unitsize(1cm);

draw(circle(c=(0,0),r=2),1bp+yellow);

shipout(bbox(1cm,Fill));

/* 
frame bbox(picture pic=currentpicture, 
           real xmargin=0, real ymargin=xmargin,
           pen p=currentpen, filltype filltype=NoFill)

shipout(string prefix=defaultfilename, 
        frame f,
        string format="", 
        bool wait=false, bool view=true,
        string options="", string script="",
        projection P=currentprojection)
*/