MBR(Master Boot Record) please help!

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Hi
I want to disassemble a MBR file and understand what that code does,
In particular I want to know the role of each line of the code (or
each logically connected fragment of it) in the boot sequence and why
that line is needed.
You can download the MBR file that I choose from the following link:
   http://s15.quicksharing.com/v/3894883/final.bin.html

Here is the disassembled of the .bin file
00000000  EB48              jmp short 0x4a
00000002  90                nop
00000003  108ED0BC          adc [bp+0xbcd0],cl
00000007  00B0B800          add [bx+si+0xb8],dh
0000000B  008ED88E          add [bp+0x8ed8],cl
0000000F  C0FBBE            sar bl,0xbe
00000012  007CBF            add [si-0x41],bh
00000015  0006B900          add [0xb9],al
00000019  02F3              add dh,bl
0000001B  A4                movsb
0000001C  EA21060000        jmp 0x0:0x621
00000021  BEBE07            mov si,0x7be
00000024  3804              cmp [si],al
00000026  750B              jnz 0x33
00000028  83C610            add si,byte +0x10
0000002B  81FEFE07          cmp si,0x7fe
0000002F  75F3              jnz 0x24
00000031  EB16              jmp short 0x49
00000033  B402              mov ah,0x2
00000035  B001              mov al,0x1
00000037  BB007C            mov bx,0x7c00
0000003A  B280              mov dl,0x80
0000003C  8A7403            mov dh,[si+0x3]
0000003F  02800000          add al,[bx+si+0x0]
00000043  808B820000        or byte [bp+di+0x82],0x0
00000048  0008              add [bx+si],cl
0000004A  FA                cli
0000004B  EA507C0000        jmp 0x0:0x7c50
00000050  31C0              xor ax,ax
00000052  8ED8              mov ds,ax
00000054  8ED0              mov ss,ax
00000056  BC0020            mov sp,0x2000
00000059  FB                sti
0000005A  A0407C            mov al,[0x7c40]
0000005D  3CFF              cmp al,0xff
0000005F  7402              jz 0x63
00000061  88C2              mov dl,al
00000063  52                push dx
00000064  BE767D            mov si,0x7d76
00000067  E83401            call 0x19e
0000006A  F6C280            test dl,0x80
0000006D  7454              jz 0xc3
0000006F  B441              mov ah,0x41
00000071  BBAA55            mov bx,0x55aa
00000074  CD13              int 0x13
00000076  5A                pop dx
00000077  52                push dx
00000078  7249              jc 0xc3
0000007A  81FB55AA          cmp bx,0xaa55
0000007E  7543              jnz 0xc3
00000080  A0417C            mov al,[0x7c41]
00000083  84C0              test al,al
00000085  7505              jnz 0x8c
00000087  83E101            and cx,byte +0x1
0000008A  7437              jz 0xc3
0000008C  668B4C10          mov ecx,[si+0x10]
00000090  BE057C            mov si,0x7c05
00000093  C644FF01          mov byte [si-0x1],0x1
00000097  668B1E447C        mov ebx,[0x7c44]
0000009C  C7041000          mov word [si],0x10
000000A0  C744020100        mov word [si+0x2],0x1
000000A5  66895C08          mov [si+0x8],ebx
000000A9  C744060070        mov word [si+0x6],0x7000
000000AE  6631C0            xor eax,eax
000000B1  894404            mov [si+0x4],ax
000000B4  6689440C          mov [si+0xc],eax
000000B8  B442              mov ah,0x42
000000BA  CD13              int 0x13
000000BC  7205              jc 0xc3
000000BE  BB0070            mov bx,0x7000
000000C1  EB7D              jmp short 0x140
000000C3  B408              mov ah,0x8
000000C5  CD13              int 0x13
000000C7  730A              jnc 0xd3
000000C9  F6C280            test dl,0x80
000000CC  0F84F300          jz near 0x1c3
000000D0  E98D00            jmp 0x160
000000D3  BE057C            mov si,0x7c05
000000D6  C644FF00          mov byte [si-0x1],0x0
000000DA  6631C0            xor eax,eax
000000DD  88F0              mov al,dh
000000DF  40                inc ax
000000E0  66894404          mov [si+0x4],eax
000000E4  31D2              xor dx,dx
000000E6  88CA              mov dl,cl
000000E8  C1E202            shl dx,0x2
000000EB  88E8              mov al,ch
000000ED  88F4              mov ah,dh
000000EF  40                inc ax
000000F0  894408            mov [si+0x8],ax
000000F3  31C0              xor ax,ax
000000F5  88D0              mov al,dl
000000F7  C0E802            shr al,0x2
000000FA  668904            mov [si],eax
000000FD  66A1447C          mov eax,[0x7c44]
00000101  6631D2            xor edx,edx
00000104  66F734            div dword [si]
00000107  88540A            mov [si+0xa],dl
0000010A  6631D2            xor edx,edx
0000010D  66F77404          div dword [si+0x4]
00000111  88540B            mov [si+0xb],dl
00000114  89440C            mov [si+0xc],ax
00000117  3B4408            cmp ax,[si+0x8]
0000011A  7D3C              jnl 0x158
0000011C  8A540D            mov dl,[si+0xd]
0000011F  C0E206            shl dl,0x6
00000122  8A4C0A            mov cl,[si+0xa]
00000125  FEC1              inc cl
00000127  08D1              or cl,dl
00000129  8A6C0C            mov ch,[si+0xc]
0000012C  5A                pop dx
0000012D  8A740B            mov dh,[si+0xb]
00000130  BB0070            mov bx,0x7000
00000133  8EC3              mov es,bx
00000135  31DB              xor bx,bx
00000137  B80102            mov ax,0x201
0000013A  CD13              int 0x13
0000013C  722A              jc 0x168
0000013E  8CC3              mov bx,es
00000140  8E06487C          mov es,[0x7c48]
00000144  60                pusha
00000145  1E                push ds
00000146  B90001            mov cx,0x100
00000149  8EDB              mov ds,bx
0000014B  31F6              xor si,si
0000014D  31FF              xor di,di
0000014F  FC                cld
00000150  F3A5              rep movsw
00000152  1F                pop ds
00000153  61                popa
00000154  FF26427C          jmp near [0x7c42]
00000158  BE7C7D            mov si,0x7d7c
0000015B  E84000            call 0x19e
0000015E  EB0E              jmp short 0x16e
00000160  BE817D            mov si,0x7d81
00000163  E83800            call 0x19e
00000166  EB06              jmp short 0x16e
00000168  BE8B7D            mov si,0x7d8b
0000016B  E83000            call 0x19e
0000016E  BE907D            mov si,0x7d90
00000171  E82A00            call 0x19e
00000174  EBFE              jmp short 0x174
00000176  47                inc di
00000177  52                push dx
00000178  55                push bp
00000179  42                inc dx
0000017A  2000              and [bx+si],al
0000017C  47                inc di
0000017D  656F              gs outsw
0000017F  6D                insw
00000180  004861            add [bx+si+0x61],cl
00000183  7264              jc 0x1e9
00000185  204469            and [si+0x69],al
00000188  736B              jnc 0x1f5
0000018A  005265            add [bp+si+0x65],dl
0000018D  61                popa
0000018E  640020            add [fs:bx+si],ah
00000191  45                inc bp
00000192  7272              jc 0x206
00000194  6F                outsw
00000195  7200              jc 0x197
00000197  BB0100            mov bx,0x1
0000019A  B40E              mov ah,0xe
0000019C  CD10              int 0x10
0000019E  AC                lodsb
0000019F  3C00              cmp al,0x0
000001A1  75F4              jnz 0x197
000001A3  C3                ret
000001A4  0000              add [bx+si],al
000001A6  0000              add [bx+si],al
000001A8  0000              add [bx+si],al
000001AA  0000              add [bx+si],al
000001AC  0000              add [bx+si],al
000001AE  0000              add [bx+si],al
000001B0  0000              add [bx+si],al
000001B2  0000              add [bx+si],al
000001B4  0000              add [bx+si],al
000001B6  0000              add [bx+si],al
000001B8  0000              add [bx+si],al
000001BA  0000              add [bx+si],al
000001BC  0000              add [bx+si],al
000001BE  800101            add byte [bx+di],0x1
000001C1  0083FE3F          add [bp+di+0x3ffe],al
000001C5  0C3F              or al,0x3f
000001C7  0000              add [bx+si],al
000001C9  008E2F03          add [bp+0x32f],cl
000001CD  0000              add [bx+si],al
000001CF  0001              add [bx+di],al
000001D1  0D83FE            or ax,0xfe83
000001D4  BFDECD            mov di,0xcdde
000001D7  2F                das
000001D8  0300              add ax,[bx+si]
000001DA  52                push dx
000001DB  FC                cld
000001DC  B000              mov al,0x0
000001DE  0000              add [bx+si],al
000001E0  81DF82FE          sbb di,0xfe82
000001E4  FF0E1F2C          dec word [0x2c1f]
000001E8  B400              mov ah,0x0
000001EA  30C4              xor ah,al
000001EC  0B00              or ax,[bx+si]
000001EE  0000              add [bx+si],al
000001F0  0000              add [bx+si],al
000001F2  0000              add [bx+si],al
000001F4  0000              add [bx+si],al
000001F6  0000              add [bx+si],al
000001F8  0000              add [bx+si],al
000001FA  0000              add [bx+si],al
000001FC  0000              add [bx+si],al
000001FE  55                push bp
000001FF  AA                stosb


Re: MBR(Master Boot Record) please help!
Hello,

Quoted text here. Click to load it

Then you are wrong here.
There are many web sites dedicated to operating system programming, e.g.
http://www.osdever.net /. These are more appropriate for you.

Quoted text here. Click to load it

I see many lines with

add [bx+si],al

and I think that means you have disassembled the file without taking care of
strings or other variables. This way you can destroy everything, because
instructions have different opcode lengths on x86. Then a charachter (which
isn't executed, of course) may be recognized as a two-byte opcode,
disassembled, and disturb the following opcode (which began one byte ago).
So your disassembly is useless, and I wouldn't expect to get a solution
here.

Regards,
Sebastian



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