circle / oval / semicircle ?

hi:  where would I find facilities to draw circles, ovals, and
semicircles?   (or should I construct them myself using curve?)

The latter probably, but using lines rather than curve (these being 
implicit/parametric functions). e.g. 

tt = seq(0, 2 * pi, length = 50)
plot(cos(tt), sin(tt), type = 'l')

Deepayan