC/2  ALL     root_dev        Default root device number
 149 01FE/2  ALL     boot_flag       0xAA55 magic number
 150 0200/2  2.00+   jump            Jump instruction
 151 0202/4  2.00+   header          Magic signature "HdrS"
 152 0206/2  2.00+   version         Boot protocol version supported
 153 0208/4  2.00+   realmode_swtch  Boot loader hook (see below)
 154 020C/2  2.00+   start_sys       The load-low segment (0x1000) (obsolete)
 155 020E/2  2.00+   kernel_version  Pointer to kernel version string
 156 0210/1  2.00+   type_of_loader  Boot loader identifier
 157 0211/1  2.00+   loadflags       Boot protocol option flags
 158 0212/2  2.00+   setup_move_size Move to high memory size (used with hooks)
 159 0214/4  2.00+   code32_start    Boot loader hook (see below)
 160 0218/4  2.00+   ramdisk_image   initrd load address (set by boot loader)
 161 021C/4  2.00+   ramdisk_size    initrd size (set by boot loader)
 162 0220/4  2.00+   bootsect_kludge DO NOT USE - for bootsect.S use only
 163 0224/2  2.01+   heap_end_ptr    Free memory after setup end
 164 0226/2  N/A     pad1            Unused
 165 0228/4  2.02+   cmd_line_ptr    32-bit pointer to the kernel command line
 166 022C/4  2.03+   initrd_addr_max Highest legal initrd address
 167 0230/4  2.05+   kernel_alignment Physical addr alignment required for kernel
 168 0234/1  2.05+   relocatable_kernel Whether kernel is relocatable or not
 169 0235/3  N/A     pad2            Unused
 170 0238/4  2.06+   cmdline_size    Maximum size of the kernel command line
 171 023C/4  2.07+   hardware_subarch Hardware subarchitecture
 172 0240/8  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
 181 If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
 182 the boot protocol version is "old".  Loading an old kernel, the
 183 following parameters should be assumed:
 184
 185         Image type = zImage
 186         initrd not supported
 187         Real-mode kernel must be located at 0x90000.
 188
 189 Otherwise, the "version" field contains the protocol version,
 190 e.g. protocol version 2.01 will contain 0x0201 in this field.  When
 191 setting fields in the header, you must make sure only to set fields
 192 supported by the protocol version in use.
 193
 194
 195 **** DETAILS OF HEADER FIELDS
 196
 197 For 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
 200 bootloader ("modify").
 201
 202 All general purpose boot loaders should write the fields marked
 203 (obligatory).  Boot loaders who want to load the kernel at a
 204 nonstandard address should fill in the fields marked (reloc); other
 205 boot loaders can ignore those fields.
 206
 207 The byte order of all fields is littleendian (this is x86, after all.)
 208
 209 Field name:     setup_sects
 210 Type:           read
 211 Offset/size:    0x1f1/1
 212 Protocol:       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
 218 Field name:      root_flags
 219 Type:            modify (optional)
 220 Offset/size:     0x1f2/2
 221 Protocol:        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
 227 Field name:     syssize
 228 Type:           read
 229 Offset/size:    0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
 230 Protocol:       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
 237 Field name:     ram_size
 238 Type:           kernel internal
 239 Offset/size:    0x1f8/2
 240 Protocol:       ALL
 241
 242   This field is obsolete.
 243
 244 Field name:     vid_mode
 245 Type:           modify (obligatory)
 246 Offset/size:    0x1fa/2
 247
 248   Please see the section on SPECIAL COMMAND LINE OPTIONS.
 249
 250 Field name:     root_dev
 251 Type:           modify (optional)
 252 Offset/size:    0x1fc/2
 253 Protocol:       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
 258 Field name:     boot_flag
 259 Type:           read
 260 Offset/size:    0x1fe/2
 261 Protocol:       ALL
 262
 263   Contains 0xAA55.  This is the closest thing old Linux kernels have
 264   to a magic number.
 265
 266 Field name:     jump
 267 Type:           read
 268 Offset/size:    0x200/2
 269 Protocol:       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
 275 Field name:     header
 276 Type:           read
 277 Offset/size:    0x202/4
 278 Protocol:       2.00+
 279
 280   Contains the magic number "HdrS" (0x53726448).
 281
 282 Field name:     version
 283 Type:           read
 284 Offset/size:    0x206/2
 285 Protocol:       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
 291 Field name:     readmode_swtch
 292 Type:           modify (optional)
 293 Offset/size:    0x208/4
 294 Protocol:       2.00+
 295
 296   Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
 297
 298 Field name:     start_sys
 299 Type:           read
 300 Offset/size:    0x20c/4
 301 Protocol:       2.00+
 302
 303   The load low segment (0x1000).  Obsolete.
 304
 305 Field name:     kernel_version
 306 Type:           read
 307 Offset/size:    0x20e/2
 308 Protocol:       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
 325 Field name:     type_of_loader
 326 Type:           write (obligatory)
 327 Offset/size:    0x210/1
 328 Protocol:       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
 350 Field name:     loadflags
 351 Type:           modify (obligatory)
 352 Offset/size:    0x211/1
 353 Protocol:       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
 373 Field name:     setup_move_size
 374 Type:           modify (obligatory)
 375 Offset/size:    0x212/2
 376 Protocol:       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
 389 Field name:     code32_start
 390 Type:           modify (optional, reloc)
 391 Offset/size:    0x214/4
 392 Protocol:       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
 406 Field name:     ramdisk_image
 407 Type:           write (obligatory)
 408 Offset/size:    0x218/4
 409 Protocol:       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
 414 Field name:     ramdisk_size
 415 Type:           write (obligatory)
 416 Offset/size:    0x21c/4
 417 Protocol:       2.00+
 418
 419   Size of the initial ramdisk or ramfs.  Leave at zero if there is no
 420   initial ramdisk/ramfs.
 421
 422 Field name:     bootsect_kludge
 423 Type:           kernel internal
 424 Offset/size:    0x220/4
 425 Protocol:       2.00+
 426
 427   This field is obsolete.
 428
 429 Field name:     heap_end_ptr
 430 Type:           write (obligatory)
 431 Offset/size:    0x224/2
 432 Protocol:       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
 437 Field name:     cmd_line_ptr
 438 Type:           write (obligatory)
 439 Offset/size:    0x228/4
 440 Protocol:       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
 453 Field name:     initrd_addr_max
 454 Type:           read
 455 Offset/size:    0x22c/4
 456 Protocol:       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
 465 Field name:     kernel_alignment
 466 Type:           read (reloc)
 467 Offset/size:    0x230/4
 468 Protocol:       2.05+
 469
 470   Alignment unit required by the kernel (if relocatable_kernel is true.)
 471
 472 Field name:     relocatable_kernel
 473 Type:           read (reloc)
 474 Offset/size:    0x234/1
 475 Protocol:       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
 482 Field name:     cmdline_size
 483 Type:           read
 484 Offset/size:    0x238/4
 485 Protocol:       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
 492 Field name:     hardware_subarch
 493 Type:           write
 494 Offset/size:    0x23c/4
 495 Protocol:       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
 508 Field name:     hardware_subarch_data
 509 Type:           write
 510 Offset/size:    0x240/8
 511 Protocol:       2.07+
 512
 513   A pointer to data that is specific to hardware subarch
 514
 515
 516 **** THE KERNEL COMMAND LINE
 517
 518 The kernel command line has become an important way for the boot
 519 loader to communicate with the kernel.  Some of its options are also
 520 relevant to the boot loader itself, see "special command line options"
 521 below.
 522
 523 The kernel command line is a null-terminated string. The maximum
 524 length can be retrieved from the field cmdline_size.  Before protocol
 525 version 2.06, the maximum was 255 characters.  A string that is too
 526 long will be automatically truncated by the kernel.
 527
 528 If the boot protocol version is 2.02 or later, the address of the
 529 kernel command line is given by the header field cmd_line_ptr (see
 530 above.)  This address can be anywhere between the end of the setup
 531 heap and 0xA0000.
 532
 533 If the protocol version is *not* 2.02 or higher, the kernel
 534 command 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
 550 The real-mode code requires a stack/heap to be set up, as well as
 551 memory allocated for the kernel command line.  This needs to be done
 552 in the real-mode accessible memory in bottom megabyte.
 553
 554 It should be noted that modern machines often have a sizable Extended
 555 BIOS Data Area (EBDA).  As a result, it is advisable to use as little
 556 of the low megabyte as possible.
 557
 558 Unfortunately, under the following circumstances the 0x90000 memory
 559 segment 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
 569 When loading at 0x90000, avoid using memory above 0x9a000.
 570
 571 For boot protocol 2.02 or higher, the command line does not have to be
 572 located in the same 64K segment as the real-mode setup code; it is
 573 thus permitted to give the stack/heap the full 64K segment and locate
 574 the command line above it.
 575
 576 The kernel command line should not be located below the real-mode
 577 code, nor should it be located in high memory.
 578
 579
 580 **** SAMPLE BOOT CONFIGURATION
 581
 582 As a sample configuration, assume the following layout of the real
 583 mode 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
 597 Such 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
 658 The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
 659 in the kernel file (again, if setup_sects == 0 the real value is 4.)
 660 It should be loaded at address 0x10000 for Image/zImage kernels and
 661 0x100000 for bzImage kernels.
 662
 663 The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
 664 bit (LOAD_HIGH) in the loadflags field is set:
 665
 666         is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
 667         load_address = is_bzImage ? 0x100000 : 0x10000;
 668
 669 Note that Image/zImage kernels can be up to 512K in size, and thus use
 670 the entire 0x10000-0x90000 range of memory.  This means it is pretty
 671 much a requirement for these kernels to load the real-mode part at
 672 0x90000.  bzImage kernels allow much more flexibility.
 673
 674
 675 **** SPECIAL COMMAND LINE OPTIONS
 676
 677 If the command line provided by the boot loader is entered by the
 678 user, the user may expect the following command line options to work.
 679 They should normally not be deleted from the kernel command line even
 680 though not all of them are actually meaningful to the kernel.  Boot
 681 loader authors who need additional command line options for the boot
 682 loader itself should get them registered in
 683 Documentation/kernel-parameters.txt to make sure they will not
 684 conflict 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
 708 In addition, some boot loaders add the following options to the
 709 user-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
 718 If these options are added by the boot loader, it is highly
 719 recommended that they are located *first*, before the user-specified
 720 or configuration-specified command line.  Otherwise, "init=/bin/sh"
 721 gets confused by the "auto" option.
 722
 723
 724 **** RUNNING THE KERNEL
 725
 726 The kernel is started by jumping to the kernel entry point, which is
 727 located at *segment* offset 0x20 from the start of the real mode
 728 kernel.  This means that if you loaded your real-mode kernel code at
 729 0x90000, the kernel entry point is 9020:0000.
 730
 731 At entry, ds = es = ss should point to the start of the real-mode
 732 kernel code (0x9000 if the code is loaded at 0x90000), sp should be
 733 set up properly, normally pointing to the top of the heap, and
 734 interrupts should be disabled.  Furthermore, to guard against bugs in
 735 the kernel, it is recommended that the boot loader sets fs = gs = ds =
 736 es = ss.
 737
 738 In 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
 754 If your boot sector accesses a floppy drive, it is recommended to
 755 switch off the floppy motor before running the kernel, since the
 756 kernel boot leaves interrupts off and thus the motor will not be
 757 switched off, especially if the loaded kernel has the floppy driver as
 758 a demand-loaded module!
 759
 760
 761 **** ADVANCED BOOT LOADER HOOKS
 762
 763 If the boot loader runs in a particularly hostile environment (such as
 764 LOADLIN, which runs under DOS) it may be impossible to follow the
 765 standard memory location requirements.  Such a boot loader may use the
 766 following hooks that, if set, are invoked by the kernel at the
 767 appropriate time.  The use of these hooks should probably be
 768 considered an absolutely last resort!
 769
 770 IMPORTANT: 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
 792 For machine with some new BIOS other than legacy BIOS, such as EFI,
 793 LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
 794 based on legacy BIOS can not be used, so a 32-bit boot protocol needs
 795 to be defined.
 796
 797 In 32-bit boot protocol, the first step in loading a Linux kernel
 798 should be to setup the boot parameters (struct boot_params,
 799 traditionally known as "zero page"). The memory for struct boot_params
 800 should be allocated and initialized to all zero. Then the setup header
 801 from offset 0x01f1 of kernel image on should be loaded into struct
 802 boot_params and examined. The end of setup header can be calculated as
 803 follow:
 804
 805         0x0202 + byte value at offset 0x0201
 806
 807 In addition to read/modify/write the setup header of the struct
 808 boot_params as that of 16-bit boot protocol, the boot loader should
 809 also fill the additional fields of the struct boot_params as that
 810 described in zero-page.txt.
 811
 812 After setupping the struct boot_params, the boot loader can load the
 813 32/64-bit kernel in the same way as that of 16-bit boot protocol.
 814
 815 In 32-bit boot protocol, the kernel is started by jumping to the
 816 32-bit kernel entry point, which is the start address of loaded
 817 32/64-bit kernel.
 818
 819 At entry, the CPU must be in 32-bit protected mode with paging
 820 disabled; a GDT must be loaded with the descriptors for selectors
 821 __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
 822 segment; __BOOS_CS must have execute/read permission, and __BOOT_DS
 823 must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
 824 must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
 825 address of the struct boot_params; %ebp, %edi and %ebx must be zero.