Mainframe floating point math implementation.

Charles Anthony at
Sun Mar 13 17:47:39 CDT 2016

On Sun, Mar 13, 2016 at 3:16 PM, Lawrence Wilkinson <ljw-cctech at>

> I haven't gone through your code in great detail. In particular I'm
> not sure what your rounding algorithm is (towards zero, round to
> nearest, etc.) I don't see what you're using the guard bit for, I
> would expect it to be appended to the mantissa prior to addition
> and rounding. Whether that's significant would depend on how you
> expect the rounding to work.
I was experimenting with the IEEE rounding algorithms, which are driven by
guard bit, last bit, sticky bit; I left that data collection in place.
Using sticky bit got the best results.

You're not sure what the rounding algorithm is because I don't know what it
should be.

Two suggestions (which you may well have tried):
> 1. Try an extra guard bit.
> if mantissa negative and the last 3 bits shifted out are ones, then round
(for this case, set the mantissa to 0)?

> 2. Convert to sign+magnitude so you don't have to worry about negative
> mantissas (i.e. negate a negative mantissa before shifting and back
> again after alignment).
[So far] the only test failures have been with negative mantissas, so I
don't think that I gain anything by going to sign-and-magnitude; I'd be
tracking zeros being shifted off instead of ones, which it isn't going to
change the rounding algorithm, just the implementation.

-- Charles

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