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

Question

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.}

Answer 1

If you want to do it yourself, I'd say just do it.

I guess you wont find too many resources or tutorial because there is not much to it.

Here is an outline:

• adding/subtracting:
  if exponents differ too much (more than the mantissa has bits):
  just return the value with larger exponent (if this is the subtrahend: negate)

  if exponents are similar:
  shift the mantissa of smaller value by difference of exponents and add to/subtract from other mantissa (using fixed point arithmetic)
  if result is not 0: shift mantissa up until MSBit of result is 1 and decrement exponent the same amount

• multiplication/division:
  multiply/divide mantissas (using fixed point arithmetic) and add/subtract exponents

Answer 2

You should be able to use this multiple-precision floating-point library on the Propeller, with either Catalina C or gcc. It might be too slow for many applications, however.

Answer 3

It looks like this question has taken on the slant of seeking understanding more than solving the problem, so this may not be the most useful answer, but just for sake of completeness

http://code.google.com/p/propgcc/wiki/PropGccCompileOptions

Implies that the propeller version of GCC has floating point support, including doubles.

GCC and its compiler libraries are of course open source, though there may be a real learning curve before you could start to see what the code is doing.