[AlecS]

Although G-code itself isn't even fully standardized, there is no standard command for ellipse's, parabola's, etc. The closest thing you may find is NURBs interpolation, but this varies from controller to controller. NURB's commands work kind of like the spline tool in CAD programs, but again the exact implementation will vary, if it even exists on your machine. They are however, not very common in most basic machine work, and only really come into use in large die/mold work.

The most common way to do this is to use a CAM program (HSMworks is free) to output the curve. The CAM program will use either lines or arcs to approximate the curve. Sadly, if you're trying to write your own programs, you are pretty much out of luck when it comes to complex curves.