1 THE LINUX/I386 BOOT PROTOCOL 2 ---------------------------- 3 4 H. Peter Anvin <hpa@zytor.com> 5 Last update 2007-05-23 6 7On the i386 platform, the Linux kernel uses a rather complicated boot 8convention. This has evolved partially due to historical aspects, as 9well as the desire in the early days to have the kernel itself be a 10bootable image, the complicated PC memory model and due to changed 11expectations in the PC industry caused by the effective demise of 12real-mode DOS as a mainstream operating system. 13 14Currently, the following versions of the Linux/i386 boot protocol exist. 15 16Old kernels: zImage/Image support only. Some very early kernels 17 may not even support a command line. 18 19Protocol 2.00: (Kernel 1.3.73) Added bzImage and initrd support, as 20 well as a formalized way to communicate between the 21 boot loader and the kernel. setup.S made relocatable, 22 although the traditional setup area still assumed 23 writable. 24 25Protocol 2.01: (Kernel 1.3.76) Added a heap overrun warning. 26 27Protocol 2.02: (Kernel 2.4.0-test3-pre3) New command line protocol. 28 Lower the conventional memory ceiling. No overwrite 29 of the traditional setup area, thus making booting 30 safe for systems which use the EBDA from SMM or 32-bit 31 BIOS entry points. zImage deprecated but still 32 supported. 33 34Protocol 2.03: (Kernel 2.4.18-pre1) Explicitly makes the highest possible 35 initrd address available to the bootloader. 36 37Protocol 2.04: (Kernel 2.6.14) Extend the syssize field to four bytes. 38 39Protocol 2.05: (Kernel 2.6.20) Make protected mode kernel relocatable. 40 Introduce relocatable_kernel and kernel_alignment fields. 41 42Protocol 2.06: (Kernel 2.6.22) Added a field that contains the size of 43 the boot command line 44 45 46**** MEMORY LAYOUT 47 48The traditional memory map for the kernel loader, used for Image or 49zImage kernels, typically looks like: 50 51 | | 520A0000 +------------------------+ 53 | Reserved for BIOS | Do not use. Reserved for BIOS EBDA. 5409A000 +------------------------+ 55 | Command line | 56 | Stack/heap | For use by the kernel real-mode code. 57098000 +------------------------+ 58 | Kernel setup | The kernel real-mode code. 59090200 +------------------------+ 60 | Kernel boot sector | The kernel legacy boot sector. 61090000 +------------------------+ 62 | Protected-mode kernel | The bulk of the kernel image. 63010000 +------------------------+ 64 | Boot loader | <- Boot sector entry point 0000:7C00 65001000 +------------------------+ 66 | Reserved for MBR/BIOS | 67000800 +------------------------+ 68 | Typically used by MBR | 69000600 +------------------------+ 70 | BIOS use only | 71000000 +------------------------+ 72 73 74When using bzImage, the protected-mode kernel was relocated to 750x100000 ("high memory"), and the kernel real-mode block (boot sector, 76setup, and stack/heap) was made relocatable to any address between 770x10000 and end of low memory. Unfortunately, in protocols 2.00 and 782.01 the 0x90000+ memory range is still used internally by the kernel; 79the 2.02 protocol resolves that problem. 80 81It is desirable to keep the "memory ceiling" -- the highest point in 82low memory touched by the boot loader -- as low as possible, since 83some newer BIOSes have begun to allocate some rather large amounts of 84memory, called the Extended BIOS Data Area, near the top of low 85memory. The boot loader should use the "INT 12h" BIOS call to verify 86how much low memory is available. 87 88Unfortunately, if INT 12h reports that the amount of memory is too 89low, there is usually nothing the boot loader can do but to report an 90error to the user. The boot loader should therefore be designed to 91take up as little space in low memory as it reasonably can. For 92zImage or old bzImage kernels, which need data written into the 930x90000 segment, the boot loader should make sure not to use memory 94above the 0x9A000 point; too many BIOSes will break above that point. 95 96For a modern bzImage kernel with boot protocol version >= 2.02, a 97memory layout like the following is suggested: 98 99 ~ ~ 100 | Protected-mode kernel | 101100000 +------------------------+ 102 | I/O memory hole | 1030A0000 +------------------------+ 104 | Reserved for BIOS | Leave as much as possible unused 105 ~ ~ 106 | Command line | (Can also be below the X+10000 mark) 107X+10000 +------------------------+ 108 | Stack/heap | For use by the kernel real-mode code. 109X+08000 +------------------------+ 110 | Kernel setup | The kernel real-mode code. 111 | Kernel boot sector | The kernel legacy boot sector. 112X +------------------------+ 113 | Boot loader | <- Boot sector entry point 0000:7C00 114001000 +------------------------+ 115 | Reserved for MBR/BIOS | 116000800 +------------------------+ 117 | Typically used by MBR | 118000600 +------------------------+ 119 | BIOS use only | 120000000 +------------------------+ 121 122... where the address X is as low as the design of the boot loader 123permits. 124 125 126**** THE REAL-MODE KERNEL HEADER 127 128In the following text, and anywhere in the kernel boot sequence, "a 129sector" refers to 512 bytes. It is independent of the actual sector 130size of the underlying medium. 131 132The first step in loading a Linux kernel should be to load the 133real-mode code (boot sector and setup code) and then examine the 134following header at offset 0x01f1. The real-mode code can total up to 13532K, although the boot loader may choose to load only the first two 136sectors (1K) and then examine the bootup sector size. 137 138The header looks like: 139 140Offset Proto Name Meaning 141/Size 142 14301F1/1 ALL(1 setup_sects The size of the setup in sectors 14401F2/2 ALL root_flags If set, the root is mounted readonly 14501F4/4 2.04+(2 syssize The size of the 32-bit code in 16-byte paras 14601F8/2 ALL ram_size DO NOT USE - for bootsect.S use only 14701FA/2 ALL vid_mode Video mode control 14801FC/2 ALL root_dev Default root device number 14901FE/2 ALL boot_flag 0xAA55 magic number 1500200/2 2.00+ jump Jump instruction 1510202/4 2.00+ header Magic signature "HdrS" 1520206/2 2.00+ version Boot protocol version supported 1530208/4 2.00+ realmode_swtch Boot loader hook (see below) 154020C/2 2.00+ start_sys The load-low segment (0x1000) (obsolete) 155020E/2 2.00+ kernel_version Pointer to kernel version string 1560210/1 2.00+ type_of_loader Boot loader identifier 1570211/1 2.00+ loadflags Boot protocol option flags 1580212/2 2.00+ setup_move_size Move to high memory size (used with hooks) 1590214/4 2.00+ code32_start Boot loader hook (see below) 1600218/4 2.00+ ramdisk_image initrd load address (set by boot loader) 161021C/4 2.00+ ramdisk_size initrd size (set by boot loader) 1620220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only 1630224/2 2.01+ heap_end_ptr Free memory after setup end 1640226/2 N/A pad1 Unused 1650228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line 166022C/4 2.03+ initrd_addr_max Highest legal initrd address 1670230/4 2.05+ kernel_alignment Physical addr alignment required for kernel 1680234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not 1690235/3 N/A pad2 Unused 1700238/4 2.06+ cmdline_size Maximum size of the kernel command line 171 172(1) For backwards compatibility, if the setup_sects field contains 0, the 173 real value is 4. 174 175(2) For boot protocol prior to 2.04, the upper two bytes of the syssize 176 field are unusable, which means the size of a bzImage kernel 177 cannot be determined. 178 179If the "HdrS" (0x53726448) magic number is not found at offset 0x202, 180the boot protocol version is "old". Loading an old kernel, the 181following parameters should be assumed: 182 183 Image type = zImage 184 initrd not supported 185 Real-mode kernel must be located at 0x90000. 186 187Otherwise, the "version" field contains the protocol version, 188e.g. protocol version 2.01 will contain 0x0201 in this field. When 189setting fields in the header, you must make sure only to set fields 190supported by the protocol version in use. 191 192 193**** DETAILS OF HEADER FIELDS 194 195For each field, some are information from the kernel to the bootloader 196("read"), some are expected to be filled out by the bootloader 197("write"), and some are expected to be read and modified by the 198bootloader ("modify"). 199 200All general purpose boot loaders should write the fields marked 201(obligatory). Boot loaders who want to load the kernel at a 202nonstandard address should fill in the fields marked (reloc); other 203boot loaders can ignore those fields. 204 205The byte order of all fields is littleendian (this is x86, after all.) 206 207Field name: setup_secs 208Type: read 209Offset/size: 0x1f1/1 210Protocol: ALL 211 212 The size of the setup code in 512-byte sectors. If this field is 213 0, the real value is 4. The real-mode code consists of the boot 214 sector (always one 512-byte sector) plus the setup code. 215 216Field name: root_flags 217Type: modify (optional) 218Offset/size: 0x1f2/2 219Protocol: ALL 220 221 If this field is nonzero, the root defaults to readonly. The use of 222 this field is deprecated; use the "ro" or "rw" options on the 223 command line instead. 224 225Field name: syssize 226Type: read 227Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL) 228Protocol: 2.04+ 229 230 The size of the protected-mode code in units of 16-byte paragraphs. 231 For protocol versions older than 2.04 this field is only two bytes 232 wide, and therefore cannot be trusted for the size of a kernel if 233 the LOAD_HIGH flag is set. 234 235Field name: ram_size 236Type: kernel internal 237Offset/size: 0x1f8/2 238Protocol: ALL 239 240 This field is obsolete. 241 242Field name: vid_mode 243Type: modify (obligatory) 244Offset/size: 0x1fa/2 245 246 Please see the section on SPECIAL COMMAND LINE OPTIONS. 247 248Field name: root_dev 249Type: modify (optional) 250Offset/size: 0x1fc/2 251Protocol: ALL 252 253 The default root device device number. The use of this field is 254 deprecated, use the "root=" option on the command line instead. 255 256Field name: boot_flag 257Type: read 258Offset/size: 0x1fe/2 259Protocol: ALL 260 261 Contains 0xAA55. This is the closest thing old Linux kernels have 262 to a magic number. 263 264Field name: jump 265Type: read 266Offset/size: 0x200/2 267Protocol: 2.00+ 268 269 Contains an x86 jump instruction, 0xEB followed by a signed offset 270 relative to byte 0x202. This can be used to determine the size of 271 the header. 272 273Field name: header 274Type: read 275Offset/size: 0x202/4 276Protocol: 2.00+ 277 278 Contains the magic number "HdrS" (0x53726448). 279 280Field name: version 281Type: read 282Offset/size: 0x206/2 283Protocol: 2.00+ 284 285 Contains the boot protocol version, in (major << 8)+minor format, 286 e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version 287 10.17. 288 289Field name: readmode_swtch 290Type: modify (optional) 291Offset/size: 0x208/4 292Protocol: 2.00+ 293 294 Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.) 295 296Field name: start_sys 297Type: read 298Offset/size: 0x20c/4 299Protocol: 2.00+ 300 301 The load low segment (0x1000). Obsolete. 302 303Field name: kernel_version 304Type: read 305Offset/size: 0x20e/2 306Protocol: 2.00+ 307 308 If set to a nonzero value, contains a pointer to a NUL-terminated 309 human-readable kernel version number string, less 0x200. This can 310 be used to display the kernel version to the user. This value 311 should be less than (0x200*setup_sects). 312 313 For example, if this value is set to 0x1c00, the kernel version 314 number string can be found at offset 0x1e00 in the kernel file. 315 This is a valid value if and only if the "setup_sects" field 316 contains the value 15 or higher, as: 317 318 0x1c00 < 15*0x200 (= 0x1e00) but 319 0x1c00 >= 14*0x200 (= 0x1c00) 320 321 0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15. 322 323Field name: type_of_loader 324Type: write (obligatory) 325Offset/size: 0x210/1 326Protocol: 2.00+ 327 328 If your boot loader has an assigned id (see table below), enter 329 0xTV here, where T is an identifier for the boot loader and V is 330 a version number. Otherwise, enter 0xFF here. 331 332 Assigned boot loader ids: 333 0 LILO (0x00 reserved for pre-2.00 bootloader) 334 1 Loadlin 335 2 bootsect-loader (0x20, all other values reserved) 336 3 SYSLINUX 337 4 EtherBoot 338 5 ELILO 339 7 GRuB 340 8 U-BOOT 341 9 Xen 342 A Gujin 343 B Qemu 344 345 Please contact <hpa@zytor.com> if you need a bootloader ID 346 value assigned. 347 348Field name: loadflags 349Type: modify (obligatory) 350Offset/size: 0x211/1 351Protocol: 2.00+ 352 353 This field is a bitmask. 354 355 Bit 0 (read): LOADED_HIGH 356 - If 0, the protected-mode code is loaded at 0x10000. 357 - If 1, the protected-mode code is loaded at 0x100000. 358 359 Bit 7 (write): CAN_USE_HEAP 360 Set this bit to 1 to indicate that the value entered in the 361 heap_end_ptr is valid. If this field is clear, some setup code 362 functionality will be disabled. 363 364Field name: setup_move_size 365Type: modify (obligatory) 366Offset/size: 0x212/2 367Protocol: 2.00-2.01 368 369 When using protocol 2.00 or 2.01, if the real mode kernel is not 370 loaded at 0x90000, it gets moved there later in the loading 371 sequence. Fill in this field if you want additional data (such as 372 the kernel command line) moved in addition to the real-mode kernel 373 itself. 374 375 The unit is bytes starting with the beginning of the boot sector. 376 377 This field is can be ignored when the protocol is 2.02 or higher, or 378 if the real-mode code is loaded at 0x90000. 379 380Field name: code32_start 381Type: modify (optional, reloc) 382Offset/size: 0x214/4 383Protocol: 2.00+ 384 385 The address to jump to in protected mode. This defaults to the load 386 address of the kernel, and can be used by the boot loader to 387 determine the proper load address. 388 389 This field can be modified for two purposes: 390 391 1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.) 392 393 2. if a bootloader which does not install a hook loads a 394 relocatable kernel at a nonstandard address it will have to modify 395 this field to point to the load address. 396 397Field name: ramdisk_image 398Type: write (obligatory) 399Offset/size: 0x218/4 400Protocol: 2.00+ 401 402 The 32-bit linear address of the initial ramdisk or ramfs. Leave at 403 zero if there is no initial ramdisk/ramfs. 404 405Field name: ramdisk_size 406Type: write (obligatory) 407Offset/size: 0x21c/4 408Protocol: 2.00+ 409 410 Size of the initial ramdisk or ramfs. Leave at zero if there is no 411 initial ramdisk/ramfs. 412 413Field name: bootsect_kludge 414Type: kernel internal 415Offset/size: 0x220/4 416Protocol: 2.00+ 417 418 This field is obsolete. 419 420Field name: heap_end_ptr 421Type: write (obligatory) 422Offset/size: 0x224/2 423Protocol: 2.01+ 424 425 Set this field to the offset (from the beginning of the real-mode 426 code) of the end of the setup stack/heap, minus 0x0200. 427 428Field name: cmd_line_ptr 429Type: write (obligatory) 430Offset/size: 0x228/4 431Protocol: 2.02+ 432 433 Set this field to the linear address of the kernel command line. 434 The kernel command line can be located anywhere between the end of 435 the setup heap and 0xA0000; it does not have to be located in the 436 same 64K segment as the real-mode code itself. 437 438 Fill in this field even if your boot loader does not support a 439 command line, in which case you can point this to an empty string 440 (or better yet, to the string "auto".) If this field is left at 441 zero, the kernel will assume that your boot loader does not support 442 the 2.02+ protocol. 443 444Field name: initrd_addr_max 445Type: read 446Offset/size: 0x22c/4 447Protocol: 2.03+ 448 449 The maximum address that may be occupied by the initial 450 ramdisk/ramfs contents. For boot protocols 2.02 or earlier, this 451 field is not present, and the maximum address is 0x37FFFFFF. (This 452 address is defined as the address of the highest safe byte, so if 453 your ramdisk is exactly 131072 bytes long and this field is 454 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.) 455 456Field name: kernel_alignment 457Type: read (reloc) 458Offset/size: 0x230/4 459Protocol: 2.05+ 460 461 Alignment unit required by the kernel (if relocatable_kernel is true.) 462 463Field name: relocatable_kernel 464Type: read (reloc) 465Offset/size: 0x234/1 466Protocol: 2.05+ 467 468 If this field is nonzero, the protected-mode part of the kernel can 469 be loaded at any address that satisfies the kernel_alignment field. 470 After loading, the boot loader must set the code32_start field to 471 point to the loaded code, or to a boot loader hook. 472 473Field name: cmdline_size 474Type: read 475Offset/size: 0x238/4 476Protocol: 2.06+ 477 478 The maximum size of the command line without the terminating 479 zero. This means that the command line can contain at most 480 cmdline_size characters. With protocol version 2.05 and earlier, the 481 maximum size was 255. 482 483 484**** THE KERNEL COMMAND LINE 485 486The kernel command line has become an important way for the boot 487loader to communicate with the kernel. Some of its options are also 488relevant to the boot loader itself, see "special command line options" 489below. 490 491The kernel command line is a null-terminated string. The maximum 492length can be retrieved from the field cmdline_size. Before protocol 493version 2.06, the maximum was 255 characters. A string that is too 494long will be automatically truncated by the kernel. 495 496If the boot protocol version is 2.02 or later, the address of the 497kernel command line is given by the header field cmd_line_ptr (see 498above.) This address can be anywhere between the end of the setup 499heap and 0xA0000. 500 501If the protocol version is *not* 2.02 or higher, the kernel 502command line is entered using the following protocol: 503 504 At offset 0x0020 (word), "cmd_line_magic", enter the magic 505 number 0xA33F. 506 507 At offset 0x0022 (word), "cmd_line_offset", enter the offset 508 of the kernel command line (relative to the start of the 509 real-mode kernel). 510 511 The kernel command line *must* be within the memory region 512 covered by setup_move_size, so you may need to adjust this 513 field. 514 515 516**** MEMORY LAYOUT OF THE REAL-MODE CODE 517 518The real-mode code requires a stack/heap to be set up, as well as 519memory allocated for the kernel command line. This needs to be done 520in the real-mode accessible memory in bottom megabyte. 521 522It should be noted that modern machines often have a sizable Extended 523BIOS Data Area (EBDA). As a result, it is advisable to use as little 524of the low megabyte as possible. 525 526Unfortunately, under the following circumstances the 0x90000 memory 527segment has to be used: 528 529 - When loading a zImage kernel ((loadflags & 0x01) == 0). 530 - When loading a 2.01 or earlier boot protocol kernel. 531 532 -> For the 2.00 and 2.01 boot protocols, the real-mode code 533 can be loaded at another address, but it is internally 534 relocated to 0x90000. For the "old" protocol, the 535 real-mode code must be loaded at 0x90000. 536 537When loading at 0x90000, avoid using memory above 0x9a000. 538 539For boot protocol 2.02 or higher, the command line does not have to be 540located in the same 64K segment as the real-mode setup code; it is 541thus permitted to give the stack/heap the full 64K segment and locate 542the command line above it. 543 544The kernel command line should not be located below the real-mode 545code, nor should it be located in high memory. 546 547 548**** SAMPLE BOOT CONFIGURATION 549 550As a sample configuration, assume the following layout of the real 551mode segment: 552 553 When loading below 0x90000, use the entire segment: 554 555 0x0000-0x7fff Real mode kernel 556 0x8000-0xdfff Stack and heap 557 0xe000-0xffff Kernel command line 558 559 When loading at 0x90000 OR the protocol version is 2.01 or earlier: 560 561 0x0000-0x7fff Real mode kernel 562 0x8000-0x97ff Stack and heap 563 0x9800-0x9fff Kernel command line 564 565Such a boot loader should enter the following fields in the header: 566 567 unsigned long base_ptr; /* base address for real-mode segment */ 568 569 if ( setup_sects == 0 ) { 570 setup_sects = 4; 571 } 572 573 if ( protocol >= 0x0200 ) { 574 type_of_loader = <type code>; 575 if ( loading_initrd ) { 576 ramdisk_image = <initrd_address>; 577 ramdisk_size = <initrd_size>; 578 } 579 580 if ( protocol >= 0x0202 && loadflags & 0x01 ) 581 heap_end = 0xe000; 582 else 583 heap_end = 0x9800; 584 585 if ( protocol >= 0x0201 ) { 586 heap_end_ptr = heap_end - 0x200; 587 loadflags |= 0x80; /* CAN_USE_HEAP */ 588 } 589 590 if ( protocol >= 0x0202 ) { 591 cmd_line_ptr = base_ptr + heap_end; 592 strcpy(cmd_line_ptr, cmdline); 593 } else { 594 cmd_line_magic = 0xA33F; 595 cmd_line_offset = heap_end; 596 setup_move_size = heap_end + strlen(cmdline)+1; 597 strcpy(base_ptr+cmd_line_offset, cmdline); 598 } 599 } else { 600 /* Very old kernel */ 601 602 heap_end = 0x9800; 603 604 cmd_line_magic = 0xA33F; 605 cmd_line_offset = heap_end; 606 607 /* A very old kernel MUST have its real-mode code 608 loaded at 0x90000 */ 609 610 if ( base_ptr != 0x90000 ) { 611 /* Copy the real-mode kernel */ 612 memcpy(0x90000, base_ptr, (setup_sects+1)*512); 613 base_ptr = 0x90000; /* Relocated */ 614 } 615 616 strcpy(0x90000+cmd_line_offset, cmdline); 617 618 /* It is recommended to clear memory up to the 32K mark */ 619 memset(0x90000 + (setup_sects+1)*512, 0, 620 (64-(setup_sects+1))*512); 621 } 622 623 624**** LOADING THE REST OF THE KERNEL 625 626The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512 627in the kernel file (again, if setup_sects == 0 the real value is 4.) 628It should be loaded at address 0x10000 for Image/zImage kernels and 6290x100000 for bzImage kernels. 630 631The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01 632bit (LOAD_HIGH) in the loadflags field is set: 633 634 is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01); 635 load_address = is_bzImage ? 0x100000 : 0x10000; 636 637Note that Image/zImage kernels can be up to 512K in size, and thus use 638the entire 0x10000-0x90000 range of memory. This means it is pretty 639much a requirement for these kernels to load the real-mode part at 6400x90000. bzImage kernels allow much more flexibility. 641 642 643**** SPECIAL COMMAND LINE OPTIONS 644 645If the command line provided by the boot loader is entered by the 646user, the user may expect the following command line options to work. 647They should normally not be deleted from the kernel command line even 648though not all of them are actually meaningful to the kernel. Boot 649loader authors who need additional command line options for the boot 650loader itself should get them registered in 651Documentation/kernel-parameters.txt to make sure they will not 652conflict with actual kernel options now or in the future. 653 654 vga=<mode> 655 <mode> here is either an integer (in C notation, either 656 decimal, octal, or hexadecimal) or one of the strings 657 "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask" 658 (meaning 0xFFFD). This value should be entered into the 659 vid_mode field, as it is used by the kernel before the command 660 line is parsed. 661 662 mem=<size> 663 <size> is an integer in C notation optionally followed by 664 (case insensitive) K, M, G, T, P or E (meaning << 10, << 20, 665 << 30, << 40, << 50 or << 60). This specifies the end of 666 memory to the kernel. This affects the possible placement of 667 an initrd, since an initrd should be placed near end of 668 memory. Note that this is an option to *both* the kernel and 669 the bootloader! 670 671 initrd=<file> 672 An initrd should be loaded. The meaning of <file> is 673 obviously bootloader-dependent, and some boot loaders 674 (e.g. LILO) do not have such a command. 675 676In addition, some boot loaders add the following options to the 677user-specified command line: 678 679 BOOT_IMAGE=<file> 680 The boot image which was loaded. Again, the meaning of <file> 681 is obviously bootloader-dependent. 682 683 auto 684 The kernel was booted without explicit user intervention. 685 686If these options are added by the boot loader, it is highly 687recommended that they are located *first*, before the user-specified 688or configuration-specified command line. Otherwise, "init=/bin/sh" 689gets confused by the "auto" option. 690 691 692**** RUNNING THE KERNEL 693 694The kernel is started by jumping to the kernel entry point, which is 695located at *segment* offset 0x20 from the start of the real mode 696kernel. This means that if you loaded your real-mode kernel code at 6970x90000, the kernel entry point is 9020:0000. 698 699At entry, ds = es = ss should point to the start of the real-mode 700kernel code (0x9000 if the code is loaded at 0x90000), sp should be 701set up properly, normally pointing to the top of the heap, and 702interrupts should be disabled. Furthermore, to guard against bugs in 703the kernel, it is recommended that the boot loader sets fs = gs = ds = 704es = ss. 705 706In our example from above, we would do: 707 708 /* Note: in the case of the "old" kernel protocol, base_ptr must 709 be == 0x90000 at this point; see the previous sample code */ 710 711 seg = base_ptr >> 4; 712 713 cli(); /* Enter with interrupts disabled! */ 714 715 /* Set up the real-mode kernel stack */ 716 _SS = seg; 717 _SP = heap_end; 718 719 _DS = _ES = _FS = _GS = seg; 720 jmp_far(seg+0x20, 0); /* Run the kernel */ 721 722If your boot sector accesses a floppy drive, it is recommended to 723switch off the floppy motor before running the kernel, since the 724kernel boot leaves interrupts off and thus the motor will not be 725switched off, especially if the loaded kernel has the floppy driver as 726a demand-loaded module! 727 728 729**** ADVANCED BOOT LOADER HOOKS 730 731If the boot loader runs in a particularly hostile environment (such as 732LOADLIN, which runs under DOS) it may be impossible to follow the 733standard memory location requirements. Such a boot loader may use the 734following hooks that, if set, are invoked by the kernel at the 735appropriate time. The use of these hooks should probably be 736considered an absolutely last resort! 737 738IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and 739%edi across invocation. 740 741 realmode_swtch: 742 A 16-bit real mode far subroutine invoked immediately before 743 entering protected mode. The default routine disables NMI, so 744 your routine should probably do so, too. 745 746 code32_start: 747 A 32-bit flat-mode routine *jumped* to immediately after the 748 transition to protected mode, but before the kernel is 749 uncompressed. No segments, except CS, are guaranteed to be 750 set up (current kernels do, but older ones do not); you should 751 set them up to BOOT_DS (0x18) yourself. 752 753 After completing your hook, you should jump to the address 754 that was in this field before your boot loader overwrote it 755 (relocated, if appropriate.) 756