Binary decimal format

description.txt

Bit 63 = sign

Subnormal 
0.0-9.999 999 999 999 999E-511
Stored as uint64

Normal fp
Bit 62-53 = exponent
Bit 52-0 = mantissa
Mantissa range 0-8 999 999 999 999 999
Add 1 000 000 000 000 000 to cover 1.0-9.99eX

Exponent 0 and 1 used for subnormal

Exponent 0x3FF used for ±infinity 

Remaining: 2-0x3FE (1021 values), subnormal adds 1 exponent

That is a range -511 to +510

Can make it symmetrical, -511 to +511 with some shifting. 

Subnormal values don't actually use the upper range of exponent = 1.
Infinity doesn't actually need the lower range of exponent 0x3FF.

So by taking away 0x00100000 00000000 the normal exponent can take values 1-0xFE, add 0 for subnormal and there are now 1023 values (-511 to 511)