64-bit floating point

Norsk Data 64-bit double precision format: Exponent has a bias of +256. (Subtract 256 to get true exponent)

Special values for exponent:

Mantissa has an implicit 55th bit of 1 (unless mantissa is 0). This is the 0.5 bit, i.e. mantissa is normalised to be 0.5 < mantissa < 1.0

Decimal point is thus to the left of the implicit 55th bit.

Unlike IEEE the ND 64-bit double precision format is just the 32-bit single precision format extended with more bits in the mantissa.

The 64-bit floating point format is supported in hardware on ND-500/5000 models with FPU hardware. On the ND-100 double precision floating point processing had to be done in software.