1.. SPDX-License-Identifier: GPL-2.0 2 3=========================== 4The Linux/x86 Boot Protocol 5=========================== 6 7On the x86 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/x86 boot protocol exist. 15 16============= ============================================================ 17Old kernels zImage/Image support only. Some very early kernels 18 may not even support a command line. 19 20Protocol 2.00 (Kernel 1.3.73) Added bzImage and initrd support, as 21 well as a formalized way to communicate between the 22 boot loader and the kernel. setup.S made relocatable, 23 although the traditional setup area still assumed 24 writable. 25 26Protocol 2.01 (Kernel 1.3.76) Added a heap overrun warning. 27 28Protocol 2.02 (Kernel 2.4.0-test3-pre3) New command line protocol. 29 Lower the conventional memory ceiling. No overwrite 30 of the traditional setup area, thus making booting 31 safe for systems which use the EBDA from SMM or 32-bit 32 BIOS entry points. zImage deprecated but still 33 supported. 34 35Protocol 2.03 (Kernel 2.4.18-pre1) Explicitly makes the highest possible 36 initrd address available to the bootloader. 37 38Protocol 2.04 (Kernel 2.6.14) Extend the syssize field to four bytes. 39 40Protocol 2.05 (Kernel 2.6.20) Make protected mode kernel relocatable. 41 Introduce relocatable_kernel and kernel_alignment fields. 42 43Protocol 2.06 (Kernel 2.6.22) Added a field that contains the size of 44 the boot command line. 45 46Protocol 2.07 (Kernel 2.6.24) Added paravirtualised boot protocol. 47 Introduced hardware_subarch and hardware_subarch_data 48 and KEEP_SEGMENTS flag in load_flags. 49 50Protocol 2.08 (Kernel 2.6.26) Added crc32 checksum and ELF format 51 payload. Introduced payload_offset and payload_length 52 fields to aid in locating the payload. 53 54Protocol 2.09 (Kernel 2.6.26) Added a field of 64-bit physical 55 pointer to single linked list of struct setup_data. 56 57Protocol 2.10 (Kernel 2.6.31) Added a protocol for relaxed alignment 58 beyond the kernel_alignment added, new init_size and 59 pref_address fields. Added extended boot loader IDs. 60 61Protocol 2.11 (Kernel 3.6) Added a field for offset of EFI handover 62 protocol entry point. 63 64Protocol 2.12 (Kernel 3.8) Added the xloadflags field and extension fields 65 to struct boot_params for loading bzImage and ramdisk 66 above 4G in 64bit. 67 68Protocol 2.13 (Kernel 3.14) Support 32- and 64-bit flags being set in 69 xloadflags to support booting a 64-bit kernel from 32-bit 70 EFI 71 72Protocol 2.14 BURNT BY INCORRECT COMMIT 73 ae7e1238e68f2a472a125673ab506d49158c1889 74 (x86/boot: Add ACPI RSDP address to setup_header) 75 DO NOT USE!!! ASSUME SAME AS 2.13. 76 77Protocol 2.15 (Kernel 5.5) Added the kernel_info and kernel_info.setup_type_max. 78============= ============================================================ 79 80.. note:: 81 The protocol version number should be changed only if the setup header 82 is changed. There is no need to update the version number if boot_params 83 or kernel_info are changed. Additionally, it is recommended to use 84 xloadflags (in this case the protocol version number should not be 85 updated either) or kernel_info to communicate supported Linux kernel 86 features to the boot loader. Due to very limited space available in 87 the original setup header every update to it should be considered 88 with great care. Starting from the protocol 2.15 the primary way to 89 communicate things to the boot loader is the kernel_info. 90 91 92Memory Layout 93============= 94 95The traditional memory map for the kernel loader, used for Image or 96zImage kernels, typically looks like:: 97 98 | | 99 0A0000 +------------------------+ 100 | Reserved for BIOS | Do not use. Reserved for BIOS EBDA. 101 09A000 +------------------------+ 102 | Command line | 103 | Stack/heap | For use by the kernel real-mode code. 104 098000 +------------------------+ 105 | Kernel setup | The kernel real-mode code. 106 090200 +------------------------+ 107 | Kernel boot sector | The kernel legacy boot sector. 108 090000 +------------------------+ 109 | Protected-mode kernel | The bulk of the kernel image. 110 010000 +------------------------+ 111 | Boot loader | <- Boot sector entry point 0000:7C00 112 001000 +------------------------+ 113 | Reserved for MBR/BIOS | 114 000800 +------------------------+ 115 | Typically used by MBR | 116 000600 +------------------------+ 117 | BIOS use only | 118 000000 +------------------------+ 119 120When using bzImage, the protected-mode kernel was relocated to 1210x100000 ("high memory"), and the kernel real-mode block (boot sector, 122setup, and stack/heap) was made relocatable to any address between 1230x10000 and end of low memory. Unfortunately, in protocols 2.00 and 1242.01 the 0x90000+ memory range is still used internally by the kernel; 125the 2.02 protocol resolves that problem. 126 127It is desirable to keep the "memory ceiling" -- the highest point in 128low memory touched by the boot loader -- as low as possible, since 129some newer BIOSes have begun to allocate some rather large amounts of 130memory, called the Extended BIOS Data Area, near the top of low 131memory. The boot loader should use the "INT 12h" BIOS call to verify 132how much low memory is available. 133 134Unfortunately, if INT 12h reports that the amount of memory is too 135low, there is usually nothing the boot loader can do but to report an 136error to the user. The boot loader should therefore be designed to 137take up as little space in low memory as it reasonably can. For 138zImage or old bzImage kernels, which need data written into the 1390x90000 segment, the boot loader should make sure not to use memory 140above the 0x9A000 point; too many BIOSes will break above that point. 141 142For a modern bzImage kernel with boot protocol version >= 2.02, a 143memory layout like the following is suggested:: 144 145 ~ ~ 146 | Protected-mode kernel | 147 100000 +------------------------+ 148 | I/O memory hole | 149 0A0000 +------------------------+ 150 | Reserved for BIOS | Leave as much as possible unused 151 ~ ~ 152 | Command line | (Can also be below the X+10000 mark) 153 X+10000 +------------------------+ 154 | Stack/heap | For use by the kernel real-mode code. 155 X+08000 +------------------------+ 156 | Kernel setup | The kernel real-mode code. 157 | Kernel boot sector | The kernel legacy boot sector. 158 X +------------------------+ 159 | Boot loader | <- Boot sector entry point 0000:7C00 160 001000 +------------------------+ 161 | Reserved for MBR/BIOS | 162 000800 +------------------------+ 163 | Typically used by MBR | 164 000600 +------------------------+ 165 | BIOS use only | 166 000000 +------------------------+ 167 168 ... where the address X is as low as the design of the boot loader permits. 169 170 171The Real-Mode Kernel Header 172=========================== 173 174In the following text, and anywhere in the kernel boot sequence, "a 175sector" refers to 512 bytes. It is independent of the actual sector 176size of the underlying medium. 177 178The first step in loading a Linux kernel should be to load the 179real-mode code (boot sector and setup code) and then examine the 180following header at offset 0x01f1. The real-mode code can total up to 18132K, although the boot loader may choose to load only the first two 182sectors (1K) and then examine the bootup sector size. 183 184The header looks like: 185 186=========== ======== ===================== ============================================ 187Offset/Size Proto Name Meaning 188=========== ======== ===================== ============================================ 18901F1/1 ALL(1) setup_sects The size of the setup in sectors 19001F2/2 ALL root_flags If set, the root is mounted readonly 19101F4/4 2.04+(2) syssize The size of the 32-bit code in 16-byte paras 19201F8/2 ALL ram_size DO NOT USE - for bootsect.S use only 19301FA/2 ALL vid_mode Video mode control 19401FC/2 ALL root_dev Default root device number 19501FE/2 ALL boot_flag 0xAA55 magic number 1960200/2 2.00+ jump Jump instruction 1970202/4 2.00+ header Magic signature "HdrS" 1980206/2 2.00+ version Boot protocol version supported 1990208/4 2.00+ realmode_swtch Boot loader hook (see below) 200020C/2 2.00+ start_sys_seg The load-low segment (0x1000) (obsolete) 201020E/2 2.00+ kernel_version Pointer to kernel version string 2020210/1 2.00+ type_of_loader Boot loader identifier 2030211/1 2.00+ loadflags Boot protocol option flags 2040212/2 2.00+ setup_move_size Move to high memory size (used with hooks) 2050214/4 2.00+ code32_start Boot loader hook (see below) 2060218/4 2.00+ ramdisk_image initrd load address (set by boot loader) 207021C/4 2.00+ ramdisk_size initrd size (set by boot loader) 2080220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only 2090224/2 2.01+ heap_end_ptr Free memory after setup end 2100226/1 2.02+(3) ext_loader_ver Extended boot loader version 2110227/1 2.02+(3) ext_loader_type Extended boot loader ID 2120228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line 213022C/4 2.03+ initrd_addr_max Highest legal initrd address 2140230/4 2.05+ kernel_alignment Physical addr alignment required for kernel 2150234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not 2160235/1 2.10+ min_alignment Minimum alignment, as a power of two 2170236/2 2.12+ xloadflags Boot protocol option flags 2180238/4 2.06+ cmdline_size Maximum size of the kernel command line 219023C/4 2.07+ hardware_subarch Hardware subarchitecture 2200240/8 2.07+ hardware_subarch_data Subarchitecture-specific data 2210248/4 2.08+ payload_offset Offset of kernel payload 222024C/4 2.08+ payload_length Length of kernel payload 2230250/8 2.09+ setup_data 64-bit physical pointer to linked list 224 of struct setup_data 2250258/8 2.10+ pref_address Preferred loading address 2260260/4 2.10+ init_size Linear memory required during initialization 2270264/4 2.11+ handover_offset Offset of handover entry point 2280268/4 2.15+ kernel_info_offset Offset of the kernel_info 229=========== ======== ===================== ============================================ 230 231.. note:: 232 (1) For backwards compatibility, if the setup_sects field contains 0, the 233 real value is 4. 234 235 (2) For boot protocol prior to 2.04, the upper two bytes of the syssize 236 field are unusable, which means the size of a bzImage kernel 237 cannot be determined. 238 239 (3) Ignored, but safe to set, for boot protocols 2.02-2.09. 240 241If the "HdrS" (0x53726448) magic number is not found at offset 0x202, 242the boot protocol version is "old". Loading an old kernel, the 243following parameters should be assumed:: 244 245 Image type = zImage 246 initrd not supported 247 Real-mode kernel must be located at 0x90000. 248 249Otherwise, the "version" field contains the protocol version, 250e.g. protocol version 2.01 will contain 0x0201 in this field. When 251setting fields in the header, you must make sure only to set fields 252supported by the protocol version in use. 253 254 255Details of Header Fields 256======================== 257 258For each field, some are information from the kernel to the bootloader 259("read"), some are expected to be filled out by the bootloader 260("write"), and some are expected to be read and modified by the 261bootloader ("modify"). 262 263All general purpose boot loaders should write the fields marked 264(obligatory). Boot loaders who want to load the kernel at a 265nonstandard address should fill in the fields marked (reloc); other 266boot loaders can ignore those fields. 267 268The byte order of all fields is littleendian (this is x86, after all.) 269 270============ =========== 271Field name: setup_sects 272Type: read 273Offset/size: 0x1f1/1 274Protocol: ALL 275============ =========== 276 277 The size of the setup code in 512-byte sectors. If this field is 278 0, the real value is 4. The real-mode code consists of the boot 279 sector (always one 512-byte sector) plus the setup code. 280 281============ ================= 282Field name: root_flags 283Type: modify (optional) 284Offset/size: 0x1f2/2 285Protocol: ALL 286============ ================= 287 288 If this field is nonzero, the root defaults to readonly. The use of 289 this field is deprecated; use the "ro" or "rw" options on the 290 command line instead. 291 292============ =============================================== 293Field name: syssize 294Type: read 295Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL) 296Protocol: 2.04+ 297============ =============================================== 298 299 The size of the protected-mode code in units of 16-byte paragraphs. 300 For protocol versions older than 2.04 this field is only two bytes 301 wide, and therefore cannot be trusted for the size of a kernel if 302 the LOAD_HIGH flag is set. 303 304============ =============== 305Field name: ram_size 306Type: kernel internal 307Offset/size: 0x1f8/2 308Protocol: ALL 309============ =============== 310 311 This field is obsolete. 312 313============ =================== 314Field name: vid_mode 315Type: modify (obligatory) 316Offset/size: 0x1fa/2 317============ =================== 318 319 Please see the section on SPECIAL COMMAND LINE OPTIONS. 320 321============ ================= 322Field name: root_dev 323Type: modify (optional) 324Offset/size: 0x1fc/2 325Protocol: ALL 326============ ================= 327 328 The default root device device number. The use of this field is 329 deprecated, use the "root=" option on the command line instead. 330 331============ ========= 332Field name: boot_flag 333Type: read 334Offset/size: 0x1fe/2 335Protocol: ALL 336============ ========= 337 338 Contains 0xAA55. This is the closest thing old Linux kernels have 339 to a magic number. 340 341============ ======= 342Field name: jump 343Type: read 344Offset/size: 0x200/2 345Protocol: 2.00+ 346============ ======= 347 348 Contains an x86 jump instruction, 0xEB followed by a signed offset 349 relative to byte 0x202. This can be used to determine the size of 350 the header. 351 352============ ======= 353Field name: header 354Type: read 355Offset/size: 0x202/4 356Protocol: 2.00+ 357============ ======= 358 359 Contains the magic number "HdrS" (0x53726448). 360 361============ ======= 362Field name: version 363Type: read 364Offset/size: 0x206/2 365Protocol: 2.00+ 366============ ======= 367 368 Contains the boot protocol version, in (major << 8)+minor format, 369 e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version 370 10.17. 371 372============ ================= 373Field name: realmode_swtch 374Type: modify (optional) 375Offset/size: 0x208/4 376Protocol: 2.00+ 377============ ================= 378 379 Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.) 380 381============ ============= 382Field name: start_sys_seg 383Type: read 384Offset/size: 0x20c/2 385Protocol: 2.00+ 386============ ============= 387 388 The load low segment (0x1000). Obsolete. 389 390============ ============== 391Field name: kernel_version 392Type: read 393Offset/size: 0x20e/2 394Protocol: 2.00+ 395============ ============== 396 397 If set to a nonzero value, contains a pointer to a NUL-terminated 398 human-readable kernel version number string, less 0x200. This can 399 be used to display the kernel version to the user. This value 400 should be less than (0x200*setup_sects). 401 402 For example, if this value is set to 0x1c00, the kernel version 403 number string can be found at offset 0x1e00 in the kernel file. 404 This is a valid value if and only if the "setup_sects" field 405 contains the value 15 or higher, as:: 406 407 0x1c00 < 15*0x200 (= 0x1e00) but 408 0x1c00 >= 14*0x200 (= 0x1c00) 409 410 0x1c00 >> 9 = 14, So the minimum value for setup_secs is 15. 411 412============ ================== 413Field name: type_of_loader 414Type: write (obligatory) 415Offset/size: 0x210/1 416Protocol: 2.00+ 417============ ================== 418 419 If your boot loader has an assigned id (see table below), enter 420 0xTV here, where T is an identifier for the boot loader and V is 421 a version number. Otherwise, enter 0xFF here. 422 423 For boot loader IDs above T = 0xD, write T = 0xE to this field and 424 write the extended ID minus 0x10 to the ext_loader_type field. 425 Similarly, the ext_loader_ver field can be used to provide more than 426 four bits for the bootloader version. 427 428 For example, for T = 0x15, V = 0x234, write:: 429 430 type_of_loader <- 0xE4 431 ext_loader_type <- 0x05 432 ext_loader_ver <- 0x23 433 434 Assigned boot loader ids (hexadecimal): 435 436 == ======================================= 437 0 LILO 438 (0x00 reserved for pre-2.00 bootloader) 439 1 Loadlin 440 2 bootsect-loader 441 (0x20, all other values reserved) 442 3 Syslinux 443 4 Etherboot/gPXE/iPXE 444 5 ELILO 445 7 GRUB 446 8 U-Boot 447 9 Xen 448 A Gujin 449 B Qemu 450 C Arcturus Networks uCbootloader 451 D kexec-tools 452 E Extended (see ext_loader_type) 453 F Special (0xFF = undefined) 454 10 Reserved 455 11 Minimal Linux Bootloader 456 <http://sebastian-plotz.blogspot.de> 457 12 OVMF UEFI virtualization stack 458 13 barebox 459 == ======================================= 460 461 Please contact <hpa@zytor.com> if you need a bootloader ID value assigned. 462 463============ =================== 464Field name: loadflags 465Type: modify (obligatory) 466Offset/size: 0x211/1 467Protocol: 2.00+ 468============ =================== 469 470 This field is a bitmask. 471 472 Bit 0 (read): LOADED_HIGH 473 474 - If 0, the protected-mode code is loaded at 0x10000. 475 - If 1, the protected-mode code is loaded at 0x100000. 476 477 Bit 1 (kernel internal): KASLR_FLAG 478 479 - Used internally by the compressed kernel to communicate 480 KASLR status to kernel proper. 481 482 - If 1, KASLR enabled. 483 - If 0, KASLR disabled. 484 485 Bit 5 (write): QUIET_FLAG 486 487 - If 0, print early messages. 488 - If 1, suppress early messages. 489 490 This requests to the kernel (decompressor and early 491 kernel) to not write early messages that require 492 accessing the display hardware directly. 493 494 Bit 6 (obsolete): KEEP_SEGMENTS 495 496 Protocol: 2.07+ 497 498 - This flag is obsolete. 499 500 Bit 7 (write): CAN_USE_HEAP 501 502 Set this bit to 1 to indicate that the value entered in the 503 heap_end_ptr is valid. If this field is clear, some setup code 504 functionality will be disabled. 505 506 507============ =================== 508Field name: setup_move_size 509Type: modify (obligatory) 510Offset/size: 0x212/2 511Protocol: 2.00-2.01 512============ =================== 513 514 When using protocol 2.00 or 2.01, if the real mode kernel is not 515 loaded at 0x90000, it gets moved there later in the loading 516 sequence. Fill in this field if you want additional data (such as 517 the kernel command line) moved in addition to the real-mode kernel 518 itself. 519 520 The unit is bytes starting with the beginning of the boot sector. 521 522 This field is can be ignored when the protocol is 2.02 or higher, or 523 if the real-mode code is loaded at 0x90000. 524 525============ ======================== 526Field name: code32_start 527Type: modify (optional, reloc) 528Offset/size: 0x214/4 529Protocol: 2.00+ 530============ ======================== 531 532 The address to jump to in protected mode. This defaults to the load 533 address of the kernel, and can be used by the boot loader to 534 determine the proper load address. 535 536 This field can be modified for two purposes: 537 538 1. as a boot loader hook (see Advanced Boot Loader Hooks below.) 539 540 2. if a bootloader which does not install a hook loads a 541 relocatable kernel at a nonstandard address it will have to modify 542 this field to point to the load address. 543 544============ ================== 545Field name: ramdisk_image 546Type: write (obligatory) 547Offset/size: 0x218/4 548Protocol: 2.00+ 549============ ================== 550 551 The 32-bit linear address of the initial ramdisk or ramfs. Leave at 552 zero if there is no initial ramdisk/ramfs. 553 554============ ================== 555Field name: ramdisk_size 556Type: write (obligatory) 557Offset/size: 0x21c/4 558Protocol: 2.00+ 559============ ================== 560 561 Size of the initial ramdisk or ramfs. Leave at zero if there is no 562 initial ramdisk/ramfs. 563 564============ =============== 565Field name: bootsect_kludge 566Type: kernel internal 567Offset/size: 0x220/4 568Protocol: 2.00+ 569============ =============== 570 571 This field is obsolete. 572 573============ ================== 574Field name: heap_end_ptr 575Type: write (obligatory) 576Offset/size: 0x224/2 577Protocol: 2.01+ 578============ ================== 579 580 Set this field to the offset (from the beginning of the real-mode 581 code) of the end of the setup stack/heap, minus 0x0200. 582 583============ ================ 584Field name: ext_loader_ver 585Type: write (optional) 586Offset/size: 0x226/1 587Protocol: 2.02+ 588============ ================ 589 590 This field is used as an extension of the version number in the 591 type_of_loader field. The total version number is considered to be 592 (type_of_loader & 0x0f) + (ext_loader_ver << 4). 593 594 The use of this field is boot loader specific. If not written, it 595 is zero. 596 597 Kernels prior to 2.6.31 did not recognize this field, but it is safe 598 to write for protocol version 2.02 or higher. 599 600============ ===================================================== 601Field name: ext_loader_type 602Type: write (obligatory if (type_of_loader & 0xf0) == 0xe0) 603Offset/size: 0x227/1 604Protocol: 2.02+ 605============ ===================================================== 606 607 This field is used as an extension of the type number in 608 type_of_loader field. If the type in type_of_loader is 0xE, then 609 the actual type is (ext_loader_type + 0x10). 610 611 This field is ignored if the type in type_of_loader is not 0xE. 612 613 Kernels prior to 2.6.31 did not recognize this field, but it is safe 614 to write for protocol version 2.02 or higher. 615 616============ ================== 617Field name: cmd_line_ptr 618Type: write (obligatory) 619Offset/size: 0x228/4 620Protocol: 2.02+ 621============ ================== 622 623 Set this field to the linear address of the kernel command line. 624 The kernel command line can be located anywhere between the end of 625 the setup heap and 0xA0000; it does not have to be located in the 626 same 64K segment as the real-mode code itself. 627 628 Fill in this field even if your boot loader does not support a 629 command line, in which case you can point this to an empty string 630 (or better yet, to the string "auto".) If this field is left at 631 zero, the kernel will assume that your boot loader does not support 632 the 2.02+ protocol. 633 634============ =============== 635Field name: initrd_addr_max 636Type: read 637Offset/size: 0x22c/4 638Protocol: 2.03+ 639============ =============== 640 641 The maximum address that may be occupied by the initial 642 ramdisk/ramfs contents. For boot protocols 2.02 or earlier, this 643 field is not present, and the maximum address is 0x37FFFFFF. (This 644 address is defined as the address of the highest safe byte, so if 645 your ramdisk is exactly 131072 bytes long and this field is 646 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.) 647 648============ ============================ 649Field name: kernel_alignment 650Type: read/modify (reloc) 651Offset/size: 0x230/4 652Protocol: 2.05+ (read), 2.10+ (modify) 653============ ============================ 654 655 Alignment unit required by the kernel (if relocatable_kernel is 656 true.) A relocatable kernel that is loaded at an alignment 657 incompatible with the value in this field will be realigned during 658 kernel initialization. 659 660 Starting with protocol version 2.10, this reflects the kernel 661 alignment preferred for optimal performance; it is possible for the 662 loader to modify this field to permit a lesser alignment. See the 663 min_alignment and pref_address field below. 664 665============ ================== 666Field name: relocatable_kernel 667Type: read (reloc) 668Offset/size: 0x234/1 669Protocol: 2.05+ 670============ ================== 671 672 If this field is nonzero, the protected-mode part of the kernel can 673 be loaded at any address that satisfies the kernel_alignment field. 674 After loading, the boot loader must set the code32_start field to 675 point to the loaded code, or to a boot loader hook. 676 677============ ============= 678Field name: min_alignment 679Type: read (reloc) 680Offset/size: 0x235/1 681Protocol: 2.10+ 682============ ============= 683 684 This field, if nonzero, indicates as a power of two the minimum 685 alignment required, as opposed to preferred, by the kernel to boot. 686 If a boot loader makes use of this field, it should update the 687 kernel_alignment field with the alignment unit desired; typically:: 688 689 kernel_alignment = 1 << min_alignment 690 691 There may be a considerable performance cost with an excessively 692 misaligned kernel. Therefore, a loader should typically try each 693 power-of-two alignment from kernel_alignment down to this alignment. 694 695============ ========== 696Field name: xloadflags 697Type: read 698Offset/size: 0x236/2 699Protocol: 2.12+ 700============ ========== 701 702 This field is a bitmask. 703 704 Bit 0 (read): XLF_KERNEL_64 705 706 - If 1, this kernel has the legacy 64-bit entry point at 0x200. 707 708 Bit 1 (read): XLF_CAN_BE_LOADED_ABOVE_4G 709 710 - If 1, kernel/boot_params/cmdline/ramdisk can be above 4G. 711 712 Bit 2 (read): XLF_EFI_HANDOVER_32 713 714 - If 1, the kernel supports the 32-bit EFI handoff entry point 715 given at handover_offset. 716 717 Bit 3 (read): XLF_EFI_HANDOVER_64 718 719 - If 1, the kernel supports the 64-bit EFI handoff entry point 720 given at handover_offset + 0x200. 721 722 Bit 4 (read): XLF_EFI_KEXEC 723 724 - If 1, the kernel supports kexec EFI boot with EFI runtime support. 725 726 727============ ============ 728Field name: cmdline_size 729Type: read 730Offset/size: 0x238/4 731Protocol: 2.06+ 732============ ============ 733 734 The maximum size of the command line without the terminating 735 zero. This means that the command line can contain at most 736 cmdline_size characters. With protocol version 2.05 and earlier, the 737 maximum size was 255. 738 739============ ==================================== 740Field name: hardware_subarch 741Type: write (optional, defaults to x86/PC) 742Offset/size: 0x23c/4 743Protocol: 2.07+ 744============ ==================================== 745 746 In a paravirtualized environment the hardware low level architectural 747 pieces such as interrupt handling, page table handling, and 748 accessing process control registers needs to be done differently. 749 750 This field allows the bootloader to inform the kernel we are in one 751 one of those environments. 752 753 ========== ============================== 754 0x00000000 The default x86/PC environment 755 0x00000001 lguest 756 0x00000002 Xen 757 0x00000003 Moorestown MID 758 0x00000004 CE4100 TV Platform 759 ========== ============================== 760 761============ ========================= 762Field name: hardware_subarch_data 763Type: write (subarch-dependent) 764Offset/size: 0x240/8 765Protocol: 2.07+ 766============ ========================= 767 768 A pointer to data that is specific to hardware subarch 769 This field is currently unused for the default x86/PC environment, 770 do not modify. 771 772============ ============== 773Field name: payload_offset 774Type: read 775Offset/size: 0x248/4 776Protocol: 2.08+ 777============ ============== 778 779 If non-zero then this field contains the offset from the beginning 780 of the protected-mode code to the payload. 781 782 The payload may be compressed. The format of both the compressed and 783 uncompressed data should be determined using the standard magic 784 numbers. The currently supported compression formats are gzip 785 (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA 786 (magic number 5D 00), XZ (magic number FD 37), LZ4 (magic number 787 02 21) and ZSTD (magic number 28 B5). The uncompressed payload is 788 currently always ELF (magic number 7F 45 4C 46). 789 790============ ============== 791Field name: payload_length 792Type: read 793Offset/size: 0x24c/4 794Protocol: 2.08+ 795============ ============== 796 797 The length of the payload. 798 799============ =============== 800Field name: setup_data 801Type: write (special) 802Offset/size: 0x250/8 803Protocol: 2.09+ 804============ =============== 805 806 The 64-bit physical pointer to NULL terminated single linked list of 807 struct setup_data. This is used to define a more extensible boot 808 parameters passing mechanism. The definition of struct setup_data is 809 as follow:: 810 811 struct setup_data { 812 u64 next; 813 u32 type; 814 u32 len; 815 u8 data[0]; 816 }; 817 818 Where, the next is a 64-bit physical pointer to the next node of 819 linked list, the next field of the last node is 0; the type is used 820 to identify the contents of data; the len is the length of data 821 field; the data holds the real payload. 822 823 This list may be modified at a number of points during the bootup 824 process. Therefore, when modifying this list one should always make 825 sure to consider the case where the linked list already contains 826 entries. 827 828 The setup_data is a bit awkward to use for extremely large data objects, 829 both because the setup_data header has to be adjacent to the data object 830 and because it has a 32-bit length field. However, it is important that 831 intermediate stages of the boot process have a way to identify which 832 chunks of memory are occupied by kernel data. 833 834 Thus setup_indirect struct and SETUP_INDIRECT type were introduced in 835 protocol 2.15:: 836 837 struct setup_indirect { 838 __u32 type; 839 __u32 reserved; /* Reserved, must be set to zero. */ 840 __u64 len; 841 __u64 addr; 842 }; 843 844 The type member is a SETUP_INDIRECT | SETUP_* type. However, it cannot be 845 SETUP_INDIRECT itself since making the setup_indirect a tree structure 846 could require a lot of stack space in something that needs to parse it 847 and stack space can be limited in boot contexts. 848 849 Let's give an example how to point to SETUP_E820_EXT data using setup_indirect. 850 In this case setup_data and setup_indirect will look like this:: 851 852 struct setup_data { 853 __u64 next = 0 or <addr_of_next_setup_data_struct>; 854 __u32 type = SETUP_INDIRECT; 855 __u32 len = sizeof(setup_indirect); 856 __u8 data[sizeof(setup_indirect)] = struct setup_indirect { 857 __u32 type = SETUP_INDIRECT | SETUP_E820_EXT; 858 __u32 reserved = 0; 859 __u64 len = <len_of_SETUP_E820_EXT_data>; 860 __u64 addr = <addr_of_SETUP_E820_EXT_data>; 861 } 862 } 863 864.. note:: 865 SETUP_INDIRECT | SETUP_NONE objects cannot be properly distinguished 866 from SETUP_INDIRECT itself. So, this kind of objects cannot be provided 867 by the bootloaders. 868 869============ ============ 870Field name: pref_address 871Type: read (reloc) 872Offset/size: 0x258/8 873Protocol: 2.10+ 874============ ============ 875 876 This field, if nonzero, represents a preferred load address for the 877 kernel. A relocating bootloader should attempt to load at this 878 address if possible. 879 880 A non-relocatable kernel will unconditionally move itself and to run 881 at this address. 882 883============ ======= 884Field name: init_size 885Type: read 886Offset/size: 0x260/4 887============ ======= 888 889 This field indicates the amount of linear contiguous memory starting 890 at the kernel runtime start address that the kernel needs before it 891 is capable of examining its memory map. This is not the same thing 892 as the total amount of memory the kernel needs to boot, but it can 893 be used by a relocating boot loader to help select a safe load 894 address for the kernel. 895 896 The kernel runtime start address is determined by the following algorithm:: 897 898 if (relocatable_kernel) 899 runtime_start = align_up(load_address, kernel_alignment) 900 else 901 runtime_start = pref_address 902 903============ =============== 904Field name: handover_offset 905Type: read 906Offset/size: 0x264/4 907============ =============== 908 909 This field is the offset from the beginning of the kernel image to 910 the EFI handover protocol entry point. Boot loaders using the EFI 911 handover protocol to boot the kernel should jump to this offset. 912 913 See EFI HANDOVER PROTOCOL below for more details. 914 915============ ================== 916Field name: kernel_info_offset 917Type: read 918Offset/size: 0x268/4 919Protocol: 2.15+ 920============ ================== 921 922 This field is the offset from the beginning of the kernel image to the 923 kernel_info. The kernel_info structure is embedded in the Linux image 924 in the uncompressed protected mode region. 925 926 927The kernel_info 928=============== 929 930The relationships between the headers are analogous to the various data 931sections: 932 933 setup_header = .data 934 boot_params/setup_data = .bss 935 936What is missing from the above list? That's right: 937 938 kernel_info = .rodata 939 940We have been (ab)using .data for things that could go into .rodata or .bss for 941a long time, for lack of alternatives and -- especially early on -- inertia. 942Also, the BIOS stub is responsible for creating boot_params, so it isn't 943available to a BIOS-based loader (setup_data is, though). 944 945setup_header is permanently limited to 144 bytes due to the reach of the 9462-byte jump field, which doubles as a length field for the structure, combined 947with the size of the "hole" in struct boot_params that a protected-mode loader 948or the BIOS stub has to copy it into. It is currently 119 bytes long, which 949leaves us with 25 very precious bytes. This isn't something that can be fixed 950without revising the boot protocol entirely, breaking backwards compatibility. 951 952boot_params proper is limited to 4096 bytes, but can be arbitrarily extended 953by adding setup_data entries. It cannot be used to communicate properties of 954the kernel image, because it is .bss and has no image-provided content. 955 956kernel_info solves this by providing an extensible place for information about 957the kernel image. It is readonly, because the kernel cannot rely on a 958bootloader copying its contents anywhere, but that is OK; if it becomes 959necessary it can still contain data items that an enabled bootloader would be 960expected to copy into a setup_data chunk. 961 962All kernel_info data should be part of this structure. Fixed size data have to 963be put before kernel_info_var_len_data label. Variable size data have to be put 964after kernel_info_var_len_data label. Each chunk of variable size data has to 965be prefixed with header/magic and its size, e.g.:: 966 967 kernel_info: 968 .ascii "LToP" /* Header, Linux top (structure). */ 969 .long kernel_info_var_len_data - kernel_info 970 .long kernel_info_end - kernel_info 971 .long 0x01234567 /* Some fixed size data for the bootloaders. */ 972 kernel_info_var_len_data: 973 example_struct: /* Some variable size data for the bootloaders. */ 974 .ascii "0123" /* Header/Magic. */ 975 .long example_struct_end - example_struct 976 .ascii "Struct" 977 .long 0x89012345 978 example_struct_end: 979 example_strings: /* Some variable size data for the bootloaders. */ 980 .ascii "ABCD" /* Header/Magic. */ 981 .long example_strings_end - example_strings 982 .asciz "String_0" 983 .asciz "String_1" 984 example_strings_end: 985 kernel_info_end: 986 987This way the kernel_info is self-contained blob. 988 989.. note:: 990 Each variable size data header/magic can be any 4-character string, 991 without \0 at the end of the string, which does not collide with 992 existing variable length data headers/magics. 993 994 995Details of the kernel_info Fields 996================================= 997 998============ ======== 999Field name: header 1000Offset/size: 0x0000/4 1001============ ======== 1002 1003 Contains the magic number "LToP" (0x506f544c). 1004 1005============ ======== 1006Field name: size 1007Offset/size: 0x0004/4 1008============ ======== 1009 1010 This field contains the size of the kernel_info including kernel_info.header. 1011 It does not count kernel_info.kernel_info_var_len_data size. This field should be 1012 used by the bootloaders to detect supported fixed size fields in the kernel_info 1013 and beginning of kernel_info.kernel_info_var_len_data. 1014 1015============ ======== 1016Field name: size_total 1017Offset/size: 0x0008/4 1018============ ======== 1019 1020 This field contains the size of the kernel_info including kernel_info.header 1021 and kernel_info.kernel_info_var_len_data. 1022 1023============ ============== 1024Field name: setup_type_max 1025Offset/size: 0x000c/4 1026============ ============== 1027 1028 This field contains maximal allowed type for setup_data and setup_indirect structs. 1029 1030 1031The Image Checksum 1032================== 1033 1034From boot protocol version 2.08 onwards the CRC-32 is calculated over 1035the entire file using the characteristic polynomial 0x04C11DB7 and an 1036initial remainder of 0xffffffff. The checksum is appended to the 1037file; therefore the CRC of the file up to the limit specified in the 1038syssize field of the header is always 0. 1039 1040 1041The Kernel Command Line 1042======================= 1043 1044The kernel command line has become an important way for the boot 1045loader to communicate with the kernel. Some of its options are also 1046relevant to the boot loader itself, see "special command line options" 1047below. 1048 1049The kernel command line is a null-terminated string. The maximum 1050length can be retrieved from the field cmdline_size. Before protocol 1051version 2.06, the maximum was 255 characters. A string that is too 1052long will be automatically truncated by the kernel. 1053 1054If the boot protocol version is 2.02 or later, the address of the 1055kernel command line is given by the header field cmd_line_ptr (see 1056above.) This address can be anywhere between the end of the setup 1057heap and 0xA0000. 1058 1059If the protocol version is *not* 2.02 or higher, the kernel 1060command line is entered using the following protocol: 1061 1062 - At offset 0x0020 (word), "cmd_line_magic", enter the magic 1063 number 0xA33F. 1064 1065 - At offset 0x0022 (word), "cmd_line_offset", enter the offset 1066 of the kernel command line (relative to the start of the 1067 real-mode kernel). 1068 1069 - The kernel command line *must* be within the memory region 1070 covered by setup_move_size, so you may need to adjust this 1071 field. 1072 1073 1074Memory Layout of The Real-Mode Code 1075=================================== 1076 1077The real-mode code requires a stack/heap to be set up, as well as 1078memory allocated for the kernel command line. This needs to be done 1079in the real-mode accessible memory in bottom megabyte. 1080 1081It should be noted that modern machines often have a sizable Extended 1082BIOS Data Area (EBDA). As a result, it is advisable to use as little 1083of the low megabyte as possible. 1084 1085Unfortunately, under the following circumstances the 0x90000 memory 1086segment has to be used: 1087 1088 - When loading a zImage kernel ((loadflags & 0x01) == 0). 1089 - When loading a 2.01 or earlier boot protocol kernel. 1090 1091.. note:: 1092 For the 2.00 and 2.01 boot protocols, the real-mode code 1093 can be loaded at another address, but it is internally 1094 relocated to 0x90000. For the "old" protocol, the 1095 real-mode code must be loaded at 0x90000. 1096 1097When loading at 0x90000, avoid using memory above 0x9a000. 1098 1099For boot protocol 2.02 or higher, the command line does not have to be 1100located in the same 64K segment as the real-mode setup code; it is 1101thus permitted to give the stack/heap the full 64K segment and locate 1102the command line above it. 1103 1104The kernel command line should not be located below the real-mode 1105code, nor should it be located in high memory. 1106 1107 1108Sample Boot Configuartion 1109========================= 1110 1111As a sample configuration, assume the following layout of the real 1112mode segment. 1113 1114 When loading below 0x90000, use the entire segment: 1115 1116 ============= =================== 1117 0x0000-0x7fff Real mode kernel 1118 0x8000-0xdfff Stack and heap 1119 0xe000-0xffff Kernel command line 1120 ============= =================== 1121 1122 When loading at 0x90000 OR the protocol version is 2.01 or earlier: 1123 1124 ============= =================== 1125 0x0000-0x7fff Real mode kernel 1126 0x8000-0x97ff Stack and heap 1127 0x9800-0x9fff Kernel command line 1128 ============= =================== 1129 1130Such a boot loader should enter the following fields in the header:: 1131 1132 unsigned long base_ptr; /* base address for real-mode segment */ 1133 1134 if ( setup_sects == 0 ) { 1135 setup_sects = 4; 1136 } 1137 1138 if ( protocol >= 0x0200 ) { 1139 type_of_loader = <type code>; 1140 if ( loading_initrd ) { 1141 ramdisk_image = <initrd_address>; 1142 ramdisk_size = <initrd_size>; 1143 } 1144 1145 if ( protocol >= 0x0202 && loadflags & 0x01 ) 1146 heap_end = 0xe000; 1147 else 1148 heap_end = 0x9800; 1149 1150 if ( protocol >= 0x0201 ) { 1151 heap_end_ptr = heap_end - 0x200; 1152 loadflags |= 0x80; /* CAN_USE_HEAP */ 1153 } 1154 1155 if ( protocol >= 0x0202 ) { 1156 cmd_line_ptr = base_ptr + heap_end; 1157 strcpy(cmd_line_ptr, cmdline); 1158 } else { 1159 cmd_line_magic = 0xA33F; 1160 cmd_line_offset = heap_end; 1161 setup_move_size = heap_end + strlen(cmdline)+1; 1162 strcpy(base_ptr+cmd_line_offset, cmdline); 1163 } 1164 } else { 1165 /* Very old kernel */ 1166 1167 heap_end = 0x9800; 1168 1169 cmd_line_magic = 0xA33F; 1170 cmd_line_offset = heap_end; 1171 1172 /* A very old kernel MUST have its real-mode code 1173 loaded at 0x90000 */ 1174 1175 if ( base_ptr != 0x90000 ) { 1176 /* Copy the real-mode kernel */ 1177 memcpy(0x90000, base_ptr, (setup_sects+1)*512); 1178 base_ptr = 0x90000; /* Relocated */ 1179 } 1180 1181 strcpy(0x90000+cmd_line_offset, cmdline); 1182 1183 /* It is recommended to clear memory up to the 32K mark */ 1184 memset(0x90000 + (setup_sects+1)*512, 0, 1185 (64-(setup_sects+1))*512); 1186 } 1187 1188 1189Loading The Rest of The Kernel 1190============================== 1191 1192The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512 1193in the kernel file (again, if setup_sects == 0 the real value is 4.) 1194It should be loaded at address 0x10000 for Image/zImage kernels and 11950x100000 for bzImage kernels. 1196 1197The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01 1198bit (LOAD_HIGH) in the loadflags field is set:: 1199 1200 is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01); 1201 load_address = is_bzImage ? 0x100000 : 0x10000; 1202 1203Note that Image/zImage kernels can be up to 512K in size, and thus use 1204the entire 0x10000-0x90000 range of memory. This means it is pretty 1205much a requirement for these kernels to load the real-mode part at 12060x90000. bzImage kernels allow much more flexibility. 1207 1208Special Command Line Options 1209============================ 1210 1211If the command line provided by the boot loader is entered by the 1212user, the user may expect the following command line options to work. 1213They should normally not be deleted from the kernel command line even 1214though not all of them are actually meaningful to the kernel. Boot 1215loader authors who need additional command line options for the boot 1216loader itself should get them registered in 1217Documentation/admin-guide/kernel-parameters.rst to make sure they will not 1218conflict with actual kernel options now or in the future. 1219 1220 vga=<mode> 1221 <mode> here is either an integer (in C notation, either 1222 decimal, octal, or hexadecimal) or one of the strings 1223 "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask" 1224 (meaning 0xFFFD). This value should be entered into the 1225 vid_mode field, as it is used by the kernel before the command 1226 line is parsed. 1227 1228 mem=<size> 1229 <size> is an integer in C notation optionally followed by 1230 (case insensitive) K, M, G, T, P or E (meaning << 10, << 20, 1231 << 30, << 40, << 50 or << 60). This specifies the end of 1232 memory to the kernel. This affects the possible placement of 1233 an initrd, since an initrd should be placed near end of 1234 memory. Note that this is an option to *both* the kernel and 1235 the bootloader! 1236 1237 initrd=<file> 1238 An initrd should be loaded. The meaning of <file> is 1239 obviously bootloader-dependent, and some boot loaders 1240 (e.g. LILO) do not have such a command. 1241 1242In addition, some boot loaders add the following options to the 1243user-specified command line: 1244 1245 BOOT_IMAGE=<file> 1246 The boot image which was loaded. Again, the meaning of <file> 1247 is obviously bootloader-dependent. 1248 1249 auto 1250 The kernel was booted without explicit user intervention. 1251 1252If these options are added by the boot loader, it is highly 1253recommended that they are located *first*, before the user-specified 1254or configuration-specified command line. Otherwise, "init=/bin/sh" 1255gets confused by the "auto" option. 1256 1257 1258Running the Kernel 1259================== 1260 1261The kernel is started by jumping to the kernel entry point, which is 1262located at *segment* offset 0x20 from the start of the real mode 1263kernel. This means that if you loaded your real-mode kernel code at 12640x90000, the kernel entry point is 9020:0000. 1265 1266At entry, ds = es = ss should point to the start of the real-mode 1267kernel code (0x9000 if the code is loaded at 0x90000), sp should be 1268set up properly, normally pointing to the top of the heap, and 1269interrupts should be disabled. Furthermore, to guard against bugs in 1270the kernel, it is recommended that the boot loader sets fs = gs = ds = 1271es = ss. 1272 1273In our example from above, we would do:: 1274 1275 /* Note: in the case of the "old" kernel protocol, base_ptr must 1276 be == 0x90000 at this point; see the previous sample code */ 1277 1278 seg = base_ptr >> 4; 1279 1280 cli(); /* Enter with interrupts disabled! */ 1281 1282 /* Set up the real-mode kernel stack */ 1283 _SS = seg; 1284 _SP = heap_end; 1285 1286 _DS = _ES = _FS = _GS = seg; 1287 jmp_far(seg+0x20, 0); /* Run the kernel */ 1288 1289If your boot sector accesses a floppy drive, it is recommended to 1290switch off the floppy motor before running the kernel, since the 1291kernel boot leaves interrupts off and thus the motor will not be 1292switched off, especially if the loaded kernel has the floppy driver as 1293a demand-loaded module! 1294 1295 1296Advanced Boot Loader Hooks 1297========================== 1298 1299If the boot loader runs in a particularly hostile environment (such as 1300LOADLIN, which runs under DOS) it may be impossible to follow the 1301standard memory location requirements. Such a boot loader may use the 1302following hooks that, if set, are invoked by the kernel at the 1303appropriate time. The use of these hooks should probably be 1304considered an absolutely last resort! 1305 1306IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and 1307%edi across invocation. 1308 1309 realmode_swtch: 1310 A 16-bit real mode far subroutine invoked immediately before 1311 entering protected mode. The default routine disables NMI, so 1312 your routine should probably do so, too. 1313 1314 code32_start: 1315 A 32-bit flat-mode routine *jumped* to immediately after the 1316 transition to protected mode, but before the kernel is 1317 uncompressed. No segments, except CS, are guaranteed to be 1318 set up (current kernels do, but older ones do not); you should 1319 set them up to BOOT_DS (0x18) yourself. 1320 1321 After completing your hook, you should jump to the address 1322 that was in this field before your boot loader overwrote it 1323 (relocated, if appropriate.) 1324 1325 132632-bit Boot Protocol 1327==================== 1328 1329For machine with some new BIOS other than legacy BIOS, such as EFI, 1330LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel 1331based on legacy BIOS can not be used, so a 32-bit boot protocol needs 1332to be defined. 1333 1334In 32-bit boot protocol, the first step in loading a Linux kernel 1335should be to setup the boot parameters (struct boot_params, 1336traditionally known as "zero page"). The memory for struct boot_params 1337should be allocated and initialized to all zero. Then the setup header 1338from offset 0x01f1 of kernel image on should be loaded into struct 1339boot_params and examined. The end of setup header can be calculated as 1340follow:: 1341 1342 0x0202 + byte value at offset 0x0201 1343 1344In addition to read/modify/write the setup header of the struct 1345boot_params as that of 16-bit boot protocol, the boot loader should 1346also fill the additional fields of the struct boot_params as 1347described in chapter Documentation/arch/x86/zero-page.rst. 1348 1349After setting up the struct boot_params, the boot loader can load the 135032/64-bit kernel in the same way as that of 16-bit boot protocol. 1351 1352In 32-bit boot protocol, the kernel is started by jumping to the 135332-bit kernel entry point, which is the start address of loaded 135432/64-bit kernel. 1355 1356At entry, the CPU must be in 32-bit protected mode with paging 1357disabled; a GDT must be loaded with the descriptors for selectors 1358__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat 1359segment; __BOOT_CS must have execute/read permission, and __BOOT_DS 1360must have read/write permission; CS must be __BOOT_CS and DS, ES, SS 1361must be __BOOT_DS; interrupt must be disabled; %esi must hold the base 1362address of the struct boot_params; %ebp, %edi and %ebx must be zero. 1363 136464-bit Boot Protocol 1365==================== 1366 1367For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader 1368and we need a 64-bit boot protocol. 1369 1370In 64-bit boot protocol, the first step in loading a Linux kernel 1371should be to setup the boot parameters (struct boot_params, 1372traditionally known as "zero page"). The memory for struct boot_params 1373could be allocated anywhere (even above 4G) and initialized to all zero. 1374Then, the setup header at offset 0x01f1 of kernel image on should be 1375loaded into struct boot_params and examined. The end of setup header 1376can be calculated as follows:: 1377 1378 0x0202 + byte value at offset 0x0201 1379 1380In addition to read/modify/write the setup header of the struct 1381boot_params as that of 16-bit boot protocol, the boot loader should 1382also fill the additional fields of the struct boot_params as described 1383in chapter Documentation/arch/x86/zero-page.rst. 1384 1385After setting up the struct boot_params, the boot loader can load 138664-bit kernel in the same way as that of 16-bit boot protocol, but 1387kernel could be loaded above 4G. 1388 1389In 64-bit boot protocol, the kernel is started by jumping to the 139064-bit kernel entry point, which is the start address of loaded 139164-bit kernel plus 0x200. 1392 1393At entry, the CPU must be in 64-bit mode with paging enabled. 1394The range with setup_header.init_size from start address of loaded 1395kernel and zero page and command line buffer get ident mapping; 1396a GDT must be loaded with the descriptors for selectors 1397__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat 1398segment; __BOOT_CS must have execute/read permission, and __BOOT_DS 1399must have read/write permission; CS must be __BOOT_CS and DS, ES, SS 1400must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base 1401address of the struct boot_params. 1402 1403EFI Handover Protocol (deprecated) 1404================================== 1405 1406This protocol allows boot loaders to defer initialisation to the EFI 1407boot stub. The boot loader is required to load the kernel/initrd(s) 1408from the boot media and jump to the EFI handover protocol entry point 1409which is hdr->handover_offset bytes from the beginning of 1410startup_{32,64}. 1411 1412The boot loader MUST respect the kernel's PE/COFF metadata when it comes 1413to section alignment, the memory footprint of the executable image beyond 1414the size of the file itself, and any other aspect of the PE/COFF header 1415that may affect correct operation of the image as a PE/COFF binary in the 1416execution context provided by the EFI firmware. 1417 1418The function prototype for the handover entry point looks like this:: 1419 1420 efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp) 1421 1422'handle' is the EFI image handle passed to the boot loader by the EFI 1423firmware, 'table' is the EFI system table - these are the first two 1424arguments of the "handoff state" as described in section 2.3 of the 1425UEFI specification. 'bp' is the boot loader-allocated boot params. 1426 1427The boot loader *must* fill out the following fields in bp:: 1428 1429 - hdr.cmd_line_ptr 1430 - hdr.ramdisk_image (if applicable) 1431 - hdr.ramdisk_size (if applicable) 1432 1433All other fields should be zero. 1434 1435NOTE: The EFI Handover Protocol is deprecated in favour of the ordinary PE/COFF 1436 entry point, combined with the LINUX_EFI_INITRD_MEDIA_GUID based initrd 1437 loading protocol (refer to [0] for an example of the bootloader side of 1438 this), which removes the need for any knowledge on the part of the EFI 1439 bootloader regarding the internal representation of boot_params or any 1440 requirements/limitations regarding the placement of the command line 1441 and ramdisk in memory, or the placement of the kernel image itself. 1442 1443[0] https://github.com/u-boot/u-boot/commit/ec80b4735a593961fe701cc3a5d717d4739b0fd0 1444