If you REALLY want to do double calcs on a 8051 - either find a compiler that has a double library or write your own. Writing your own is not a trivial exercise but not too onerous. One would need to do a bit of research and maybe port a library from another architecture. The easiest way would just be to chose a more suitable processor for your task - something like a LPC21xx from Philips should be a better choice as its a 32 bit micro at a 8051 price. The arm gcc compiler has double support (I think).