plotting implicit functions



Hi everyone, I'm sorry if my english is not perfect but I'll try my best!
What I was trying to do is plotting a function which is defined as 
implicit, such as x^2+y^2=1.
I searched the web and found a method, which is:

load(draw)
draw2d(implicit(x^2+y^2=1, x, -2, 2, y, -2, 2))

and this works nicely. Anyway I would like a higher precision since the 
function is plotted with some angles and straight lines.
Is there a way to obtain this?
Thank you, bye!