I used to play with graphics in dos quite a lot on pc's, using assembler. I found interrupts were hopeless. There is an interrupt for drawing a pixel or reading a pixel, and it works in any normal screen mode, but it is so slow it is painful to watch. There's not a lot of point in bursting into assembler and then sacrificing speed. The bios graphic interrupts are more useful for things like setting a mode, which you only do once at the start of the programme, and which otherwise involves quite a detailed knowledge of a VGA card. But all this is a bit obsolete now.