Re: [PATCH 1/2] softfloat: m68k: infinity is a valid encoding

[KONRAD Frederic]

Le 4/29/20 à 10:42 AM, Laurent Vivier a écrit :
> Le 28/04/2020 à 19:17, KONRAD Frederic a écrit :
> > The MC68881 say about infinities (3.2.4):
> >
> > "*For the extended precision format, the most significant bit of the
> > mantissa (the integer bit) is a don't care."
> >
> > https://www.nxp.com/docs/en/reference-manual/MC68881UM.pdf
>
> As we use 68040 I refer to:
>
> https://www.nxp.com/files-static/archives/doc/ref_manual/M68000PRM.pdf

[...]

> This is denormalized numbers and should generate an exception.
>
> I tried something like that in the past:
>
> https://patchew.org/QEMU/address@hidden/address@hidden/
>
> Pierre tried recently:
> https://patchew.org/QEMU/address@hidden/

Arg, yes that's almost the same!  Sorry Pierre I missed this one :(.

> See "1.6.2 Denormalized Numbers" in M68000 FAMILY PROGRAMMER’S REFERENCE
> MANUAL.
>
> "Since the extended-precision data format has an explicit integer bit, a
> number can be formatted with a nonzero exponent, less than the maximum
> value, and a zero integer bit. The IEEE 754 standard does not define a
> zero integer bit. Such a number is an unnormalized number. Hardware does
> not directly support denormalized and unnormalized numbers, but
> implicitly supports them by trapping them as unimplemented data types,
> allowing efficient conversion in software."
>
> But m68k FPU exceptions are not currently implemented in QEMU.

Hmm ok, I don't have any m68k with an FPU at hand for testing.  I just tested
with an other simulator and it seems to trap when I load the value in the
register.  So I'm probably chasing the wrong bug here.

Thanks for the tips Laurent!
Fred

> Thanks,
> Laurent