MBR says Invalid partition table - But system works! WHY?

Question

I have been given an end of term project to write an assembly code to read the MBR and save it to floppy. I think I managed to read it with INT 13h in DOS in a virtualBox machine. The machine has only one hard disk with one partition with XP installed. When I read the MBR and print it, it gives me a whole lot of junk and amongst the junk it says: Invalid partition table. Error loading operating system... just like in this website: http://mbr.adamsatoms.com But my system boots fine. Did I read the MBR correctly. Is it really the MBR? Why does this happen? This is part of my code for reading if it helps:

        mov dx,80h
        mov cx,1
        mov bx,ds
        mov es,bx
        mov bx,offset result
        mov ax,0201h
        int 13h     

Thanks in advance.

Answer 1

You're seeing part of the boot loader. This is a piece of program stored in the first sectors of your drive, which' job it is to read the partition table and start booting the OS. If this fails, it shows the error messages for which you just found the source... The rest of the 'junk' will be code of the bootloader (in machine code), you'll need to disassemble it for it to make sense.

Answer 2

It's normal, the "garbage" is the machine code that composes the MBR, and the various error strings are there to be displayed if the MBR code encounters some problems while trying to boot the PC. The full analysis of the MBR code is exactly at the page you said.