Author: David Zimmer
Date: 08.01.11 - 7:26pm

Finally getting around to trying my hand at some bootloader development stuff. The world of 16 bit real mode is new to me so its some interesting research. Originally i was hoping to write a shellcode to embed a 16bit com screen saver to the MBR as a fun test, so far I have only found one or two which work in the bootloader envirnoment. (out of about 20 tested) are some cool links that I have stubled across so far. Will update this post as i find more worth mentioning...

Another article that has some interesting bits about segmented programming and how the selectors work (for those of us who have never had to do it) is here:

The Real, Protected, Long mode assembly tutorial for PCs

When DI is used as an index, ES holds the default segment. When BP is used as an index, SS is the default segment. In all other cases, DS is the default segment. Note that not every register can be used as an index in real mode, for example, mov ax,[dx] is not valid in real mode.

One other note, writing to the MBR on XP was more fickle than i initially thought. You have to use just the right flags, and even just the right buffer size for it to work. Probably should have written that in C to start with so I didnt have to debug all that in asm shellcode :-\

Since i had to write a larger block than i expected at once, I also ended up wiping out the partition table on first attempts, which means /fixmbr was no help to remove it. oops..I dont know what I would do without vmware.

Although i will not be making that shellcode available. here is a hexdump of one of the bootloaders i made to play with it. (doesnt run the same as a .COM file have to run as bootloader)


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