Write MBR Code

Question

I am an electrical engineer who has recently discovered the need to modify the code in the MBR. Basically I need the ability to execute code on the HDD before, the OS starts up

Answer 1

If you can make a floppy, cd or memory stick that will boot to a MS command prompt, and have a matching version of MS debug, you can read and write to the MBR as below. A machine running win95 or win98 should be able to create a boot floppy for you. Just copy debug from the windows\command directory to the floppy.

inside debug: use the r command to change register values. set ax to 0201 for read, or 0301 for write. set es:bx to the starting address of the memory (buffer) you wish to use. 0000:7C00 might work, as this is typically the area that your next sector gets read to in the boot process. set cx to 0001 to read / write one sector of 512 bytes. set dx to 0080 for first physical hard drive.

use the "a" command to assemble the one line of code: INT 13h

use the "p" command to proceed. The data will be read or written, based on your choice of AX.

You could read to memory, "n" to name a file, "w" to write the file, and then edit a copy of the mbr in some other program. Once complete, use debug's "n" and "L" to name and load the edited MBR file, and call int 13h using ax= 0301h to write the image to the correct sector.