Lilo的实现

书承上文：http://www.cnblogs.com/long123king/p/3549267.html

我们找一份Lilo的源码来看一下

http://freecode.com/projects/lilo

ftp://metalab.unc.edu/pub/Linux/system/boot/lilo/lilo-22.8.src.tar.gz

first.S就是写到MBR中的，我们刚刚分析了一部分的汇编代码文件

   1: #if 0

   2: ;  first.S  -  LILO first stage boot loader with LBA32 support */

   3: Copyright 1992-1998 Werner Almesberger.

   4: Copyright 1999-2005 John Coffman.

   5: All rights reserved.

   6:  

   7: Licensed under the terms contained in the file 'COPYING' in the 

   8: source directory.

   9:  

  10: #endif

  11: #define LILO_ASM

  12: #include "lilo.h"

  13: get common.s        /* as86 "include" will bypass the CPP */

  14:  

  15: #define DEBUG 0

  16:  

  17: #if VERSION_MINOR>=50

  18: # define DEBUG_NEW

  19:  

  20: # undef VIDEO_ENABLE

  21: # define VIDEO_ENABLE 3

  22:  

  23: # define VALIDATE !DEBUG    /* adds 0Dh bytes */

  24: # define SECOND_CHECK !DEBUG    /* adds  5h bytes */

  25: # define CYL1023 DEBUG        /* subs  8h bytes */

  26: # define GEOMETRIC !DEBUG    /* adds  1h byte  */

  27:  

  28: # if DEBUG

  29: #  define DEBUG_LARGE

  30: # endif

  31: #else

  32:  

  33: # define VALIDATE 1        /* adds 0Dh bytes */

  34: # define SECOND_CHECK 1        /* adds  5h bytes */

  35: # define CYL1023 0        /* subs  8h bytes */

  36: # define GEOMETRIC 1        /* adds  1h byte  */

  37: #endif

  38:  

  39:  

  40: !  VIDEO_ENABLE    for those systems that disable the video on boot

  41: !    = 0        first stage does not enable video

  42: !    = 1        use get vid mode/set vid mode to enable

  43: !    = 2        use VGA enable call to enable video

  44: !            (cannot use, as code gets too big)

  45: !    = 3        use direct mode set (mode 3, CGA, EGA, VGA)

  46: !    = 7        use direct mode set (mode 7, MDA)

  47: !

  48: #ifndef VIDEO_ENABLE

  49: # if VALIDATE==0

  50: #  define VIDEO_ENABLE 2

  51: # else

  52: #  define VIDEO_ENABLE 2

  53: # endif

  54: #endif

  55:  

  56: ! do not change the following -- it must correspond to the code in bsect.c

  57: #define RELOCATABLE -1

  58:  

  59:  

  60:     .text

  61:  

  62:     .globl    _main

  63:  

  64:     .org    0

  65:  

  66: zero:

  67: _main:    cli            ! NT 4 blows up if this is missing

  68:     jmp    start

  69:  

  70: stage:    .byte    STAGE_FIRST

  71:     .org    4

  72: reloc:

  73: #if RELOCATABLE

  74:     .word    theend-zero    ! size of the code & params

  75: #else

  76:     .word    0        ! no size indication

  77: #endif

  78:     .org    6

  79:  

  80: ! Boot device parameters. They are set by the installer.

  81:  

  82: sig:    .ascii    "LILO"

  83: vers:    .word    VERSION

  84: mapstamp: .long 0        ! map timestamp

  85:  

  86: length    =  *-sig        ! for the stage 1 vs stage 2 comparison

  87:  

  88: raid:    .long    0        ! raid sector offset

  89: tstamp:    .long    0        ! timestamp

  90: map_serial_no:    .long    0    ! volume S/N containing map file

  91: prompt:    .word    0        ! indicates whether to always enter prompt

  92:                 ! contains many other flags, too

  93:  

  94: d_dev:    .byte    0x80        ! map file device code

  95: d_flag:    .byte    0        ! disk addressing flags

  96: d_addr:    .long    0        ! disk addr of second stage index sector

  97:  

  98:  

  99: edd_packet    =  0

 100: ;;;    .word    16        ! size of packet

 101: ;;;    .word    1        ! count of bytes to read

 102:  

 103: edd_addr    =  4

 104: ;;;    .word    map2        ! where to read

 105: ;;;    .word    *-*        ! segment where to read

 106:  

 107: edd_d_addr    =  8

 108: ;;;    .long    1        ! low address or CX,DX (geometric)

 109:                 ! start at sector 1 for search in geo mode

 110:  

 111: ;;;    .long    0        ! hi address

 112:  

 113: #if 0

 114: !  These locations are referenced as EX_OFF 

 115: !                    (they used to be at CODE_START_1)

 116: ext_si:    .word    0        ! external interface

 117: ext_es:    .word    0        ! these locations are referenced in second.S

 118: ext_bx:    .word    0        ! do not disturb the ordering

 119: ext_dl:    .byte    0        ! second.S will check this magic number

 120: ext_dh:    .byte    0        ! not referenced, but must align stack

 121: ext_stack:

 122: #endif

 123:     

 124: /***************************************************/

 125: !    The following instruction MUST be

 126: !    first instruction after the CLI/JMP short

 127: !    at the start of the file; otherwise

 128: !    the boot sector relocation fails.

 129: !

 130: start:

 131:     mov    ax,#BOOTSEG    ! use DS,ES,SS = 0x07C0

 132: /***************************************************/

 133:  

 134:     mov    ss,ax

 135:     mov    sp,#SETUP_STACKSIZE    ! set the stack for First Stage

 136:     sti            ! now it is safe

 137:  

 138:     push    dx        ! set ext_dl (and ext_dh, which is not used)

 139:     push    bx        ! WATCH the order of pushes

 140:     push    es        ! set ext_es

 141:     push    si        ! set ext_si

 142:  

 143: #ifdef DEBUG_NEW

 144:     push    ds

 145:     push    es    ! just not enough space with debug turned on

 146: #endif

 147:  

 148: #define JRC_DS_EQ_SS

 149:  

 150:     cld            ! do not forget to do this !!!

 151:     mov    ds,ax        ! address data area

 152:     xor    bp,bp        ! shorted addressing

 153:  

 154: #if VIDEO_ENABLE

 155: ! a BIOS has been found where the video interrupt (0x10) trashes DX

 156: !   so, we had better be very paranoid about DX

 157: !

 158: # if VIDEO_ENABLE==2

 159:     pusha            ! protect DX

 160: # endif

 161: # if VIDEO_ENABLE > 2

 162:     mov    ax,#VIDEO_ENABLE    ! set video mode 3 or 7

 163: # elif VIDEO_ENABLE==1

 164:     mov    ah,#15        ! get video mode

 165:     int    0x10

 166:     cbw

 167: # else  /* VIDEO_ENABLE==2 */

 168:     mov    ax,#0x1200    ! enable video (VGA)

 169:     mov    bl,#0x36    ! (probably a nop on EGA or MDA)

 170: # endif

 171:     int    0x10        ! video call

 172: # if VIDEO_ENABLE==2

 173:     popa            ! restore DX

 174: # endif

 175: #endif

 176:  

 177: #if (VIDEO_ENABLE&1) == 0

 178:     mov    al,#0x0d    ! gimme a CR ...

 179:     call    display

 180: ; the suspect call for trashing DX on one BIOS:

 181:     mov    al,#0x0a    ! ... an LF ...

 182:     call    display

 183: #endif

 184:  

 185: #if defined(DEBUG_NEW)

 186:     mov    ah,dl

 187:     call    bout        ! code in AH

 188: #endif

 189:     mov    al,#0x4c    ! ... an 'L' ...

 190:     call    display

 191:  

 192: lagain:

 193:     pusha            ! preserve all the registers for restart

 194:  

 195:     push    ds

 196:     pop    es        ! use buffer at end of boot sector

 197:  

 198:     cmp    dl,#EX_DL_MAG    ! possible boot command line (chain.S)

 199:     jne    boot_in_dl

 200:     mov    dl,dh        ! code passed in DH instead

 201: boot_in_dl:

 202:  

 203:     mov    bx,#map2    ! buffer for volume search

 204:     mov    dh,[d_dev](bp)  ! map device to DH

 205:  

 206: #if VALIDATE

 207:     mov    ax,dx        ! copy device code to AL

 208:     and    ah,#0x80    ! AH = 00 or 80

 209:     xor    al,ah        ! hi-bits must be the same

 210:     js    use_installed

 211:     cmp    al,#MAX_BIOS_DEVICES    ! limit the device code

 212:     jae    use_installed    ! jump if DL is not valid

 213: #endif

 214:  

 215: ! map is on boot device for RAID1, and if so marked; viz.,

 216:  

 217:     test    byte ptr [prompt](bp),#FLAG_MAP_ON_BOOT

 218:     jnz    use_boot    ! as passed in from BIOS or MBR loader

 219:  

 220: use_installed:

 221:     mov    dl,dh        ! device code to DL

 222:     mov    esi,[map_serial_no](bp)    ! to search for

 223:     or    esi,esi

 224:     jz    done_search

 225:  

 226:     push    dx        ! save flags

 227:  

 228:     mov    ah,#8        ! get number of hard disks

 229:     mov    dl,#0x80

 230:     push    bx        ! paranoia

 231:     int    0x13

 232:     pop    bx

 233:     jc    error

 234:  

 235:     movzx    cx,dl        ! extend to word in CX

 236:  

 237: #if GEOMETRIC

 238:     mov    dx,#0x80-1    ! device 80, flags=0

 239: #else

 240:     mov    dx,#LBA32_FLAG*256+0x80-1    ! device 80, flags=LBA32

 241: #endif

 242:  

 243: vagain:

 244:     inc    dx

 245:     xor    eax,eax

 246: #if GEOMETRIC

 247:     inc    ax        ! geometric addressing

 248: #endif

 249:     call    disk_read    ! read 

 250:  

 251:     cmp    esi,[PART_TABLE_OFFSET-6](bx)

 252:     je    vol_found

 253:     loop    vagain

 254:  

 255:     pop    dx        ! restore specified BIOS code

 256:                 ! AX and DX are identical at this point

 257:  

 258: vol_found:

 259:         ! uses value in DX, stack may have extra value

 260:  

 261: done_search:    

 262: use_boot:

 263:     push    bx        ! save map2 for later

 264:  

 265:     mov    dh,[d_flag](bp)    ! get device flags to DH

 266:     mov    si,#d_addr

 267:     call    pread        ! increments BX

 268:  

 269:     mov    ah,#0x99    ! possible error code

 270:     cmp    dword (bx-4),#EX_MAG_HL    ! "LILO"

 271:     jne    error

 272:  

 273:     pop    si        ! point at #map2

 274:  

 275: #if 1

 276:     push    #SETUP_STACKSIZE/16 + BOOTSEG + SECTOR_SIZE/16*2

 277:     pop    es

 278: #else

 279:     mov    ax,ds        ! get segment

 280:     add    ax,#SETUP_STACKSIZE/16    !   + SECTOR_SIZE/16*2

 281:     mov    es,ax

 282: #endif

 283:     xor    bx,bx

 284:  

 285: sload:

 286:     call    pread        ! read using map at DS:SI

 287:     jnz    sload        ! into memory at ES:BX (auto increment)

 288:  

 289: ! Verify second stage loader signature

 290:     

 291:     mov    si,#sig        ! pointer to signature area

 292:     mov    di,si

 293:     mov    cx,#length    ! number of bytes to compare

 294:     mov    ah,#0x9A    ! possible error code

 295:     repe

 296:       cmpsb            ! check Signature 1 & 2

 297:     jne    error    ! check Signature 2

 298:  

 299: #if SECOND_CHECK

 300: /* it would be nice to re-incorporate this check */

 301:     mov    al,#STAGE_SECOND    ! do not touch AH (error code)

 302:     scasb

 303:     jne    error

 304: #endif

 305:  

 306: ! Start the second stage loader     DS=location of Params

 307:  

 308:     push    es        ! segment of second stage

 309:     push    bp        ! BP==0

 310:  

 311:     mov    al,#0x49    ! display an 'I'

 312:     call    display

 313:  

 314:     retf            ! Jump to ES:BP

 315:  

 316:  

 317:  

 318:  

 319: disk_error2:

 320:     mov    ah,#0x40    ; signal seek error

 321:  

 322: ! no return from error

 323: error:

 324:  

 325: #ifndef LCF_NO1STDIAG

 326:         mov    al,#32          ! display a space

 327:     call    display0

 328:  

 329:     call    bout

 330: #endif

 331:  

 332: #ifndef DEBUG_LARGE

 333:     dec    byte [zero](bp)        !  CLI == 0xFA == 250

 334:     jz    zzz

 335:  

 336: #ifndef DEBUG_NEW

 337:     mov    sp,#SETUP_STACKSIZE-4*2-8*2    ! set the stack for First Stage

 338: #else

 339:     mov    sp,#SETUP_STACKSIZE-4*2-2*2-8*2    ! set the stack for First Stage

 340: #endif

 341:     popa                ! restore registers for restart

 342:     jmp    near lagain        ! redo from start

 343: #endif

 344:  

 345:  

 346: zzz:

 347: #ifndef DEBUG_NEW

 348:     hlt

 349: #endif

 350:     jmp    zzz        ! spin; wait for Ctrl-Alt-Del

 351:  

 352:  

 353:  

 354:  

 355: ! packet read routine

 356:  

 357: disk_read:

 358: #ifndef JRC_DS_EQ_SS

 359:     push    ds

 360: #endif

 361:     pusha

 362:  

 363: #ifndef JRC_DS_EQ_SS

 364:     push    ss

 365:     pop    ds

 366: #endif

 367:  

 368:     push    bp        ! BP==0

 369:     push    bp        ! BP==0

 370:  

 371:     push    eax        ! low order disk address

 372: #ifdef DEBUG_LARGE

 373:     xchg    ax,dx

 374:     call    wout

 375:     xchg    ax,dx

 376:     call    dout        ! print out disk address

 377: #endif

 378:     push    es        ! memory segment ES

 379:     push    bx        ! memory offset BX

 380:     push    #1        ! sector count

 381:     push    #16        ! size of packet = 16 bytes

 382:     mov    si,sp        ! address of packet  DS:SI

 383:  

 384:     push    bx

 385:  

 386:     test    dh,#LINEAR_FLAG|LBA32_FLAG

 387:     jz    disk_geometric

 388:     

 389:     test    dh,#LBA32_FLAG

 390:     jz    disk_convert    ; it must be LINEAR

 391:  

 392:     mov    bx,#0x55AA    ;magic number

 393:     mov    ah,#0x41

 394:     int    0x13

 395:     jc    disk_convert

 396:     cmp    bx,#0xAA55    ;changed?

 397:     jne    disk_convert

 398:     test    cl,#EDD_PACKET    ;EDD packet calls supported

 399:     jnz    disk_edd

 400:  

 401: disk_convert:

 402:     push    dx

 403:     push    es        ! protect on floppies

 404:     mov    ah,#8        ! get geometry

 405:     int    0x13

 406:     pop    es

 407: disk_error3:            ! transfer through on CF=1

 408:     jc    error        ! disk_error12

 409:  

 410: #if !CYL1023

 411:     push    cx

 412:     shr    cl,#6        ;;;;

 413:     xchg    cl,ch       ;CX is max cylinder number

 414:     mov    di,cx       ;DI saves it

 415:     pop    cx

 416: #endif

 417:     shr    dx,#8

 418:     xchg    ax,dx        ;AX <- DX

 419:     inc    ax        ;AX is number of heads (256 allowed)

 420:  

 421: ; compensate for Davide BIOS bug

 422:     dec    cx        ; 1..63 -> 0..62;  0->63

 423:     and    cx,#0x003f    ;CX is number of sectors

 424:     inc    cx        ; allow Sectors==0 to mean 64

 425:  

 426:     mul    cx        ; kills DX also

 427:     xchg    ax,bx           ;save in BX

 428:  

 429:     mov    ax,[edd_d_addr](si)    ;low part of address

 430:     mov    dx,[edd_d_addr+2](si)    ;hi part of address

 431:  

 432:     cmp    dx,bx

 433:     jae    disk_error2    ;prevent division error

 434:  

 435:     div    bx        ;AX is cyl, DX is head/sect

 436: #if CYL1023

 437:     cmp    ax,#1023

 438: #else

 439:     cmp    ax,di

 440: #endif

 441:     ja    disk_error2    ;cyl is too big

 442:  

 443:     shl    ah,#6        ; save hi 2 bits

 444:     xchg    al,ah

 445:     xchg    ax,dx

 446:     div    cl        ;AH = sec-1, AL = head

 447:     or    dl,ah       ;form Cyl/Sec

 448:     mov    cx,dx

 449:     inc    cx        ; sector is 1 based

 450:  

 451:     pop    dx        ! restore device code

 452:     mov    dh,al        ! set head#

 453:     jmp    disk_read2

 454:  

 455:  

 456:  

 457: disk_edd:

 458:     mov    ah,#0x42

 459: disk_int13:

 460:     pop    bx

 461:  

 462:     mov    bp,#5

 463: disk_retry:

 464:     pusha

 465:     int    0x13

 466: #if 0

 467:     stc

 468:     mov    ah,#0x9C

 469: #endif

 470:     jnc    disk_okay

 471:  

 472:     dec    bp        ! does not alter CF, already 0

 473:     jz    disk_error3    ! go to "jc" with CF=1 & ZF=1

 474:  

 475:     xor    ax,ax        ! reset the disk controller

 476:     int    0x13

 477:     popa            ! reset AX,BX,CX,DX,SI

 478:     dec    bp        ! fix up BP count

 479:     jmp    disk_retry

 480:  

 481:  

 482: disk_geometric:

 483:     push    eax

 484:     pop    cx

 485:     pop    ax

 486:     mov    dh,ah

 487:  

 488: disk_read2:

 489:     mov    ax,#0x201    ;read, count of 1

 490:     jmp    disk_int13

 491:  

 492:  

 493: disk_okay:

 494: ; the pusha block is implicitly removed below

 495: ;;;    mov    (si+2*16-1),ah    ! set error code

 496: ;   the error code is never checked

 497:     lea    sp,(si+16)    ! do not touch carry; 

 498:     popa

 499: #ifndef JRC_DS_EQ_SS

 500:     pop    ds

 501: #endif

 502:     ret

 503:  

 504:  

 505:  

 506: ! Pointer Read -- read using pointer in DS:SI

 507:  

 508: pread:

 509:     lodsd            ! get address

 510:     or    eax,eax

 511:     jz    done

 512:     add    eax,[raid](bp)    ! reloc is 0 on non-raid

 513:     call    disk_read    

 514:  

 515:     add    bh,#SECTOR_SIZE/256        ! next sector

 516: done:

 517:     ret

 518:  

 519:  

 520:  

 521:  

 522: #if !defined(LCF_NO1STDIAG) || defined(DEBUG_NEW)

 523: bout:    rol     ax,#4        ! bring hi-nibble to position

 524:     call    nout

 525:     rol     ax,#4        ! bring lo-nibble to position

 526: nout:    and    al,#0x0F    ! display one nibble

 527:     daa            ! shorter conversion routine

 528:     add    al,#0xF0

 529:     adc    al,#0x40    ! is now a hex char 0..9A..F

 530: #endif

 531: ; display - write byte in AL to console

 532: ;    preserves all register contents

 533: ; 

 534: display0:

 535: #ifndef LCF_NOVGA

 536: display:

 537: #endif

 538:     pusha        ! make sure no register is changed

 539:     mov    bx,#7        !  BH=0, BL=07

 540:     mov    ah,#14

 541:     int    0x10

 542:     popa        ! restore all the registers

 543: #ifdef LCF_NOVGA

 544: display:

 545: #endif

 546:     ret

 547:  

 548: #ifdef DEBUG_LARGE

 549:  

 550: dout:    pushad

 551:     ror    eax,#16

 552:     call    wout

 553:     ror    eax,#16

 554:     call    wout

 555:     mov    al,#0x20    ! space

 556:     call    display

 557:     popad

 558:     ret

 559:  

 560: wout:    push    ax

 561:     call    bout    ! put out AH

 562:     pop    ax

 563:     push    ax

 564:     xchg    al,ah

 565:     call    bout    ! put out AL (now in AH)

 566:     pop    ax

 567:     ret

 568: #endif

 569:  

 570: theend:

 571:  

 572: !

 573: !   If 'first' loads as the MBR, then there must be space for the partition

 574: !   table.  If 'first' loads as the boot record of some partition, then

 575: !   the space reserved below is not used.  But we must reserve the area

 576: !   as a hedge against the first case.

 577: !

 578: !

 579:     .org    MAX_BOOT_SIZE    !

 580:     .word    0,0,0,0        ! space for NT, DRDOS, and LiLO volume S/N

 581:  

 582: !    .org    0x1be        ! spot for the partition table

 583: p_table:

 584:     .blkb    16

 585:     .blkb    16

 586:     .blkb    16

 587:     .blkb    16

 588: #ifdef FIRST

 589:     .org    *-2

 590:     .long    FIRST        ! boot block check

 591: #else

 592:     .word    0xAA55        ! boot block signature

 593: #endif

 594:  

 595: ! Better be exactly 0x200

 596:  

 597: map2    equ    *        ! addressed as ES:[map2]

.text

.globl    _main

.org    0

zero:
_main:    cli            ! NT 4 blows up if this is missing
    jmp    start

#if (VIDEO_ENABLE&1) == 0
    mov    al,#0x0d    ! gimme a CR ...
    call    display
; the suspect call for trashing DX on one BIOS:
    mov    al,#0x0a    ! ... an LF ...
    call    display
#endif

#if defined(DEBUG_NEW)
    mov    ah,dl
    call    bout        ! code in AH
#endif
    mov    al,#0x4c    ! ... an 'L' ...
    call    display

second.S是主要的加载文件

我们找到下面代码：

jmpi    0,SETUPSEG        ; segment part is a variable

SYSSEG    =  0x1000    ; DEF_SYSSEG
SETUPSEG = 0x9020    ; DEF_SETUPSEG

因此：

   1: lb 0x90200

   2: c

因此，对于调试Linux内核来说，只要在0x90200处设置断点，即可以直接进入到Setup程序部分，而不需要考虑Lilo的具体实现。