How can i create a line drawing method?

i am making an os in cosmos and for the gui i need a way to draw a line with a given start point, x and y, and an end point, xEnd and yEnd, and then draw a straight line from one point to the other. so if the Xs are the same it would draw a vertical line but if Xs are different and Ys are different it will draw smooth and equal. ill need the raw source code so like a bunch of loops and algorithms