Re: [hatari-devel] FPU update

[Andreas Grabher <andreas.grabher@xxxxxxxxxxxx>]

Am 26.01.2017 um 18:24 schrieb Toni Wilen <twilen@xxxxxxxxxx>:

>> A question has raised while checking if all unnormals are caught
>> before used for calculation: Theoretically it is possible to write an
>> unnormal number to an FP register using FMOVEM. Then using that
>> register as source (or destination for dyadic operations) will input
>> an unnormal value to the arithmetic operation. At least that
>> currently happens in emulation. I wonder how this is handled by real
>> hardware. Does anybody know?
>> 
>> It could be tested by moving an unnormal number like: 0x40000000
>> 0x08000000 0x00000000 to a register using FMOVEM and then applying
>> some arithmetic operation (e.g. FADD) on it with the second operand
>> being a normal number like 0x40000000 0x80000000 0x00000000.
>> Furthermore it would be interesting if it makes a difference if the
>> source or destination operand, or both operands are unnormal. In
>> addition to the result it would be interesting if any exception bit
>> is set (most likely operand error).
> 
> Quick test results:
> 
> FMOVEM does not do anything (as documented), FMOVEM to register and
> FMOVEM from same register back to memory: same bit pattern.
> 
> Any arithmetic operation seems to normalize it automatically. I tried
> FNEG, FABS, FADD (with another register containing zero), register
> content was normalized even if it was source for FADD.
> 
Thank you for the quick test! In this case I need to add that automatic normalization to fp_arithmetic.
I guess 68040/68060 will raise unimplemented datatype (pre) exception. That would make sense, wouldn't it?