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in general you always draw in system memory and every now and then ( say 50 times or 25 times a second)

you order sdl (or whatever thing) to draw your buffer on the screen.

yes this takes some cpu power.
but some intellegent programming

(like keeping the background in a buffer copying it each time (then drawing the characters on it) ) will safe quite a lot of power.)

copying of data goes very fast. (ram to ram)

with opengl you can probably save the background in a few textures and overlay the image on it.

It's also not very difficult, but that depends on your programming experience.