[libre-riscv-dev] [Bug 44] IEEE754 FPU inverse-sqrt

[bugzilla-daemon at libre-riscv.org]

http://bugs.libre-riscv.org/show_bug.cgi?id=44

--- Comment #16 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Jacob Lifshay from comment #15)
> (In reply to Luke Kenneth Casson Leighton from comment #14)
> > looking at the diagram from ep08-21.pdf on page 36 (figure 3.15)
> > 
> > * q[j+1] in radix 2 is a 2-bit signed binary number, -1 0 or +1
> >   (top of p35), in binary -1 = 0b10, 0 = 0b00, 1 = 0b01
> shouldn't that be -1 = 0b11 assuming it's in 2s complement?

err... the paper does say -1, 0 or +1 (as does Tanako), this will need
to be checked.

> > 
> > * |q[j+1]| probably means "absolute value" of q[j+1], in other words
> >   when q[j+1] == 0b00, return 0b00, and when q[j+1] == 0b10 (-1),
> >   return 0b10 (+1).
> I think you meant 0b11 (-1), return 0b01 (+1). 
> Even if they're not 2s complement, 1 and -1 shouldn't be identical.

yep got it, well spotted.

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