China DOS Union

Sender: Netguy (Dream Maker), Board: DOS
Title: Analysis of the Hard Disk Master Boot Program
Posted from: Shuimu Tsinghua BBS (Wed Aug 27 13:32:41 1997)

If booting from the hard disk, then
when the machine starts, the ROM BIOS program loads the master boot program from cylinder 0, head 0, sector 1 of the hard disk into memory at 0000:7C00, and
begins executing it. Therefore, the absolute address of the first instruction CLI below is 0000:7C00H.
By modifying the master boot sector, a certain encryption effect can be achieved; if used by a virus...

Offset Machine code Symbolic instruction Explanation
==============================================================================
0000 FA CLI ;disable interrupts
0001 33C0 XOR AX,AX
0003 8ED0 MOV SS,AX ;(SS)=0000H
0005 BC007C MOV SP,7C00 ;(SP)=7C00H
0008 8BF4 MOV SI,SP ;(SI)=7C00H
000A 50 PUSH AX
000B 07 POP ES ;(ES)=0000H
000C 50 PUSH AX
000D 1F POP DS ;(DS)=0000H
000E FB STI
000F FC CLD
0010 BF0006 MOV DI,0600
0013 B90001 MOV CX,0100 ;512 bytes total
0016 F2 REPNZ
0017 A5 MOVSW ;the master boot program moves itself from 0000:7C00
;to 0000:0600, making room for the DOS partition's boot
;program
0018 EA1D060000 JMP 0000:061D ;jump to 0000:061D to continue execution, actually
;executing the MOV instruction below (at offset 001D)
001D BEBE07 MOV SI,07BE ;07BE-0600=01BE, 01BE is the starting address of the partition table
0020 B304 MOV BL,04 ;the partition table has at most 4 entries, i.e. at most 4 partitions
0022 803C80 CMP BYTE PTR ,80 ;80H indicates the active partition
0025 740E JZ 0035 ;if the active partition is found, jump away
0027 803C00 CMP BYTE PTR ,00 ;00H is the marker for a valid partition
002A 751C JNZ 0048 ;if it is neither 80H nor 00H, then the partition table is invalid
002C 83C610 ADD SI,+10 ;next partition table entry, 16 bytes per entry
002F FECB DEC BL ;decrement loop counter
0031 75EF JNZ 0022 ;check the next partition table entry
0033 CD18 INT 18 ;if none of the 4 can boot, enter ROM Basic
0035 8B14 MOV DX,
0037 8B4C02 MOV CX, ;get the head, cylinder, and sector of the active partition's boot sector
003A 8BEE MOV BP,SI ;then continue checking the remaining partition table entries
003C 83C610 ADD SI,+10
003F FECB DEC BL
0041 741A JZ 005D ;if all 4 have been checked, go boot the active partition
0043 803C00 CMP BYTE PTR ,00 ;00H is the valid-partition marker
0046 74F4 JZ 003C ;if this partition table entry is valid, continue checking the next one
0048 BE8B06 MOV SI,068B ;068B-0600=018B, fetch the "invalid partition" string
004B AC LODSB ;fetch one character from the string
004C 3C00 CMP AL,00 ;00H indicates end of string
004E 740B JZ 005B ;if the string has been fully displayed, enter an infinite loop
0050 56 PUSH SI
0051 BB0700 MOV BX,0007
0054 B40E MOV AH,0E
0056 CD10 INT 10 ;display one character
0058 5E POP SI
0059 EBF0 JMP 004B ;loop to display the next character
005B EBFE JMP 005B ;infinite loop here
005D BF0500 MOV DI,0005 ;read the active partition's boot sector, try at most 5 times
0060 BB007C MOV BX,7C00
0063 B80102 MOV AX,0201
0066 57 PUSH DI
0067 CD13 INT 13 ;read
0069 5F POP DI
006A 730C JNB 0078 ;if disk read succeeds, jump away
006C 33C0 XOR AX,AX
006E CD13 INT 13 ;if read fails, reset the disk
0070 4F DEC DI
0071 75ED JNZ 0060 ;if fewer than 5 times, try reading again
0073 BEA306 MOV SI,06A3 ;06A3-0600=00A3, i.e. the "Error loading" string
0076 EBD3 JMP 004B ;go display the string, then enter an infinite loop
0078 BEC206 MOV SI,06C2 ;06C2-0600=00C2, i.e. the "Missing.." string
0076 EBD3 JMP 004B ;go display the string, then enter an infinite loop
0078 BEC206 MOV SI,06C2 ;06C2-0600=00C2, i.e. the "Missing.." string
007B BFFE7D MOV DI,7DFE ;7DFE-7C00=01FE, i.e. the starting address of the last
;two bytes of the active partition's boot sector
007E 813D55AA CMP WORD PTR ,AA55;the last two bytes are valid if they are AA55H
0082 75C7 JNZ 004B ;if invalid, display the string and enter an infinite loop
0084 8BF5 MOV SI,BP
0086 EA007C0000 JMP 0000:7C00 ;if valid, jump to boot that partition
0080 49 6E 76 61 6C Inval
0090 69 64 20 70 61 72 74 69-74 69 6F 6E 20 74 61 62 id partition tab
00A0 6C 65 00 45 72 72 6F 72-20 6C 6F 61 64 69 6E 67 le.Error loading
00B0 20 6F 70 65 72 61 74 69-6E 67 20 73 79 73 74 65 operating syste
00C0 6D 00 4D 69 73 73 69 6E-67 20 6F 70 65 72 61 74 m.Missing operat
00D0 69 6E 67 20 73 79 73 74-65 6D 00 00 FB 4C 38 1D ing system...L8.
00E0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
00F0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
0100 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
0110 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
0120 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
0130 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
0140 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
0150 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
0160 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
0170 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
0180 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
0190 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
01A0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
01B0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 80 01 ................;partition table
01C0 01 00 06 0F 7F 9C 3F 00-00 00 F1 59 06 00 00 00 ......?....Y....
01D0 41 9D 05 0F FF 38 30 5A-06 00 40 56 06 00 00 00 A....80Z..@V....
01E0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
01F0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 55 AA ..............U.

I still need to look up some materials on the DOS boot program, maybe next time.
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※ Source:·Shuimu Tsinghua BBS bbs.net.tsinghua.edu.cn·

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