Although I probably get flamed for telling you this, it is the truth:
TSR's[certainly big ones involving DOS) are written in asm.
One reason is size, but more important reason is that TSR's
take over code that originally is also written in asm(like bios code
or other hardware interactions).
A screen saver TSR is probably one of the most easy, and most
done. You can look up plenty of sources at this place:

http://www.*-*-*.com/

If you want to understand TSR's really then the art of assembly
language has an excellent chapter on it, and also the MASM
documentation has a big chapter devoted to it.

Last preleminary note: If you are running windows you can make
your TSR appear in every DOSbox by installing it BEFORE loading
vmm.inc(which in fact is the working horse of win) either manually
or in autoexec.bat.

Rick
[Not reading is your problem, but you share it with a lot;)) ]