[libre-riscv-dev] [Bug 74] preliminary exploratory software emulation of FP SQRT

[bugzilla-daemon at libre-riscv.org]

http://bugs.libre-riscv.org/show_bug.cgi?id=74

--- Comment #43 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Aleksandar Kostovic from comment #42)
> >i added some comments that will help you to transition it from the
> > IEEE754 format that z is in, into s/e/m format.
> Thanks very much. I think i understood ;)
> 
> https://git.libre-riscv.org/?p=ieee754fpu.git;a=blobdiff;f=src/ieee754/
> fpsqrt/fsqrt.py;h=f7410703643e763c0ad5c43c57e6c39b3c12e71e;
> hp=5ae2bb253fa1356ec89f2960930b884faf51a2db;
> hb=75f9bd28e14c789872060997a655335bbdaed1aa;
> hpb=5690d1f94f158cbc8e715b9943d3eaba32cec9e3
> 
> Take a look

        s = 1           # sign (so, s=1)
        e = 255         # exponent (minus 128, so e = 127

note the comment that the format of IEEE offsets by 128?  so when this value
255 goes through the IEEE format processing, 128 will be added and the value
attempted to be stored will be 255+128.


        m = 1<<22     # high bit of mantissa, so m = 1<<22 i think

good...

        m = 1

... so... err... why are you trying to then destroy the value?

        m = 1<<22     # rest of mantissa is zero, so m = 1<<22 is good.

... and then set it again....

        m = 0

... and then destroy it again?

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