Jan-Erik Soderholm escreveu:
> solarwind wrote:
>   =

>> HI-TECH C is using 24 bit float (I'm having problems with their 32 bit
>> option) and in their manual it says they have a 15 bit mantissa and
>> the MSB is implied, therefore I should get at least 5 digits of
>> precision. When I compute sin 60 degrees, the answer should be
>> 0.866025404 but I'm only getting 0.8660xxx... where the x are
>> incorrect digits. Odd...
>>
>>     =

>
> Probaly "just enough" for the applications that
> the actual tools (PIC and the compiler) was ment
> for in the first place. That is, *not* a "calculator".
>
> I'd think that most calculators uses some fixed
> point BCD or decimal arithmetic to get a constant
> and predictable precision and known (or none?)
> rounding errors.
>
> Jan-Erik.
>   =

The old 8-bit personal computers used packed BCD floating point math. It
is a good option for a calculator because it will preserve the precision
of the user input.

For instance, a binary floating point type cannot represent 0.1 (one
tenth) exactly.
Try this:

long i;
float f;
for( i =3D 1000000, f =3D 0.0; i; i-- )
    f +=3D 0.1;
printf( "1000000 x 0.1 =3D %f", f );



With packed BCD floats you will get exactly two decimal digits per byte
in the mantissa.

Best regards,

Isaac

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