Editing IBM hexadecimal floating-point (section)

== Double-precision 64-bit ==
The [[double-precision floating-point format|double-precision]] HFP format (called "long" by IBM) is the same as the "short" format except that the fraction field is wider and the double-precision number is stored in a double word (8 bytes):

:{|
|- style="text-align:center"
|style="width:20px"|1
|style="width:20px"|
|style="width:50px"|7
|style="width:20px"|
|style="width:20px"|
|style="width:320px"|56
|style="width:20px"|
|style="text-align:left"|''(width in bits)''
|- style="text-align:center"
|colspan="1" style="text-align:center;background-color:#FC9"|S
|colspan="3" style="text-align:center;background-color:#99F"|Exp
|colspan="3" style="text-align:center;background-color:#9F9"|Fraction
|colspan="1" style="text-align:center;background-color:#FFF"|&nbsp;
|- style="text-align:center"
|63
|62
|...
|56
|55
|...
|0
|align="left"|''(bit index)''*
|-
|colspan="8"| ''* IBM documentation numbers the bits from left to right, so that the most significant bit is designated as bit number 0.''
|}

The exponent for this format covers only about a quarter of the range as the corresponding IEEE binary format.

14 hexadecimal digits of precision is roughly equivalent to 17 decimal digits. A conversion of double precision hexadecimal float to decimal string would require at least 18 significant digits in order to convert back to the same hexadecimal float value.