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

#if 0
;  first.S  -  LILO first stage boot loader with LBA32 support */
Copyright 1992-1998 Werner Almesberger.
Copyright 1999-2005 John Coffman.
All rights reserved.

Licensed under the terms contained in the file 'COPYING' in the
source directory.

#endif
#define LILO_ASM
#include "lilo.h"
get common.s        /* as86 "include" will bypass the CPP */

#define DEBUG 0

#if VERSION_MINOR>=50
# define DEBUG_NEW

# undef VIDEO_ENABLE
# define VIDEO_ENABLE 3

# define VALIDATE !DEBUG    /* adds 0Dh bytes */
# define SECOND_CHECK !DEBUG    /* adds  5h bytes */
# define CYL1023 DEBUG        /* subs  8h bytes */
# define GEOMETRIC !DEBUG    /* adds  1h byte  */

# if DEBUG
#  define DEBUG_LARGE
# endif
#else

# define VALIDATE 1        /* adds 0Dh bytes */
# define SECOND_CHECK 1        /* adds  5h bytes */
# define CYL1023 0        /* subs  8h bytes */
# define GEOMETRIC 1        /* adds  1h byte  */
#endif


!  VIDEO_ENABLE    for those systems that disable the video on boot
!    = 0        first stage does not enable video
!    = 1        use get vid mode/set vid mode to enable
!    = 2        use VGA enable call to enable video
!            (cannot use, as code gets too big)
!    = 3        use direct mode set (mode 3, CGA, EGA, VGA)
!    = 7        use direct mode set (mode 7, MDA)
!
#ifndef VIDEO_ENABLE
# if VALIDATE==0
#  define VIDEO_ENABLE 2
# else
#  define VIDEO_ENABLE 2
# endif
#endif

! do not change the following -- it must correspond to the code in bsect.c
#define RELOCATABLE -1


    .text

    .globl    _main

    .org    0

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

stage:    .byte    STAGE_FIRST
    .org    4
reloc:
#if RELOCATABLE
    .word    theend-zero    ! size of the code & params
#else
    .word    0        ! no size indication
#endif
    .org    6

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

sig:    .ascii    "LILO"
vers:    .word    VERSION
mapstamp: .long 0        ! map timestamp

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

raid:    .long    0        ! raid sector offset
tstamp:    .long    0        ! timestamp
map_serial_no:    .long    0    ! volume S/N containing map file
prompt:    .word    0        ! indicates whether to always enter prompt
                ! contains many other flags, too

d_dev:    .byte    0x80        ! map file device code
d_flag:    .byte    0        ! disk addressing flags
d_addr:    .long    0        ! disk addr of second stage index sector


edd_packet    =  0
;;;    .word    16        ! size of packet
;;;    .word    1        ! count of bytes to read

edd_addr    =  4
;;;    .word    map2        ! where to read
;;;    .word    *-*        ! segment where to read

edd_d_addr    =  8
;;;    .long    1        ! low address or CX,DX (geometric)
                ! start at sector 1 for search in geo mode

;;;    .long    0        ! hi address

#if 0
!  These locations are referenced as EX_OFF
!                    (they used to be at CODE_START_1)
ext_si:    .word    0        ! external interface
ext_es:    .word    0        ! these locations are referenced in second.S
ext_bx:    .word    0        ! do not disturb the ordering
ext_dl:    .byte    0        ! second.S will check this magic number
ext_dh:    .byte    0        ! not referenced, but must align stack
ext_stack:
#endif

/***************************************************/
!    The following instruction MUST be
!    first instruction after the CLI/JMP short
!    at the start of the file; otherwise
!    the boot sector relocation fails.
!
start:
    mov    ax,#BOOTSEG    ! use DS,ES,SS = 0x07C0
/***************************************************/

    mov    ss,ax
    mov    sp,#SETUP_STACKSIZE    ! set the stack for First Stage
    sti            ! now it is safe

    push    dx        ! set ext_dl (and ext_dh, which is not used)
    push    bx        ! WATCH the order of pushes
    push    es        ! set ext_es
    push    si        ! set ext_si

#ifdef DEBUG_NEW
    push    ds
    push    es    ! just not enough space with debug turned on
#endif

#define JRC_DS_EQ_SS

    cld            ! do not forget to do this !!!
    mov    ds,ax        ! address data area
    xor    bp,bp        ! shorted addressing

#if VIDEO_ENABLE
! a BIOS has been found where the video interrupt (0x10) trashes DX
!   so, we had better be very paranoid about DX
!
# if VIDEO_ENABLE==2
    pusha            ! protect DX
# endif
# if VIDEO_ENABLE > 2
    mov    ax,#VIDEO_ENABLE    ! set video mode 3 or 7
# elif VIDEO_ENABLE==1
    mov    ah,#15        ! get video mode
    int    0x10
    cbw
# else  /* VIDEO_ENABLE==2 */
    mov    ax,#0x1200    ! enable video (VGA)
    mov    bl,#0x36    ! (probably a nop on EGA or MDA)
# endif
    int    0x10        ! video call
# if VIDEO_ENABLE==2
    popa            ! restore DX
# endif
#endif

#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

lagain:
    pusha            ! preserve all the registers for restart

    push    ds
    pop    es        ! use buffer at end of boot sector

    cmp    dl,#EX_DL_MAG    ! possible boot command line (chain.S)
    jne    boot_in_dl
    mov    dl,dh        ! code passed in DH instead
boot_in_dl:

    mov    bx,#map2    ! buffer for volume search
    mov    dh,[d_dev](bp)  ! map device to DH

#if VALIDATE
    mov    ax,dx        ! copy device code to AL
    and    ah,#0x80    ! AH = 00 or 80
    xor    al,ah        ! hi-bits must be the same
    js    use_installed
    cmp    al,#MAX_BIOS_DEVICES    ! limit the device code
    jae    use_installed    ! jump if DL is not valid
#endif

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

    test    byte ptr [prompt](bp),#FLAG_MAP_ON_BOOT
    jnz    use_boot    ! as passed in from BIOS or MBR loader

use_installed:
    mov    dl,dh        ! device code to DL
    mov    esi,[map_serial_no](bp)    ! to search for
    or    esi,esi
    jz    done_search

    push    dx        ! save flags

    mov    ah,#8        ! get number of hard disks
    mov    dl,#0x80
    push    bx        ! paranoia
    int    0x13
    pop    bx
    jc    error

    movzx    cx,dl        ! extend to word in CX

#if GEOMETRIC
    mov    dx,#0x80-1    ! device 80, flags=0
#else
    mov    dx,#LBA32_FLAG*256+0x80-1    ! device 80, flags=LBA32
#endif

vagain:
    inc    dx
    xor    eax,eax
#if GEOMETRIC
    inc    ax        ! geometric addressing
#endif
    call    disk_read    ! read

    cmp    esi,[PART_TABLE_OFFSET-6](bx)
    je    vol_found
    loop    vagain

    pop    dx        ! restore specified BIOS code
                ! AX and DX are identical at this point

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

done_search:
use_boot:
    push    bx        ! save map2 for later

    mov    dh,[d_flag](bp)    ! get device flags to DH
    mov    si,#d_addr
    call    pread        ! increments BX

    mov    ah,#0x99    ! possible error code
    cmp    dword (bx-4),#EX_MAG_HL    ! "LILO"
    jne    error

    pop    si        ! point at #map2

#if 1
    push    #SETUP_STACKSIZE/16 + BOOTSEG + SECTOR_SIZE/16*2
    pop    es
#else
    mov    ax,ds        ! get segment
    add    ax,#SETUP_STACKSIZE/16    !   + SECTOR_SIZE/16*2
    mov    es,ax
#endif
    xor    bx,bx

sload:
    call    pread        ! read using map at DS:SI
    jnz    sload        ! into memory at ES:BX (auto increment)

! Verify second stage loader signature

    mov    si,#sig        ! pointer to signature area
    mov    di,si
    mov    cx,#length    ! number of bytes to compare
    mov    ah,#0x9A    ! possible error code
    repe
      cmpsb            ! check Signature 1 & 2
    jne    error    ! check Signature 2

#if SECOND_CHECK
/* it would be nice to re-incorporate this check */
    mov    al,#STAGE_SECOND    ! do not touch AH (error code)
    scasb
    jne    error
#endif

! Start the second stage loader     DS=location of Params

    push    es        ! segment of second stage
    push    bp        ! BP==0

    mov    al,#0x49    ! display an 'I'
    call    display

    retf            ! Jump to ES:BP




disk_error2:
    mov    ah,#0x40    ; signal seek error

! no return from error
error:

#ifndef LCF_NO1STDIAG
        mov    al,#32          ! display a space
    call    display0

    call    bout
#endif

#ifndef DEBUG_LARGE
    dec    byte [zero](bp)        !  CLI == 0xFA == 250
    jz    zzz

#ifndef DEBUG_NEW
    mov    sp,#SETUP_STACKSIZE-4*2-8*2    ! set the stack for First Stage
#else
    mov    sp,#SETUP_STACKSIZE-4*2-2*2-8*2    ! set the stack for First Stage
#endif
    popa                ! restore registers for restart
    jmp    near lagain        ! redo from start
#endif


zzz:
#ifndef DEBUG_NEW
    hlt
#endif
    jmp    zzz        ! spin; wait for Ctrl-Alt-Del




! packet read routine

disk_read:
#ifndef JRC_DS_EQ_SS
    push    ds
#endif
    pusha

#ifndef JRC_DS_EQ_SS
    push    ss
    pop    ds
#endif

    push    bp        ! BP==0
    push    bp        ! BP==0

    push    eax        ! low order disk address
#ifdef DEBUG_LARGE
    xchg    ax,dx
    call    wout
    xchg    ax,dx
    call    dout        ! print out disk address
#endif
    push    es        ! memory segment ES
    push    bx        ! memory offset BX
    push    #1        ! sector count
    push    #16        ! size of packet = 16 bytes
    mov    si,sp        ! address of packet  DS:SI

    push    bx

    test    dh,#LINEAR_FLAG|LBA32_FLAG
    jz    disk_geometric

    test    dh,#LBA32_FLAG
    jz    disk_convert    ; it must be LINEAR

    mov    bx,#0x55AA    ;magic number
    mov    ah,#0x41
    int    0x13
    jc    disk_convert
    cmp    bx,#0xAA55    ;changed?
    jne    disk_convert
    test    cl,#EDD_PACKET    ;EDD packet calls supported
    jnz    disk_edd

disk_convert:
    push    dx
    push    es        ! protect on floppies
    mov    ah,#8        ! get geometry
    int    0x13
    pop    es
disk_error3:            ! transfer through on CF=1
    jc    error        ! disk_error12

#if !CYL1023
    push    cx
    shr    cl,#6        ;;;;
    xchg    cl,ch       ;CX is max cylinder number
    mov    di,cx       ;DI saves it
    pop    cx
#endif
    shr    dx,#8
    xchg    ax,dx        ;AX <- DX
    inc    ax        ;AX is number of heads (256 allowed)

; compensate for Davide BIOS bug
    dec    cx        ; 1..63 -> 0..62;  0->63
    and    cx,#0x003f    ;CX is number of sectors
    inc    cx        ; allow Sectors==0 to mean 64

    mul    cx        ; kills DX also
    xchg    ax,bx           ;save in BX

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

    cmp    dx,bx
    jae    disk_error2    ;prevent division error

    div    bx        ;AX is cyl, DX is head/sect
#if CYL1023
    cmp    ax,#1023
#else
    cmp    ax,di
#endif
    ja    disk_error2    ;cyl is too big

    shl    ah,#6        ; save hi 2 bits
    xchg    al,ah
    xchg    ax,dx
    div    cl        ;AH = sec-1, AL = head
    or    dl,ah       ;form Cyl/Sec
    mov    cx,dx
    inc    cx        ; sector is 1 based

    pop    dx        ! restore device code
    mov    dh,al        ! set head#
    jmp    disk_read2



disk_edd:
    mov    ah,#0x42
disk_int13:
    pop    bx

    mov    bp,#5
disk_retry:
    pusha
    int    0x13
#if 0
    stc
    mov    ah,#0x9C
#endif
    jnc    disk_okay

    dec    bp        ! does not alter CF, already 0
    jz    disk_error3    ! go to "jc" with CF=1 & ZF=1

    xor    ax,ax        ! reset the disk controller
    int    0x13
    popa            ! reset AX,BX,CX,DX,SI
    dec    bp        ! fix up BP count
    jmp    disk_retry


disk_geometric:
    push    eax
    pop    cx
    pop    ax
    mov    dh,ah

disk_read2:
    mov    ax,#0x201    ;read, count of 1
    jmp    disk_int13


disk_okay:
; the pusha block is implicitly removed below
;;;    mov    (si+2*16-1),ah    ! set error code
;   the error code is never checked
    lea    sp,(si+16)    ! do not touch carry;
    popa
#ifndef JRC_DS_EQ_SS
    pop    ds
#endif
    ret



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

pread:
    lodsd            ! get address
    or    eax,eax
    jz    done
    add    eax,[raid](bp)    ! reloc is 0 on non-raid
    call    disk_read

    add    bh,#SECTOR_SIZE/256        ! next sector
done:
    ret




#if !defined(LCF_NO1STDIAG) || defined(DEBUG_NEW)
bout:    rol     ax,#4        ! bring hi-nibble to position
    call    nout
    rol     ax,#4        ! bring lo-nibble to position
nout:    and    al,#0x0F    ! display one nibble
    daa            ! shorter conversion routine
    add    al,#0xF0
    adc    al,#0x40    ! is now a hex char 0..9A..F
#endif
; display - write byte in AL to console
;    preserves all register contents
;
display0:
#ifndef LCF_NOVGA
display:
#endif
    pusha        ! make sure no register is changed
    mov    bx,#7        !  BH=0, BL=07
    mov    ah,#14
    int    0x10
    popa        ! restore all the registers
#ifdef LCF_NOVGA
display:
#endif
    ret

#ifdef DEBUG_LARGE

dout:    pushad
    ror    eax,#16
    call    wout
    ror    eax,#16
    call    wout
    mov    al,#0x20    ! space
    call    display
    popad
    ret

wout:    push    ax
    call    bout    ! put out AH
    pop    ax
    push    ax
    xchg    al,ah
    call    bout    ! put out AL (now in AH)
    pop    ax
    ret
#endif

theend:

!
!   If 'first' loads as the MBR, then there must be space for the partition
!   table.  If 'first' loads as the boot record of some partition, then
!   the space reserved below is not used.  But we must reserve the area
!   as a hedge against the first case.
!
!
    .org    MAX_BOOT_SIZE    !
    .word    0,0,0,0        ! space for NT, DRDOS, and LiLO volume S/N

!    .org    0x1be        ! spot for the partition table
p_table:
    .blkb    16
    .blkb    16
    .blkb    16
    .blkb    16
#ifdef FIRST
    .org    *-2
    .long    FIRST        ! boot block check
#else
    .word    0xAA55        ! boot block signature
#endif

! Better be exactly 0x200

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：

lb 0x90200
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。