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

Are you sure you need to write the MBR? I a disk with partitions, you can also modify the partition's VBR (Volume Boot Record). That may be easier/safer, as you don't need to touch the MBR and your machine will still able to boot to other partitions (and OS), even if you totally destroy your test partition.