looking for basic exercises

[grasshopperbe]

i m looking for a link to some exercises for
dos's debug. i need it for school exams, but my course is kinda worthless
(bad teacher) .
does anyone know a good link?
i just need to be able to
use int 20 and int21, and print letters, strings on screen.
does anyone know of a good link?
maybe with some extra explanations?