BootLoaders


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)

Anyway..here 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

[clip]
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.
[/clip]

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)

6800A00731C08ED88ED0BCFE81FBB412B336B000CD10B405B000CD10C42F
B80300CD10B30FC706BA7C00000F318A16BA7C80FA507C03E855008A36BB
7C80FE197C03E8400080FE0C750D80FA257508BEBC7CE84600EB1780FA25
751280FE0B740780FE0D7402EB06BEC77CE82D002405F6E4B700B402CD10
B409B90100CD10FE06BA7CEBA6C706BA7C000031D2C3B2008816BA7CFE06
BB7CFEC6C3B30EAC84C07415B700B402CD10B409B90100CD10FE06BA7CFE
C2EBE6B30FC3000020202046554B54202020002020202020202020202000





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