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. A relocatable kernel will move itself to this address if it 882 loaded below this address. 883 884============ ======= 885Field name: init_size 886Type: read 887Offset/size: 0x260/4 888============ ======= 889 890 This field indicates the amount of linear contiguous memory starting 891 at the kernel runtime start address that the kernel needs before it 892 is capable of examining its memory map. This is not the same thing 893 as the total amount of memory the kernel needs to boot, but it can 894 be used by a relocating boot loader to help select a safe load 895 address for the kernel. 896 897 The kernel runtime start address is determined by the following algorithm:: 898 899 if (relocatable_kernel) { 900 if (load_address < pref_address) 901 load_address = pref_address; 902 runtime_start = align_up(load_address, kernel_alignment); 903 } else { 904 runtime_start = pref_address; 905 } 906 907Hence the necessary memory window location and size can be estimated by 908a boot loader as:: 909 910 memory_window_start = runtime_start; 911 memory_window_size = init_size; 912 913============ =============== 914Field name: handover_offset 915Type: read 916Offset/size: 0x264/4 917============ =============== 918 919 This field is the offset from the beginning of the kernel image to 920 the EFI handover protocol entry point. Boot loaders using the EFI 921 handover protocol to boot the kernel should jump to this offset. 922 923 See EFI HANDOVER PROTOCOL below for more details. 924 925============ ================== 926Field name: kernel_info_offset 927Type: read 928Offset/size: 0x268/4 929Protocol: 2.15+ 930============ ================== 931 932 This field is the offset from the beginning of the kernel image to the 933 kernel_info. The kernel_info structure is embedded in the Linux image 934 in the uncompressed protected mode region. 935 936 937The kernel_info 938=============== 939 940The relationships between the headers are analogous to the various data 941sections: 942 943 setup_header = .data 944 boot_params/setup_data = .bss 945 946What is missing from the above list? That's right: 947 948 kernel_info = .rodata 949 950We have been (ab)using .data for things that could go into .rodata or .bss for 951a long time, for lack of alternatives and -- especially early on -- inertia. 952Also, the BIOS stub is responsible for creating boot_params, so it isn't 953available to a BIOS-based loader (setup_data is, though). 954 955setup_header is permanently limited to 144 bytes due to the reach of the 9562-byte jump field, which doubles as a length field for the structure, combined 957with the size of the "hole" in struct boot_params that a protected-mode loader 958or the BIOS stub has to copy it into. It is currently 119 bytes long, which 959leaves us with 25 very precious bytes. This isn't something that can be fixed 960without revising the boot protocol entirely, breaking backwards compatibility. 961 962boot_params proper is limited to 4096 bytes, but can be arbitrarily extended 963by adding setup_data entries. It cannot be used to communicate properties of 964the kernel image, because it is .bss and has no image-provided content. 965 966kernel_info solves this by providing an extensible place for information about 967the kernel image. It is readonly, because the kernel cannot rely on a 968bootloader copying its contents anywhere, but that is OK; if it becomes 969necessary it can still contain data items that an enabled bootloader would be 970expected to copy into a setup_data chunk. 971 972All kernel_info data should be part of this structure. Fixed size data have to 973be put before kernel_info_var_len_data label. Variable size data have to be put 974after kernel_info_var_len_data label. Each chunk of variable size data has to 975be prefixed with header/magic and its size, e.g.:: 976 977 kernel_info: 978 .ascii "LToP" /* Header, Linux top (structure). */ 979 .long kernel_info_var_len_data - kernel_info 980 .long kernel_info_end - kernel_info 981 .long 0x01234567 /* Some fixed size data for the bootloaders. */ 982 kernel_info_var_len_data: 983 example_struct: /* Some variable size data for the bootloaders. */ 984 .ascii "0123" /* Header/Magic. */ 985 .long example_struct_end - example_struct 986 .ascii "Struct" 987 .long 0x89012345 988 example_struct_end: 989 example_strings: /* Some variable size data for the bootloaders. */ 990 .ascii "ABCD" /* Header/Magic. */ 991 .long example_strings_end - example_strings 992 .asciz "String_0" 993 .asciz "String_1" 994 example_strings_end: 995 kernel_info_end: 996 997This way the kernel_info is self-contained blob. 998 999.. note:: 1000 Each variable size data header/magic can be any 4-character string, 1001 without \0 at the end of the string, which does not collide with 1002 existing variable length data headers/magics. 1003 1004 1005Details of the kernel_info Fields 1006================================= 1007 1008============ ======== 1009Field name: header 1010Offset/size: 0x0000/4 1011============ ======== 1012 1013 Contains the magic number "LToP" (0x506f544c). 1014 1015============ ======== 1016Field name: size 1017Offset/size: 0x0004/4 1018============ ======== 1019 1020 This field contains the size of the kernel_info including kernel_info.header. 1021 It does not count kernel_info.kernel_info_var_len_data size. This field should be 1022 used by the bootloaders to detect supported fixed size fields in the kernel_info 1023 and beginning of kernel_info.kernel_info_var_len_data. 1024 1025============ ======== 1026Field name: size_total 1027Offset/size: 0x0008/4 1028============ ======== 1029 1030 This field contains the size of the kernel_info including kernel_info.header 1031 and kernel_info.kernel_info_var_len_data. 1032 1033============ ============== 1034Field name: setup_type_max 1035Offset/size: 0x000c/4 1036============ ============== 1037 1038 This field contains maximal allowed type for setup_data and setup_indirect structs. 1039 1040 1041The Image Checksum 1042================== 1043 1044From boot protocol version 2.08 onwards the CRC-32 is calculated over 1045the entire file using the characteristic polynomial 0x04C11DB7 and an 1046initial remainder of 0xffffffff. The checksum is appended to the 1047file; therefore the CRC of the file up to the limit specified in the 1048syssize field of the header is always 0. 1049 1050 1051The Kernel Command Line 1052======================= 1053 1054The kernel command line has become an important way for the boot 1055loader to communicate with the kernel. Some of its options are also 1056relevant to the boot loader itself, see "special command line options" 1057below. 1058 1059The kernel command line is a null-terminated string. The maximum 1060length can be retrieved from the field cmdline_size. Before protocol 1061version 2.06, the maximum was 255 characters. A string that is too 1062long will be automatically truncated by the kernel. 1063 1064If the boot protocol version is 2.02 or later, the address of the 1065kernel command line is given by the header field cmd_line_ptr (see 1066above.) This address can be anywhere between the end of the setup 1067heap and 0xA0000. 1068 1069If the protocol version is *not* 2.02 or higher, the kernel 1070command line is entered using the following protocol: 1071 1072 - At offset 0x0020 (word), "cmd_line_magic", enter the magic 1073 number 0xA33F. 1074 1075 - At offset 0x0022 (word), "cmd_line_offset", enter the offset 1076 of the kernel command line (relative to the start of the 1077 real-mode kernel). 1078 1079 - The kernel command line *must* be within the memory region 1080 covered by setup_move_size, so you may need to adjust this 1081 field. 1082 1083 1084Memory Layout of The Real-Mode Code 1085=================================== 1086 1087The real-mode code requires a stack/heap to be set up, as well as 1088memory allocated for the kernel command line. This needs to be done 1089in the real-mode accessible memory in bottom megabyte. 1090 1091It should be noted that modern machines often have a sizable Extended 1092BIOS Data Area (EBDA). As a result, it is advisable to use as little 1093of the low megabyte as possible. 1094 1095Unfortunately, under the following circumstances the 0x90000 memory 1096segment has to be used: 1097 1098 - When loading a zImage kernel ((loadflags & 0x01) == 0). 1099 - When loading a 2.01 or earlier boot protocol kernel. 1100 1101.. note:: 1102 For the 2.00 and 2.01 boot protocols, the real-mode code 1103 can be loaded at another address, but it is internally 1104 relocated to 0x90000. For the "old" protocol, the 1105 real-mode code must be loaded at 0x90000. 1106 1107When loading at 0x90000, avoid using memory above 0x9a000. 1108 1109For boot protocol 2.02 or higher, the command line does not have to be 1110located in the same 64K segment as the real-mode setup code; it is 1111thus permitted to give the stack/heap the full 64K segment and locate 1112the command line above it. 1113 1114The kernel command line should not be located below the real-mode 1115code, nor should it be located in high memory. 1116 1117 1118Sample Boot Configuration 1119========================= 1120 1121As a sample configuration, assume the following layout of the real 1122mode segment. 1123 1124 When loading below 0x90000, use the entire segment: 1125 1126 ============= =================== 1127 0x0000-0x7fff Real mode kernel 1128 0x8000-0xdfff Stack and heap 1129 0xe000-0xffff Kernel command line 1130 ============= =================== 1131 1132 When loading at 0x90000 OR the protocol version is 2.01 or earlier: 1133 1134 ============= =================== 1135 0x0000-0x7fff Real mode kernel 1136 0x8000-0x97ff Stack and heap 1137 0x9800-0x9fff Kernel command line 1138 ============= =================== 1139 1140Such a boot loader should enter the following fields in the header:: 1141 1142 unsigned long base_ptr; /* base address for real-mode segment */ 1143 1144 if ( setup_sects == 0 ) { 1145 setup_sects = 4; 1146 } 1147 1148 if ( protocol >= 0x0200 ) { 1149 type_of_loader = <type code>; 1150 if ( loading_initrd ) { 1151 ramdisk_image = <initrd_address>; 1152 ramdisk_size = <initrd_size>; 1153 } 1154 1155 if ( protocol >= 0x0202 && loadflags & 0x01 ) 1156 heap_end = 0xe000; 1157 else 1158 heap_end = 0x9800; 1159 1160 if ( protocol >= 0x0201 ) { 1161 heap_end_ptr = heap_end - 0x200; 1162 loadflags |= 0x80; /* CAN_USE_HEAP */ 1163 } 1164 1165 if ( protocol >= 0x0202 ) { 1166 cmd_line_ptr = base_ptr + heap_end; 1167 strcpy(cmd_line_ptr, cmdline); 1168 } else { 1169 cmd_line_magic = 0xA33F; 1170 cmd_line_offset = heap_end; 1171 setup_move_size = heap_end + strlen(cmdline)+1; 1172 strcpy(base_ptr+cmd_line_offset, cmdline); 1173 } 1174 } else { 1175 /* Very old kernel */ 1176 1177 heap_end = 0x9800; 1178 1179 cmd_line_magic = 0xA33F; 1180 cmd_line_offset = heap_end; 1181 1182 /* A very old kernel MUST have its real-mode code 1183 loaded at 0x90000 */ 1184 1185 if ( base_ptr != 0x90000 ) { 1186 /* Copy the real-mode kernel */ 1187 memcpy(0x90000, base_ptr, (setup_sects+1)*512); 1188 base_ptr = 0x90000; /* Relocated */ 1189 } 1190 1191 strcpy(0x90000+cmd_line_offset, cmdline); 1192 1193 /* It is recommended to clear memory up to the 32K mark */ 1194 memset(0x90000 + (setup_sects+1)*512, 0, 1195 (64-(setup_sects+1))*512); 1196 } 1197 1198 1199Loading The Rest of The Kernel 1200============================== 1201 1202The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512 1203in the kernel file (again, if setup_sects == 0 the real value is 4.) 1204It should be loaded at address 0x10000 for Image/zImage kernels and 12050x100000 for bzImage kernels. 1206 1207The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01 1208bit (LOAD_HIGH) in the loadflags field is set:: 1209 1210 is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01); 1211 load_address = is_bzImage ? 0x100000 : 0x10000; 1212 1213Note that Image/zImage kernels can be up to 512K in size, and thus use 1214the entire 0x10000-0x90000 range of memory. This means it is pretty 1215much a requirement for these kernels to load the real-mode part at 12160x90000. bzImage kernels allow much more flexibility. 1217 1218Special Command Line Options 1219============================ 1220 1221If the command line provided by the boot loader is entered by the 1222user, the user may expect the following command line options to work. 1223They should normally not be deleted from the kernel command line even 1224though not all of them are actually meaningful to the kernel. Boot 1225loader authors who need additional command line options for the boot 1226loader itself should get them registered in 1227Documentation/admin-guide/kernel-parameters.rst to make sure they will not 1228conflict with actual kernel options now or in the future. 1229 1230 vga=<mode> 1231 <mode> here is either an integer (in C notation, either 1232 decimal, octal, or hexadecimal) or one of the strings 1233 "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask" 1234 (meaning 0xFFFD). This value should be entered into the 1235 vid_mode field, as it is used by the kernel before the command 1236 line is parsed. 1237 1238 mem=<size> 1239 <size> is an integer in C notation optionally followed by 1240 (case insensitive) K, M, G, T, P or E (meaning << 10, << 20, 1241 << 30, << 40, << 50 or << 60). This specifies the end of 1242 memory to the kernel. This affects the possible placement of 1243 an initrd, since an initrd should be placed near end of 1244 memory. Note that this is an option to *both* the kernel and 1245 the bootloader! 1246 1247 initrd=<file> 1248 An initrd should be loaded. The meaning of <file> is 1249 obviously bootloader-dependent, and some boot loaders 1250 (e.g. LILO) do not have such a command. 1251 1252In addition, some boot loaders add the following options to the 1253user-specified command line: 1254 1255 BOOT_IMAGE=<file> 1256 The boot image which was loaded. Again, the meaning of <file> 1257 is obviously bootloader-dependent. 1258 1259 auto 1260 The kernel was booted without explicit user intervention. 1261 1262If these options are added by the boot loader, it is highly 1263recommended that they are located *first*, before the user-specified 1264or configuration-specified command line. Otherwise, "init=/bin/sh" 1265gets confused by the "auto" option. 1266 1267 1268Running the Kernel 1269================== 1270 1271The kernel is started by jumping to the kernel entry point, which is 1272located at *segment* offset 0x20 from the start of the real mode 1273kernel. This means that if you loaded your real-mode kernel code at 12740x90000, the kernel entry point is 9020:0000. 1275 1276At entry, ds = es = ss should point to the start of the real-mode 1277kernel code (0x9000 if the code is loaded at 0x90000), sp should be 1278set up properly, normally pointing to the top of the heap, and 1279interrupts should be disabled. Furthermore, to guard against bugs in 1280the kernel, it is recommended that the boot loader sets fs = gs = ds = 1281es = ss. 1282 1283In our example from above, we would do:: 1284 1285 /* Note: in the case of the "old" kernel protocol, base_ptr must 1286 be == 0x90000 at this point; see the previous sample code */ 1287 1288 seg = base_ptr >> 4; 1289 1290 cli(); /* Enter with interrupts disabled! */ 1291 1292 /* Set up the real-mode kernel stack */ 1293 _SS = seg; 1294 _SP = heap_end; 1295 1296 _DS = _ES = _FS = _GS = seg; 1297 jmp_far(seg+0x20, 0); /* Run the kernel */ 1298 1299If your boot sector accesses a floppy drive, it is recommended to 1300switch off the floppy motor before running the kernel, since the 1301kernel boot leaves interrupts off and thus the motor will not be 1302switched off, especially if the loaded kernel has the floppy driver as 1303a demand-loaded module! 1304 1305 1306Advanced Boot Loader Hooks 1307========================== 1308 1309If the boot loader runs in a particularly hostile environment (such as 1310LOADLIN, which runs under DOS) it may be impossible to follow the 1311standard memory location requirements. Such a boot loader may use the 1312following hooks that, if set, are invoked by the kernel at the 1313appropriate time. The use of these hooks should probably be 1314considered an absolutely last resort! 1315 1316IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and 1317%edi across invocation. 1318 1319 realmode_swtch: 1320 A 16-bit real mode far subroutine invoked immediately before 1321 entering protected mode. The default routine disables NMI, so 1322 your routine should probably do so, too. 1323 1324 code32_start: 1325 A 32-bit flat-mode routine *jumped* to immediately after the 1326 transition to protected mode, but before the kernel is 1327 uncompressed. No segments, except CS, are guaranteed to be 1328 set up (current kernels do, but older ones do not); you should 1329 set them up to BOOT_DS (0x18) yourself. 1330 1331 After completing your hook, you should jump to the address 1332 that was in this field before your boot loader overwrote it 1333 (relocated, if appropriate.) 1334 1335 133632-bit Boot Protocol 1337==================== 1338 1339For machine with some new BIOS other than legacy BIOS, such as EFI, 1340LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel 1341based on legacy BIOS can not be used, so a 32-bit boot protocol needs 1342to be defined. 1343 1344In 32-bit boot protocol, the first step in loading a Linux kernel 1345should be to setup the boot parameters (struct boot_params, 1346traditionally known as "zero page"). The memory for struct boot_params 1347should be allocated and initialized to all zero. Then the setup header 1348from offset 0x01f1 of kernel image on should be loaded into struct 1349boot_params and examined. The end of setup header can be calculated as 1350follow:: 1351 1352 0x0202 + byte value at offset 0x0201 1353 1354In addition to read/modify/write the setup header of the struct 1355boot_params as that of 16-bit boot protocol, the boot loader should 1356also fill the additional fields of the struct boot_params as 1357described in chapter Documentation/arch/x86/zero-page.rst. 1358 1359After setting up the struct boot_params, the boot loader can load the 136032/64-bit kernel in the same way as that of 16-bit boot protocol. 1361 1362In 32-bit boot protocol, the kernel is started by jumping to the 136332-bit kernel entry point, which is the start address of loaded 136432/64-bit kernel. 1365 1366At entry, the CPU must be in 32-bit protected mode with paging 1367disabled; a GDT must be loaded with the descriptors for selectors 1368__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat 1369segment; __BOOT_CS must have execute/read permission, and __BOOT_DS 1370must have read/write permission; CS must be __BOOT_CS and DS, ES, SS 1371must be __BOOT_DS; interrupt must be disabled; %esi must hold the base 1372address of the struct boot_params; %ebp, %edi and %ebx must be zero. 1373 137464-bit Boot Protocol 1375==================== 1376 1377For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader 1378and we need a 64-bit boot protocol. 1379 1380In 64-bit boot protocol, the first step in loading a Linux kernel 1381should be to setup the boot parameters (struct boot_params, 1382traditionally known as "zero page"). The memory for struct boot_params 1383could be allocated anywhere (even above 4G) and initialized to all zero. 1384Then, the setup header at offset 0x01f1 of kernel image on should be 1385loaded into struct boot_params and examined. The end of setup header 1386can be calculated as follows:: 1387 1388 0x0202 + byte value at offset 0x0201 1389 1390In addition to read/modify/write the setup header of the struct 1391boot_params as that of 16-bit boot protocol, the boot loader should 1392also fill the additional fields of the struct boot_params as described 1393in chapter Documentation/arch/x86/zero-page.rst. 1394 1395After setting up the struct boot_params, the boot loader can load 139664-bit kernel in the same way as that of 16-bit boot protocol, but 1397kernel could be loaded above 4G. 1398 1399In 64-bit boot protocol, the kernel is started by jumping to the 140064-bit kernel entry point, which is the start address of loaded 140164-bit kernel plus 0x200. 1402 1403At entry, the CPU must be in 64-bit mode with paging enabled. 1404The range with setup_header.init_size from start address of loaded 1405kernel and zero page and command line buffer get ident mapping; 1406a GDT must be loaded with the descriptors for selectors 1407__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat 1408segment; __BOOT_CS must have execute/read permission, and __BOOT_DS 1409must have read/write permission; CS must be __BOOT_CS and DS, ES, SS 1410must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base 1411address of the struct boot_params. 1412 1413EFI Handover Protocol (deprecated) 1414================================== 1415 1416This protocol allows boot loaders to defer initialisation to the EFI 1417boot stub. The boot loader is required to load the kernel/initrd(s) 1418from the boot media and jump to the EFI handover protocol entry point 1419which is hdr->handover_offset bytes from the beginning of 1420startup_{32,64}. 1421 1422The boot loader MUST respect the kernel's PE/COFF metadata when it comes 1423to section alignment, the memory footprint of the executable image beyond 1424the size of the file itself, and any other aspect of the PE/COFF header 1425that may affect correct operation of the image as a PE/COFF binary in the 1426execution context provided by the EFI firmware. 1427 1428The function prototype for the handover entry point looks like this:: 1429 1430 efi_stub_entry(void *handle, efi_system_table_t *table, struct boot_params *bp) 1431 1432'handle' is the EFI image handle passed to the boot loader by the EFI 1433firmware, 'table' is the EFI system table - these are the first two 1434arguments of the "handoff state" as described in section 2.3 of the 1435UEFI specification. 'bp' is the boot loader-allocated boot params. 1436 1437The boot loader *must* fill out the following fields in bp:: 1438 1439 - hdr.cmd_line_ptr 1440 - hdr.ramdisk_image (if applicable) 1441 - hdr.ramdisk_size (if applicable) 1442 1443All other fields should be zero. 1444 1445NOTE: The EFI Handover Protocol is deprecated in favour of the ordinary PE/COFF 1446 entry point, combined with the LINUX_EFI_INITRD_MEDIA_GUID based initrd 1447 loading protocol (refer to [0] for an example of the bootloader side of 1448 this), which removes the need for any knowledge on the part of the EFI 1449 bootloader regarding the internal representation of boot_params or any 1450 requirements/limitations regarding the placement of the command line 1451 and ramdisk in memory, or the placement of the kernel image itself. 1452 1453[0] https://github.com/u-boot/u-boot/commit/ec80b4735a593961fe701cc3a5d717d4739b0fd0 1454