At 9:34 PM +1300 20/12/98, Osamu Shigematsu wrote:
>I have not read FB's BCD manual well, but IEEE special condition is a more
>complicated procedure, I think.
>
>Sure, the exponent which all of it bits are 1 means NAN (Not a number),
>however, this must be significant MUST NOT BE 0.
>If significant was 0, this means infinity.
>
>And one more, all bits of exponent is 0, normally this means 0 (zero), but
>this must be all bits significant also must be 0.
>if significant was not 0, this means not normalized, which is caused
>underflow of exponent, for example 1/infinity.

The code that I posted for converting an IEEE float to FB BCD handles these
cases correctly, except perhaps for Infinity and NaN (Not_a_Number), which
have no special representation in an FB BCD floating point variable. I
chose to represent both of them as 1E999. This is an arbitrary decision,
but I think not a silly one.

BTW, denormalised numbers (roughly from 1E-38 down to 1E-45) arise not from
1/Infinity but from subtraction of two very small and nearly-equal numbers
(such as 2.0001E-38 - 2E-38). This clever convention was adopted so that in
IEEE arithmetic A-B=0 if (and only if) A=B.

Robert Purves

>'convert IEEE 32 bit float to FB BCD double precision
>LOCAL FN IEEE2BCD#(IEEESing&)
> DIM expon, fr&, value#,shift
> expon = (IEEESing& AND &7F800000)>>23  ' bits 1-8
> fr&=IEEESing& AND &7FFFFF              ' bits 9-31
> SELECT expon
>  CASE 0
>   LONG IF fr&=0   'zero
>    value#=0#
>   XELSE    ' denormalised
>    'constant in next line is 2^-149
>    value#= fr& * 1.4012984643248e-45
>   END IF
>  CASE 255  'signed Infinity or NaN (can't represent; use Infinity)
>   value#=1e9999
>  CASE ELSE      'normalised
>   shift=expon-150
>   value#=fr&+8388608   ' 2^23
>   LONG IF shift>=0
>    value#=value#<<shift
>   XELSE
>    value#=value#>>-shift
>   END IF
> END SELECT
> IF IEEESing&<0 THEN value#=-value# ' sign
>END FN=value#