No Active Partition Found
(Pathways through the Windows 7 MBR)

Copyright©2013 by Daniel B. Sedory

This Page is New and Under Construction !!!

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

When each of the four 16-byte Partition Table entries (there can only be 4 entries under the standard 'Basic Disk'/MBR Partitioning Scheme) has only zero (00 hex) as its first byte; that is, no Active partition is found, the PC's BIOS will automatically jump to Interrupt 18h (often labeled as "ROM BASIC" since the original IBM® PC's XT and AT models contained the BASIC Programming Language in ROM chips). Starting back at instruction 0x0623, we show a 00 byte at Memory location 0x07BE:

The code just put 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=0000  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=00

Since we're comparing zero with zero, the result of the CMP instruction is also zero, and the Zero and Parity flags both remain set (ZF=1; PF=1); with no change in the Sign flag either (sf=0). So when we execute the JL (Jump if Less than zero) instruction:

AX=0000  BX=0000  CX=0004  DX=0000  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
                                              NV UP EI PL ZR NA PE NC
0627 7C0B          JL      0634

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.

Date Page Added: July 5, 2013 (05.07.2013).

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