1 THE LINUX/x86 BOOT PROTOCOL 2 --------------------------- 3 4On the x86 platform, the Linux kernel uses a rather complicated boot 5convention. This has evolved partially due to historical aspects, as 6well as the desire in the early days to have the kernel itself be a 7bootable image, the complicated PC memory model and due to changed 8expectations in the PC industry caused by the effective demise of 9real-mode DOS as a mainstream operating system. 10 11Currently, the following versions of the Linux/x86 boot protocol exist. 12 13Old kernels: zImage/Image support only. Some very early kernels 14 may not even support a command line. 15 16Protocol 2.00: (Kernel 1.3.73) Added bzImage and initrd support, as 17 well as a formalized way to communicate between the 18 boot loader and the kernel. setup.S made relocatable, 19 although the traditional setup area still assumed 20 writable. 21 22Protocol 2.01: (Kernel 1.3.76) Added a heap overrun warning. 23 24Protocol 2.02: (Kernel 2.4.0-test3-pre3) New command line protocol. 25 Lower the conventional memory ceiling. No overwrite 26 of the traditional setup area, thus making booting 27 safe for systems which use the EBDA from SMM or 32-bit 28 BIOS entry points. zImage deprecated but still 29 supported. 30 31Protocol 2.03: (Kernel 2.4.18-pre1) Explicitly makes the highest possible 32 initrd address available to the bootloader. 33 34Protocol 2.04: (Kernel 2.6.14) Extend the syssize field to four bytes. 35 36Protocol 2.05: (Kernel 2.6.20) Make protected mode kernel relocatable. 37 Introduce relocatable_kernel and kernel_alignment fields. 38 39Protocol 2.06: (Kernel 2.6.22) Added a field that contains the size of 40 the boot command line. 41 42Protocol 2.07: (Kernel 2.6.24) Added paravirtualised boot protocol. 43 Introduced hardware_subarch and hardware_subarch_data 44 and KEEP_SEGMENTS flag in load_flags. 45 46Protocol 2.08: (Kernel 2.6.26) Added crc32 checksum and ELF format 47 payload. Introduced payload_offset and payload_length 48 fields to aid in locating the payload. 49 50Protocol 2.09: (Kernel 2.6.26) Added a field of 64-bit physical 51 pointer to single linked list of struct setup_data. 52 53Protocol 2.10: (Kernel 2.6.31) Added a protocol for relaxed alignment 54 beyond the kernel_alignment added, new init_size and 55 pref_address fields. Added extended boot loader IDs. 56 57Protocol 2.11: (Kernel 3.6) Added a field for offset of EFI handover 58 protocol entry point. 59 60Protocol 2.12: (Kernel 3.8) Added the xloadflags field and extension fields 61 to struct boot_params for for loading bzImage and ramdisk 62 above 4G in 64bit. 63 64**** MEMORY LAYOUT 65 66The traditional memory map for the kernel loader, used for Image or 67zImage kernels, typically looks like: 68 69 | | 700A0000 +------------------------+ 71 | Reserved for BIOS | Do not use. Reserved for BIOS EBDA. 7209A000 +------------------------+ 73 | Command line | 74 | Stack/heap | For use by the kernel real-mode code. 75098000 +------------------------+ 76 | Kernel setup | The kernel real-mode code. 77090200 +------------------------+ 78 | Kernel boot sector | The kernel legacy boot sector. 79090000 +------------------------+ 80 | Protected-mode kernel | The bulk of the kernel image. 81010000 +------------------------+ 82 | Boot loader | <- Boot sector entry point 0000:7C00 83001000 +------------------------+ 84 | Reserved for MBR/BIOS | 85000800 +------------------------+ 86 | Typically used by MBR | 87000600 +------------------------+ 88 | BIOS use only | 89000000 +------------------------+ 90 91 92When using bzImage, the protected-mode kernel was relocated to 930x100000 ("high memory"), and the kernel real-mode block (boot sector, 94setup, and stack/heap) was made relocatable to any address between 950x10000 and end of low memory. Unfortunately, in protocols 2.00 and 962.01 the 0x90000+ memory range is still used internally by the kernel; 97the 2.02 protocol resolves that problem. 98 99It is desirable to keep the "memory ceiling" -- the highest point in 100low memory touched by the boot loader -- as low as possible, since 101some newer BIOSes have begun to allocate some rather large amounts of 102memory, called the Extended BIOS Data Area, near the top of low 103memory. The boot loader should use the "INT 12h" BIOS call to verify 104how much low memory is available. 105 106Unfortunately, if INT 12h reports that the amount of memory is too 107low, there is usually nothing the boot loader can do but to report an 108error to the user. The boot loader should therefore be designed to 109take up as little space in low memory as it reasonably can. For 110zImage or old bzImage kernels, which need data written into the 1110x90000 segment, the boot loader should make sure not to use memory 112above the 0x9A000 point; too many BIOSes will break above that point. 113 114For a modern bzImage kernel with boot protocol version >= 2.02, a 115memory layout like the following is suggested: 116 117 ~ ~ 118 | Protected-mode kernel | 119100000 +------------------------+ 120 | I/O memory hole | 1210A0000 +------------------------+ 122 | Reserved for BIOS | Leave as much as possible unused 123 ~ ~ 124 | Command line | (Can also be below the X+10000 mark) 125X+10000 +------------------------+ 126 | Stack/heap | For use by the kernel real-mode code. 127X+08000 +------------------------+ 128 | Kernel setup | The kernel real-mode code. 129 | Kernel boot sector | The kernel legacy boot sector. 130X +------------------------+ 131 | Boot loader | <- Boot sector entry point 0000:7C00 132001000 +------------------------+ 133 | Reserved for MBR/BIOS | 134000800 +------------------------+ 135 | Typically used by MBR | 136000600 +------------------------+ 137 | BIOS use only | 138000000 +------------------------+ 139 140... where the address X is as low as the design of the boot loader 141permits. 142 143 144**** THE REAL-MODE KERNEL HEADER 145 146In the following text, and anywhere in the kernel boot sequence, "a 147sector" refers to 512 bytes. It is independent of the actual sector 148size of the underlying medium. 149 150The first step in loading a Linux kernel should be to load the 151real-mode code (boot sector and setup code) and then examine the 152following header at offset 0x01f1. The real-mode code can total up to 15332K, although the boot loader may choose to load only the first two 154sectors (1K) and then examine the bootup sector size. 155 156The header looks like: 157 158Offset Proto Name Meaning 159/Size 160 16101F1/1 ALL(1 setup_sects The size of the setup in sectors 16201F2/2 ALL root_flags If set, the root is mounted readonly 16301F4/4 2.04+(2 syssize The size of the 32-bit code in 16-byte paras 16401F8/2 ALL ram_size DO NOT USE - for bootsect.S use only 16501FA/2 ALL vid_mode Video mode control 16601FC/2 ALL root_dev Default root device number 16701FE/2 ALL boot_flag 0xAA55 magic number 1680200/2 2.00+ jump Jump instruction 1690202/4 2.00+ header Magic signature "HdrS" 1700206/2 2.00+ version Boot protocol version supported 1710208/4 2.00+ realmode_swtch Boot loader hook (see below) 172020C/2 2.00+ start_sys_seg The load-low segment (0x1000) (obsolete) 173020E/2 2.00+ kernel_version Pointer to kernel version string 1740210/1 2.00+ type_of_loader Boot loader identifier 1750211/1 2.00+ loadflags Boot protocol option flags 1760212/2 2.00+ setup_move_size Move to high memory size (used with hooks) 1770214/4 2.00+ code32_start Boot loader hook (see below) 1780218/4 2.00+ ramdisk_image initrd load address (set by boot loader) 179021C/4 2.00+ ramdisk_size initrd size (set by boot loader) 1800220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only 1810224/2 2.01+ heap_end_ptr Free memory after setup end 1820226/1 2.02+(3 ext_loader_ver Extended boot loader version 1830227/1 2.02+(3 ext_loader_type Extended boot loader ID 1840228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line 185022C/4 2.03+ ramdisk_max Highest legal initrd address 1860230/4 2.05+ kernel_alignment Physical addr alignment required for kernel 1870234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not 1880235/1 2.10+ min_alignment Minimum alignment, as a power of two 1890236/2 2.12+ xloadflags Boot protocol option flags 1900238/4 2.06+ cmdline_size Maximum size of the kernel command line 191023C/4 2.07+ hardware_subarch Hardware subarchitecture 1920240/8 2.07+ hardware_subarch_data Subarchitecture-specific data 1930248/4 2.08+ payload_offset Offset of kernel payload 194024C/4 2.08+ payload_length Length of kernel payload 1950250/8 2.09+ setup_data 64-bit physical pointer to linked list 196 of struct setup_data 1970258/8 2.10+ pref_address Preferred loading address 1980260/4 2.10+ init_size Linear memory required during initialization 1990264/4 2.11+ handover_offset Offset of handover entry point 200 201(1) For backwards compatibility, if the setup_sects field contains 0, the 202 real value is 4. 203 204(2) For boot protocol prior to 2.04, the upper two bytes of the syssize 205 field are unusable, which means the size of a bzImage kernel 206 cannot be determined. 207 208(3) Ignored, but safe to set, for boot protocols 2.02-2.09. 209 210If the "HdrS" (0x53726448) magic number is not found at offset 0x202, 211the boot protocol version is "old". Loading an old kernel, the 212following parameters should be assumed: 213 214 Image type = zImage 215 initrd not supported 216 Real-mode kernel must be located at 0x90000. 217 218Otherwise, the "version" field contains the protocol version, 219e.g. protocol version 2.01 will contain 0x0201 in this field. When 220setting fields in the header, you must make sure only to set fields 221supported by the protocol version in use. 222 223 224**** DETAILS OF HEADER FIELDS 225 226For each field, some are information from the kernel to the bootloader 227("read"), some are expected to be filled out by the bootloader 228("write"), and some are expected to be read and modified by the 229bootloader ("modify"). 230 231All general purpose boot loaders should write the fields marked 232(obligatory). Boot loaders who want to load the kernel at a 233nonstandard address should fill in the fields marked (reloc); other 234boot loaders can ignore those fields. 235 236The byte order of all fields is littleendian (this is x86, after all.) 237 238Field name: setup_sects 239Type: read 240Offset/size: 0x1f1/1 241Protocol: ALL 242 243 The size of the setup code in 512-byte sectors. If this field is 244 0, the real value is 4. The real-mode code consists of the boot 245 sector (always one 512-byte sector) plus the setup code. 246 247Field name: root_flags 248Type: modify (optional) 249Offset/size: 0x1f2/2 250Protocol: ALL 251 252 If this field is nonzero, the root defaults to readonly. The use of 253 this field is deprecated; use the "ro" or "rw" options on the 254 command line instead. 255 256Field name: syssize 257Type: read 258Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL) 259Protocol: 2.04+ 260 261 The size of the protected-mode code in units of 16-byte paragraphs. 262 For protocol versions older than 2.04 this field is only two bytes 263 wide, and therefore cannot be trusted for the size of a kernel if 264 the LOAD_HIGH flag is set. 265 266Field name: ram_size 267Type: kernel internal 268Offset/size: 0x1f8/2 269Protocol: ALL 270 271 This field is obsolete. 272 273Field name: vid_mode 274Type: modify (obligatory) 275Offset/size: 0x1fa/2 276 277 Please see the section on SPECIAL COMMAND LINE OPTIONS. 278 279Field name: root_dev 280Type: modify (optional) 281Offset/size: 0x1fc/2 282Protocol: ALL 283 284 The default root device device number. The use of this field is 285 deprecated, use the "root=" option on the command line instead. 286 287Field name: boot_flag 288Type: read 289Offset/size: 0x1fe/2 290Protocol: ALL 291 292 Contains 0xAA55. This is the closest thing old Linux kernels have 293 to a magic number. 294 295Field name: jump 296Type: read 297Offset/size: 0x200/2 298Protocol: 2.00+ 299 300 Contains an x86 jump instruction, 0xEB followed by a signed offset 301 relative to byte 0x202. This can be used to determine the size of 302 the header. 303 304Field name: header 305Type: read 306Offset/size: 0x202/4 307Protocol: 2.00+ 308 309 Contains the magic number "HdrS" (0x53726448). 310 311Field name: version 312Type: read 313Offset/size: 0x206/2 314Protocol: 2.00+ 315 316 Contains the boot protocol version, in (major << 8)+minor format, 317 e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version 318 10.17. 319 320Field name: realmode_swtch 321Type: modify (optional) 322Offset/size: 0x208/4 323Protocol: 2.00+ 324 325 Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.) 326 327Field name: start_sys_seg 328Type: read 329Offset/size: 0x20c/2 330Protocol: 2.00+ 331 332 The load low segment (0x1000). Obsolete. 333 334Field name: kernel_version 335Type: read 336Offset/size: 0x20e/2 337Protocol: 2.00+ 338 339 If set to a nonzero value, contains a pointer to a NUL-terminated 340 human-readable kernel version number string, less 0x200. This can 341 be used to display the kernel version to the user. This value 342 should be less than (0x200*setup_sects). 343 344 For example, if this value is set to 0x1c00, the kernel version 345 number string can be found at offset 0x1e00 in the kernel file. 346 This is a valid value if and only if the "setup_sects" field 347 contains the value 15 or higher, as: 348 349 0x1c00 < 15*0x200 (= 0x1e00) but 350 0x1c00 >= 14*0x200 (= 0x1c00) 351 352 0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15. 353 354Field name: type_of_loader 355Type: write (obligatory) 356Offset/size: 0x210/1 357Protocol: 2.00+ 358 359 If your boot loader has an assigned id (see table below), enter 360 0xTV here, where T is an identifier for the boot loader and V is 361 a version number. Otherwise, enter 0xFF here. 362 363 For boot loader IDs above T = 0xD, write T = 0xE to this field and 364 write the extended ID minus 0x10 to the ext_loader_type field. 365 Similarly, the ext_loader_ver field can be used to provide more than 366 four bits for the bootloader version. 367 368 For example, for T = 0x15, V = 0x234, write: 369 370 type_of_loader <- 0xE4 371 ext_loader_type <- 0x05 372 ext_loader_ver <- 0x23 373 374 Assigned boot loader ids (hexadecimal): 375 376 0 LILO (0x00 reserved for pre-2.00 bootloader) 377 1 Loadlin 378 2 bootsect-loader (0x20, all other values reserved) 379 3 Syslinux 380 4 Etherboot/gPXE/iPXE 381 5 ELILO 382 7 GRUB 383 8 U-Boot 384 9 Xen 385 A Gujin 386 B Qemu 387 C Arcturus Networks uCbootloader 388 D kexec-tools 389 E Extended (see ext_loader_type) 390 F Special (0xFF = undefined) 391 10 Reserved 392 11 Minimal Linux Bootloader <http://sebastian-plotz.blogspot.de> 393 12 OVMF UEFI virtualization stack 394 395 Please contact <hpa@zytor.com> if you need a bootloader ID 396 value assigned. 397 398Field name: loadflags 399Type: modify (obligatory) 400Offset/size: 0x211/1 401Protocol: 2.00+ 402 403 This field is a bitmask. 404 405 Bit 0 (read): LOADED_HIGH 406 - If 0, the protected-mode code is loaded at 0x10000. 407 - If 1, the protected-mode code is loaded at 0x100000. 408 409 Bit 5 (write): QUIET_FLAG 410 - If 0, print early messages. 411 - If 1, suppress early messages. 412 This requests to the kernel (decompressor and early 413 kernel) to not write early messages that require 414 accessing the display hardware directly. 415 416 Bit 6 (write): KEEP_SEGMENTS 417 Protocol: 2.07+ 418 - If 0, reload the segment registers in the 32bit entry point. 419 - If 1, do not reload the segment registers in the 32bit entry point. 420 Assume that %cs %ds %ss %es are all set to flat segments with 421 a base of 0 (or the equivalent for their environment). 422 423 Bit 7 (write): CAN_USE_HEAP 424 Set this bit to 1 to indicate that the value entered in the 425 heap_end_ptr is valid. If this field is clear, some setup code 426 functionality will be disabled. 427 428Field name: setup_move_size 429Type: modify (obligatory) 430Offset/size: 0x212/2 431Protocol: 2.00-2.01 432 433 When using protocol 2.00 or 2.01, if the real mode kernel is not 434 loaded at 0x90000, it gets moved there later in the loading 435 sequence. Fill in this field if you want additional data (such as 436 the kernel command line) moved in addition to the real-mode kernel 437 itself. 438 439 The unit is bytes starting with the beginning of the boot sector. 440 441 This field is can be ignored when the protocol is 2.02 or higher, or 442 if the real-mode code is loaded at 0x90000. 443 444Field name: code32_start 445Type: modify (optional, reloc) 446Offset/size: 0x214/4 447Protocol: 2.00+ 448 449 The address to jump to in protected mode. This defaults to the load 450 address of the kernel, and can be used by the boot loader to 451 determine the proper load address. 452 453 This field can be modified for two purposes: 454 455 1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.) 456 457 2. if a bootloader which does not install a hook loads a 458 relocatable kernel at a nonstandard address it will have to modify 459 this field to point to the load address. 460 461Field name: ramdisk_image 462Type: write (obligatory) 463Offset/size: 0x218/4 464Protocol: 2.00+ 465 466 The 32-bit linear address of the initial ramdisk or ramfs. Leave at 467 zero if there is no initial ramdisk/ramfs. 468 469Field name: ramdisk_size 470Type: write (obligatory) 471Offset/size: 0x21c/4 472Protocol: 2.00+ 473 474 Size of the initial ramdisk or ramfs. Leave at zero if there is no 475 initial ramdisk/ramfs. 476 477Field name: bootsect_kludge 478Type: kernel internal 479Offset/size: 0x220/4 480Protocol: 2.00+ 481 482 This field is obsolete. 483 484Field name: heap_end_ptr 485Type: write (obligatory) 486Offset/size: 0x224/2 487Protocol: 2.01+ 488 489 Set this field to the offset (from the beginning of the real-mode 490 code) of the end of the setup stack/heap, minus 0x0200. 491 492Field name: ext_loader_ver 493Type: write (optional) 494Offset/size: 0x226/1 495Protocol: 2.02+ 496 497 This field is used as an extension of the version number in the 498 type_of_loader field. The total version number is considered to be 499 (type_of_loader & 0x0f) + (ext_loader_ver << 4). 500 501 The use of this field is boot loader specific. If not written, it 502 is zero. 503 504 Kernels prior to 2.6.31 did not recognize this field, but it is safe 505 to write for protocol version 2.02 or higher. 506 507Field name: ext_loader_type 508Type: write (obligatory if (type_of_loader & 0xf0) == 0xe0) 509Offset/size: 0x227/1 510Protocol: 2.02+ 511 512 This field is used as an extension of the type number in 513 type_of_loader field. If the type in type_of_loader is 0xE, then 514 the actual type is (ext_loader_type + 0x10). 515 516 This field is ignored if the type in type_of_loader is not 0xE. 517 518 Kernels prior to 2.6.31 did not recognize this field, but it is safe 519 to write for protocol version 2.02 or higher. 520 521Field name: cmd_line_ptr 522Type: write (obligatory) 523Offset/size: 0x228/4 524Protocol: 2.02+ 525 526 Set this field to the linear address of the kernel command line. 527 The kernel command line can be located anywhere between the end of 528 the setup heap and 0xA0000; it does not have to be located in the 529 same 64K segment as the real-mode code itself. 530 531 Fill in this field even if your boot loader does not support a 532 command line, in which case you can point this to an empty string 533 (or better yet, to the string "auto".) If this field is left at 534 zero, the kernel will assume that your boot loader does not support 535 the 2.02+ protocol. 536 537Field name: ramdisk_max 538Type: read 539Offset/size: 0x22c/4 540Protocol: 2.03+ 541 542 The maximum address that may be occupied by the initial 543 ramdisk/ramfs contents. For boot protocols 2.02 or earlier, this 544 field is not present, and the maximum address is 0x37FFFFFF. (This 545 address is defined as the address of the highest safe byte, so if 546 your ramdisk is exactly 131072 bytes long and this field is 547 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.) 548 549Field name: kernel_alignment 550Type: read/modify (reloc) 551Offset/size: 0x230/4 552Protocol: 2.05+ (read), 2.10+ (modify) 553 554 Alignment unit required by the kernel (if relocatable_kernel is 555 true.) A relocatable kernel that is loaded at an alignment 556 incompatible with the value in this field will be realigned during 557 kernel initialization. 558 559 Starting with protocol version 2.10, this reflects the kernel 560 alignment preferred for optimal performance; it is possible for the 561 loader to modify this field to permit a lesser alignment. See the 562 min_alignment and pref_address field below. 563 564Field name: relocatable_kernel 565Type: read (reloc) 566Offset/size: 0x234/1 567Protocol: 2.05+ 568 569 If this field is nonzero, the protected-mode part of the kernel can 570 be loaded at any address that satisfies the kernel_alignment field. 571 After loading, the boot loader must set the code32_start field to 572 point to the loaded code, or to a boot loader hook. 573 574Field name: min_alignment 575Type: read (reloc) 576Offset/size: 0x235/1 577Protocol: 2.10+ 578 579 This field, if nonzero, indicates as a power of two the minimum 580 alignment required, as opposed to preferred, by the kernel to boot. 581 If a boot loader makes use of this field, it should update the 582 kernel_alignment field with the alignment unit desired; typically: 583 584 kernel_alignment = 1 << min_alignment 585 586 There may be a considerable performance cost with an excessively 587 misaligned kernel. Therefore, a loader should typically try each 588 power-of-two alignment from kernel_alignment down to this alignment. 589 590Field name: xloadflags 591Type: read 592Offset/size: 0x236/2 593Protocol: 2.12+ 594 595 This field is a bitmask. 596 597 Bit 0 (read): XLF_KERNEL_64 598 - If 1, this kernel has the legacy 64-bit entry point at 0x200. 599 600 Bit 1 (read): XLF_CAN_BE_LOADED_ABOVE_4G 601 - If 1, kernel/boot_params/cmdline/ramdisk can be above 4G. 602 603 Bit 2 (read): XLF_EFI_HANDOVER_32 604 - If 1, the kernel supports the 32-bit EFI handoff entry point 605 given at handover_offset. 606 607 Bit 3 (read): XLF_EFI_HANDOVER_64 608 - If 1, the kernel supports the 64-bit EFI handoff entry point 609 given at handover_offset + 0x200. 610 611Field name: cmdline_size 612Type: read 613Offset/size: 0x238/4 614Protocol: 2.06+ 615 616 The maximum size of the command line without the terminating 617 zero. This means that the command line can contain at most 618 cmdline_size characters. With protocol version 2.05 and earlier, the 619 maximum size was 255. 620 621Field name: hardware_subarch 622Type: write (optional, defaults to x86/PC) 623Offset/size: 0x23c/4 624Protocol: 2.07+ 625 626 In a paravirtualized environment the hardware low level architectural 627 pieces such as interrupt handling, page table handling, and 628 accessing process control registers needs to be done differently. 629 630 This field allows the bootloader to inform the kernel we are in one 631 one of those environments. 632 633 0x00000000 The default x86/PC environment 634 0x00000001 lguest 635 0x00000002 Xen 636 0x00000003 Moorestown MID 637 0x00000004 CE4100 TV Platform 638 639Field name: hardware_subarch_data 640Type: write (subarch-dependent) 641Offset/size: 0x240/8 642Protocol: 2.07+ 643 644 A pointer to data that is specific to hardware subarch 645 This field is currently unused for the default x86/PC environment, 646 do not modify. 647 648Field name: payload_offset 649Type: read 650Offset/size: 0x248/4 651Protocol: 2.08+ 652 653 If non-zero then this field contains the offset from the beginning 654 of the protected-mode code to the payload. 655 656 The payload may be compressed. The format of both the compressed and 657 uncompressed data should be determined using the standard magic 658 numbers. The currently supported compression formats are gzip 659 (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA 660 (magic number 5D 00), and XZ (magic number FD 37). The uncompressed 661 payload is currently always ELF (magic number 7F 45 4C 46). 662 663Field name: payload_length 664Type: read 665Offset/size: 0x24c/4 666Protocol: 2.08+ 667 668 The length of the payload. 669 670Field name: setup_data 671Type: write (special) 672Offset/size: 0x250/8 673Protocol: 2.09+ 674 675 The 64-bit physical pointer to NULL terminated single linked list of 676 struct setup_data. This is used to define a more extensible boot 677 parameters passing mechanism. The definition of struct setup_data is 678 as follow: 679 680 struct setup_data { 681 u64 next; 682 u32 type; 683 u32 len; 684 u8 data[0]; 685 }; 686 687 Where, the next is a 64-bit physical pointer to the next node of 688 linked list, the next field of the last node is 0; the type is used 689 to identify the contents of data; the len is the length of data 690 field; the data holds the real payload. 691 692 This list may be modified at a number of points during the bootup 693 process. Therefore, when modifying this list one should always make 694 sure to consider the case where the linked list already contains 695 entries. 696 697Field name: pref_address 698Type: read (reloc) 699Offset/size: 0x258/8 700Protocol: 2.10+ 701 702 This field, if nonzero, represents a preferred load address for the 703 kernel. A relocating bootloader should attempt to load at this 704 address if possible. 705 706 A non-relocatable kernel will unconditionally move itself and to run 707 at this address. 708 709Field name: init_size 710Type: read 711Offset/size: 0x260/4 712 713 This field indicates the amount of linear contiguous memory starting 714 at the kernel runtime start address that the kernel needs before it 715 is capable of examining its memory map. This is not the same thing 716 as the total amount of memory the kernel needs to boot, but it can 717 be used by a relocating boot loader to help select a safe load 718 address for the kernel. 719 720 The kernel runtime start address is determined by the following algorithm: 721 722 if (relocatable_kernel) 723 runtime_start = align_up(load_address, kernel_alignment) 724 else 725 runtime_start = pref_address 726 727Field name: handover_offset 728Type: read 729Offset/size: 0x264/4 730 731 This field is the offset from the beginning of the kernel image to 732 the EFI handover protocol entry point. Boot loaders using the EFI 733 handover protocol to boot the kernel should jump to this offset. 734 735 See EFI HANDOVER PROTOCOL below for more details. 736 737 738**** THE IMAGE CHECKSUM 739 740From boot protocol version 2.08 onwards the CRC-32 is calculated over 741the entire file using the characteristic polynomial 0x04C11DB7 and an 742initial remainder of 0xffffffff. The checksum is appended to the 743file; therefore the CRC of the file up to the limit specified in the 744syssize field of the header is always 0. 745 746 747**** THE KERNEL COMMAND LINE 748 749The kernel command line has become an important way for the boot 750loader to communicate with the kernel. Some of its options are also 751relevant to the boot loader itself, see "special command line options" 752below. 753 754The kernel command line is a null-terminated string. The maximum 755length can be retrieved from the field cmdline_size. Before protocol 756version 2.06, the maximum was 255 characters. A string that is too 757long will be automatically truncated by the kernel. 758 759If the boot protocol version is 2.02 or later, the address of the 760kernel command line is given by the header field cmd_line_ptr (see 761above.) This address can be anywhere between the end of the setup 762heap and 0xA0000. 763 764If the protocol version is *not* 2.02 or higher, the kernel 765command line is entered using the following protocol: 766 767 At offset 0x0020 (word), "cmd_line_magic", enter the magic 768 number 0xA33F. 769 770 At offset 0x0022 (word), "cmd_line_offset", enter the offset 771 of the kernel command line (relative to the start of the 772 real-mode kernel). 773 774 The kernel command line *must* be within the memory region 775 covered by setup_move_size, so you may need to adjust this 776 field. 777 778 779**** MEMORY LAYOUT OF THE REAL-MODE CODE 780 781The real-mode code requires a stack/heap to be set up, as well as 782memory allocated for the kernel command line. This needs to be done 783in the real-mode accessible memory in bottom megabyte. 784 785It should be noted that modern machines often have a sizable Extended 786BIOS Data Area (EBDA). As a result, it is advisable to use as little 787of the low megabyte as possible. 788 789Unfortunately, under the following circumstances the 0x90000 memory 790segment has to be used: 791 792 - When loading a zImage kernel ((loadflags & 0x01) == 0). 793 - When loading a 2.01 or earlier boot protocol kernel. 794 795 -> For the 2.00 and 2.01 boot protocols, the real-mode code 796 can be loaded at another address, but it is internally 797 relocated to 0x90000. For the "old" protocol, the 798 real-mode code must be loaded at 0x90000. 799 800When loading at 0x90000, avoid using memory above 0x9a000. 801 802For boot protocol 2.02 or higher, the command line does not have to be 803located in the same 64K segment as the real-mode setup code; it is 804thus permitted to give the stack/heap the full 64K segment and locate 805the command line above it. 806 807The kernel command line should not be located below the real-mode 808code, nor should it be located in high memory. 809 810 811**** SAMPLE BOOT CONFIGURATION 812 813As a sample configuration, assume the following layout of the real 814mode segment: 815 816 When loading below 0x90000, use the entire segment: 817 818 0x0000-0x7fff Real mode kernel 819 0x8000-0xdfff Stack and heap 820 0xe000-0xffff Kernel command line 821 822 When loading at 0x90000 OR the protocol version is 2.01 or earlier: 823 824 0x0000-0x7fff Real mode kernel 825 0x8000-0x97ff Stack and heap 826 0x9800-0x9fff Kernel command line 827 828Such a boot loader should enter the following fields in the header: 829 830 unsigned long base_ptr; /* base address for real-mode segment */ 831 832 if ( setup_sects == 0 ) { 833 setup_sects = 4; 834 } 835 836 if ( protocol >= 0x0200 ) { 837 type_of_loader = <type code>; 838 if ( loading_initrd ) { 839 ramdisk_image = <initrd_address>; 840 ramdisk_size = <initrd_size>; 841 } 842 843 if ( protocol >= 0x0202 && loadflags & 0x01 ) 844 heap_end = 0xe000; 845 else 846 heap_end = 0x9800; 847 848 if ( protocol >= 0x0201 ) { 849 heap_end_ptr = heap_end - 0x200; 850 loadflags |= 0x80; /* CAN_USE_HEAP */ 851 } 852 853 if ( protocol >= 0x0202 ) { 854 cmd_line_ptr = base_ptr + heap_end; 855 strcpy(cmd_line_ptr, cmdline); 856 } else { 857 cmd_line_magic = 0xA33F; 858 cmd_line_offset = heap_end; 859 setup_move_size = heap_end + strlen(cmdline)+1; 860 strcpy(base_ptr+cmd_line_offset, cmdline); 861 } 862 } else { 863 /* Very old kernel */ 864 865 heap_end = 0x9800; 866 867 cmd_line_magic = 0xA33F; 868 cmd_line_offset = heap_end; 869 870 /* A very old kernel MUST have its real-mode code 871 loaded at 0x90000 */ 872 873 if ( base_ptr != 0x90000 ) { 874 /* Copy the real-mode kernel */ 875 memcpy(0x90000, base_ptr, (setup_sects+1)*512); 876 base_ptr = 0x90000; /* Relocated */ 877 } 878 879 strcpy(0x90000+cmd_line_offset, cmdline); 880 881 /* It is recommended to clear memory up to the 32K mark */ 882 memset(0x90000 + (setup_sects+1)*512, 0, 883 (64-(setup_sects+1))*512); 884 } 885 886 887**** LOADING THE REST OF THE KERNEL 888 889The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512 890in the kernel file (again, if setup_sects == 0 the real value is 4.) 891It should be loaded at address 0x10000 for Image/zImage kernels and 8920x100000 for bzImage kernels. 893 894The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01 895bit (LOAD_HIGH) in the loadflags field is set: 896 897 is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01); 898 load_address = is_bzImage ? 0x100000 : 0x10000; 899 900Note that Image/zImage kernels can be up to 512K in size, and thus use 901the entire 0x10000-0x90000 range of memory. This means it is pretty 902much a requirement for these kernels to load the real-mode part at 9030x90000. bzImage kernels allow much more flexibility. 904 905 906**** SPECIAL COMMAND LINE OPTIONS 907 908If the command line provided by the boot loader is entered by the 909user, the user may expect the following command line options to work. 910They should normally not be deleted from the kernel command line even 911though not all of them are actually meaningful to the kernel. Boot 912loader authors who need additional command line options for the boot 913loader itself should get them registered in 914Documentation/kernel-parameters.txt to make sure they will not 915conflict with actual kernel options now or in the future. 916 917 vga=<mode> 918 <mode> here is either an integer (in C notation, either 919 decimal, octal, or hexadecimal) or one of the strings 920 "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask" 921 (meaning 0xFFFD). This value should be entered into the 922 vid_mode field, as it is used by the kernel before the command 923 line is parsed. 924 925 mem=<size> 926 <size> is an integer in C notation optionally followed by 927 (case insensitive) K, M, G, T, P or E (meaning << 10, << 20, 928 << 30, << 40, << 50 or << 60). This specifies the end of 929 memory to the kernel. This affects the possible placement of 930 an initrd, since an initrd should be placed near end of 931 memory. Note that this is an option to *both* the kernel and 932 the bootloader! 933 934 initrd=<file> 935 An initrd should be loaded. The meaning of <file> is 936 obviously bootloader-dependent, and some boot loaders 937 (e.g. LILO) do not have such a command. 938 939In addition, some boot loaders add the following options to the 940user-specified command line: 941 942 BOOT_IMAGE=<file> 943 The boot image which was loaded. Again, the meaning of <file> 944 is obviously bootloader-dependent. 945 946 auto 947 The kernel was booted without explicit user intervention. 948 949If these options are added by the boot loader, it is highly 950recommended that they are located *first*, before the user-specified 951or configuration-specified command line. Otherwise, "init=/bin/sh" 952gets confused by the "auto" option. 953 954 955**** RUNNING THE KERNEL 956 957The kernel is started by jumping to the kernel entry point, which is 958located at *segment* offset 0x20 from the start of the real mode 959kernel. This means that if you loaded your real-mode kernel code at 9600x90000, the kernel entry point is 9020:0000. 961 962At entry, ds = es = ss should point to the start of the real-mode 963kernel code (0x9000 if the code is loaded at 0x90000), sp should be 964set up properly, normally pointing to the top of the heap, and 965interrupts should be disabled. Furthermore, to guard against bugs in 966the kernel, it is recommended that the boot loader sets fs = gs = ds = 967es = ss. 968 969In our example from above, we would do: 970 971 /* Note: in the case of the "old" kernel protocol, base_ptr must 972 be == 0x90000 at this point; see the previous sample code */ 973 974 seg = base_ptr >> 4; 975 976 cli(); /* Enter with interrupts disabled! */ 977 978 /* Set up the real-mode kernel stack */ 979 _SS = seg; 980 _SP = heap_end; 981 982 _DS = _ES = _FS = _GS = seg; 983 jmp_far(seg+0x20, 0); /* Run the kernel */ 984 985If your boot sector accesses a floppy drive, it is recommended to 986switch off the floppy motor before running the kernel, since the 987kernel boot leaves interrupts off and thus the motor will not be 988switched off, especially if the loaded kernel has the floppy driver as 989a demand-loaded module! 990 991 992**** ADVANCED BOOT LOADER HOOKS 993 994If the boot loader runs in a particularly hostile environment (such as 995LOADLIN, which runs under DOS) it may be impossible to follow the 996standard memory location requirements. Such a boot loader may use the 997following hooks that, if set, are invoked by the kernel at the 998appropriate time. The use of these hooks should probably be 999considered an absolutely last resort! 1000 1001IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and 1002%edi across invocation. 1003 1004 realmode_swtch: 1005 A 16-bit real mode far subroutine invoked immediately before 1006 entering protected mode. The default routine disables NMI, so 1007 your routine should probably do so, too. 1008 1009 code32_start: 1010 A 32-bit flat-mode routine *jumped* to immediately after the 1011 transition to protected mode, but before the kernel is 1012 uncompressed. No segments, except CS, are guaranteed to be 1013 set up (current kernels do, but older ones do not); you should 1014 set them up to BOOT_DS (0x18) yourself. 1015 1016 After completing your hook, you should jump to the address 1017 that was in this field before your boot loader overwrote it 1018 (relocated, if appropriate.) 1019 1020 1021**** 32-bit BOOT PROTOCOL 1022 1023For machine with some new BIOS other than legacy BIOS, such as EFI, 1024LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel 1025based on legacy BIOS can not be used, so a 32-bit boot protocol needs 1026to be defined. 1027 1028In 32-bit boot protocol, the first step in loading a Linux kernel 1029should be to setup the boot parameters (struct boot_params, 1030traditionally known as "zero page"). The memory for struct boot_params 1031should be allocated and initialized to all zero. Then the setup header 1032from offset 0x01f1 of kernel image on should be loaded into struct 1033boot_params and examined. The end of setup header can be calculated as 1034follow: 1035 1036 0x0202 + byte value at offset 0x0201 1037 1038In addition to read/modify/write the setup header of the struct 1039boot_params as that of 16-bit boot protocol, the boot loader should 1040also fill the additional fields of the struct boot_params as that 1041described in zero-page.txt. 1042 1043After setting up the struct boot_params, the boot loader can load the 104432/64-bit kernel in the same way as that of 16-bit boot protocol. 1045 1046In 32-bit boot protocol, the kernel is started by jumping to the 104732-bit kernel entry point, which is the start address of loaded 104832/64-bit kernel. 1049 1050At entry, the CPU must be in 32-bit protected mode with paging 1051disabled; a GDT must be loaded with the descriptors for selectors 1052__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat 1053segment; __BOOT_CS must have execute/read permission, and __BOOT_DS 1054must have read/write permission; CS must be __BOOT_CS and DS, ES, SS 1055must be __BOOT_DS; interrupt must be disabled; %esi must hold the base 1056address of the struct boot_params; %ebp, %edi and %ebx must be zero. 1057 1058**** 64-bit BOOT PROTOCOL 1059 1060For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader 1061and we need a 64-bit boot protocol. 1062 1063In 64-bit boot protocol, the first step in loading a Linux kernel 1064should be to setup the boot parameters (struct boot_params, 1065traditionally known as "zero page"). The memory for struct boot_params 1066could be allocated anywhere (even above 4G) and initialized to all zero. 1067Then, the setup header at offset 0x01f1 of kernel image on should be 1068loaded into struct boot_params and examined. The end of setup header 1069can be calculated as follows: 1070 1071 0x0202 + byte value at offset 0x0201 1072 1073In addition to read/modify/write the setup header of the struct 1074boot_params as that of 16-bit boot protocol, the boot loader should 1075also fill the additional fields of the struct boot_params as described 1076in zero-page.txt. 1077 1078After setting up the struct boot_params, the boot loader can load 107964-bit kernel in the same way as that of 16-bit boot protocol, but 1080kernel could be loaded above 4G. 1081 1082In 64-bit boot protocol, the kernel is started by jumping to the 108364-bit kernel entry point, which is the start address of loaded 108464-bit kernel plus 0x200. 1085 1086At entry, the CPU must be in 64-bit mode with paging enabled. 1087The range with setup_header.init_size from start address of loaded 1088kernel and zero page and command line buffer get ident mapping; 1089a GDT must be loaded with the descriptors for selectors 1090__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat 1091segment; __BOOT_CS must have execute/read permission, and __BOOT_DS 1092must have read/write permission; CS must be __BOOT_CS and DS, ES, SS 1093must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base 1094address of the struct boot_params. 1095 1096**** EFI HANDOVER PROTOCOL 1097 1098This protocol allows boot loaders to defer initialisation to the EFI 1099boot stub. The boot loader is required to load the kernel/initrd(s) 1100from the boot media and jump to the EFI handover protocol entry point 1101which is hdr->handover_offset bytes from the beginning of 1102startup_{32,64}. 1103 1104The function prototype for the handover entry point looks like this, 1105 1106 efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp) 1107 1108'handle' is the EFI image handle passed to the boot loader by the EFI 1109firmware, 'table' is the EFI system table - these are the first two 1110arguments of the "handoff state" as described in section 2.3 of the 1111UEFI specification. 'bp' is the boot loader-allocated boot params. 1112 1113The boot loader *must* fill out the following fields in bp, 1114 1115 o hdr.code32_start 1116 o hdr.cmd_line_ptr 1117 o hdr.cmdline_size 1118 o hdr.ramdisk_image (if applicable) 1119 o hdr.ramdisk_size (if applicable) 1120 1121All other fields should be zero. 1122