DIY FP - Implementing floating point math on a microcontroller without a hardware FPU

I'm working on a little personal project ^{_{(targeting a Parallax Propeller, but this should be pretty generalizable)}} that needs floating point math.

The micro-controller I am working with does not have native floating point, or a stock floating-point library.

Are there any good resources or tutorials on implementing software floating-point math? Preferably on really limited resources (I don't even have a stack!).

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^{I'd like to support double sized floats. The propeller is a 32 bit MCU, so I'll have to use multiple variables for each float.}

^{I know there is one software-floating-point library for the propeller out there, but it only supports single sized floats.}

^{No, I probably don't really need double sized floats, but this sounds like a really interesting project. Half the reason I want to do this is because I'll learn a lot along the way.}