Spiral Draw

[Cezna]

This is a program I made to experiment with drawing circles manually (as opposed to using the drawoval procedure)
Right now it draws a spiral, but you can make it draw a circle by setting rchange to 0.
Pretty much all of the variables of consequence are at the top where they are easy to change.
If r becomes less than or equal to x, it draws a straight vertical line instead of continuing to spiral. If anyone can figure out why (I have a vague idea, but I need to know exactly why), please tell me.
It's not very efficient either, of that I am sure, so if anyone can improve, please post the code.

So here it is, feel free to do whatever you want with it:
EDIT: just learned that I just use the code tag, so I fixed that to make it more readable.

code:

var x, y, r, xprev, yprev, xchange, rchange, linethickness, centx, centy, minr : real var clr : int r := 50 x := 50 y := 200 xprev := x + 320 yprev := y centx := 320 centy := 200 xchange := .002 % this determines how fast the is drawn rchange := .0005 % this determines how fast the spiral's radius expands minr := 10 linethickness := 20 clr := 52 colourback (black) cls loop     % quad 1     if x > 0 and y < 0 then         x += xchange         if (r ** 2 - x ** 2) > 0 then             y := (sqrt (r ** 2 - x ** 2) * -1)         else             y := (sqrt ((r ** 2 - x ** 2) * -1))         end if         % quad 2     elsif x > 0 and y > 0 then         x -= xchange         if (r ** 2 - x ** 2) > 0 then             y := sqrt (r ** 2 - x ** 2)         else             y := (sqrt ((r ** 2 - x ** 2) * -1) * -1)         end if         % quad 3     elsif x < 0 and y > 0 then         x -= xchange         if (r ** 2 - x ** 2) > 0 then             y := sqrt (r ** 2 - x ** 2)         else             y := ((sqrt ((r ** 2 - x ** 2) * -1)) * -1)         end if         % quad 4     elsif x < 0 and y < 0 then         x += xchange         if (r ** 2 - x ** 2) > 0 then             y := (sqrt (r ** 2 - x ** 2) * -1)         else             y := ((sqrt ((r ** 2 - x ** 2) * -1)) * -1)         end if     end if     if r > minr and r > x then         r += rchange     end if     if r <= minr then         r := minr     end if     if r <= x then         xchange += .0001     end if     Draw.ThickLine (round (xprev), round (yprev), round (x + centx), round (y + centy), round (linethickness), clr)     xprev := x + centx     yprev := y + centy end loop