64-bit floating point
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Norsk Data 64-bit double precision format:
| bit 63 | (1 bit) | Sign bit |
| bit 62-54 | (9 bits) | Exponent |
| bit 53-0 | (54 bits) | Mantissa |
Exponent has a bias of +256. (Subtract 256 to get true exponent)
Special values for exponent:
| All bits zero except sign bit: | Floating underflow. |
| 0 (binary 0, aka 0x00): | Zero |
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.