Invalid Partition Entry - Pathways through Win 7 MBR

Invalid Partition Entry Found
(Pathways through the Windows 7 MBR)

Copyright©2013 by Daniel B. Sedory

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Error Messages Display Routine

If any of the four 16-byte Partition Table entries (there can only be 4 entries under the standard 'Basic Disk'/MBR Partitioning Scheme) has a 01h through 7Fh byte as its first byte, the MBR code will notify the user it has encountered an "Invalid" partition entry; this will halt execution, even if a valid Active partition entry follows the invalid one! Starting back at instruction 0x623, we show a 01 byte at Memory location 0x7BE:

 The code places 07be into the Base Pointer (BP) Register to check
  for 80h at the correct offset in the first Partition Table entry:

AX=0000  BX=0000  CX=0004  DX=0080  SP=7C00  BP=07BE  SI=7E00  DI=0800
DS=0000  ES=0000  SS=0000  CS=0000  IP=0623   of df IF sf ZF af PF cf
                                              NV UP EI PL ZR NA PE NC
0623 807E0000      CMP     BYTE PTR [BP+00],00    SS:07BE=01

So our CMP (CoMPare) instruction proceeds by subtracting the second operand (the Zero) from the first operand (the value found at Memory location 0x7BE; the 01h). Since 1 - 0 = 1, there's no change in the Sign Flag; it's still positive (sf=0), but the Zero Flag is reset (zf=0) since the result is not all zero bits and the Parity Flag is also reset (pf=0) since there's a single 1's bit (an odd number) in the result; all other flags are unaffected. This would also be true for any of the other values from 02h through 7Fh; with the only exceptions being those values where the Parity would be even (PF=1): 03, 05, 06, 09, 0Ah, 0Ch, 0Fh, etc. However, parity never affects the pathways taken by this MBR code.

AX=0000  BX=0000  CX=0004  DX=0080  SP=7C00  BP=07BE  SI=7E00  DI=0800
DS=0000  ES=0000  SS=0000  CS=0000  IP=0627   of df IF sf zf af pf cf
0627 7C0B          JL      0634               NV UP EI PL NZ NA PO NC

And since we still have a positive value (sf=0), excution simply proceeds to the next instruction:

AX=0000  BX=0000  CX=0004  DX=0080  SP=7C00  BP=07BE  SI=7E00  DI=0800
DS=0000  ES=0000  SS=0000  CS=0000  IP=0629   of df IF sf zf af pf cf
0629 0F850E01      JNZ     073B               NV UP EI PL NZ NA PO NC

Since any value from 01h through 7Fh is both positive (sf=0) and non-zero (zf=0), having such a value as a Partition Table entry's Boot Indicator byte means execution will eventually display the "Invalid partition table" message. So, the JNZ (Jump if Not Zero) instruction jumps to 0x73B:

As was shown on our Windows 7 MBR page, the following instructions set up the CPU registers used in displaying the "Invalid partition table" Error Message stored (by this time) at linear Memory location 0x763h (which is also 0000:0763 in terms of Segment:Offset) and following.

AX=0000  BX=0000  CX=0004  DX=0080  SP=7C00  BP=07BE  SI=7E00  DI=0800
DS=0000  ES=0000  SS=0000  CS=0000  IP=073B   of df IF sf zf af pf cf
                                              NV UP EI PL NZ NA PO NC
073B A0B507        MOV     AL,[07B5]            DS:07B5=63

 mov al,[07B5] copies the contents of this Memory location (a 63h in
  this case) into the Lower half (the L in AL) of the AX Register:

AX=0063  BX=0000  CX=0004  DX=0080  SP=7C00  BP=07BE  SI=7E00  DI=0800
DS=0000  ES=0000  SS=0000  CS=0000  IP=073E   of df IF sf zf af pf cf
073E 32E4          XOR     AH,AH              NV UP EI PL NZ NA PE NC

 xor ah, ah doesn't make any real change, since the High byte of AX
 is already zeroed out:

AX=0063  BX=0000  CX=0004  DX=0080  SP=7C00  BP=07BE  SI=7E00  DI=0800
DS=0000  ES=0000  SS=0000  CS=0000  IP=0740   of df IF sf zf af pf cf
0740 050007        ADD     AX,0700            NV UP EI PL NZ NA PE NC

 add ax, 0700 results in the Offset 0763h; where MBR Error Messages
    begin in Memory; this one points to: "Invalid partition table".

AX=0763  BX=0000  CX=0004  DX=0080  SP=7C00  BP=07BE  SI=7E00  DI=0800
DS=0000  ES=0000  SS=0000  CS=0000  IP=0743   of df IF sf zf af pf cf
0743 8BF0          MOV     SI,AX              NV UP EI PL NZ NA PE NC

 mov si, ax sets up the Source Index location for the routine which
   will eventaully DISPLAY the Error Message on the PC screen:

AX=0763  BX=0000  CX=0004  DX=0080  SP=7C00  BP=07BE  SI=0763  DI=0800
DS=0000  ES=0000  SS=0000  CS=0000  IP=0745   of df IF sf zf af pf cf
0745 AC            LODSB                      NV UP EI PL NZ NA PE NC

Location of English Error Messages and
Message Offsets in Memory

                3  4  5  6  7  8  9  A  B  C  D  E  F
0763           49 6E 76 61 6C 69 64 20 70 61 72 74 69     Invalid parti
0770  74 69 6F 6E 20 74 61 62 6C 65 00 45 72 72 6F 72  tion Table.Error
0780  20 6C 6F 61 64 69 6E 67 20 6F 70 65 72 61 74 69   loading operati
0790  6E 67 20 73 79 73 74 65 6D 00 4D 69 73 73 69 6E  ng system.Missin
07A0  67 20 6F 70 65 72 61 74 69 6E 67 20 73 79 73 74  g operating syst
07B0  65 6D 00 00 00 63 7B 9A                          em...c{.
       0  1  2  3  4  5  6  7  8  9  A  B  C  D  E  F

Error Messages Display Routine

This LODSB (LOaD String Byte) instruction causes data to be loaded into the AL register, one byte at a time, from a string of bytes pointed at by the Memory location currently stored in the DS:SI (Data Segment:Source Index) registers. After a byte has been copied into AL, the value in SI will be incremented by 1, since the Direction Flag bit is "UP" (df=0). The routine uses a 'Compare' and 'Conditional Jump' (JZ) to run through a string of bytes; depending on locating a ZERO byte at the end of the string, in order to exit the loop formed by the non-conditional Jump at Memory location 0x751 back to LODSB at 0x745.

Since the AL register will contain whatever byte has just been copied to it by the LODSB instruction, we've decided to represent these varying character values with 'cc' in the figures below; we are not going to step through every character of each possible message string (not even for English), since there are many possible strings depending upon the language of the MBR code being examined (i.e., French, German, Italian, Spanish, etc.). Likewise, the increasing values of the SI register are represented by 'nn'; it will vary anywhere from 63h to B3h (one byte beyond the ending zero byte of the last message string):

AX=07cc  BX=0000  CX=0004  DX=0080  SP=7C00  BP=07BE  SI=07nn  DI=0800
DS=0000  ES=0000  SS=0000  CS=0000  IP=0745   of df IF sf zf af[PF]cf
0745 AC            LODSB                      NV UP EI PL NZ NA[PE]NC

AX=07cc  BX=0000  CX=0004  DX=0080  SP=7C00  BP=07BE  SI=07nn  DI=0800
DS=0000  ES=0000  SS=0000  CS=0000  IP=0746   of df IF sf zf af[pf]cf
0746 3C00          CMP     AL,00              NV UP EI PL NZ NA[PO]NC

 cmp al,00 

AX=07cc  BX=0000  CX=0004  DX=0080  SP=7C00  BP=07BE  SI=07nn  DI=0800
DS=0000  ES=0000  SS=0000  CS=0000  IP=0748   of df IF sf[zf]af[pf]cf
0748 7409          JZ     753                 NV UP EI PL[NZ]NA[PO]NC



AX=07cc  BX=0000  CX=0004  DX=0080  SP=7C00  BP=07BE  SI=07nn  DI=0800
DS=0000  ES=0000  SS=0000  CS=0000  IP=074A   of df IF sf zf af[pf]cf
074A BB0700        MOV     BX,0007            NV UP EI PL NZ NA[PO]NC



AX=07cc  BX=0007  CX=0004  DX=0080  SP=7C00  BP=07BE  SI=07nn  DI=0800
DS=0000  ES=0000  SS=0000  CS=0000  IP=074D   of df IF sf zf af[pf]cf
074D B40E          MOV     AH,0E              NV UP EI PL NZ NA[PO]NC



AX=0Ecc  BX=0007  CX=0004  DX=0080  SP=7C00  BP=07BE  SI=07nn  DI=0800
DS=0000  ES=0000  SS=0000  CS=0000  IP=074F   of df IF sf zf af[pf]cf
074F CD10          INT     10                 NV UP EI PL NZ NA[PO]NC

More will be posted in the near future... this page is still Under Construction !!!

If you need any help in setting up the Bochs Debugger, please email us.

First Published: July 5, 2013 (05.07.2013).
Updated:.
Last Update: July 11, 2013. (11.07.2013)

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