『第
29 楼』:
FAT16 的引导扇区剖析
关键字：boot sector, boot record, asm, assembly, FAT16 file system, MS-DOS, 引导扇区，引导记录，FAT16 文件系统，汇编语言，反汇编
从这个引导过程的分析可以知道，IO.SYS 在接管控制后，进入点是第二扇区的开头。
IO.SYS 被 boot sector 装入了 4 个扇区，装入的地址是 0000:0700，控制转移的 CS:IP=0070:0200，也就是 IO.SYS 的第二扇区的开头。另外，BP,SP,SI,DI,SS,DS,ES 以及堆栈内容都是有意义的，在下面的注释中都作了详细说明。
2181:7C00 EB 3C 90 4D-53 57 49 4E-34 2E 31 00-02 04 04 00 .<.MSWIN4.1.....
2181:7C10 02 00 02 00-00 F8 F8 00-20 00 40 00-20 00 00 00 ........ .@. ...
2181:7C20 E0 DF 03 00-80 00 29 FA-6F 0B 00 55-53 42 5A 49 ......).o..USBZI
2181:7C30 50 2D 42 4F-4F 54 46 41-54 31 36 00-00 00 33 C9 P-BOOTFAT16...3.
2181:7C40 8E D1 BC FC-7B 16 07 BD-78 00 C5 76-00 1E 56 16 ....{...x..v..V.
2181:7C50 55 BF 22 05-89 7E 00 89-4E 02 B1 0B-FC F3 A4 06 U."..~..N.......
2181:7C60 1F BD 00 7C-C6 45 FE 0F-38 4E 24 7D-20 8B C1 99 ...|.E..8N$} ...
2181:7C70 E8 7E 01 83-EB 3A 66 A1-1C 7C 66 3B-07 8A 57 FC .~...:f..|f;..W.
2181:7C80 75 06 80 CA-02 88 56 02-80 C3 10 73-ED 33 C9 FE u.....V....s.3..
2181:7C90 06 D8 7D 8A-46 10 98 F7-66 16 03 46-1C 13 56 1E ..}.F...f..F..V.
2181:7CA0 03 46 0E 13-D1 8B 76 11-60 89 46 FC-89 56 FE B8 .F....v.`.F..V..
2181:7CB0 20 00 F7 E6-8B 5E 0B 03-C3 48 F7 F3-01 46 FC 11 ....^...H...F..
2181:7CC0 4E FE 61 BF-00 07 E8 28-01 72 3E 38-2D 74 17 60 N.a....(.r>8-t.`
2181:7CD0 B1 0B BE D8-7D F3 A6 61-74 3D 4E 74-09 83 C7 20 ....}..at=Nt...
2181:7CE0 3B FB 72 E7-EB DD FE 0E-D8 7D 7B A7-BE 7F 7D AC ;.r......}{..}.
2181:7CF0 98 03 F0 AC-98 40 74 0C-48 74 13 B4-0E BB 07 00 .....@t.Ht......
2181:7D00 CD 10 EB EF-BE 82 7D EB-E6 BE 80 7D-EB E1 CD 16 ......}....}....
2181:7D10 5E 1F 66 8F-04 CD 19 BE-81 7D 8B 7D-1A 8D 45 FE ^.f......}.}..E.
2181:7D20 8A 4E 0D F7-E1 03 46 FC-13 56 FE B1-04 E8 C2 00 .N....F..V......
2181:7D30 72 D7 EA 00-02 70 00 52-50 06 53 6A-01 6A 10 91 r....p.RP.Sj.j..
2181:7D40 8B 46 18 A2-26 05 96 92-33 D2 F7 F6-91 F7 F6 42 .F..&...3......B
2181:7D50 87 CA F7 76-1A 8A F2 8A-E8 C0 CC 02-0A CC B8 01 ...v............
2181:7D60 02 80 7E 02-0E 75 04 B4-42 8B F4 8A-56 24 CD 13 ..~..u..B...V$..
2181:7D70 61 61 72 0A-40 75 01 42-03 5E 0B 49-75 77 C3 03 aar.@u.B.^.Iuw..
2181:7D80 18 01 27 0D-0A 49 6E 76-61 6C 69 64-20 73 79 73 ..'..Invalid sys
2181:7D90 74 65 6D 20-64 69 73 6B-FF 0D 0A 44-69 73 6B 20 tem disk...Disk
2181:7DA0 49 2F 4F 20-65 72 72 6F-72 FF 0D 0A-52 65 70 6C I/O error...Repl
2181:7DB0 61 63 65 20-74 68 65 20-64 69 73 6B-2C 20 61 6E ace the disk, an
2181:7DC0 64 20 74 68-65 6E 20 70-72 65 73 73-20 61 6E 79 d then press any
2181:7DD0 20 6B 65 79-0D 0A 00 00-49 4F 20 20-20 20 20 20 key....IO
2181:7DE0 53 59 53 4D-53 44 4F 53-20 20 20 53-59 53 7F 01 SYSMSDOS SYS.
2181:7DF0 00 41 BB 00-07 60 66 6A-00 E9 3B FF-00 00 55 AA .A...`fj..;...U.
2181:7C00 EB 3C jmp 7C3E
2181:7C02 90 4D-53 57 49 4E-34 2E 31 00-02 04 04 00 .MSWIN4.1.....
2181:7C10 02 00 02 00-00 F8 F8 00-20 00 40 00-20 00 00 00 ........ .@. ...
2181:7C20 E0 DF 03 00-80 00 29 FA-6F 0B 00 55-53 42 5A 49 ......).o..USBZI
2181:7C30 50 2D 42 4F-4F 54 46 41-54 31 36 00-00 00 P-BOOTFAT16...
2181:7C3E 33 C9 xor cx,cx ;CX=0
2181:7C40 8E D1 mov ss,cx ;SS=0
2181:7C42 BC FC 7B mov sp,7BFC ;SP=7BFC
2181:7C45 16 push ss
2181:7C46 07 pop es ;ES=0
2181:7C47 BD 78 00 mov bp,0078 ;BP=78
2181:7C4A C5 76 00 lds si,[bp+00] ;DS:SI->floppy param
2181:7C4D 1E push ds
2181:7C4E 56 push si ;Save floppy param
2181:7C4F 16 push ss
2181:7C50 55 push bp ;Save 0000:0078
2181:7C51 BF 22 05 mov di,0522 ;DI=0522
2181:7C54 89 7E 00 mov [bp+00],di ;Set 0000:0522 as...
2181:7C57 89 4E 02 mov [bp+02],cx ;...new floppy param
2181:7C5A B1 0B mov cl,000B ;CX=000B
2181:7C5C FC cld
2181:7C5D F3 A4 repz movsb ;copy old param to new
2181:7C5F 06 push es
2181:7C60 1F pop ds ;DS=0
2181:7C61 BD 00 7C mov bp,7C00 ;BP=7C00
2181:7C64 C6 45 FE 0F mov byte [di-02],000F ;modify floppy param
2181:7C68 38 4E 24 cmp [bp+24],cl ;drive is floppy?
2181:7C6B 7D 20 jge 7C8D ;yes. jump.
hard drive: check partition table for EBIOS indicator
2181:7C6D 8B C1 mov ax,cx ;AX=CX=0
2181:7C6F 99 cwd ;DX=0
2181:7C70 E8 7E 01 call 7DF1 ;read the MBR sector
2181:7C73 83 EB 3A sub bx,003A ;startLBA of entry 0
2181:7C76 66 A1 1C 7C mov eax,[7C1C] ;EAX=hidden sectors
2181:7C7A 66 3B 07 cmp eax,[bx] ;match?
2181:7C7D 8A 57 FC mov dl,[bx-04] ;DL=partition type
2181:7C80 75 06 jnz 7C88 ;not match, skip
2181:7C82 80 CA 02 or dl,0002 ;if the type is 0C
;let it be 0E
2181:7C85 88 56 02 mov [bp+02],dl ;set EBIOS flag in 7C02
2181:7C88 80 C3 10 add bl,0010 ;next entry
2181:7C8B 73 ED jnb 7C7A ;carry set if done
the real deal begins here
2181:7C8D 33 C9 xor cx,cx ;CX=0
2181:7C8F FE 06 D8 7D inc byte [7DD8] ;Change IO to JO
2181:7C93 8A 46 10 mov al,[bp+10] ;AL=number of FATs
2181:7C96 98 cbw ;AX=number of FATs
2181:7C97 F7 66 16 mul word [bp+16] ;sectors per FAT
;DX:AX=sectors in FATs
2181:7C9A 03 46 1C add ax,[bp+1C]
2181:7C9D 13 56 1E adc dx,[bp+1E] ;Add hidden sectors
2181:7CA0 03 46 0E add ax,[bp+0E] ;Add reserved sectors
2181:7CA3 13 D1 adc dx,cx ;DX:AX->root dir
2181:7CA5 8B 76 11 mov si,[bp+11] ;SI=root dir entries
2181:7CA8 60 pushaw
2181:7CA9 89 46 FC mov [bp-04],ax ;
2181:7CAC 89 56 FE mov [bp-02],dx ;save DX:AX on stack
2181:7CAF B8 20 00 mov ax,0020 ;AX=dir entry length
2181:7CB2 F7 E6 mul si ;DX:AX=root dir size
2181:7CB4 8B 5E 0B mov bx,[bp+0B] ;BX=sector size
2181:7CB7 03 C3 add ax,bx
2181:7CB9 48 dec ax
2181:7CBA F7 F3 div bx ;AX=root dir sectors
2181:7CBC 01 46 FC add [bp-04],ax
2181:7CBF 11 4E FE adc [bp-02],cx ;stack->Data area
2181:7CC2 61 popaw
2181:7CC3 BF 00 07 mov di,0700 ;DI=0700
2181:7CC6 E8 28 01 call 7DF1 ;read root dir
2181:7CC9 72 3E jb 7D09
2181:7CCB 38 2D cmp [di],ch
2181:7CCD 74 17 jz 7CE6 ;failure find file
2181:7CCF 60 pushaw
2181:7CD0 B1 0B mov cl,000B ;find kernel filename
2181:7CD2 BE D8 7D mov si,7DD8
2181:7CD5 F3 A6 repz cmpsb
2181:7CD7 61 popaw
2181:7CD8 74 3D jz 7D17 ;found, jump
2181:7CDA 4E dec si ;SI=rest entries
2181:7CDB 74 09 jz 7CE6 ;not found, failure
2181:7CDD 83 C7 20 add di,0020 ;next entry
2181:7CE0 3B FB cmp di,bx ;end?
2181:7CE2 72 E7 jb 7CCB ;no, check the entry
2181:7CE4 EB DD jmp 7CC3 ;yes, load 1 sector
2181:7CE6 FE 0E D8 7D dec byte [7DD8] ;Change JO to IO
2181:7CEA 7B A7 jnp 7C93 ;find again
2181:7CEC BE 7F 7D mov si,7D7F ;not found
print error message:
SI=7D7F "invalid system disk"
SI=7D80 "Disk I/O error"
SI=7D82 "Replace the disk, and then press any key..."
2181:7CEF AC lodsb
2181:7CF0 98 cbw
2181:7CF1 03 F0 add si,ax
display string DS:SI, if encountered FF, then
display "Replace the disk, and then press any key..."
if encountered 00, wait for a key press and reboot.
2181:7CF3 AC lodsb
2181:7CF4 98 cbw
2181:7CF5 40 inc ax
2181:7CF6 74 0C jz 7D04
2181:7CF8 48 dec ax
2181:7CF9 74 13 jz 7D0E
display char in AL
2181:7CFB B4 0E mov ah,000E
2181:7CFD BB 07 00 mov bx,0007
2181:7D00 CD 10 int 10
2181:7D02 EB EF jmp 7CF3
display "Replace the disk, and then press any key..."
2181:7D04 BE 82 7D mov si,7D82
2181:7D07 EB E6 jmp 7CEF
display "Disk I/O error"
2181:7D09 BE 80 7D mov si,7D80
2181:7D0C EB E1 jmp 7CEF
2181:7D0E CD 16 int 16
2181:7D10 5E pop si
2181:7D11 1F pop ds ;DS:SI=0000:0078
2181:7D12 66 8F 04 pop dword [si] ;restore original...
;...floppy params
2181:7D15 CD 19 int 19
the kernel file was found
2181:7D17 BE 81 7D mov si,7D81 ;error message
2181:7D1A 8B 7D 1A mov di,[di+1A] ;start cluster
2181:7D1D 8D 45 FE lea ax,[di-02]
2181:7D20 8A 4E 0D mov cl,[bp+0D] ;sectors per cluster
2181:7D23 F7 E1 mul cx ;DX:AX=sectors
2181:7D25 03 46 FC add ax,[bp-04]
2181:7D28 13 56 FE adc dx,[bp-02] ;DX:AX=LBA
2181:7D2B B1 04 mov cl,0004 ;read 4 sectors
2181:7D2D E8 C2 00 call 7DF2 ;... to 0000:0700
2181:7D30 72 D7 jb 7D09
Transfer control to the second sector of IO.SYS, with
SS=DS=ES=0000
SP=7BF4
BP=7C00
SI=error message "invalid system disk"
DI=start cluster number of IO.SYS
[SP+00]=[7BF4]=0078
[SP+02]=[7BF6]=0000
[SP+04]=[7BF8]=OFFSET of FLOPPY PARAMS
[SP+06]=[7BFA]=SEGMENT of FLOPPY PARAMS
[SP+08]=[7BFC]=[BP-4]=LBA of data area
2181:7D32 EA 00 02 70 00 jmp 0070:0200 ;CS:IP=0070:0200
2181:7D37 52 push dx ;DX:AX=LBA lo 32 bits
2181:7D38 50 push ax
2181:7D39 06 push es ;ES:BX=buffer
2181:7D3A 53 push bx
2181:7D3B 6A 01 push 0001 ;read 1 sector
2181:7D3D 6A 10 push 0010 ;EBIOS packet size
begin geometry translation: LBA to CHS
2181:7D3F 91 xchg ax,cx ;save AX to CX
2181:7D40 8B 46 18 mov ax,[bp+18] ;AX=sectors per track
2181:7D43 A2 26 05 mov [0526],al ;modify floppy param
2181:7D46 96 xchg ax,si ;SI=sectors per track
2181:7D47 92 xchg ax,dx ;AX=DX
2181:7D48 33 D2 xor dx,dx
2181:7D4A F7 F6 div si ;AX=quo, DX=res
2181:7D4C 91 xchg ax,cx ;restore AX from CX
;CX=quo
;DX:AX=the rest sectors
2181:7D4D F7 F6 div si ;AX=quo, DX=res
2181:7D4F 42 inc dx ;DX=sector
2181:7D50 87 CA xchg cx,dx ;CX=sector, DX=old quo
;DX:AX=quo
2181:7D52 F7 76 1A div word [bp+1A] ;heads
;AX=cyl, DX=head
2181:7D55 8A F2 mov dh,dl ;DH=head
2181:7D57 8A E8 mov ch,al ;CH=cyl. lo 8 bits
2181:7D59 C0 CC 02 ror ah,02 ;AH=cyl. hi 2 bits
2181:7D5C 0A CC or cl,ah ;put into CL
end geometry translation: LBA to CHS
2181:7D5E B8 01 02 mov ax,0201 ;read 1 sector
2181:7D61 80 7E 02 0E cmp byte [bp+02],000E ;USE EBIOS?
2181:7D65 75 04 jnz 7D6B ;no, skip
2181:7D67 B4 42 mov ah,0042 ;yes, set AH=42
2181:7D69 8B F4 mov si,sp ;DS:SI->packet
2181:7D6B 8A 56 24 mov dl,[bp+24] ;drive number
2181:7D6E CD 13 int 13 ;BIOS disk call
2181:7D70 61 popaw ;discard packet
2181:7D71 61 popaw ;restore regs
2181:7D72 72 0A jb 7D7E ;failure jump
2181:7D74 40 inc ax ;next sector
2181:7D75 75 01 jnz 7D78
2181:7D77 42 inc dx
2181:7D78 03 5E 0B add bx,[bp+0B] ;buffer
2181:7D7B 49 dec cx
2181:7D7C 75 77 jnz 7DF5 ;continue to read
;until done
2181:7D7E C3 ret
2181:7D7F 03 .
2181:7D80 18 01 27 0D-0A 49 6E 76-61 6C 69 64-20 73 79 73 ..'..Invalid sys
2181:7D90 74 65 6D 20-64 69 73 6B-FF 0D 0A 44-69 73 6B 20 tem disk...Disk
2181:7DA0 49 2F 4F 20-65 72 72 6F-72 FF 0D 0A-52 65 70 6C I/O error...Repl
2181:7DB0 61 63 65 20-74 68 65 20-64 69 73 6B-2C 20 61 6E ace the disk, an
2181:7DC0 64 20 74 68-65 6E 20 70-72 65 73 73-20 61 6E 79 d then press any
2181:7DD0 20 6B 65 79-0D 0A 00 00-49 4F 20 20-20 20 20 20 key....IO
2181:7DE0 53 59 53 4D-53 44 4F 53-20 20 20 53-59 53 7F 01 SYSMSDOS SYS.
2181:7DF0 00 .
subroutine: read sectors
2181:7DF1 41 inc cx ;CX=1
CX=sectors to read
2181:7DF2 BB 00 07 mov bx, 0700 ;ES:BX=0000:0700
BX=buffer
2181:7DF5 60 pushaw
2181:7DF6 66 6A 00 push 00000000
2181:7DF9 E9 3B FF jmp 7D37
2181:7DFC 00 00 55 AA ..U.
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