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IBM PC x86 BIOS code: https://github.com/kaneton/appendix-bios https://github.com/kaneton/appendix-bios OT: This brings back memories of tinkering with the MS
by msravi 8y ago
IBM PC x86 BIOS code:
https://github.com/kaneton/appendix-bios https://github.com/kaneton/appendix-bios
OT: This brings back memories of tinkering with the MS-DOS boot process. Back then, the BIOS would read the MBR and copy its contents to 0x7C00 and start execution from there. So you could assemble your own code (using no less than MS-DOS debug) and plonk it into the MBR. I remember doing things like fooling the boot loader into thinking there's less ram than there actually was (639kB instead of 640kB) and using the unaccounted 1kB for placing your own code that could be triggered by a captured interrupt... Fun times!
- mschaef 8y agoWhat was the advantage of this over 'Terminate and Stay Resident'?
- linuxlizard 8y agoMaybe stealthiness? TSRs could be viewed with the right tools, IIRC. This seems like a technique to hide code?
- msravi 8y agoSo yes, this was a TSR, except that it resided in unaccounted for memory. Also, since you put it into the MBR, it would get loaded into ram automatically with every boot, and no fiddling with autoexec.bat or anything where it could be easily discovered. All this of course, in trying to understand the fascinating world of viruses.